Wastewater treatment by pond systems: experiences in Catalonia, Spain

An evaluation of the 24 existing pond systems for treatment of urban wastew ater in Catalonia (north-eastern Spain) was conducted in 1997-1998. The eva luation covered 13 aerated ponds, 7 waste stabilization ponds (WSP), and 4 maturation ponds added to conventional biological wastewater treatment plan ts (WWTP). The technical characteristics of the pond systems were obtained through a survey completed during several visits to the plants. Performance data was supplied by the Catalonia Wastewater Agency and the monthly monit oring campaigns conducted during 12 months in five WWTP. The average ratio of water surface to population equivalent (p-e) is 1.3 m(2)/p-e for aerated ponds, 9.6 m(2)/p-e for WSP and 1.7 m(2)/p-e for maturation ponds added to conventional WWTP. Average electrical energy consumption is 0.5 kW.h/m(3) for aerated ponds and 0.04 kW.h/m(3) for WSP (when wastewater has to be pum ped from the sewer). Most of the WSP show an inadequate level O & M, becaus e the very limited operator attendance (0.6 h/day on average) as compared t o aerated ponds (3.2 h/day on average). Effluent quality of aerated ponds ( 31 mg SS/I and 21 mg BOD,II, on average) is usually better that of WSP (100 mg SS/I and 67 mg BOD5/I, on average). Most of the WSP are overloaded, wit h only two of the pond systems receiving less than 50 kg BOD5/ha. Facultati ve WSP show a better performance than anaerobic WSP; anaerobic WSP have ver y high overloading conditions (with more than 250 ka BOD5/ha.d). Average af fluent quality of all the pond systems studied comply with European Union s tandards.