Sand filtration of naturally treated wastewater for irrigation reuse

The performance of a slow sand-filter was evaluated in a pilot-scale wastewater stabilization pond system, near Thessaloniki city, in Greece. Samples were taken over a period of 3 years, and the efficiency for irrigation reuse was determined based on TSS, BOD5, pH and FC measurements. Sand filtration reduced the TSS, BOD5 and pH values, up to 16%, 25% and 5%, respectively, thus resulting in their highly significant improvement. Simple linear models were determined to predict TSS, BOD5 and pH at the sand-filter outlet. During the irrigation period, TSS values were less than 50 me, (70% of the outlet values). For drip irrigation, however, an additional filtration was necessary to guarantee a constant effluent quality under this limit. BOD5 and pH were less than 15 mg O-2/L and 8.5 (75% and 95% of the measurements), thus resulting in satisfactory levels for irrigation. Sand-filter operation had no influence on FC concentration during winter period, while a rise, up to 2 log units, was observed in summer, thus reaching the limit of 1000 FC/ 100 ml. Disinfection was necessary to meet the stipulated effluent standards for unrestricted agricultural use.