4.1.1 Soil-aquifer treatment (SAT)
4.1.2 SAT system layouts
4.1.3 Soil requirements
Where soil and groundwater conditions are favourable for artificial recharge of groundwater through infiltration basins, a high degree of upgrading can be achieved by allowing partially-treated sewage effluent to infiltrate into the soil and move down to the groundwater. The unsaturated or "vadose" zone then acts as a natural filter and can remove essentially all suspended solids, biodegradable materials, bacteria, viruses, and other microorganisms. Significant reductions in nitrogen, phosphorus, and heavy metals concentrations can also be achieved.
After the sewage, treated in passage through the vadose zone, has reached the groundwater it is usually allowed to flow some distance through the aquifer before it is collected (Figure 11). This additional movement through the aquifer can produce further purification (removal of microorganisms, precipitation of phosphates, adsorption of synthetic organics, etc.) of the sewage. Since the soil and aquifer are used as natural treatment, systems such as those in Figure 11 are called soil-aquifer treatment systems or SAT systems. Soil-aquifer treatment is, essentially, a low-technology, advanced wastewater treatment system. It also has an aesthetic advantage over conventionally treated sewage in that water recovered from an SAT system is not only clear and odour-free but it comes from a well, drain, or via natural drainage to a stream or low area, rather than from a sewer or sewage treatment plant. Thus, the water has lost its connotation of sewage and the public see it water more as coming out of the ground (groundwater) than as sewage effluent. This could be an important factor in the public acceptance of sewage reuse schemes.
Various types of SAT system are shown in Figure 11, the simplest being where the sewage effluent is applied to infiltration basins on high ground from where it moves down to the groundwater and eventually drains naturally through an aquifer into a lower area (Figure 11A). This lower area can be a natural depression or seepage area, a stream or lake, or a surface drain. SAT systems as in Figure 11A also serve to reduce the pollution of surface waters. Instead of discharging wastewater directly into streams or lakes, it is applied to infiltration basins at a higher elevation so that it receives soil-aquifer treatment before entering the stream or lake. The system shown in Figure 11B is similar to that shown in 11A but the treated sewage water, after SAT, is collected by underground, agricultural-type drains. Systems 11A and 11B have the advantage that the entire SAT process is accomplished without pumping.
Where the groundwater is too deep to collect the renovated sewage water by gravity, pumped wells must be used and there are two basic layouts. In one (Figure 11C), the infiltration basins are arranged in two parallel strips and the wells are located on the line midway between the two strips. In the other (Figure 11D), the infiltration basins are located close together in a cluster and the wells are on a circle around this cluster. The system of Figure 11C can be designed and managed so that the wells pump essentially all renovated sewage water and no native groundwater from the aquifer outside the SAT system. Systems as in Figure 11D are more likely to deliver a mixture of renovated sewage water and native groundwater. Systems 11C and 11D can be used both for seasonal underground storage of sewage water, allowing the groundwater mound to rise during periods of low irrigation water demand (winter), and for pumping the groundwater mound down in periods of high irrigation water demands (summer). The type of SAT system shown in Figure 11C would be suitable for small systems where there are only a few basins around a centrally located well (Figure 12).
Figure 11: Schematic of soil-aquifer treatment systems (Bouwer 1987)
While SAT systems give considerable water quality improvement to the sewage effluent, the quality of the resulting renovated water is not often as good as that of the native groundwater. Thus, SAT systems should normally be designed and managed to prevent encroachment of sewage water into the aquifer outside the portion of the aquifer used for soil-aquifer treatment. For systems A and B in Figure 11, this could be achieved by ensuring that all the renovated water is intercepted by the surface or subsurface drain, which would result from excavating or installing the drain deeply enough to make sure that groundwater on the other side of the drain also moves toward the drain.
For system C in Figure 11, movement of renovated sewage water to the aquifer outside the SAT system can be prevented by managing infiltration and pumping rates so that the groundwater table below the outer boundaries of the infiltration strips never rises higher than the groundwater table outside the SAT system. This requires groundwater-level monitoring in a few observation wells installed at the outer edges of the infiltration strips (Figure 11C). In the case of system D in Figure 11, movement of renovated sewage water into the aquifer outside the circle of wells can be prevented by pumping the wells at sufficient rate so that there is movement of native groundwater outside the SAT system toward the wells.
Sewage water should travel sufficient distance through the soil and aquifer, and residence times in the SAT system should be long enough, to produce renovated water of the desired quality. While 100 m underground travel and one month underground retention time have been suggested as rule-of-thumb values, the actually required values depend on the quality of sewage effluent infiltrating into the ground, the soil types in the vadose zone and aquifer, the depth to groundwater, and the desired quality of the renovated water. Most of the quality improvement of sewage effluent moving through an SAT system occurs in the top 1m of soil. However, longer travel is desirable because it gives more complete removal of microorganisms and "polishing" treatment.
Figure 12: Schematic of four small infiltration basins with well in centre for pumping renovated sewage water from aquifer (Bouwer 1987)
Infiltration basins for SAT systems should be located in soils that are permeable enough to give high infiltration rates. This requirement is important where sewage flows are relatively large, where excessive basin areas should be avoided (due to land cost) and where evaporation losses from the basins should be minimized. The soils, however, should also be fine enough to provide good filtration and quality improvement of the effluent as it passes through. Thus, the best surface soils for SAT systems are in the fine sand, loamy sand, and sandy loam range. Materials deeper in the vadose zone should be granular and preferably coarser than the surface soils. Soil profiles consisting of coarse-textured material on top and finer-textured material deeper down should be avoided because of the danger that fine suspended material in the sewage will move through the coarse upper material and accumulate on the deeper, finer material. This could cause clogging of the soil profile at some depth, where removal of the clogging material would be very difficult.
Vadose zones should not contain clay layers or other soils that could restrict the downward movement of water and form perched groundwater mounds. Aquifers should be sufficiently deep and transmissive to prevent excessive rises of the groundwater table (mounding) due to infiltration. Groundwater tables should be at least 1 m below the bottom of the infiltration basins during flooding. Above all, soil and aquifer materials should be granular. Fractured-rock aquifers should be protected by a soil mantle of adequate texture and thickness (at least a few metres). Shallow soils underlain by fractured rock are not suitable for SAT systems.
4.2.1 Hydraulic capacity and evaporation
4.2.2 Basin management
Infiltration basins in SAT systems are intermittently flooded to provide regular drying periods, for restoration of infiltration rates and for aeration of the soil. Flooding schedules typically vary from 8 hours dry-16 hours flooding to 2 weeks dry-2 weeks flooding. Therefore, SAT systems should have a number of basins so that some basins can be flooded while others are drying. Annual infiltration amounts or "hydraulic loading rates" typically vary from 15 m/year to 100 m/year, depending on soil, climate, quality of sewage effluent, and frequency of basin cleaning. Thus, assuming a sewage production of 100 I/person day, a city of 100,000 people, and a hydraulic loading rate of 50 m/year, an SAT system for the entire sewage flow would require about 7.3 ha of infiltration basins. This shows that SAT systems do not necessarily require very large land areas, provided, of course, that the soils are permeable enough and the sewage is of such a quality (low suspended solids content) so as to allow high hydraulic loading rates to be maintained.
Evaporation losses from free water surfaces in dry, warm areas typically range between 1 and 2 m/year. Since the soil of infiltration basins will be mostly wet during drying, evaporation from intermittently flooded basins will be almost the same as that under continuous flooding conditions. Assuming an SAT system with a hydraulic loading rate of 50 m/year and evaporation losses of 1.5 m/year, evaporation losses would be 3% of all the sewage applied which would cause a 3 % increase in the concentration of dissolved salts in the sewage water.
Bare soil is often the best condition for the bottom of infiltration basins in SAT systems. Occasional weeds are no problem but too many weeds can hamper the soil drying process, which delays recovery of infiltration rates. Dense weeds can also aggravate mosquito and other insect problems. Low water depths (about 20 cm) may be preferable to large water depths (about 1 m) because the turnover rate of sewage applied to shallow basins is faster than for deep basins of the same infiltration rate, thus giving suspended algae less time to develop in shallow basins. Suspended algae can produce low infiltration rates because they are filtered out on the basin bottom, where they clog the soil. Also, algae, being photosynthetic, remove dissolved carbon dioxide from the water, which increases the pH of the water. At high algal concentrations, this can cause the pH to rise to 9 or 10 which, in turn, causes precipitation of calcium carbonate. This cements the soil surface and results in further soil clogging and reduction of infiltration rates. Because suspended algae and soil clogging problems are reduced, shallow basins generally yield higher hydraulic loading rates than deep basins.
During flooding, organic and other suspended solids in the sewage effluent accumulate on the bottom of the basins, producing a clogging layer which causes infiltration rates to decline. Drying of the basins causes the clogging layer to dry, crack, and form curled-up flakes; the organic material also decomposes. These processes restore the hydraulic capacity so that when the basins are flooded again, infiltration rates are close to the original, high levels. However, as flooding continues, infiltration rates decrease again until they become so low that another drying period is necessary.
Depending on how much material accumulates on the bottom of infiltration basins, periodic removal of this material is necessary. Removing the material by raking or scraping is much better than mixing it with the soil with, for example, a disk harrow. The latter practice will lead to gradual accumulation of clogging materials in the top 10 or 20 cm of the soil, eventually necessitating complete removal of this layer, which could be expensive.
For clean secondary sewage effluent with suspended solids concentration of 10 to 20 mg/l, flooding and drying periods can be as long as 2 weeks each, and cleaning of basin bottoms may be necessary only once a year or once every 2 years. Primary effluent, with much higher suspended solids concentration, will require a schedule which might be 2 days flooding-8 days drying, and basin bottoms might be expected to require cleaning at the end of almost every drying period. The best schedule of flooding, drying, and cleaning of basins in a given system must be evaluated by on-site experimentation.
The main constituent that must be removed from raw sewage before it is applied to an SAT system is suspended solids. Reductions in BOD and bacteria are also desirable, but less essential. In the USA, there are several hundred SAT systems and, prior to land application, the sewage typically receives conventional primary and secondary treatment because that is the treatment normally prescribed before anything can be done with the effluent. Secondary treatment removes mostly biodegradable material, as expressed by the BOD, but bacteria in the soil can also degrade organic material and reduce the BOD of the sewage to essentially zero. Thus, where pretreatment is followed by SAT, primary treatment would normally be sufficient. The primary effluent would have a higher BOD and suspended solids content than secondary effluent and this would result in somewhat lower hydraulic loading rates for the SAT system and would require more frequent basin cleaning (Rice and Bouwer 1984). However, elimination of the secondary step in conventional pretreatment of the effluent would result in very significant cost savings for the overall system.
4.3.1 Suspended solids
4.3.2 Organic compounds
4.3.3 Bacteria and viruses
4.3.6 Trace elements and salts
As mentioned previously, the main constitutents that must be removed from sewage effluent before it can be used for unrestricted irrigation are pathogenic organisms. Nitrogen concentration might also have to be reduced and suspended solids and biodegradable materials should perhaps be removed to protect the irrigation system or for aesthetic reasons. If the renovated water is to be used for recreational lakes or discharged into surface water, phosphorus should also be removed to prevent algal growth in the receiving water. The following sections describe how these constituents are removed or reduced in SAT systems.
After appropriate pretreatment, the suspended solids in sewage effluent are usually relatively fine and in organic form (sewage sludge, bacteria, floes, algal cells, etc.). These solids accumulate on the soil in the infiltration basins, requiring regular drying for infiltration recovery and periodic removal from the soil by raking or scraping. For loamy sands and sandy loams, few suspended solids will penetrate into the soil and then, usually, only for a short distance (a few cm, for example). In dune sands and other coarser soils, fine and colloidal suspended solids (including algal cells) can penetrate much greater distances. Except for medium and coarse uniform sands, soils are very effective filters, and suspended solids will be essentially completely removed from the sewage effluent after about 1m of percolation through the vadose zone. Additional details regarding suspended solids removal and clogging are given in Bouwer (1985) and Bouwer and Chaney (1974).
Most organic compounds of human, animal or plant origin in sewage effluent are rapidly decomposed in the soil. Under aerobic conditions (intermittent flooding), breakdown is generally faster and more complete (to carbon dioxide, minerals and water) than under anaerobic conditions. The latter prevail in the soil profile during continuous or long-term flooding. Stable, non-toxic organic compounds such as humic and fulvic acids can be formed as products of reactions between proteins and carbohydrates (cellulose or lignin).
The BOD5 of sewage varies from several hundred to about 1000 mg/l for raw sewage, and from about 10 to 20 mg/l for good quality secondary effluent. SAT systems can handle high BOD-loadings, probably hundreds of kg/ha day (Bouwer and Chaney 1974), and BOD levels are generally reduced to essentially zero after a few metres (often less) of percolation through soil. However, the final product water from SAT systems still contains some organic carbon, usually a few mg/l. This is probably mostly due to humic and fulvic acids but also to synthetic organic compounds in the sewage effluent that do not break down in the underground environment.
Halogenated hydrocarbons tend to be more resistant to biodegradation than non-halogenated hydrocarbons (Bouwer et al. 1984; Bouwer and Rice 1984). Synthetic organic compounds in the renovated water from SAT systems are generally present at very low concentrations, usually at the ppb (micrograms/l) level, and are not considered a problem when the water is used for irrigation. If it were to be used for drinking, however, additional treatment of the water by, for example, carbon filtration and reverse osmosis, would be necessary to remove the organic compounds. Additional details regarding BOD removal in SAT systems are given in Bouwer (1985) and Bouwer and Chaney (1974).
Pathogenic organisms in sewage effluent include salmonella, shigella, mycobacterium, and vibrio comma. Specific tests for these bacteria are not routinely carried out but, instead, the numbers of faecal coliform bacteria are normally determined. Escherichia coli are indicator organisms that are widely used to detect faecal contamination of water and the assumption is that if faecal coliform bacteria are present in a sample, then human pathogenic bacteria could also exist. It is also inferred that if faecal coliform bacteria are no longer present, pathogenic bacteria are also absent. Viruses in sewage effluent include entero- and adeno-viruses. Hepatitis viruses are of special concern. Viruses in renovated water from SAT systems are tested for by passing large volumes (1000 to 2000 l) through positively-charged filters to trap the viruses. Subsequently the viruses are determined in the laboratory as plaque-forming units (PFU's), which usually represent clusters of viruses. Specific viruses are tested for serologically. Other pathogens in sewage effluent include protozoa and helminth parasites, which are discussed elsewhere.
Soil is an effective filter to remove microorganisms from sewage effluent (except, of course, coarse soils such as sands and gravels, or fractured rock). Bacteria are physically strained from the water, whereas the much smaller viruses are usually adsorbed. This adsorption is favored by a low pH, a high salt concentration in the sewage, and high relative concentrations of calcium and magnesium over monovalent cations such as sodium and potassium. Human bacteria and viruses immobilized in the soil do not reproduce, and eventually die. Most bacteria and viruses die in a few weeks to a few months, but much longer survival times have also been reported. Many studies indicate essentially complete faecal coliform removal after percolation of 1 to a few metres through the soil. However, much longer distances of underground travel of microorganisms have also been reported. Usually, these long distances are associated with macropores, as may be found in gravelly or other coarse materials, structured or cracked clay soils, fractured rock, cavernous limestones, etc.
The best protection against breakthrough of pathogenic microorganisms in the renovated sewage water from SAT systems is to reduce bacterial levels in the sewage effluent before infiltration, to avoid coarse textured materials in the SAT systems, and to allow long underground travel distances and retention times. Additional information on this subject is provided in Bouwer (1985), Bouwer and Chaney (1974) and Gerba and Goyal (1985).
Nitrogen levels in sewage can range from 20 to more than 100 mg/l, depending on in-house water use and diet of the local people and on the treatment of the sewage effluent prior to SAT. Nitrogen is primarily present as organic, ammonium, and nitrate nitrogen. The relative amounts of these nitrogen forms depend on the form of treatment prior to SAT. For secondary effluent, much of the nitrogen will often be in the ammonium form but some processes are designed to achieve nitrification and the effluent will then contain primarily nitrate-nitrogen. Raw sewage has considerable organic nitrogen.
The desirable form and concentration of nitrogen in the renovated sewage water from an SAT system depends on the nitrogen and water requirements of the crops to be irrigated, the need for preventing nitrate pollution of groundwater in the irrigated area due to excess nitrogen application to the crops, and on other possible uses of the water (including fish ponds, for which low concentrations of ammonium are required).
Control of the form and concentration of the nitrogen in renovated water from an SAT system is possible by properly selecting hydraulic loading rates and flooding and drying periods for the infiltration basins. For example, if the nitrogen in the sewage effluent is mostly in the ammonium form, short flooding periods and frequent drying of the infiltration basins (for example, 2 days flooding-5 days drying) will cause essentially complete nitrification of the ammonium in the soil, due to frequent aeration of the soil profile and resulting aerobic conditions. Thus, almost all the nitrogen in the renovated water from the SAT system will then be in the nitrate form and at concentrations about equal to the total nitrogen concentration in the sewage effluent applied to the basin. Long flooding and drying periods (for example, 1 month flooding-1 month drying) would eventually lead to complete breakthrough of ammonium in the renovated water because of anaerobic conditions in the soil and absence of nitrification. If flooding and drying periods are of intermediate length (for example, 1 to 2 weeks flooding-1 to 2 weeks drying), there will be a succession of aerobic and anaerobic conditions in the upper part of the soil profile, which stimulates nitrification and denitrification. The latter is an anaerobic bacterial process that reduces nitrate to free nitrogen gas and oxides of nitrogen that return to the atmosphere. With this process, about 75% of the nitrogen in sewage has been removed in an SAT system in Arizona, USA, with almost all of the remaining nitrogen in the renovated water occurring in the nitrate form.
Denitrification requires the presence of nitrate and organic carbon (an energy source for denitrifying bacteria) under anaerobic conditions. About 1 mg/l of organic carbon is required for each mg of nitrate nitrogen to be denitrified. If the nitrogen in the sewage is already mostly in the nitrate form and the water quite stabilized, organic carbon (as primary effluent, for example) may have to be added to the sewage effluent to achieve sufficient denitrification when the system goes anaerobic. Local experimentation is usually required to find the optimum schedule for flooding and drying, hydraulic loading, and organic carbon addition for stimulating denitrification. More information can be found in Bouwer (1985) and Bouwer and Chaney (1974).
Sewage effluent can contain 5 to 50 mg/l phosphorus, depending on diet and water use of the local population. During pretreatment of the sewage, and in passage through the soil of the SAT system, organic phosphorus is biologically converted to phosphate. In calcareous soils and at alkaline pH, phosphate precipitates with calcium to form calcium phosphate. In acid soils, phosphate reacts with iron and aluminium oxides in the soil to form insoluble compounds. Sometimes, phosphate is initially immobilized by adsorption to the soil and then slowly reverts to insoluble forms, allowing more adsorption of mobile phosphate, etc. In clean sands with about neutral pH, phosphate can be relatively mobile. Further information is given in Bouwer (1985) and Bouwer and Chaney (1974).
Sewage effluent contains a wide spectrum of other chemicals at low concentrations. These include heavy metals, fluorine, and boron. Unless these elements were already present in large concentrations in the drinking water or added to the sewage in significant amounts by industrial discharges, their concentrations in sewage are usually below the maximum limits for irrigation water (FAO 1985).
Metals are significantly retained in most soils but a high pH favours immobilization. Fluoride can form calcium fluoride, which has a very low solubility, in the soil and is also adsorbed by various soil components, especially hydrous aluminium oxides. Boron is mobile in sands and gravels but can be adsorbed on clay. Thus, SAT systems can significantly reduce the concentrations of trace elements in sewage effluent (Bouwer 1985; Bouwer and Chaney 1974).
Total salt concentrations in sewage effluent can be several hundred mg/l higher than in drinking water. Since SAT systems generally have sandy soils, hydraulic loading rates will be much higher than evaporation losses (for example, 50 m/yr vs 1.5 m/yr). Hence, the salt concentration in the renovated water from SAT systems will be about the same as (or slightly higher than) that of the sewage effluent. If clay or organic matter is present in the soil, there will be cation adsorption and ion exchange when the SAT system is first put into operation. However, eventually, the ionic composition of the renovated sewage water will be essentially the same as that of the sewage effluent going into the ground. SAT systems do not remove salts from sewage.