Soil Aquifer Treatment as Disinfection Unit

The Shafdan wastewater treatment system treats the municipal wastewater of the Tel-Aviv metropolitan area, which contains 22 cities with a population of about two million. The conventional mechanical-biological treatment is followed by managed groundwater recharge. The effluent undergoes soil aquifer treatment (SAT) by controlled passage through the unsaturated zone and an average retention time of 960 days in the aquifer. The SAT provides effluent polishing treatment as well as seasonal and multiannual storage, which are much needed in the semiarid Mediterranean climate. The Shafdan SAT operates without disinfection prior to the land spreading of the secondary effluents or after the water is reclaimed from the aquifer. Despite that, since 1995 there has not been even a single positive test of fecal coliforms (FC) in any of the reclamation wells, and since 2001, all of the enterovirus tests in an observation well located midway between the infiltration basins and the reclamation wells were negative. The few positive cases of fecal coliforms that were reported before 1995 are well within the anticipated rate of the method's false positives. The reclaimed water complies with all of the water quality specifications of drinking water except for the provision of a disinfection step. The SAT provides efficient reagentless physical pathogen removal, superior to chlorination and without its adverse health effects. It is therefore maintained that even without predisinfection and post-disinfection, the reclaimed water is safe for unrestricted irrigation of crops that are consumed raw and also for unrestricted nonpotable urban reuse. The high stability provided by the long retention time and reagentless disinfection obviates the need for frequent sampling, though assurance of no bacterial regrowth in the distribution system should be effected by monitoring and/or maintaining residual disinfectant. The endorsement of leading water reuse guidelines should be considered in view of these findings.