Role of Acinetobacter johnsonii S35 isolate in floc-formation in activated sludge process

Activated sludge process is widely used for removing organic and inorganic substances from municipal as well as industrial wastewaters. The process depends on the settleability of flocs formed by microorganisms (mainly bacteria). Therefore, the bioflocculation, mentioned mainly to the isolation and flocculation mechanism of floc-forming bacteria, have been investigated by many workers. Meanwhile, many nonflocculating bacteria are known tobepresent in any sludge samples.Wepreviously reported that nonflocculating bacterium, Acinetobacter johnsonii S35 isolate, coaggregated with several kinds of Gram-positive and -negative bacteria, obtained from the same sewage activated sludge. In the present study, the coaggregation behaviour of A. johnsonii S35 isolate with sewage bacteria was investigated using different samples from a municipal wastewater treatment plant and a community plant. Thecoaggregationexperimentwasassessedbya spectrophotometric assay and the mixing ratios of A. johnsonii S35 and settled sewage sample were 0.2:1–1.5:1 on the basis of their turbidity. The relationship between coaggregation behaviour and cell surface characteristics was also studied using the microbial adhesion to hydrocarbon (MATH) test and adsorption to octyl-Sepharose CL4B. Population and role of Acinetobacter spp. were also examined using fluorescence in situ hybridization (FISH) technique. The results showedthatA. johnsoniiS35coaggregatedwithother free-floating bacteria or microflocs at the mixing ratios of 0.2:10.6:1 of A. johnsonii S35 and sewage samples. In addition, the size of coaggregates became larger under the same conditions. These indicated that A. johnsonii S35 played a role as a bridging bacterium in the coaggregation. A. johnsonii S35 cells were highly adsorbed onto sludge samples. MATH test and adsorption to octyl-Sepharose CL4B showed that A. johnsonii S35 cells and sludge samples had a hydrophobic character. According to the result of FISH, the population of Acinetobacter spp. presented in settled sludge and settled sewage in a primary settling tank as well as in mixed liquor in an aeration tank of the municipal wastewater treatment plant were 6.7%, 4.0%, and2.7%, respectively. On the other hand, the population ofAcinetobacter spp. in influent andmixed liquor in anaeration tank of the community plantwere 6.2% 2.2%, respectively. These results suggest that Acinetobacter spp. were derived from the influent and play an important role in the formation of sludge flocs. The overall results demonstrated that A. johnsonii S35 isolate had a high affinity for many of settled sewage bacteria and activatedsludgeflocs.A. johnsoniiS35cells andsewageactivatedsludge used showed a hydrophobic character. The population of Acinetobacter spp. were higher in primary settling tank, suggesting that Acinetobacter spp. were derived from the influent. These suggest that bacteriawith a hydrophobic cell surface like Acinetobacter spp. isolates may play similar function as bridging organisms and contribute in solid–liquid separation in activated sludge process.