Involvement of Acinetobacter sp. in the floc-formation in activated sludge process

► We study coaggregation of Acinetobacter johnsonii S35 isolate with sewage bacteria. ► A. johnsonii S35 had a high affinity for many of sewage bacteria and activated sludge flocs. ► A. johnsonii S35 cells and sewage activated sludge showed a hydrophobic character. ► Hydrophobic interaction might play an important role in the coaggregation. ► Acinetobacter spp. were derived from the influent. The coaggregation behavior of Acinetobacter johnsonii S35 isolate with sewage bacteria was assessed by a spectrophotometric assay using different samples from a municipal wastewater treatment plant and a community plant. A. johnsonii S35 coaggregated well with other free bacteria and microflocs at the mixing ratios of 0.2:1–0.6:1 of A. johnsonii S35 and sewage samples. In addition, the size of coaggregates became larger (100μm or more) under the same conditions. A. johnsonii S35 cells were highly adsorbed (adsorption=93–99%) onto sludge samples. Microbial adhesion to hydrocarbon (MATH) test and adsorption to octyl-Sepharose CL-4B showed that A. johnsonii S35 cells and sludge samples had a hydrophobic character. The population of Acinetobacter spp. in sewage treatment plants was 2–7% and its role in bioflocculation was discussed. The present study revealed that A. johnsonii S35 isolate can play as a bridging organism and contribute in floc-formation in activated sludge process.