WASTEWATER RENOVATION THROUGH SOIL AQUIFER TREATMENT SYSTEM

Abstract

In recent times, land treatment of domestic wastewater by slow infiltration has emerged as an effective and economic alternative to the conventional systems oftreatment. In land treatment, appropriately pretreated wastewater is allowed to infiltrate through vadose zone where it undergoes purification through unit operations and processes, viz. filtration, adsorption, ion exchange and biodegradation. Soil treated wastewater further moves laterally for some distance through the underlying saturated zone(aquifer) where it receives a further degree of purification by dispersion and dilution. Since both soil and aquifer participate in the renovation process, such a land treatment system is commonly known as Soil-Aquifer-Treatment(SAT) system. Several studies on tertiary treatment from existing wastewater treatment systems using SAT have been conducted in developed countries. Wastewater treated upto varying degree of pretreatment, viz. primary, and secondary have been used as influent to SAT system. However, the system's response to different input pollutants, viz. suspended solids, organics, nutrients, bacteria, viruses, etc. has not so far been studied in sufficient details and, still not understood well. With the ever increasing water pollution loads, the available dilution capacities in surface waters have gone down significantly all the world over. This situation has become much worse in densely populated countries like 11 India where the demand of the times is to adopt low cost alternative technologies for wastewater treatment which do not pollute the surface sources directly. The present work pertains to the study on the potential of SAT technology as an alternative wastewater treatment and disposal system in the context of Indian conditions. The present research is based on a critical review ofexisting plants in various parts ofthe world employing SAT system. The thesis presents a critical review of the available literature on SAT technology. Efficiency studies at the rapid-infiltration basin site (Flushing Meadow Project) in the Salt river- bed at Phoenix using secondary effluent as evaluated by Bouwer et al.(1980) reported COD and BOD reductions as 60 and 100 percent respectively. Removal of nitrogen and phosphorous could be achieved upto 65 and 80 percent.Rice and Bouwer (1984) demonstrated the efficacy of SAT using primary treated wastewater. The renovated wastewater quality was comparable to that obtained using secondary treated wastewater, with hydraulic loadings ranging 27-51 m/year. The study inferred that the expensive and energy- intensive secondary treatment could even be omitted without affecting the final treated effluent quality. A comparative assessment of the relative efficacies of the SAT system using primary and secondary effluent at a rapid infiltration site by Carlson et al. (1982) showed that higher infiltration rates were achievable with primary effluent in comparison to secondary effluent. Lance et al. (1980) compared the renovation of primary and secondary effluents using high rate filtration in the laboratory columns packed with soil. It was observed that the infiltration rates for primary effluent were only marginally lower (15%) in comparison to secondary effluent though the SS concentration in the two effluents were 51-181 mg/L in and 10 mg/L respectively which is contrary to the general perception. Rice (1974) reported that in soil columns fed with secondary effluent, physical clogging occurred due to high SS. However, biological clogging became a causative factor when long flooding period with low SS concentration was used. This was attributed to a decrease in hydraulic conductivity caused by entrapped gases, probably nitrogen, which accumulates in the soil pores as a result of denitrification or depletion of oxygen in soil air by microbial activity. Algiil growth on the soil surface increased the infiltration rates during flooding as the algal mat became buoyant thereby freeing the soil surface from clogging. In summary, it is seen that the land treatment of secondary treated domestic wastewater by slow infiltration has a potential to provide an economical tertiary wastewater treatment, where suitable land is available. Under Indian conditions also, the secondary treatment of municipal wastewater is not only cost prohibitive but also requires high level of operation and maintenance which is usually a constraint in setting up of adequate treatment facilities.Therefore, significant cost savings are expected only by applying primary treated effluent to the land and omitting secondary treatment at the plant. Further, all studies on SAT systems done so far lacked in a comprehensive analysis of performance data involving different operational parameters with recourse to mathematical approach in order to predict the systems behavior under different hydrogeological and climatic conditions. Also, the mechanism of removal of organics in the media has not been dealt through conceptual approach ofunit operations and processes. As an example, the effect of biological mat formed on the infiltration bed on the wastewater purification process has also not been studied as it is vital lor iv optimal perfomiance. Keeping all these features of the design of SAT systems in view, alongwith favourable hydrogeology of the aquifer system underlying the river-bed, the present research work was planned to examine the feasibility of using primary treated sewage for renovation through the SAT system on a dry river-bed. Specifically, the study addresses itself to the following programme of research: 1. Undertaking column lysimeter studies to assess the treatability and behaviour of SAT system. Towards this, the following were aimed: (i). Assessment of the efficacy of the SAT system under varying wastewater quality conditions. (ii) Determination of the upper limits of hydraulic and mass loadings for pilot SAT system. 2. On the basis of lysimeter results, a pilot study was carried out to evaluate the feasibility of the SAT concept using primary settled municipal wastewater on Sabarmati river-bed in the city of Ahmedabad which remains dry almost round the year. Following objectives of the field study have been delineated: ( i) To determine achievable hydraulic and organic loading rates of primary treated effluent under field conditions that would produce desired effluent quality. ( ii) To study the functional behaviour of infiltration basins with reference to