Co-Treatment of Septage and Domestic Sewage for the Greater Athens Area

Abstract

The paper evaluates the performance of the Metamorphosis treatment plant, designed to treat 20,000 m3.d−1 domestic sewage and 24,000 m3. d−1 septage from the Greater Athens region. During the period 1987-1989 the plant received about one third of the design load. Sewage and septage were subjected to separate preliminary and primary treatment and subsequently to joint activated sludge treatment. The performance of the treatment plant was satisfactory, resulting in an overall mean effluent BOD5 concentration of 24 mg.l−1 and an adequately stabilised sludge. However, persistent sludge bulking, which prevailed for most of the studied period, casts doubts on the effectiveness of future performance at full load, not only with respect to final effluent quality but also in relation to the degree of sludge stabilisation, due to possible short retention times in the digestors. Identification of the causes of bulking through systematic research and improvement of the sludge thickening processes are two priority aspects which should be presently addressed, in order to ensure efficient future performance of the plant.