Three large Wastewater Treatment Plants (WWTP) in Greece with occasional severe foaming were selected in order to evaluate the influence of the recycle of foaming filamentous bacteria from the solids handling processes to their foaming problems. According to the results, a range of 3–39% of the total quantity of viable foaming filamentous bacteria present in biological wastewater treatment stage of the WWTPs was found to be recycled through the underflows of thickening and dewatering processes. In parallel, the feasibility of alternative nonspecific foam control methods was assessed. Within the context of this study two foam control practices were evaluated: (a) the addition of polyaluminium chloride (PAX) to the mixed liquor and (b) the selective wasting of foam from the activated sludge system followed by dewatering. The economic feasibility of each method was assessed by evaluating and comparing the total cost (capital cost and operation and maintenance costs) of each alternative for Psyttalia Sewage Treatment Works, which is the largest WWTP in Greece serving the Greater Athens Area with a treatment capacity of approximately 3.5 million people. The estimated total costs of the two foam control methods were 0.009 and 0.002 €/m3 of wastewater for PAX addition and selective wasting of foam, respectively.

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