Appropriate technology for the treatment of wastewaters for small rural communities: report on a WHO meeting, Lyon, 7–11 June 1982

WORLD HEALTH ORI ORGANISATION MONDIAl ~\ - ' \.' 2\~ \1 ~q \ 6 ~ World Health Organization • Regional Office for Europe ~I Copenhagen EURO Reports and Studies 90 . - I j Appropriate technology for the treatment of wastewaters for small rural communities Report on a WHO meeting t 11 1 N OPY ORl'1NAIJ DUPLICATE I hL' llf FIL( No .?.".¢.f..l/1.& ,:, • The World Health Organization is a specialized agency of the United Nations with primary responsibility for international health matters and public health. Through this Organization, which was created in 1948, the health professions of some 160 countries exchange their knowledge and experience with the aim of making possible the attainment by all citizens of the world by the year 2000 of a level of health that will permit them to lead a socially and economically productive life. 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The views expressed in this publication are those of the participants in the meeting and do not necessarily represent the decisions or the stated policy of the World Health Organization. PRINTE D IN D E NMARK ISSN 0250-8710 CONTENTS Page Introduction .... ... ...... .. ... ...................................................................... .............. ... . Conventional sanitation systems....... ... ........................... ....... .. .... .... ...... ... ... ... 2 Standard methods ... ... ... ........................... ........ ....... ... ..................... .......... ........... 3 Natu ra l lagooning .... .. ... ............................. .... ..................................... ...... ....... .... 6 Land treatment .... .... ... ... .. ........ ..................... .. .. .......... ........... ....... ............. ........... 11 Modified processes .. .... ... . . . . ....... ... .. . .. . . . .... ... .. ... . . .. . . .... .. ... ... . .. .... .... .. .... .. .. . ......... ... 12 On-site waste disposa l .. .... ..... .. ..... ............... .. .. ...... .. .. ...... ..... .... ... ...... .. ....... .. ....... 13 Septic tanks and infiltrati on ................ .. ........ .. ........... ..... ... .................................. 14 Other methods ... .. .... ... .................................... .. ....... ........................ ....... ... ........... 22 Quality control .... .. ... ... ...... ....... .. ..... ... .. ...... ... .. .. ... ..... ..... ...... .... ..... ... ............. ........ 25 Protection of the waste di scharge environment .... .. ... .. ... ...... ... ... .. ..... .. ... ...... ... ...... 25 Health considerations .... ... ... ... ....... ................. .. .... .... ..... .. ... ... .... .... ...... ......... ........ 26 Reuse of wastewater and sludge in agricult ure.............................................. ...... .. 28 Operation and supervision of waste treatment facilities ... .. ... ... ...... ..... .... 29 Design of systems .... ... ............ .... ..... .. .. ...... .. ... .. ...... ... .............. ....... ...... ... ...... ....... 29 Operation of facilities ......... .......... ....... .......... .......... ........................ ....... ... ....... .... 30 Assistance to operators....... ... .. ....... ... ..... .. .. ... ............. .......................................... 31 Motiva tion of policy-makers ....... ..... ..... ..... .. .. .. ... ..... .... ... .... ... ...... ..... .. ... ...... ........ 32 Conclusions and recommendations on technical aspects ...... .. ................ 32 Choice of a sa nitation system .......................... ........................ ....................... ...... 32 Operation of waste treatment faci lities ..... ..... .. .... ... ... ...... ..... ....... ... ...... .. ...... ..... ... 33 Disinfection ....... .. ... .................................. .. ..... .... ................................... ... ........... 34 Training in the design of systems........... .... ....... .. ..... .......................................... ... 34 Recommendations for ac tion at the international level.......... .... ... ..... 35 Annex I Existing sanitation systems for small rural communities in Europe.............................................................................. 36 Annex 2 Standard methods of waste treatment for small communities ...................... ....................... .. ...................... 40 Annex 3 Comparison of sanitation technology options for small communities .................................... ..... .............. .............. 57 Annex 4 Environmental classification of excreted infections.. ......... 60 Annex 5 Participants .... .. ...... .......... ........ ... ... .... ...... .. .. ... .. .. ....... ...... 61 INTRODUCTION The meeting was organized by the WHO Regional Office for Europe in cooperat ion with the French Government, and was hosted by the National Centre for Agricultural Mechanization , Rural Engineering, Water and Forestry Management (CEMAGREF). It was convened as part of WHO's strategy for the International Drinking Water Supply and Sanitation Decade in pursuance of reso lution EUR/RC3 I/ R9 of the thirty-first ses- sion of the Regional Committee for Europe, whereby the Member States recommended the improvement and development of water supplies and sanitation in rural areas. This strategy, described in document EH E 82/29 , assigns first priority among the development tasks to appropriate technol- ogies for the treatment of wastes for the presently underserved communi- ties in rural and periurban areas. The group was composed of 21 participants from 11 countries of the WHO European Region (see Annex 5) and chaired by Mr D. Bal lay of the French Ministry of Agriculture. The discussions were conducted in Eng- lish and French, with simultaneous interpretation into each language . The group set out first to identify possibilities, limitations and con- ditions with regard to the application of appropriate technology for waste- water treatment for small rural communities. It a lso cons idered the quality control requirements that should be met in the treatment of efnuents, especia ll y from the health standpoint. Fina ll y, it examined problems in the operat ion of treatment plants and the training of the staff who have the main responsibility for ensuring that they function efficiently and without interruption. Before proceeding to the technical part of the discussions, the group heard a number of reports by participants describing the different types of sa nitation system encountered in the Region (see Annex I). In a ll countries of the Region, sanitat ion systems are designed mainly for waste treatment for urban communities, a nd include installations for: collection of wastes through a sewer network and their conveya nce to a treatment site; treatment of the wastes in a large plant permanently manned by a team of specialists; discharge of the trea tment effluents, usually into a river. Systems of this kind offer a satisfactory service and a high level of convenience to consumers in urban areas. In many countries similar systems have been developed for small rural communities. However, simply transferring an urban system to a rural area involves a number of difficulties; in particular: the cost of the sanitation service per consumer is much higher , as economies of scale are not possible; the treatment techniques are not suited to the educational level of the operators, and the plants rarely function with optimum efficiency. The problem of sanitation in rural areas therefore calls for appropriate technologies which should be developed in the light of the following considerations: the drawback of the small size of rural communities can be com- pensated by making optimum use of the natural potential: available land area, use of the soil and subsoil, agricultural reuse of waste- water or sludge; the range of sanitation options may be increased by using a combi- nation of individual sanitation and sewer systems; in some cases it may be worth while to use new technologies from the developing countries for sanitation in rural areas. CONVENTIONAL SANITATION SYSTEMS Such systems collect wastewater through a sewer network. It is then treated either by a conventional method (activated sludge, trickling filters, etc.) or an extensive or specifical ly rural method (natural lagoon- ing, land treatment) . The different waste treatment methods are described below (see also the comparative table in Annex 3). 2 Standard methods Such methods are simplified versions of those used in urban areas. In a conventional plant, wastewater is treated in several stages: physical pretreatment; primary settlement; biological treatment; secondary treatment; treatment of sludge collected from settlement tanks. By combining different types of equipment, a large number of treat- ment techniques may be used. The decision to use one or the other to deal with a particular sanitation problem must be made on a case-by-case basis , taking into account the local conditions including: characteristics of the efnuent: nature, concentration, fluctuations in now and load; degree of treatment required, depending on the waste discharge environment; the possibility of ensuring efficient operation of the system (in this respect consideration must also be given to the operational capacity and organization of the basic health services); the suitability of the site: climatic conditions, nature of the terrain and available land area, sociocultural characteristics; capita l costs. Annex 2 describes the waste treatment methods most common ly used in rural areas, with details of their applications and the factors governing their adoption and use. The following guidelines are app licable to treatment systems for rural areas. Pretreatment Even in small plants, the pretreatment unit should include at least a bar screen. The design of this device does not pose any particular problem. It is , however, important to ensure that it is reliable, i.e.: that it is designed with operational requirements in mind (ease of cleaning, inclusion of a screening trough and refuse storage con- tainer in the system); 3 that the bars in the screen are not too fine: a net spacing of 30-70 mm between the bars is preferable in most cases; that there is provision for shortcircuiting the system to prevent any overflow of effluent in the case of a temporary breakdown of the pretreatment process. Settlement The settlement process produces sludge that must then be treated. For this reason, at least where primary settlement is concerned, it is often worth while to combine this process with digestion (two-storey tanks - Imhoff or other). In cases where the secondary settlement is not carried out in a com- bined tank, consideration must be given: either to recirculation of the sludge to the top of the primary settlement tank; or to separate treatment ( on the spot, or in a central unit). Different combinations of primary or secondary settlement tanks and settlement-digestion tanks are possible, and the choice will depend on the nature of the site (gradient, presence of groundwater) and the cost of the operation (unless the equipment is standardized, two-storey tanks have the disadvantage of high cost). Biological treatment Three types of system have been found particularly suitable for wastewater treatment for small communities: trickling filters, biodiscs and modified processes, and the extended-aeration active-sludge method. The use of such systems should, however, meet the following criteria. Low- or medium-rate trickling filters. This system is particularly suit- able for wastewater treatment for very small communities. Its main dis- advantage is its high cost, but this can be offset by using rural construction techniques to which it lends itself quite well. At the top of the unit there should be an integral system for periodic flushing (syphon, tipping system) which ensures satisfactory dispersion of the effluent over the filter. The smaller the community, the more important this is. Recirculation of the effluent avoids drying of the filter and ensures more reliable treatment. It should be borne in mind, however, that recircu- lation requires the installation of a pump and the supply of power to 4 the treatment site (the other parts of the system operate without power since they are gravity-fed) . The organic load on the installation can vary from 0. 1 to 0.4 kg BOD5/m 3 of material/day, depending on the level of treatment of the effluent and the degree of stabilization required for the sludge. Biodiscs and modified processes. One of the main advantages of these systems is their modular design, which makes it easier to adjust them to increases in the number of users. They provide high quality treatment for an organic load of 7-15 g of BOD5/m 2 of disc/day. Extended-aeration active-sludge method. This process is very suitable for wastewater treatment for small communities - particularly the sequential systems in which secondary settlement takes place in one part of the aeration tank (Pasveer and similar). The treatment produces only a small quantity of sludge and also overcomes the constraints of primary settlement. At the same time it has a high energy cost compared with the other systems and its use calls for greater technical skill. Loading rates are between 0.05 and 0.1 kg of BOD/ kg of volatile solids/day, depending on the degree of sludge stabilization that is required . It should be noted that the loading rate decided upon will greatly affect the cost of construction of the system. It can be said in general, with regard to each of the treatment methods presented above, that the low-rate options seem best suited to the condi- tions in small rural communities . Sludge disposal Sludge treatment and removal are the operations that pose the greatest difficulty in small treatment plants, and preference should always be given to methods that reduce the number of such operations to a minimum, particularly the extended-aeration active-sludge method. Drying beds are frequently used prior to agricultural irrigation or dumping of dried sludge. The possibilities for using this technique and the sizing of the beds depend largely on the local climatic conditions. The current norms range from I m 2/2 users in the United Kingdom to I m2/25 users in Italy. Application of liquid sludge to agricultural land is an increasingly common technique. It requires installations for: temporary storage, in the case of sludge that is satisfactorily stabil- ized and applied to crops at little risk from the epidemiological standpoint (cereals, vines, etc.); 5 storage to enable stabilization and disinfection in cases where the sludge is applied to grazing land or vulnerable crops. A third possibility that may be envisaged is that of transferring the sludge to a central unit where inexpensive digestion (possibly including biogas production) or composting treatments may be carried out. Conclusion Convent iona l systems have been widely used in many European countries for wastewater treatment for small rural communit ies. They are sti ll suitable in many cases; before recommending them , however , it is essen- tial to ensure that the technica l resources needed to operate them are ava ilable . At the sa me time the high cost of se tting up such facilities is an obs tacle to their use . The cost may be lowered by using standard model designs, or standardized components. In that case, however, a preliminary check must be made on the quality and efficiency of the proposed equipment, and this will be the responsibility of the technical se rvices of the relevant authorities in each country. Natural lagooning Natural lagooning is a biological wastewater treatment process carried out in tanks where a constant level of water is maintained (oxidation pond). In the tanks, the pollutant loads are transformed concurrently with stabiliz- ation of the resulting sludge, through the action of microorganisms that a re generated in this environment (especially bacteria and algae). This technique has become increasingly popular for wastewater treatment in developing countries and in sma ll rural communities in Europe because of its: Design efficiency in destroying pathogens; simple operation; appropriateness for the rural environment. In Europe lagooning is carried out using many different techniques . Within this diversity, however, it is possible to identify three main principles: 6 nat ural lagooning is suitable for direct treatment of raw effl uents (a bar screen for pretreatment may be installed upstream oft he tanks); it is essentia l to di vide th e to ta l a rea of the installation into a t leas t three ta nks so as to produce efnu ent o f adequate qua lity fro m both the phys icochemical a nd the bac terio logical sta ndpoint s; the lagoons should be suffi cientl y watertight to ma inta in the desired water level, ta king into account both inOow (efnuent , ra in- wa ter) a nd losses (ra te o f discha rge , eva poration, infiltration). With regard to the dimensio ns of the ta nks, two types of sys tem should be considered : ae robic-a naero bic a nd full y aero bic. Aerobic-anaerobic sy stems (Fig. I) co nsist o f a se ri es of three to fi ve lagoons, a rranged as fo llows: a first anaerobic lagoon: depth : 2 to 4 m, theoretica l retention time: I to 5 days; a n aerobic-anae robic lagoon: depth: I to 1.5 m, theoretical retenti on time: 5 to 30 days; one to three aerobic lagoons: depth : I to 1.5 m, theoretica l retentio n time: 5 to IO days. Fully aerobic systems (Fig. 2) a re designed to produce aero bic con- ditions in a first , la rge lagoo n (50% of the total water surface in the insta ll a ti on) as well as more di ve rsified biocenoses in the remaining lagoons. Fo r thi s purpose it is so metimes considered useful to es ta blish aquatic root plants in the lagoons (m acrophyte as opposed to mic ro phyte lagoons). The dimensio ns of the lagoo ns will depend on the nature of the efnuent to be treated and the loca l climatic conditions. By way of illustratio n, the following dimensions have been fo und appropriate for temperate climatic conditions in France: first (microphyte) lagoon: depth : I to 1.4 m, organic load : 100 kg BOD/ ha of water surface, i.e. a bout 5 m2 per user, theoretical retention time: 30 days; second and third lagoons: surface a rea of each: 2.5 m2 per user. 7 Anaerobic system Fig . 1. Aero bi c-a na ero bi c lagoo n system r - 1 Aerob ic -anaerob ic system I l T 1 I l_~ _) Aerobic system u ~ --=o~ ____ c 1 1::1- Source. Kalbermatten, J .M . et al. A Planner's Guide. Appropriate Technology for Water Supp ly and Sanita ti on. Vol. 2 Washing ton. DC. World Bank. 1980. The insta ll at io ns consist either of microph yte lagoons averagi ng I min de pth o r macroph yte lagoons averaging 0.3 min depth , wi th dense growth o f aqua ti c pla nts (Scirpus. Phragmites, Typha, etc.) o r mi xed lagoons combining the cha racteri sti cs of the other two types. The no rms o n the d im ensio ns of the two types o f sys tem a re give n as an exa mple; o ther techniques requ ire different dimensio ns. The cho ice betwee n o ne system o r a nother may be based o n the fo llo wing co nsidera tio ns: 8 a naerobic sys tems a re es pecia ll y suit a ble fo r trea ting co ncentrated effluents (B0D 5 > 500mg/l); aerobic-a naero bic sys tems req uire less la nd tha n full y aero bic o nes; ma intena nce of a naero bic sys tems invo lves mo re frequent clea ns- ing, a nd the lagoons may p roduce o ffensive odo urs; macrophyte lagoons sho ul d no t be used in a reas where th ey may give ri se to breeding o f moslj uitos that wo uld constitute a nui:;a nce fo r the po pula tio n o r be vec to rs o f di seases . Fig . 2. Natural lagoon system - overhead and cross-sectional views Vegetation 1. Microphyte lagoon 2. Mixed lagoon 3. Macrophyte lagoon Source: Lagunage nature/ et lagunage aere. procedes d 'epuration des petites col- lectivites. Paris. CTGREF et Agence de Bassin Loire-Bretagne. 1979. 9 Construction Preliminary geotechnical and geological studies are needed to determine whether the site is suitable for lagooning. If the soil is not sufficiently impermeable to keep water in the lagoons, or if there is a danger of contamination of groundwater that is being or could be used for water supply, the lagoons should be made watertight. The most economical approach is to use local materials (clay), when there is a nearby source of supply. Other sealing techniques generally entail excessively high capital costs, which may mean that the lagooning option is abandoned. Embankment materials should be laid so as to achieve a satisfactory degree of stability and watertightness. Earth compaction is essential. The gradient of the embankment should be sufficiently low to prevent the sides falling away. Adequate protection against lapping of the water is also indispensable (planting of grass, enrockment, etc.). Applications Apart from the requirements mentioned above concerning the selection of a site (nature and area of land needed), the main obstacle to the expansion of natural lagooning is the fact that the process is sensitive to variations in temperature : in cold climates or harsh winters, the biological activity may fall to zero. However, even in these circumstances, the technique can be used to treat seasonal effluents (tourist areas). Also, the loss of efficiency in winter may be compensated by a greater dilution in the recipient water. At the same time, because of the improved efficiency during warm weather, it is possible, to a limited but nevertheless appreciable extent, to treat the wastes of a temporarily increased population in a tourist area without enlarging the lagoon system correspondingly. Natural lagooning is also an effective technique for microbial disinfec- tion. The results are particularly satisfactory in shallow lagoon systems. It therefore appears to be the most suitable technique for disinfection of effluents of small communities in cases where this is necessary (discharge of sewage near bathing or shellfish growing areas, epidemiological situ- ation). Furthermore, because they are of satisfactory bacteriological qual- ity, effluents from lagooning are more suitable for agricultural reuse (irrigation) than those produced by other methods. Some precautions must, however, be taken: there should be limitations with regard to the choice of crops, and the mode and duration of application. Maintenance Lagoons should be managed like other wastewater treatment facilities. Access to them should be restricted to maintenance staff and they should preferably be fenced off. IO The most important maintenance requirements involve keeping the access roads in good order, controlling vegetation on the embankments especially to prevent mosquito breeding, servicing the pretreatment units and, if necessary, periodically removing floating vegetation (which may be reused, particularly as fertilizer). Regular desludging should be carried out to prevent silting, at intervals which will depend on the design and mode of operation of the facility. The desludging procedure and the arrangements for disposal of the sludge should be specified at the design stage of the project. Conclusion Because it is suited to rural conditions, lagooning should always be con- sidered at the outset as an option for waste treatment for a small rural community. The method may prove inapplicable under the local con- ditions but it should not be ruled out without very good reason. It is also important to ensure that the community realizes that such systems, although designed for rural use, do require regular maintenance. Land treatment Land treatment involves using the purification and infiltration potential of existing or redeveloped land for sanitation purposes. However, it is not only a form of treatment but in most cases also a method of disposal (infiltration). A number ofland treatment techniques are suitable for small communities : irrigation through drainage trenches (surface irrigation); subsurface irrigation into existing or reconstituted soil (sand filters). These techniques are modified versions of individual sanitation methods, and will be described in the next chapter. Design The wastewater is infiltrated into a network of open trenches. Primary treatment is not essential from the technical standpoint since the trenches are readily accessible should silting occur. Only a bar screen seems absol- utely indispensable. In practice, however, the technique is normally used for secondary treatment. In this case primary treatment in a settlement- digestion tank is recommended, since septic tanks are not suitable (prob- lem of odour). The technique may also be used to supplement treatment in a conventional plant or lagooning. 11 No genera l rul e can be la id d o wn with regard to dimensio ns, which sho uld be wo rked o ut on a case-by-case bas is, bea ring the foll o wing po ints in mind : s il t ing is very ea~y to co rrect (by des ludging o r ma rking o ut the tre nches aga in ): met hods fo r sizi ng subsurfa ce irriga tio n sys tems (see nex t cha pter) furni sh undul y high va lues fo r thi s purpose. In prac ti ce, ma intena nce is required o nl y for the pretrea tment a nd poss ibly the prim a ry trea tment units. In the efflu ent di scha rge a rea itse lf a ll t ha t is need ed is to cut the vege ta ti o n in the trenches a nd des ludge them fro m time to time. Applications Surface irriga tio n is suit a ble fo r remo te rural co mmunities which ca nnot mee t high in vestm ent a nd o pera ting costs. It is a seco nda ry treatment tec hnique pa rti cul a rl y suited to sit es witho ut o utl ets a nd to co mmunities o f va riable size (to uri st a reas). Thus it is efficient in rura l co nditions a nd sho uld be conside red wh eneve r it see ms feasible, having regard to: the geo logica l and so il cha racteristi cs of the site; the deg ree o f pro tec tio n o f the groundwa ter; the prox imity of sources o f drinking-water. Modified processes Peat beds Attenti o n is aga in being pa id to the use o f pea t beds fo r trea ting the was tes of sma ll co mmunities. Further resea rch is needed to determine the effec- ti ve ness and suit a bilit y o f this process in rural areas. It s use may be considered , bea ring in mind the fo ll owing points : (a ) the wastewa ter must undergo prima ry treatment upstrea m of pea t beds (a se ttl ement-digestio n ta nk is suita ble for thi s purpose); 12 (b) co ntinuo us use o f pea t beds res ult s in rapid silting, i.e. fo rmatio n o f a mo re o r less impermeable scum o n the surface, silting in the beds themselves, and hence the following preca utions should be taken : a number of parallel beds sho uld be established so that some can be ta ken out o f service periodica lly for desilting, the surface of the beds must receive regula r attention (sca rificatio n), the pea t must be ch a nged at a ppro priate intervals; (c) va ri a ti o ns in th e humus content o f peat (i .e. whether it is li ght a nd o nl y slightl y deco mposed o r da rk a nd a t a mo re ad va nced stage o f deco m- pos itio n) a ffect its ca pac ity fo r in filt ra ti o n and purifica tio n, a nd hence full acco unt must be ta ken o f the type used when sizing the beds. Macrophy te techniques Studies have been ca rri ed o ut in severa l Europea n countri es o n the poss i- bil iti es fo r using the puri fica tio n potenti a l of ce rt a in aquati c roo t pl a nt s. The sys tems pro posed include: macro phyte lagoo ns (see F ig. 2); macro phyte tre nches: trenches pla nted with Scirpus o r Phragmites, which we re deve lo ped in the Netherlands fo r seco nda ry t rea tm ent of was tes of sma ll seasona l communities (ca mping sites ); macro phyte ta nks: drained wa tertight tanks pla nted with Scirpus, Phragmites o r iri ses a nd insta ll ed o n a filtering substra tum , which were develo ped in the Federa l Republic o f G erma ny fo r ma in o r seconda ry trea tment of wastes o f sma ll communities o r fo r inde- pendent systems; different des igns a ll ow fo r co mbina tio ns of ta nks in se ri es o r in para ll el. These systems p rovide a n adequate level o f trea tment. The o nl y ma in- tena nce in vo lved co nsists o f a tt enti o n to the aquati c pla nt s. Fuller deta il s of resea rch o n such methods wo uld , ho wever, be necessa ry to show how they sho uld be appl ied in sma ll rura l communities. ON-SITE WASTE DISPOSAL On-site was te di sposa l sys tems a re in co mmo n use a nd they rema in a n impo rtant mean s o f sa nit a tio n for small rural communities. Systems with septic ta nks a re by fa r the mos t frequent o ption . Technica l aspec ts o f th eir co rrect use were di scussed by the group in some detail. After trea tment in a septic tank , the effluent is usua ll y disc ha rged into the so il. T hus the satisfact ory o pera tio n o f these sys tems depends to a large extent o n the design a nd size of the infiltratio n unit s. Other techniques suitable for rural areas were al so studied . Th ey have mo re limited appli ca tion s than septic tanks in Europe but ma y be 13 appropriate in certain cases. Mention may be made in particular of techniques for localities without a piped water supply, on which consider- able research has been done in a number of developing countries in recent years. Septic tanks and infiltration This technique seems particularly suitable for treatment of the wastes of single households, institutions and small communities which have a water supply system. The associated treatment and effluent disposal system will depend on the nature of the soil and the requirement to protect the local groundwater and drinking-water sources: whenever it is feasible, drain- fields are the most simple and least expensive option. Design Septic tanks are built with watertight materials and normally comprise two compartments (see Fig. 3). Fig . 3. Design of a septic tank ventilation wastewater inlet ___ _ elbow plunger T plunger scum section sludge formation section partition wastewater outlet Source: L 'assam,ssement mdiv,duel - princ,pes et techniques actuelles. Paris. Ministere de !'Environnement et de Cadre de Vie et Agence de Bassin Loire-Bretagne, 1980 . 14 Both sullage and faecal wastewater are conveyed to the septic tank. In the first compartment the solid fraction settles to form a sludge layer which undergoes anaerobic digest ion. Further sedimentation as well as sedi- mentation of sludge that has been resuspended by peak flows takes place in the second compartment, which is generally half the size of the first. The effectiveness of the treatment depends on the local climate (and especially temperature) . BOD may be reduced by 30-50% and total sus- pended solids (TSS) by 50-70%. Generally speaking, however, the physico- chemical quality of the effluent is such that it cannot be discharged directly into surface water or an aquifer (cesspool, fissured subsoil). Most , if not all European countries have codes of practice for the sizing of septic tanks. There is considerable variation in their design rec- ommendations which may, at least partially, be explained by the different climatic and sociocultural conditions in each country. In any case, how- ever, it is accepted that the required size depends on the following factors: the influent wastewater flow; the retention time required for effective solids settlement; the sludge accumulation rate; the frequency of desludging. The influent wastewater flow depends on water consumption, and measures to reduce it (e.g. replacing conventional flush toilets by water- saving designs) allow a more economical sizing of the installation. The required retention time for effective solids settlement depends on the number of users; for instance, for tanks serving a single household it is usually taken as the equivalent of one day (i.e. a tank having a capacity of three days when empty). The sludge accumulation rate varies con- siderably, depending on climatic conditions, and ranges from 30 litres/ person/year in southern Europe to 70 litres/person/year in the north. The frequency of desludging depends on the rate of solids accumu- lation and the cost of emptying. According to the practice in different European countries, the recommended frequency of desludging varies from twice a year to once every four years, although yearly or twice-yearly intervals are usually advised. When the local conditions do not allow regular emptying, additional sludge storage capacity within the tank may be required. Subsurface irrigation systems Such systems involve the infiltration of wastewater into the soil through drains embedded in a filtering stratum. 15 Infiltration of wastewater into the soil is the best means of completing purification that has started in a septic tank, while allowing for very safe disposal of the effluent. The basic system is shown in Fig. 4. It could also include as additional options: a prefilter upstream of the distribution unit, serving as a precaution against silting of the drains since it is an indicator of the functioning of the septic tank : a flushing cistern to ensure better distribution of the wastewater in the treatment units. Effluent collector Fig. 4 . Shallow subsurface irrigation system Septic tank Drains Source: L ·assa1nissemem mdw1duel - prmc1pes et techniques ac1Uelles. Paris. Min - istere de l'Env1ronnement et du Cadre de Vie et Agence de Bassin Lo1re-Bretagne, 1980. 16 A subsurface treatment system ma y consist of a series of narrow (0.5 to I m) leaching trenches or one or more sand filters. The choice bet ween trenches or filters will depend on the nature of the soi l and the land immediately surrounding the sys tem (see Fig. 5). Fig . 5. Subsurface irrigation ,.___. embankment ---+ - • ' ' - - ■nticontamin■ntfilm - " ~....,""._---_--- :::::I or crushed stone /_,; ·:,:,r-77 r;--;·· ;; ...,___soil ~ Sand filter Source: L ·epandage des eaux usees domesr,ques. Etude prealable de 1·apt1tude des so ls et regles de d1mens1onnemen t des 1nstallat1ons . Paris. CTGRF. Etude No. 50. 1980. Trenches are more suitable when the terrain is not very permeable and is difficult to work on. They allow some storage of the effluent, the walls play a useful role in the infiltration process , and there is no need to move any equipment over the infiltration area. Sand filters are more compact and are particularly suitable when the soil is permeable, and when the site does not present any topographical problems or difficulties due to the presence of impermeable strata (risk of resurgence of the wastewater). To use these techniques, the local soil must have the necessary proper- ties for infiltration and purification of the wastewater. The soil must be sufficiently permeable to avoid the risk of stagnation or resurgence of wastewater and it must also be capable of purifying the septic tank effluent. The relative importance of the last requirement depends on the risk posed to any groundwater in the vicinity of the system. 17 Permea bility is conve nti o na ll y assessed by mea ns of clea n wa ter perco- la ti o n tes ts. Us ing thi s empiri ca l meth od it is possible: to assess the suitab ilit y of the soi l; to work o ut the size o f insta ll a ti o n required, in terms of the admi ss- ibl e wa ter press ure o r the required surface a rea per perso n se rved . T he va lid ity of th is a pproac h may be q ues tio ned sin ce in place of knowledge of t he actua l co nd itio ns gove rn ing the in filtra tio n of wastewa ter (whi ch has a silting effect) , the size of the insta lla ti o n is determined empiri- ca ll y a nd sta ti sti ca ll y fro m the result s o f a meas urement using clea n wa ter. Ex per ience has, however, shown tha t the method ca n be conside red re liab le, with ce rt a in prov isos as o utlined below. I. Meas urement of permeability must a lways be co mbined with so il stud ies showing: the thi ckness o f the so il stra tum ; the na ture of the subso il : impermea bl e rock , fi ssured roc k; th e presence of pe rma nent o r temporary groundwa ter (it is esse nti a l to ca rry o ut this in vesti ga tio n (presence of mottling) if the measure- ments a re made when the wa ter level is low). T hese studi es are a minimum requirement fo r assess ing the suita bility of the so il in cases where the ava il a ble mea ns do no t a ll ow the perfo rm a nce of permea bilit y tes ts. F urt hermo re , o nl y a des ign spec ia lis t wit h considera bl e loca l experi- ence wo ul d be ab le to ma ke gra in-size meas urement s o f the so il in place of pe rmea bility studi es, since there is no direct rela tio nship between gra in size a nd permea bil ity except in ex treme cases (sa nd , co mpac t clay). A method tha t co mbines a measurement of permea bility with soil stud ies in orde r to de termine the feas ibilit y a nd a ppro pria te size of p ro- posed ir riga tio n sys tems is now co ming into general use in Fra nce. 2. T he measurement o f permeability is conce rned o nl y with the hyd ra uli c as pec t of infi ltra ti o n o f the was tewa ter a nd no t the puri fica ti on ca pac ity of th e so il. It may th erefore a lso be necessa ry to conside r th e thi ckness a nd gra in size of the soil , the poss ible presence o f fissures which wo uld a ll o w ra pid subsurface move ment of the was tewa ter , a nd con- stra ints assoc ia ted with the ri sk to the ground wa ter. The suit a bility of so il fo r a n irriga tio n sys tem is often assessed by mea ns of studies rela ting to a give n group o f use rs: co llecti ve sys tem, housing esta te, in stitutio n . 18 Assessments are rarely undertaken, even in the minimum form pro- posed (soil studies), for a si ngle household. In some cases the preparation of maps showing the suitability of the soil for individual sanitation (on the basis of soil maps or studies) may be a worthwhile approach and provide a very useful planning tool. Alternatives to subsurface irrigation Certain more costly techniques may be used as alternatives to land treat- ment when the environment does not lend itself to the use of the methods described above, particularly: when unprotected groundwater is located near the surface; when the soil stratum is not sufficiently thick. Drained sandfilters (Fig. 6) are used when the soil is too impermeable or when groundwater at risk is located near the surface (0.5-1 m) . They should be used only in cases where the effluent can be discharged into the surface environment. It is preferable to use regularly sized grains of0.35 to I. 7 mm. A preliminary study will enable correct sizing of the filter. Undrained sand filters (Fig. 7) are a variant of the system described above and can be used when the land is not suitable for irrigation (soil stratum not sufficiently thick) but does allow infiltration of effluents after treatment (fissured substratum). Raised sand filters (Fig. 8) consist of a mound of sand placed on the natural terrain after levelling. The height of the mound should be about I m. This system may be appropriate if there is an aqu ifer close to the surface (depth of0.5 to Im) and if the effluent cannot be discharged into the environment. To use this technique the soil must be sufficiently permeable, and equipment to raise the effluent is necessary in many cases. Conclusion Combined septic tank/irrigation systems are being adopted increasingly for both individual santitation and sewerage (for up to 1000 people in some European countries). They should be used in accordance with the follow- ing guide lines: Maintenance and operation. Land treatment facilities are inexpensive to run and very easy to maintain. However, to ensure that they function satisfactori ly, desludging of the septic tanks must be carried out at the appropriate intervals (problem of possible silting of the drains) . 19 20 Fig. 6. Drain ed sand filter for a single household Distributor manhole Section "'-l.....- - . . . ~ . - .. . ... • Distribution pipe ipe Sand Gravel Crushed stone ,, Fig . 7 . Undrain ed sand filter gravel humus fissured soil Source: L ·assa,mssement mdw1duel - pnnc,pes et techntques actuelles. Paris. M 1n- 1stere de l'Env1 ron nement et du Cadre de Vie et Agence de Bassin Lo1re-Bretagne. 1980. septic tank Fig . 8 . Raised sand filter feeder chamber humus Source: L ·assain issemem in d1v1duel - prmc1p es et techniques actuelles. Pa ris. Min - is tere de !'Envi ro nnement et du Cadre de V ie et Ag ence de Bassi n Lo1 re-Bretag ne. 1980 . Quality of the effluent. Precise monitoring of the quality is possible only in the case of systems where the discharge of the effluent is localized (drained sand filters) . All irrigation systems, when correctly operated, provide satisfactory treatment from the physicochemical and bacterio- logical standpoint. The treatment facilities must, however, be located at some distance from drinking-water sources. A protective zone should be established in accordance with the hydrogeological characteristics of the site and the depth of the catchment. 21 Other methods This secti o n d esc ribes techniques which may be an a lt erna ti ve to the comb ined se ptic tank / irrigatio n sys tem, so me of which receive only part of the wastewater (su ll age or faeca l water), while o thers are suitable fo r sites that do not have a water suppl y. Cesspools In situations where none o f the efn uent ca n be di scha rged , the wastewa ter may be stored o n site in cesspoo ls, fo r subsequent remova l to a trea tment plant by a vacuum tanker a t regular interva ls. The following gu idelines sho uld be o bserved when using thi s system: it is important to ensure that the wa ll s o f the cesspool a re watertigh t; an o ptimum balance can be reached between the ca pacity of the cesspool and the frequency of emptying, taking into acco unt the local cos ts of construct io n a nd em ptying. Cesspoo ls a re nevertheless a costl y sa nita tio n option tha t sho uld be co nsidered o nl y as a las t resort. The cost may be lowered by: restr icting water consumptio n; wherever possible, providing separa te facilities for o n-site disposal of sull age, a nd usi ng the cesspool fo r faeca l water o nl y. Composting toilets These units consist either of a single compartment (continuous co mpos t- ing) o r a do ubl e compartment, one pa rt of which is used a lte rnately. Both types are water less toilets receiving o nl y excreta and they require organic ho useho ld wastes to obtain a suita bl e carbon-nitrogen bala nce and reduce the moisture co ntent to around 60% , for more efficient composting. Composting toilets have the advantage of producing no liquid efnuent a nd are the refore su itable for use in a reas where the discharge of efnuent , even after trea tment, might be a majo r haza rd (possibility of gro undwater po lluti o n , fragile aquatic ecosystems). They have , however, the following disadvantages : 22 shortcircuiting may occur in continuous day-to-day use , and the compost may be contaminated by untreated excreta; soc iocultural factors may militate against the reuse of exc reta. Compared with other methods, composting toilets require a high degree of user care and maintenance. It is therefore recommended that they be adopted as a rural sanitation option only after careful evaluation of all the factors involved. Biogas units As biogas yields from human excrement are relatively low, such units can be considered only for treating animal excrement, to which human wastes may be added (on-site treatment of effluents of a farm, for instance). However, even in these circumstances the technique may not be effective in cold climates or cold weather. Moreover, the sludge may still contain viable pathogens infective to both man and animals (Taenia, for instance) and precautions must be taken if it is to be reused. Ventilated improved pit latrines (Fig. 9) Screened pit latrines may be used in more remote areas without a piped water supply . In their simplest form they consist of a pit, a cover slab and a superstructure. Ventilation corrects two of the principal disadvantages assoc iated with traditional pit latrines, namely fly and odour nuisance. Ventilation is provided by a large diameter ( 100-200 mm) external vertical vent topped by a fly screen. Air is drawn from the storage pit and through the hole in the cover slab, thereby leaving the superstructure odour-free. The circulation of air is effective at wind speeds down to 0.5 mis. The vent pipe also has a major role in controlling flies, which are attracted to the pit where they lay eggs . After hatching, the adult insects, due to their phototropic nature, fly up the vent pipe, are trapped by the screen, and eventually fa ll down and die in the pit. In a three-month controlled trial in Zimbabwe, about 14000 flies were caught from an unventilated latrine, compared with only about 150 from a ventilated but otherwise identical latrine. The latrine may have a single pit, in which case it is dismantled and moved to another site when the pit is full, or twin pits, which are used and emptied alternately each year. After a year of storage, the sl udge does not present any major health hazard a nd may be applied to soi l. The pit size is determined by: the sludge accumulation rate (60 to 100 litres/person/year, accord- ing to climatic conditions (mean temperature)); the household size; the desired pit life (1-2 years in the case of twin pit latrines, and 5-10 years for the single pit version). 23 Fig . 9 . Ventilated improved pit latrine (cross sec ti on) dimensions in mm fly screen gc-jjh~=~ 100-200 mm_ vent pipe 100 pit 1000-- superstructure a, ::0 ·.: "' > 0 0 C") I I ~, Source: Kalbermatten, J.M. et al. A planner's gu,de: appropriate technoloyy for water supply and sa nitati on. Vol. 2 . Wa shing ton, DC , World Bank. 1980. 24 s Groundwater pollution may pose a health risk if the household obtains its water from an on-site well located too close to the pit latrine. The required distance between the well and the latrine depends on the local hydrogeology and soi l conditions. Finally, it is not recommended to use pit latrines in karst/limestone areas. Disposal of sullage In the case of sanitation systems which are not designed to treat sullage, such as pit latrines, a separate facility is required for its treatment or disposal. Sullage contains a much lower concentration of pathogens than faecal water, although its organic load may be relatively high. Depending on the risk to adjacent groundwater, the sullage may be disposed of through a soakage pit or by subsurface irrigation. In determining the size of the system, account should be taken of the fact that sullage has a lower silting effect. Before irrigation or discharge into a soakage pit, the sullage should undergo at least 12 or so hours of primary settlement, and it may also be channelled to the last compartment of a modified three-compartment septic tank. Small-bore sewers Small-bore sewers operate in the same way as conventional sewers in that the effluent is conveyed to a central treatment unit. In this case, however, the effluent is collected after treatment in a septic tank and consequently the cost of the sewers will be lower; in particular: they may be laid at a very shallow gradient, since the lower propor- tion of suspended solids in the wastewater means that it is unneces- sary to design the system to achieve self-cleansing velocities; they can be smaller in diameter. The effluent remains septic and may be further treated by anaerobic lagooning. When setting up a sewer system of this nature, it is important to arrange for desludging of septic tanks at the correct intervals. QUALITY CONTROL Protection of the waste discharge environment Protection of the waste discharge environment (watercourse, lake, aquifer, marine environment) is a key requirement of sanitation systems in some 25 areas. A number of countries have laid down effluent discharge standards which, depending on the legislation, relate to the type or size of treatment plant, or to the required qua lity of the waste discharge environment. The quality of the discharged effluent is measured in terms of its content of oxidizable organic matter (800 5 and COD tests) and of suspended solids. ln the case of discharge environments that are at greater risk (certain watercourses, aquifers used for water supplies, lakes), account must also be taken of the content of fertilizing minerals in the effluent (nitrogen and phosphorous compounds). Assessment of the potential impact of an efflu- ent on a particular environment does, however, require a knowledge of its quality and quantity in terms of pollutants , in relation to the absorption capacity of the environment. All the various sanitation options are not equally effective or reliable (see Annexes 2 and 3) and the degree of treatment to be achieved is a major factor in the choice of a method . An increase in water pressure in a facility during periods of rainfall (separate unit or drainage system) is one of the most common causes ofa temporary lowering of final effluent quality. In such cases it is necessary to install an overflow for discharge of the excess effluent into the environment, so as to ensure efficient operation of the treatment plant. To achieve greater reliability in the quality of the final effluent, it is necessary to: use treatment systems that are less sensitive to increases in water pressure (lagooning); build extra capacity into the system (e.g. to handle three times the dry weather load in the United Kingdom); incorporate settlement/storage units in the system (e.g. to handle up to ten times the dry weather load in the Federal Republic of Germany). Health considerations The principle public health hazard assoc iated with wastewater is that due to the substantial microbial content of faecal water and to a lesser extent sull age . The toxic risk cannot be wholly discounted but generally relates only to flows of nitrogen into adjacent groundwater, which may increase its nitrate concentration. 26 The microbio logical risk may be assessed in terms of: (a) the seriousness of the risk, which depends on: the nature of the microorganisms present in the wastewater: hepa- titis viruses, Shigel/a , Salmonella, Vibrio cholerae, Entamoeba dysen- teriae, or Giardia, which may cause severe infections in humans; the concept of the "minimum infective dose" according to which, all things being equal, a low concentration of cysts of parasites or viruses is more likely to cause an epidemic than the same low concentration of Salmonella or Escherichia coli; the survival and reproductive capacity of microorganisms, espe- cially in foodstuffs (this is an important factor in the case of on-site sanitation systems, since the effluent is discharged close to areas where household activities are carried out); (b) the frequency of the risk, in that the risk associated with effluents is not the same in areas affected by certain endemic or epidemic diseases, and in sedentary, remote communities sheltered from the usual outbreaks. Annex 4 gives a list of excreted infections. The pathogens are class ified according to their potential virulence. The health aspects of the transmission of diseases - especially diar- rhoea and dysentry - in relation to the bacteriological quality of water have been studied by a WHO workshop.a Risks of infection also arise both within treatment facilities (hence the need for protection of operators) and outside them, and are associated with : possible wastewater contamination of aquifers used for human water supplies; agricultural reuse of sludge which may contain highly persistent parasi tes ( e.g. Taenia) that can infect animals and then man . Disinfection At present techniques are available for bacterial disinfection, but as yet very little is known about their effectiveness against viruses. Moreover, they are never totally effective even against bacteria and it is always wise to bear in mind that all the microorganisms present in wastewater may reappear, at lower concentrations, in treated effluent. Improvement oft he bacteriological quality of final effluent, assessed in terms of a reduction of microbial indicators of contamination, may be advisable in some cases, particularly: when the final effluent is discharged into bathing or shellfish grow- ing areas or into groundwater used for human water supplies; a Surveillance and control of acute diarrhoeal diseases: report on a WHO meeting. Copenhagen, WHO Regiona l Office for Europe, 1981 (EURO Reports and Studies, No. 44). 27 when the final effluent is reused in agriculture; in certain epidemiological situations (endemic diseases, ep idemics). In the case of on-site sanitation or systems for small rural communities, adequate bacterial disinfection may be achieved using the following techniques: land treatment, provided the system is correctly planned on the basis of appropriate soil and hydrogeological studies; or natural lagooning, particularly where shallow lagoon systems are used . Chemical disinfection techniques (chlorination) are difficult to use, costly and not fully effective . They should be used in treatment systems for small communities only in very special circumstances, after ascertaining their operational feasibility (supplies of reagents, training of operators). Generally speaking it is more common to use chemical disinfection units in drinking-water treatment systems. Reuse of wastewater and sludge in agriculture If the proper precautions are taken, recycling of "by-products" of the treatment is of value economically and contributes to environmental pro- tection. The recovery of the wastes does , however, entail a potential public health risk associated with the contamination of soil, agricultural crops or groundwater. To minimize the risk in effluent reuse schemes, account must be taken at the outset of certain factors such as the nature of the terrain, the chemical composition and bacteriological quality of the wastewater and sludge, the proposed crop and the mode of application. The preliminary study of the terrain, with a view to protecting any adjacent groundwater, may be carried out using the methods developed for assessing the suit- ability of soil for irrigation systems (see page 11). Any wastewater reused in agriculture must have a sufficiently low concentration of organic matter and suspended so lids (silting of irrigation systems or soil) and of dissolved salts (toxicity for crops, damage to the structure of certain types of soil). Similarly, it should not contain toxic concentrations of organic or metallic micropollutants. Measurement of the nitrogen or phosphorous content may be of value for monitoring the fertilization effect. The optimum application rate may also be determined by measuring the nitrogen content o f the sludge. The heavy metal content must a lso be monitored. Natural lagooning, which produces a satisfactory level of bacterial disin- fection, may be considered a suitable treatment technique for wastewater 28 tha t is to be reused in agr icul ture. However, the techn ique is not re liab le in re mov ing viruses a nd res ista nt forms o f paras ites . At a la ter stage a tt en ti on will have to be give n to the ques tio n of t he d ura ti o n o f microbia l resis ta nce in soi l a nd o n plan ts (Asca ris eggs ca n survive fo r over a yea r in so il , a nd Salmonella fo r up to six mo nths o n pla nt s). The surviva l times in sludge must a lso be bo rne in mind when it is applied to crops. T he ri sks o f bacte ri a l conta mina tion ca n be grea tl y reduced by a n a ppro pria te cho ice o f the type o f crop to irriga te (pl a nt s not directl y consumed by ma n o r a nim als) a nd the fo rm o f irriga tio n (sp rinklin g o n pa rt s o f the pla nt tha t a re not consumed ). Industri a l crops (cotto n, Oax, suga r bee t, t rees fo r timber) are less subject to bacteria l co nta mina tio n tha n food crops a nd , a mo ng the la tt er, ce rea ls a nd fru it t rees a re less vulnera ble tha n vege ta bles or pas ture. As fa r as the mode of irriga tio n is concerned , gravity irriga tion is fa r prefera ble to sprinkling. In a ny case systems fo r agricultu ra l re use of the by-products of t rea t- ment sho uld be established with in a regula to ry fra mework , a nd there sho uld be provisio n fo r mo nitoring a nd in fo rmatio n o f use rs. OPERATION AND S UPERVISION OF WASTE TR EATM ENT FAC ILITI ES Whateve r technique is used , pro per ma intena nce is necessa ry fo r the effi cient opera tio n o f a trea tment fac ility. Bea ring in mind the particula r co nditio ns in ru ra l a reas, the group reco mmended the use of sys tems tha t a re as simple to o pera te as poss ib le. Neve rtheless, a n essentia l minimum of ma intena nce must be ca rri ed o ut. In ma ny Euro pea n countri es the situ- atio n is not a ltogether sa tisfacto ry a t present a nd , to improve it , effort s must be made a t a number o f leve ls: des ign o f sys tems; o pera tio n of fac ilities; assista nce to o pera to rs; motivatio n of po li cy-ma kers. These effo rts sho uld prefera bl y ta ke the form of na tiona l initia tives whose na ture will depend o n the po litica l a nd eco no mic sys tems of the co untry concerned . Design of systems At the des ign stage of a sys tem, the technica l se rvi ces must ma ke a n o bjective lo ng-term assess ment of the mo ti va tio n o f the co mmunity, it s 29 resou rces in ma npo wer and funds, the q ua lifica tio ns and tra ining potentia l of the staff, a nd the capacity of the enviro nment to absorb the by-prod ucts of the t reatmen t process . T hey m ust a lso ensure tha t the system which has been des igned ta king t hL·se fac to rs int o acco unt is acce pta ble to the a uth- ori ti es: fa iling this there is a ri sk tha t it will fa ll into gradual a nd irrevers ible uis use . Furthermo re, even a spec ifica ll y ru ra l system may be di fficult to o per- a te if it has no t bee n des igned so as to: a ll ow sa ti sfacto ry access to the different insta ll a tio ns a nd the regu- la tory equipment ; functi o n with the use of sim ple p roced ures a nd rea dil y ava il able local suppli es of co nsuma ble ma te ria ls; ensure the phys ica l safety of the o pera to rs (ra ilings, emerge ncy switc h for electri ca l a nd mecha nica l insta ll a tio ns, etc. ); ena ble the o pera to rs to wo rk in suit a bl y hygienic conditi ons (water o utl et fo r was hing, ha ndling o f wastes ). Pa rti cula r a ttentio n sho uld be pa id to these as pec ts when sta nda rd model des igns o r sys tems using sta nda rdi zed co mpo nents a re being con- side red . In thi s case a rra nge ments must be made for prio r testing a nd ha rmo niza tio n . It is beco ming increas ingly diffi cult to mo nito r the o pera ti o n of o n-site sa nita ti o n systems. Use rs sho uld where poss ible be co nsult ed a bo ut the ma intena nce requirements a t the des ign stage . If there is a suffi cientl y la rge number of o n-site trea tment sys tems in a pa rti cula r loca lit y, it may be worth whil e to o rga nize a communa l sys tem fo r opera tio ns such as empty- ing o f septi c ta nks. Operation of facilities Ro utine ma intena nce of trea tment fac ilities ca lls for sta ff who have rece ived middle-level technica l training, a nd who are mo tiva ted and ava il- a ble; thus there is a need to enha nce the prestige of thi s work . Ro utine o pera ti o n consists mai nl y of superv isio n, regulatio n , ma intena nce a nd clea ning o f the insta ll a tio ns, includ ing mino r repa irs a nd the removal of was tes (screenings, sludge). A log boo k is useful for reco rding these o per- a tio ns as well as obse rva ti o ns a bou t the running of the pla nt. Supervisio n a nd regula tio n in volve the perfo rmance of simple tes ts to mo nito r the o perati o n of the system a nd determine whether certa in pro- cedures a re necessary (dra wing o ff excess sludge, regula tion of fl o w, etc. ); the o pera to rs sho uld therefo re be give n spec ia li zed training in wastev1a ter trea tment generall y a nd in the o pera tio n of t he pa rticular sys tem used . Both 30 organized courses and training visits are suitable for this purpose. In any case simple, practical handbooks should be distributed. As far as possible, the staff should be available at all times and be assigned permanently to the same plant. Since the maintenance of small facilities does not require full-time staff it is possible, where local conditions allow (associations of communities, operation of public services by a corporation, etc.), for the same operator to supervise several plants in the same area, thereby improving his experi- ence and technical expertise. Action on the present recommendations should ensure the necessary motivation of operators, particularly in cases where the local sociocultural attitudes attach little prestige to any work connected with waste disposal. It is recommended that operators should live in the area concerned as it is felt that they are in the best position to know about the functioning of the system and the local practices: it is therefore easier for them to understand variations in the effluent, to take action if the system breaks down, to develop waste disposal methods, etc. In this way they will also be able to function as health workers in the community. The operation of a treatment plant, however simple, may nevertheless require high-level professional support from time to time. The establish- ment of technical services that can meet the needs of a large area is recommended for this purpose. Where the local conditions permit, special- ists from nearly urban treatment plants may be asked to perform this function. Assistance to operators The technical service should: provide training for operators; come to the assistance of operators at their request in cases of difficulty; perform in-depth operational assessments when necessary; collect and analyse data on the performance of treatment plants in order to improve skills and training; supply information to administrators responsible for capital invest- ment policy. The technical expertise required for this purpose includes a thorough knowledge of chemistry, biology, hydraulics and electromechanics. The service should have a flexible form of organization enabling it to provide on-site assistance (measurement and sampling equipment that is easy to transport and suitable for different purposes, etc.) while the 31 opera tors should be give n the necessary testing materi a ls, training and information. If it is to be acceptable to the local authorities, the se rvice shou ld a lways seek to func tion in an advisory rather than a superv iso ry capacity. The estab li shment o r a se rvice of thi s kind presupposes that the countries will first set up the necessary training facilities (universities, specialized schoo ls) or wi ll use the programm es in o ther countries . In de ve lo ping such train in g syste ms it may be advisable to use the resources of in stitut ions in ot her fields (local au thorit y staff training centres in so me co untries, associ- a tio ns of technici a ns). Motivation of policy-makers Renewa l of the materia ls a nd equipment used in treatment plants, as well as improvements in the light of experience and technological advances are essential for continuous and efficient o peration of a treatment plant. It is importa nt to mak e policy-makers aware of this need so that they will be prepared , in so far as economic conditions and priorities allow, to auth- orize the required investments . CONCLUSIONS AND RECOMMENDATIONS ON TECHNICAL ASPECTS Choice of a sanitation system I. Th e use o f a combination of on-site sanitation and sewer systems, ta king into account the characteristics of the environment, the pattern of ho using and future development prospects, is a promising approach to the provision o f wastewater treatment for small rural communities and the achievement of the goals of the International Drinking Water Supply and Sanitation Decade in Europe. 2. At the design stage of a sanitation sys tem , account must be taken oft he form of water supply and water co nsumption. Reducing the amounts of water used for the disposal of faecal matter facilitates and reduces the cost of its treatment. Similarly, consideration may be given to trea ting faecal water and sullage separately. 3. On-site sanitation plays and will continue to play an important role, especially in the form of combined septic tank/subsurface irrigation systems. However, many existing on-site sanitation systems are not altogether 32 satisfactory . Resea rch progra mmes sho uld be established to improve a nd refin e the design a nd sizing of such sys tems. New techno logies have emerged in the developing countri es a nd it see ms wo rth while to consider a nd even promo te their use in Eu ro pe in ce rtain situations (ventil a ted improved pit la trines). 4. Since na tural lagooning invo lves the most simple o pera ting pro- cedures and provides adequate microbia l di sinfectio n, especia ll y in sha l- lo w lagoon sys tems, it sho uld always be co nsidered as a n o ptio n fo r sma ll communities; it may be rejec ted beca use of loca l conditio ns such as cli - mate, to pograph y, so il co mpositio n, ava ilable land , etc ., but the reasons sho uld be co mpelling. 5. In view of recent develo pments in la nd treatment and infiltratio n techniques, co nsidera tio n sho uld be given to using th em for seconda ry trea tment. Whereve r poss ible, subject to appro pria te sizing of the insta l- la tio n, their use depends o n: the geo logica l a nd so il cha rac teri stics o f the site; the degree of protectio n of the groundwa ter; the p ro ximit y of sources of drinking-water (account must be ta ken o f the depth o f the ca tchment as well as the d ista nce betwee n the trea tment pla nt a nd the point of d raw-off of the wa ter). 6. If aft er studying the d iffe rent poss ibilities it is dec ided to use a conven- tio nal trea tment technique, it is preferable to o pt fo r a sta nda rd model des ign, or o ne with sta nda rd ized co mpo nents, whose q ua lity a nd effic iency have been determined in ad va nce . This ass umes that the a uth o riti es will a rra nge to mo nito r th e perfo rma nce o f sma ll pla nts a nd ma ke the res ult s widely known - whether sa ti sfacto ry o r unsa ti sfac to ry. Operation of waste treatment facilities 7. The effici ency o f waste trea tment pl a nts fo r small rural co mmunities is o ft en dependent o n o pera ti o nal facto rs. It is therefore importa nt to co n- sider the o perating conditi o ns when choos ing a trea tment system. In the past , operational co nstraints have bee n disrega rded in the design o f plants; in the future it is essentia l tha t they be ta ken into acco unt fro m the outse t. 8. Operators of sma ll trea tment pla nts in rura l a reas generall y wo rk pa rt time. They must therefore have access to professio nal suppo rt when needed. This can be provided either by specialists from nearby larger treatment plants or by a n o rganized technical se rvice for a given geo- graphical a rea containing a la rge number of plants. It is a lso important to 33 consult these technical services when organizing training courses for staff of small treatment plants. 9. Sludge handling and removal are probably the operations that pose the greatest difficulty for small plants . The form of management must be planned from the outset and the necessary technical arrangements made. Disinfection 10. Disinfection of waste effluents may be advisable in some cases, particularly: in certain epidemiological situations (endemic diseases, epidemics); when the effluent is discharged into bathing or shellfish culture areas; when the final effluent is discharged into groundwater (need for protection); when the final effluent is reused. In such cases it is preferable to use systems with lagoons. 11. Chemical disinfection of effluents from small treatment plants can only rarely be justified. Chlorination should be regarded as part of the drinking-water treatment process and not as a means of reducing the numbers of pathogens in the environment. Moreover, chemical disinfec- tion is still an expensive and difficult process, which generates undesirable by-products in the environment. 12. It should be borne in mind that it is almost impossible to achieve total disinfection of effluents: in particular, there is no guarantee that certain viruses and parasites will be destroyed, whatever the technique used (lagooning or chemical disinfection). Training in the design of systems 13. The most effective and economical systems of sanitation for small communities are generally those that make optimum use of the natural features of the site. An analysis of the socioeconomic, soil, geological and hydrological characteristics of the area is therefore an essential step in the design of a system, and sanitarians should receive special training for this purpose. 34 RECOMMENDATIONS FOR ACTION AT THE INTERNATIONAL LEVEL I. WHO should e~tablish a collaborating centre for rural sanitation. 2. A working group should be convened to discuss basic and further training for operators, as there is a need to bring the training into line with practice. 3. The correct runctioni ng of an individual sanitation system involves the regular performance or operations such as desludging or septic tanks. WHO could collate the different experience of th e organization and financ- ing or such operations. 4. WHO should promote reciprocal exchanges of information on rural sanitation technology bet ween the developed and the developing countries. 5. WHO should encourage Member States to develop infrastructure and organ ize systems that will show what results can be expected in th e oper- a tion, supervision and eva luation of the effec tiveness of small waste treat- ment plants in rural areas. 6. Numerous studies should be undertaken on the hygienic aspects of the reuse of waste efnuents in agriculture. 7. WHO sho uld support studies on the health effects of the dispersion of pathogens in the environment. 35 Annex I EXISTING SANITATION SYSTEMS FOR SMALL RURAL COMMUNITIES IN EUROPE Participants described a variety of sanitation systems that have been adopted for small rural communities, and this information is summarized in the first part of the Annex; the second part describes a number of features of sanitation systems that are common to the different countries. Situation in different European countries Czechoslovakia Some 27% of the population live in communities of less than 2000 people, which may be rural or industrial, and 58% are served by a sewer system (mainly in towns). Priority is now being given to the construction of treatment plants for communities of 100 to 5000 people. For this purpose standardized treat- ment units (using biodiscs) have been proposed in order to reduce the costs of research and development and of production. The installation of these facilities is supervised by the responsible technical services. Most of the treatment plants are operated by a public agency under the Ministry of Water and Forests. France The rural sector comprises a settled population of 20 million and a seasonal population of 9 million (second homes, camping sites, etc.), 40% of whom are served by wastewater collection and treatment systems. The aim is to achieve a coverage of 70% in 15 years. Individual systems should serve 30% of the rural population. A separate authorization is required for the discharge of all waste effluent, and it must comply with minimum quality standards depending on the nature of the discharge environment. The regulations provide for six levels of treatment for oxidizable matter, and several for nitrogen and phosphorous compounds. The smaller plants generally use activated sludge or extended-aeration active-sludge techniques. At the same time there has been a considerable increase in lagoon treatment. District services provide technical support and supervision of the treatment plants, supervise their operation, and carry out four inspections 36 per year. Many individual san itation systems are not wholly satisfactory, and steps are now being taken to specify operating conditions for them (Ministry of Agriculture, Ministry of the Environment). Germany. Federal Republic of About 25% of the population live in communities ofless than IO 000 people, and 3% in communities ofless than 1000. At present 71 % of the population are served by a sewer system and 51 % have a piped, treated water supply. Current development is directed to increasing the number of treatment facilities and improving the quality of efnuents. The present standards call for efnuent qualities of varying levels, depending on the size of the facility. Professional associations of sanitarians (A TV) assis t in the design and operation of facilities in the following ways: by publishing handbooks on the design of plants to serve different numbers of users; by training operators and providing technical support (within a given region close cooperation is maintained between the staff of large treatment plants and those of small facilities). Making allowance for stormwater is one of the problems encountered in setting quality standards for watercourses . In place of stormwater overnows, which used to be widespread, the present tendency is to install storage facilities (with a capacity ofup to ten times the dry weather now). Hungary In Hungary 34% of the population are served by a sewer system, including only 3.3% in rural areas. A great many dwellings have individual sani- tation systems, generally consisting of septic tanks or pit latrines. This situation is not wholly satisfactory, for the fo ll owing reasons: checking of such a large number of faci lit ies, which is a responsibil- ity of the Housing Board, is impossible; the hydrogeological conditions in a large part of the country are highly unsuitable for subsurface irrigation (groundwater near the surface, impermeable soil) and the present situation is leading to a significant level of pollution of the groundwater. The present tendency is to install: sewer systems and treatment units for villages; more carefu ll y supervised individual sanitation systems (combined septic tanks/irrigation). 37 The most common treatment systems are trickling filters for low popu- lation densities, and oxidation ditches. The latter system has proved par- ti cu larly reliable under the local operating conditions. Activated sludge sy~tems pose greater operational problems in rural areas. Lagoon treat- ment has been tried out, but the system does not seem well suited to the local climatic conditions. Portugal The population is 57% rural, with 2.3% not living in communities. In 1980, 40% of people were served by a sewer system and 10% by a system incorporating a treatment facility. At present it is mainly the urban popu- lation that is served by such systems, i.e. those living in towns of more than 2000 people. In rural areas only 28% of the population have a piped water supply. Consequently the sanitation problem in rural areas is very different from that in towns and in certain respects the situation is similar to that in the developing countries. The current targets give priority to water supplies and sanitation for large towns. In urban areas a coverage rate of 80% has been set within the framework of the International Drinking Water Supply and Sanitation Decade. From the technical standpoint the tendency is to install sewer systems - except for single households - and to use simple operating methods. A study is currently being made of the use of lagoons and oxidation ditches, and it is planned to develop land treatment. Major problems include technical training of operators and arrangements for technical operation of systems. United Kingdom Some 95% of the population are served by sewer systems, and the remain- ing 5% by individual systems or private facilities (camping sites, etc.). Out of the 7800 existing sanitation systems in the country: 4000 serve up to 500 people; I 000 serve 500 to I 000 people; 1500 serve I 000 to 5000 people; 1300 serve more than 5000 people. Considerable progress in wastewater treatment, monitoring of the operation of facilities by river inspectors, and efforts to improve coordi- nation between the setting up and operation of systems on the part of the river basin authorities, have resulted in a definite improvement in the quality of surface waters. These advances have been achieved within the 38 framework of a Code of Practice that was introduced in 1974 and is now being updated. The Code lays down technical criteria for the design and operation of treatment plants. Common features of sanitation systems in different countries Because of the variety of situations encountered, it is not possible to give a very precise definition of a "small rural community". However, for stat- istical purposes it has generally been agreed to adopt an upper population limit of 2000. Such communities also have the following features: a lower level of facilities in relation to urban areas, both for sani- tation and for water supply systems; a variable, but never negligible, proportion of single households; in many cases, polluting agricultural processing activities or, in some, small-scale industrial activities; limited technical and financial resources and organizational capacity. The extent of coverage of the needs for water supplies and sanitation varies considerably from country to country. However, the sanitation programmes depend on the water supply programmes which in any case always have priority. Finally, where sanitation is concerned, problems in the running of small facilities are always recognized as being a priority, and they relate both to technical training of staff and to operational and supervisory arrangements. 39 Annex 2 STANDARD METHODS OF WASTE TREATMENT FOR SMALL COMMUNITIES PRETREATMENT UNITS Features Pretreatment of domestic wastewater is carried out using techniques for physical separation of bulky objects, scum and undesirable materials. It is essential to install pretreatment units upstream of treatment plants to protect the equipment (valves, pumps, pipes, etc.) against stoppages and unnecessary wear. For communities with a small treatment plant that does not require a full-time operator, only the following forms of pretreatment should be envisaged: manual screening; grit removal; flotation. Bar screen First screen The purpose of this screen is to prevent rapid clogging of the main screen by bulky objects. The spacing of the bars should be 50-100 mm. This device is optional for small plants. Main screen The bars are generally installed at an angle of 60 ° or 80 ° from the horizontal. In the case of small communities, a coarse bar screen is prefer- able (net spacing of 30-70 mm between the bars). If there is a storm water overflow, the bar screen should be arranged so as not to hinder its operation. A device for by-passing the pretreatment unit is useful to ensure a continuous flow of wastewater to the treatment plant in the case of accidental clogging of the bar screen and thereby avoid the discharge of raw effluent into the environment. It is essential to include a screening trough and refuse storage container in the system. 40 Fi g. 1. M anu all y o perated bar sc reen Bars: 30 - 70 mm apart "" Screening trough Source· The opera1,on and mam1enance of small sewage works. National Wa ter Counc il . Occasional Technical Paper No 4 Standing Techn ical Commi ttee on Waste Wa ter Tr ea tmen t. London. 1 980 . Grit removal Wh en it is necessa ry to include a grit cha mber in the system, it may take th e fo rm o f a sma ll , easy to cl ea n unit to co ll ect g rit a nd sto nes. In ma ny cases ex tra ca pac it y may ha ve to be built int o the sys tem to ena ble it to cope e ffectivel y with large nuctuati o ns in th e waste water n o ws , which in creases the o perating cost. Fi g. 2. Grit c hamber Handstop Source: The operauon and mamrenance of small sewage works. Natio nal Wate r Cou ncil. Occasional Technical Paper No. 4 Standing Technical Commit tee on Waste Water Trea t men t. London. 1980. 41 It is preferable to dispe nse with such units in treatment plants for ve ry small communit ies . Flotation When a flotation system is necessary it may cons ist of a simple scum board installed near the wastewater inlet. The dev ice shown below by way of illustration may be located upstream of a lagoon. Influent pipe Anchor Fig . 3. Simpl e grease tr ap Source: Lagunage na1urel e1 lagunage aere. procedes d "epurauon des peu1es col - lec/lvl/es. Par,s. CTGREF et Age nce de Bass ,n Lo, re-B retagne. 1979 . PRIMARY SETTL EMENT Features For small plants (average f1 ow up to 20 m 3/ h ; po pulation under 1000- 2000), the type of se ttlement tank no rmally used is a vertical circulation unit, either cylindro-conical or in the form of a hopper (Dortmund type), with sides a t a sharp angle from the horizontal (60° ). Settlement a nd digestion of th e sludge that forms may be carried out: - in a single two-storey unit (Imhoff se ttlement-digestion tank) ; 42 in two separate units in the case of larger systems (primary settle- ment tank and unheated digestion unit). The effluent moves rapidly through the settlement tank and therefore remains "raw" as compared with that flowing from a septic tank. Fig . 4. Two-storey tank Settlement chamber Digestion chamber Wastewater inlet Internal partition Fig . 5. Settlement tank Source: L ·assainissement individuel-principes et techniques actuelles. Paris. Min - istere de !'Environnement et du Cadre de Vie et Agence de Bassin Loire -Bretagne. 19B0. 43 Description - dimensions Primary settlement unit: a unit with a scum retention device: retention time: ;;,; I hour at peak flow; upward rate of flow:~ 1.5 m/h at peak flow . Digestion unit: useful digestion capacity;;,; 120 litres per person served. The dimensions depend to a great extent on the climatic and operating conditions (intervals at which the sludge is drawn off). The digestion unit may be somewhat smaller in the case of a two-storey tank, in which contact between the sludge and raw effluent allows a sufficient temperature to be maintained . In the case of medium- and high-rate systems with separate settlement and digestion units installed upstream , the useful digestion capacity Fig . 6 . Separate settlemen t and digestion system -1/j Grit chamber i"~I Biological unit : trickling filter biodiscs or modified process Settlement tank Sludge thickener I ' I ' I ' I I I I + I I Drying beds : (temperate regions) 1 m1 per 10 people, primary sludge 1 m1 per 5 - 8 people, primary and secondary sludge I I I I I I ~ ,--- 1 I I I + I __ , Digestion tank I / / I. r----, I ~ Source. Etude compara11ve des procedes d 'epura11on app/Jcables aux effluems des pe11res er moyennes collec11v1tes. Pari s. Cen tr e techniqu e du Geni e rural des Eaux et des Forets et Agence du Bassin Lo1re-Bretagne. 19 76 . 44 required will be a bove o r equa l to 150 lit res per perso n se rved. T he raw prim a ry (a nd seconda ry) s ludge co ll ec ted fr o m the bo tto m of the se ttl e- ment unit is pumped to the digestio n unit a ft er thi ckening. Efficiency - maintenance The effi ciency o f these sys tems is in th eo ry co mpa ra ble to tha t o f se ptic tank s (remova l o f 30% o f BOD , a nd 50% o fTTS) bu t in prac ti ce tends to be lowe r beca use of the less sa ti sfac to ry hyd ra uli c co nditio ns in sma ll sys tems. Regula r ma intena nce invo lves: removing the scum a nd scra ping the s ides of the ta nk; brea kin g up the n oa ting sludge a nd d rawi ng o ff th e digested sedime nt. Applications Prim ary se ttl ement unit s: a re suit a ble fo r p rocess ing was tewa ter pri o r to aero bi c trea tment (tri ckling filt ers, b iodi scs); ca n be used in conjunctio n with surface irriga tio n; a re no t ve ry effec ti ve prior to lagoo ning o r subsurface irriga tio n. Numbe r o f use rs: betwee n 100 a nd 1000. Features TRI CKLIN G FILTERS - LOW- OR M EDI U M-R ATE Trickling filt ers o perat e by hav ing the prev io usly se ttl ed was tewa ter n o w over a bed co mposed of coa rse fill er ma teri a l. Th e ma te ri a l a ll o ws the growth o f mi croo rga ni s ms which form a slime th ro ugh whi ch the liqu o r a nd oxyge n pass. T ypes o f ma teri a l suit a bl e fo r th e purpose include pozzola n a nd c rushed sto nes of regul a r size. The n o w of wastewa ter is di scontinuo us a nd ae rati o n is ca rri ed o ut by natu ra l ve ntil a ti o n. 45 Description - dimensions Pretreatment unit: primary settlement ta nk . T ri ckling filt ers: d epth o f fill er: > 1.5 m o f materi a l of 40-80 mm ; di stributio n by sprinkle r; loading ra te: 0. 1 to 0.4 kg 8OD5/ m -' o f materi a l per day. Seco nda ry se ttl ement ta nk (o ptio nal) . Drying beds. Fig. 7. Sett lement-digest ion ta nk and tri ck lin g filt er w it ho ut seco nda ry settl emen t tank Settlement-digestion tank \ \ \ ' I Drying beds~ " '-! , • • • '---------==='-.:;;;;... __ _ Source. Etude com par at we des procedes d'epurauon appl,cables aux effluents des pe/1/es et movennes collecuv11es. Pari s. Cent re technique du Genie ru ral des Eaux et des Forets et Age nce du Bass in Lo ire-Bretagne. 1976 . 46 In g ra vity-fed sys tems, a nush ta nk fitt ed with a se lf-clos ing sypho n is insta ll ed a t the o utl et o ft he prima ry se ttl ement ta nk to ensure a n adequa te ra te of n o w to the sprinkle r. It is ad visa ble to use s imple nush dev ices which a re no t eas il y a ffected by sto ppages. Alternative designs A modular tri ckling filt er which ca n be co nstructed by a loca l builder: se ri es-n ow system ; pa rall el-n o w sys tem. Depending on the size o f the co mmunity a nd degree of trea tment required , the trickling filt er sys tem may include a seco nda ry se ttlement ta nk . Fig . 8. Settl em ent -d iges ti o n tank. tr 1ck l1ng fi lt er and secondary sett lemen t tank . Recirculation of sludge ,jffl Gr~a;ber r-------•-----------•---7 I I I i-- 1 Settlement-digestion ♦ tank I I I I ,. I I I I I I I I & ♦ I I ~ Drying beds Secondary settlement tank Source: Etude compara{lve des procedes d 'epurar,on appl,cables aux effluems des pe{ltes et movennes collecr,v,r es. Par is. Cent re te chnique du Geni e rura l des Eaux et des Fo rets et Agen ce du Ba ss in Lo1r e- Bretagne. 1 976 47 Efficiency - maintenance Efficiency of treatment: BOD5: 30-40 mg/ I; TTS: 30 mg/I (JOO mg/I or more without settlement); partial nitrification of effluent. The system is simple to operate and uses either no or very little electricity. Applications Suitable for small communities, makes use of natural gradient of the land. Not suitable for use in a cold climate or if there are sharp variations in the water pressure. Number of users: communities of 100 to 600 people: trickling filter without secondary settlement tank: 100-400 users. trickling filter with secondary settlement tank: over 500 users. The lower limit depends on economic considerations. BIODISCS AND MODIFIED PROCESSES Features The equipment consists of large-bore discs or contactors mounted horizon- tally. This assembly, which is half submerged, rotates around the central axis. A surface film varying in thickness from I to 4 mm then forms on the discs . Rotation of the discs allows both oxygenation and contact with the wastewater. Description - dimensions Pretreatment units . 48 Biodiscs rotating slowly in a basin of the same shape: at least two storeys separated by a spillway and protected by a building; useful surface of the discs sufficient to ensure that the specific loading rate is between 7 a nd 15 g BOD5/m 2 of disc per day; secondary sludge recirculated upstream of the installati o n by a pump or self-closing valve; pumping of the raw primary and secondary sludge collected from the bottom of the settlement tank to the digestion tank after thickening. Secondary settlement tank (or clarifier). Drying beds. Alternative designs Primary and secondary se ttlement units consisting of earth lagoons: primary lagoon : depth 1.5-3 m, capacity equivalent to four times the daily flow of effluent to be treated; secondary lagoon : unit with a sludge collector upstream , and a capacity equivalent to o ne day's retention . In modified processes, the contactors are lined with special materials . Efficiency - maintenance Efficiency of treatment : 8OD5: 30-40 mg/ I; TTS: 30 mg/ I. Maintenance involves operations si milar to those applicable to conven- tional treatment plants . Particular care must be taken with the electro- mechanical equipment. It is important to avoid lengthy interruptions of service that may cause an imbalance of the system (contactors out of true) in the case of axial-drive units. Applications Biodiscs and modified processes: are suitable for small communities; fit into the surroundings of dwellings and take up little space; entail high investment costs and require the installation of prefabri- cated modular units for systems serving less than 1500 people. Number of users: communities of a few hundred people or more. 49 Fig. 9. Settl eme nt-d igesti o n tank, b iodi scs Recirculation of sludge ,-----·---------·----, I I I I //// Grit chamber + J 'llll C=::J~--- ---- ,-- 1 I I I ♦ I I I I I I I I I ' ' ' ' \ ' 1 Settlement-digestion tank cjr Secondary settlement tank Dry ing bed3 ~ Source. Etude comparauve des procedes d"epurauon app!,cables aux effluems des pe/1/es e1 moyennes collecrw,1es. Paris. Cen tre technique du Genie ru ral des Eaux et des Forets et Agence du Bassin Lo1re-Bretagne. 1976 . EXTENDED-AERATION ACTIV E-SLUDGE TECHNIQUE Features This treatment process is based on the activity of a bacterial culture that is dispersed and maintained in a condition of aerobiosis in the system. The mixture of wastewater and biological floe is agitated and aerated. The activated sludge is subsequently separated from the treated was tewater in a secondary sett lement tank (clarifier) . Part of the thickened sludge is then recirculated to the aeration tank, and the excess removed from the system. 50 Description - dimensions Pretreatment unit: bar screen, grit chamber, possibly a grease trap. Aeration tank : loadi ng rates: 0.35 kg BOD,/m '/day; 0.1 kg BOD5/kg volatile so lids/day. Secondary se ttl ement tank: retention time: 2 h at peak now; recirculation of sludge by pumping, at a rate of up to 100% of the average now. Possible collect ion of excess sludge removed from the system in a thickener (20 litres per user), fo llowed by dehydration. Drying beds. Alternative designs For small communities, compact systems are used. They are construc ted with prefabricated components; and ae ratio n and sett lemen t may be car- ried out in a single (package) unit. Generally speaking, the system sho uld be of large capacity so as to handle peak nows and organic loads, and this requirement is not readily compatible with space saving. If possible the aera ti on and sett lement compartments should be quite separa te and the sl udge shou ld be circulated by pumping. Efficiency - maintenance Efficiency of trea tment : BOD, : 15-40 mg/ I; TTS: 20-30 mg/ I. Nitrificatio n of the efnuent: it is possible to ac hieve a higher level of nitrification by adopting lower loading rates than th e customary ones (0. 1-0.2 BOD 5/ m3/ day) . Energy consumption is around 100 Wh / user/ day . Regular maintenance is required, amoun ting to 3-5 hours a week a t two-day intervals . Applications These sys tems are suitab le for intensive treatment of wastewater but they ca ll for regular atten tion. 5 1 Fig . 10. Extended aeration with recirculation by pumping -----, I Secondary settlement tank Fixed sludge thickener ' ~ __ .. __ direction of flow of wastewater -•+-•- recirculation (wastewater or sludge) --♦-- direction of flow of excess sludge ----- overspill Drying beds Source: Etude com para/Ive des procedes d 'epura/lon appltcables aux effluents des petites et moyennes collec/lv1tes. Paris. Centre technique du Genie rural des Eaux et des Forets et Agence du Bassin Lo1re-Bretagne. 1976. Number of users: co mmumt1es of over 1000 population equivalent. Monobloc units may be used for smaller communities (100-500 people). Precautions must be taken with regard to foreseeable losses of sludge. AERATED LAGOONING Features The oxygen required for the biol ogical treatment is provided by an arti- ficial aeration device which both oxygenates and agitates the wastewater. 52 Description - dimensions Bar screen. Aeration tank: floor surface and sides made of compacted earth, without additional waterproofing. Protection of sides against lapping water (by a plastic film , stone packing, etc .) and the floor surface immediately below the aerators (concrete slab , for instance). One or several tanks arranged in a series: depth : 2.5-3 m; em uent retention time: 20 days; surface aerator (floating rotors , for instance); installed power capacity: 4-5 W / m .1 of tank (domestic wastewater). By using several rotors it is possible to limit the effects of a possible breakdown and to make more efficient use of energy for agitating the effluent. Settlement tank: same features; effluent retention time: 5 days (two identical tanks arranged in parallel). Fi g. 11 . Aerated lag oo n (c ross sec tion) Source: L ·assam,ssement 1nd1v1duel - p r, nc,pes er techniques actuelles. Pari s. M1n - 1s tere de l"En v1ron nemen t et du Cadre de Vie et Agence de Bass in Lo ,re-B retag ne. 1980. 53 Efficiency - maintenance Aerated lagooning with settlement: BODs: 30-40 mg/I; ITS: 30-50mg/l. Aerated lagooning without settlement: BODs: 40 mg/I (with filtered water); ITS: 50-l00mg/l. Reduction in the number of indicator microbes for faecal contamination. Population equivalent 300 400 500 600 700 800 1000 Fig . 12 . Aerated lagooning : layout of aeration rotors 4 +4 6.5 + 4 5.5 + 5.5 4 5.5 + 4 5.5 + 5 5 5.5 • -· ~ri =I l1 t~7 r-r Source: Lagunage nature/ et lagunage aere. procedes d 'epuration des petittJs col- lectivites. Paris. CTGREF et Agence de Bassin Loire-Bretagne. 1979. 54 Maintenance of the system involves: regular attention to the electromechanical equipment (supervision and regulation of daily operating times of the aerators); periodic removal of sludge which accumulates at the bottom of the settlement tank (variable: 1-5 years), either in liquid form (irri- gation o nto adjoining land), or in solid form after drying out the tank. The energy requirement for aeration is about 2 kWh per kg of 8OD5 removed, or about 100 Wh/ person/day. Applications Lagoon systems: are suitable for treating mixtures of domestic wastewater and agri- cultural or agro-industrial effluents (in which case a higher installed energy capacity is required); are little affected by septic effluents; are suitable for treating diluted effluents; are able to deal with nuctuations in load; are not unduly affected by errors of regulation and routine o peration ; do not take up much space and therefore blend into the environment; use a great deal of energy. Number of users: from a few hundred to about 3000. 55 Annex 3 COMPARISON OF SANITATION TECHNOLOGY OPTIONS FOR SMALL COMMUNITIES 57 Degree of treatment Manpower Capital of domestic requirements cost eff lu ent Nature and fr equency of maintenance operations BOD TTS < 1000 > 1000 mg/I mg/I skilled unskilled popu- popu -lat1on lat1on Trickling filters 30- 30- Drawing off sludge every medium high high very hi gh 50 100 6 months: removal of screen ings 3 times a week B1od1scs 30- 30- Drawing off sludge every medium low high very high 50 50 6 months: removal of screenings 3 times a week Extended 15- 10- Regulation of system 2- high low high high aera ti on 30 40 5 times a year: desludg1ng every 1 5 days; attention to pretreatment units every 2 days Aerated 30- 20- Regulation of system twice medium high low low lagoon1ng 50 100 a year: desludg1ng every 2 - 3 years: atten t ion to pretreatment units twice a week Natural lagoon1ng - fu ll y 30 50 - Maintenance of surround- low med ium low med iu m aerobic +100 1ngs with either system: desludg1ng every 5- 1 0 years: removal of screenings 1- 2 times a week - aerobic - 30 Desludg1ng every 2 - low medium low medium anaerobic 5 years : removal of screen - ings 1 - 2 times a week Settlement - Drawing off sludge every 4- low medium medium medium d1gest1on tank 6 months: removal of surface screenings and frothing 2 - irrigation 3 times a week: clearance of vegetation 1- 2 times a year Septic tank Desludg1ng every 2 years low medium medium high surface (every year in the case o f 1mgat1on seasonal use): c lea ran ce of vegetation Peat beds (for the record only; details not availa ble) Macrophyte Attention to pretr eatment low high medium high sys tems units twice a w eek 1n an alternate feed sys tem: changing the containers every 1- 3 days: ross1bly. c learing out the m acro- phytes 1-2 times a year 58 Suitability for coping with Suitability for Poss1bll1ty of Energy cost fluctuations 1n load treating sewage siting the system of opera ti on together with near dwellings (Wh / user / day) agro-1ndustrial (free from water pressure organic load wastewater nuisances) 20 15- 20% poor average poor average of the operating cost 40: 20- 30% good averag e average good o f the operating cost 80- 100: poor (very poor good ( poor in good good 25 - 40% 1n combined combined of the opera ting systems) systems) cost 70 very good very good very good limited nil very good very good good average nil very good very good very good l1m1ted nil average good average limited nil average good very poor very good nil good very good average good 59 Cate- gory II Ill IV V VI a Annex 4 ENVIRONM ENT AL CLASSIFICATION OF EX CRETED IN FECTIONS Ep1dem1olog1cal Infecti on Dominant tr ans- Ma1or control featur e m1 ss1on foc us measu re Nonlatent, low Enterob1as1 s Pe rso nal Domestic w ater supply 1nfec t1 ve dose Enteroviral 1nfect1 ons Domest ic Hea lth educa tion Hymeno lep1as1s Improved housing Amoeb1as1s Provision o f toilets G,ard,as,s Balant1d1as1s Non latent. medium Typhoid Persona l Domestic water supply o r high ,nfect,ve Salm onell os,s Domestic Hea lth educa ti on dose. moderately Sh,gellos,s W ater Improved housing persistent and able Cholera Crop Prov,s,on of toilets to mul tip ly Path. Eschench,a col, Trea tment prior to dis- Yers1rnos1s cha rge or reuse Campylobacrer 1nfec t1on Latent and persist- Ascar1a s1s Yard Prov,s,on o f to il ets en t with no inter- Tri churi as ,s F,eld Trea tment o f excreta mediate host Hookworm Crop prior to land appl,cat,on Latent and persis t- Taen1as1s Yard Prov,s,o n o f to il ets en t w,th cow or p,g Field Treatment of excret a ,n term ed,ate hos t Fodder prior to land appl,ca t, on Cooking. meat ,nspect, on Latent and persist - C lonorch,as,s Water Prov,s,on o f to il ets en t w,th acquat,c D1phyllobothrias1s Trea tm ent of excreta 1ntermed1ate hos tls) Fasc,ol, as,s prio r to d ischarge Fasc,ol ops,as,s Contro l of anima l Gas trod1sco1d1as,s reservoir s Heterophy1as1s Cooking Metagon,m ,as,s Paragon1m1as1s Sch,s tosom,as,s Excreta- related Bancroft1an fil arias ,s Various faeca lly lden t,f, ca t, on and el, m- in sec t vec tors (transmitted by Cu/ex contaminated ,nation o f su itab le p,p,ens). a and all the si tes ,n wh,ch breeding si tes 1nfect1ons lis ted ,n 1-V insec ts breed for which fli es and cock- roa ches ca n be vectors Cu/ex p,p,ens 1s a complex o f mosquito species and subspec ies Th e ponc1pal tr opical species. and th e vec tor o f fd arias1s 1n those t ropica l areas where the 1nfect1on 1s transmitted by Cu/ex. ,s Cu/ex qumquefasc,arus (previously also known as Cu/ex p1p1ens faugans. C p qumquefasc,atus. or C far1gans) Source Kalbermatten. J .M . et al . Appropr,a1e technology for water supply and san,rauon techn,cal and econom,c opr,ons. Vol 1 a Wa shington. DC . World Ban k. 1980 60 Annex 5 PARTICIPANTS Temporary advisers Mr J.P. Auzet, CEMAGREF,° Division of Water Quality, Fisheries and Aquaculture, Lyon Group, France Mr D. Ballay, Chief, Subdirectorate of Community Infrastructures , Directorate of Planning and Development, Ministry of Agricul- ture, Paris , France (Chairman) Dr H. Baumgart, Lippe Group, Association of Sanitarians (A TV), Essen, Federal Republic of Germany Mr P. Boutin , CEMAGREF, Water Quality Section , Bordeaux Group, France Mr A. Chaouche, Ministry of Water Supply, Algiers, Algeria Dr M. Csanady, National Institute of Hygiene, Budapest , Hungary Mr J.J . Gril , CEMAGREF, Division of Water Quality, Fisheries and Aquaculture, Antony Group, France Dr B. Havlik, Institute of Hygiene and Epidemiology, Prague, Czechoslovakia Dr D. D. Mara, Professor of Civil Engineering, University of Leeds, United Kingdom Mrs M. H. Marecos do Monte, Technical Centre for Basic Sanitation, Ministry of Public Health , Lisbon, Portugal Mr G. Martijnse, Ministry of Public Health and Environmental Protec- tion, Leidschendam, Netherlands a CEMAGREF: National Centre for Agricultural Mechanization, Rural Engineering, Water and Forestry Management. 61 Mrs A.M . Martins, Technical Centre for Basic Sanitation, Ministry of Health, Lisbon, Portugal Dr L. Mendia, Director, Department of Sanitary Engineering, Univer- sity of Naples, Italy Dr E. Mozhaev, Head of Department, A.N. Sysin Institute of General and Community Hygiene, Academy of Medical Sciences of the USSR, Moscow, USSR Mr K. Nilsson, VIAK AB, Malmo, Sweden Mr E. Olsson , Project Manager, National Institute of Building Research, Nykoping, Sweden Mr J . O'Neil, Manager, Division of Water Pollution Control, Yorkshire Water Authority, Bradford, United Kingdom ( Vice-Chairman) Mr J.M. Tetart , Directorate of Infrastructure Development for Hauts de Seine, Ministry of Town Planning and Housing, Nanterre , France Mrs Ta Thu Thuy, Project Manager, International Training Centre for Water Resources Management (CEFIGRE), Sophia Antipolis, France Mr D . Tricard, Deputy Director of Preventive Medicine, Ministry of Health , Paris, France (Vice-Chairman) Mr M. Vuillot, CEMAGREF, Division of Water Quality, Fisheries and Aquaculture, Lyon Group, France (Rapporteur) World Health Organization 62 Regional Office for Europe Dr G. Watters, Regional Officer for the International Drinking Water Supply and Sanitation Decade (Secretary) WHO publications may be obtained, direct or through booksellers, from : ALGERIA: Entreprise nationalc du Livre (ENAL). J bd Zirout Youccf. ALGIERS ARGENTINA : Cartos Hirsch. SRL. Florida 165. Galcrias Giiemcs. Escntorio 4B/46S. BUENOS AIRES AUSTRALIA : Hunter Publications. SSA Gopps Street. COLLINGWOOD. 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