Disinfection of secondary effluents by infiltration percolation

Abstract

Among the most attractive applications of reclaimed wastewater are: irrigation of public parks, sports fields, golf courses and market gardening. These uses require advanced wastewater treatment including disinfection. According to WHO guidelines (1989) and current rules and regulations in Tunisia, faecal coliform levels have to be reduced to <103 or 102 CFU/100 mL. In Tunisia, most wastewater plants are only secondary treatment and, in order to meet health related regulations, the effluents need to be disinfected. However, it is usual for secondary effluents to need filtration prior to disinfection. Effectiveness of conventional disinfection processes, such as chlorination and UV radiation, are dependent upon the oxidation level and the levels of suspended solids of the treated water. Ozonation is relatively expensive and energy consuming. The consideration of the advantages and disadvantages of conventional techniques, their reliability, investment needs and operational costs will lead to the use of less sophisticated alternative techniques for certain facilities. Among alternative techniques, soil aquifer treatment and infiltration percolation through sand beds have been studied in Arizona, Israel, France, Spain and Morocco. Infiltration percolation plants have been intermittently fed with secondary or high quality primary effluents which percolated through 1.5-2 m unsaturated coarse sand and were recovered by under-drains. In such infiltration percolation facilities, microorganisms were eliminated through numerous physical, physicochemical and biological inter-related processes (mechanical filtration, adsorption and microbial degradation respectively). Efficiency of faecal coliform removal was dependent upon the water detention times in the filtering medium and on the oxidation of the filtered water. Effluents of Sfax town aerated ponds were infiltrated through 1.5 m deep sand columns in order to determine the performance of infiltration percolation in the polishing of secondary effluents. Elimination of bacteria (total and coliforms, faecal streptococci) and their relationship with the hydraulic load and the temperature were investigated.