The performance of three waste stabilization ponds (WSP) treatment units (lines A, B, C) for a period of two years is presented. The above units were part of a research and demonstration project in North Greece concerning wastewater treatment with natural systems and effluent reuse for irrigation. Line A consisted of a deep anaerobic pond followed by a facultative and two maturation ponds. The anaerobic pond supplied with 120 m3/d was also feeding line B at a ratio 1:3 (line A: line B). Line B had the same design features with line A but with a recirculation of effluent. The third unit fed with primary treated effluent at a rate of 50m3/d from the WWTP of Thessaloniki was consisted of a facultative and two maturation ponds. In the anaerobic pond the BOD5, COD, SS, TC and FC mean reduction was 28%, 42%, 58%, 31% and 36% for the cold period (T<10°C) and 45%, 53%, 72%, 67% and 71% for the warm period (T>10°C) respectively. In line A, the BOD5, COD, SS, TC and FC total reduction was 98%, 89%, 93%, 99.99% and 99.99% for the cold and 98%, 85%, 88%, 99.99% and 99.99% for the warm period. In line B the reduction was 92%, 86%, 88%, 98.5% and 98.82% for the cold and 96%, 88%, 83%, 99.95% and 99.97% for the warm period. In line C the reduction of the same parameters was 92%, 64%, 40%, 99.88% and 99.96% for the cold and 92%, 59%, 22%, 99.99% and 99.99% for the warm period. The effluent quality was further improved with the use of a storage reservoir and a slow sand filter and reused successfully for crop irrigation.

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