From January to December 1992, the efficiency of two waste stabilization pond systems (WSPS) located in the Region of Murcia, South-eastern Spain, has been studied in terms of the physicochemistry and the microbiology of the systems. The efficiencies of the different stages of the treatment and the global performance have been compared. The WSPS of Lorquí and Ceutí (12,000 inhabitants) receives fruit processing and cannery wastes mixed with the urban sewage. This increases the load up to 95,000 equivalent inhabitants with an average BOD5 of 800 mgO2/l. Global reductions of COD and BOD5 were 79% and 73% as a yearly average. The mean of the reductions of indicator microorganisms were: coliphages (CHP), 83.14%; total coliforms (TC), 72.90%; faecal coliforms (FC), 93.84% and salmonellae (SAL), 98.29%. The WSPS of San Javier (15,000 inhabitants), situated at the border of the Mediterranean, does not receive industrial wastes but suffers from intrusions of marine waters into the sewers. BOD5 is diluted by the intrusion to less than 100 mgO2/l, even in summertime when population doubles because of the tourism. Average yearly reductions of COD and BOD5 were 32% and 21%. The means of the microbiological reductions were: CHP, 81.60%; TC, 90.77%; FC, 94.37% and SAL, 99.42%. Design constants have been calculated for both systems considering the climatic conditions of the area.

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