Comparison of physical and chemical methods for extraction of coliform from wastewater particles and flocs

Disinfection processes are hindered by the presence of particles that protect the micro-organisms from the disinfectant. To understand better the extent of particle association and its effect on disinfection, it is necessary to efficiently release and enumerate particle-associated micro-organisms. Using a wastewater from a single treatment facility, various physical and chemical methods were evaluated to extract micro-organisms from particles present in wastewater, and a method was recommended. Increases in coliform bacteria were used as the indicator to evaluate the effectiveness of extraction methods. Physical extraction methods evaluated were blending, homogenization, stirring, sonication, and centrifugation, and chemical extraction methods were EDTA and EGTA extraction together with a combination of Zwittergent 3-12, Tris buffer, EGTA, and peptone. The results of this study indicate that high shear forces are not needed to extract particle-associated coliform in wastewater, as measured by increases in bacterial counts. Low-speed blending (3,500 rpm) or high-speed stirring (1,000 rpm) were both effective in extracting coliform from wastewater particles and flocs. Homogenization, high-speed blending, and sonication at typical operational speeds and durations used in soil and activated carbon studies decreased the coliform counts in wastewater. Among the methods tested in this study, blending with EGTA, EDTA, and a combination of Zwittergent 3-12, Tris buffer, EGTA, and peptone at 3,500 rpm were found most effective in extracting particle-associated coliform from wastewater particulates.