Foaming Control in Activated Sludge Treatment Plants by Coagulants Addition

This paper presents the results of an investigation that aimed to evaluate the efficiency of various inorganic coagulants and organic polymers to combat filamentous foaming and bulking problems, caused by the proliferation of M. parvicella and/or Gordona amarae. The duration of the investigation covered 14 months. During this period foam samples were taken from the aeration basins of two full-scale wastewater treatment plants that contained high concentrations of M. parvicella and/or Gordona amarae. Bench scale batch experiments were conducted to evaluate the efficiency of the following coagulants: ferric chloride, ferrous chloride, polyaluminium chloride, hydrated aluminium sulphate, cationic polymer. In addition bench scale batch experiments (ammonia uptake rate (AUR) and oxygen uptake rate (OUR) measurements) were conducted to evaluate the toxicity effects of the most promising coagulants on nitrification and organic matter removal. Polyaluminium chloride (PAX) and cationic polymer proved to be the most efficient among all the coagulants investigated. By adding PAX (aluminium application of 6.6 – 11.5 g Al kg MLSS) a general improvement of the settling properties of the activated sludge was achieved. High foaming control was also achieved with cationic polymer addition at doses in the 3.5 to 4.5 g kg MLSS. According to microscopic analysis of the sludge samples following PAX or polymer addition, the floc strength was improved and flocs appeared more compact and dense. In addition both M. parvicella and Gordona amarae filamentous organisms were embedded inside the floc material, making access to particulate and colloidal substrates more difficult due to increased diffusional resistance. PAX or polymer addition, at the above specified doses, resulted in a 75 – 100 % reduction of the sludge foaming potential. PAX potential toxicity was investigated in the context of this study. PAX addition caused no inhibition on autotrophic and heterotrophic bacteria that perform nitrification and organic carbon removal, respectively. The estimated operational costs of PAX and polymer addition for foaming control, are approximately 0.0057 euros m and 0.0026 euros m of wastewater treated, respectively.