The effect of different sludge pretreatment methods on mitigation of anaerobic digester foaming

Abstract

In this study, waste activated sludge obtained from two full-scale treatment plants with foaming issues was pretreated with acid/alkali treatment, acid-phase fermentation, thermal treatment, sonication, and metal salt treatment to investigate their effect on foam mitigation. The major foam forming organisms were identified by microscopy and qPCR as Gordonia amarae, which are characterized as branched filaments. Waste activated sludge was characterized for foaming index prior to each pretreatment method. After carrying out different pretreatment methods, the foaming potential was again measured in batch experiments carried at different holding times per pretreatment methods. Among all pretreatment methods investigated, acid/alkali treatment and acid phase fermentation both showed a high reduction in foaming and in inactivation of G. amarae filaments and thermal treatment combined at alkaline pH enhanced the effectiveness of foam reduction. The maximum foaming reduction of 56% was achieved upon holding the sample for 2.5 h at 70oC and pH of 11. Pretreatment methods that resulted in a sharp decrease in the foaming potential concurred with numbers in dead foam formers as confirmed using live/dead staining and the PMA-qPCR technique. Pretreating with FeCl3 gave good foaming reductions with 53.5% reduction in the unstable foaming potential at a concentration of 260g/L but did not result in a decrease of foam formers as confirmed by live/dead staining. Sonication did not prove effective in lowering the foaming index or killing the G.amarae filaments.