Composition, fate, and transformation of extracellular polymers in wastewater and sludge treatment processes

Abstract

Extracellular polymers are produced by microorganisms for protection and attachment. They can be loosely-bound (slime) or firmly attached (capsule) to the cell wall. In wastewater and sludge treatment, biopolymers have important roles in flocculation, sludge dewatering, metals removal, and reuse of treated wastewater. The characterization of biopolymers in treatment processes is incomplete, with most data coming from activated sludge. This research was designed to provide a comprehensive knowledge of the composition and fate of biopolymers in wastewater and sludge treatment processes. Three sets of samples were collected in nine locations at a full-scale wastewater treatment plant during a three month period. Capsule was extracted using a cation exchange resin, while slime were separated by centrifugation. Biopolymers were analyzed for carbohydrate, protein, RNA, DNA, humic acids, total organic carbon, trace metals, and molecular weight. Protein, carbohydrate, RNA, DNA, and humic acids were all constituents of biopolymers. Protein was the major compound in both capsule and slime. The inorganic fraction comprised 10 % to 20 % of the biopolymers’ weight. Molecules with sizes greater than 2.0 x 10⁶ were present in biopolymers. The capsule content of volatile suspended solids in digested sludge was 65 % of the capsule content of VSS in activated sludge. Statistical analysis suggested that the quantity of capsular biopolymer changed in different locations, but the composition remained the same. The hypothesis that the biopolymer contents of volatile suspended solids in digested sludge, primary, and secondary effluents were the same was not rejected. The equality hypothesis for biopolymer contents between activated sludge and secondary effluent VSS was rejected. Statistical analysis suggested that the quantity of capsular biopolymer changed in different locations, but the composition remained the same. The hypothesis that the biopolymer contents of volatile suspended solids in digested sludge, primary, and secondary effluents were the same was not rejected. The equality hypothesis for biopolymer contents between activated sludge and secondary effluent VSS was rejected. These results, together with the observation that digested sludge VSS contained less biopolymer than activated sludge VSS, suggest that particles from anaerobic digester were significantly present in effluents, or there was a preferential settling of particles holding more biopolymers in the sedimentation tanks. Mass balances showed production of capsular biopolymer constituents in the activated sludge process, and destruction during anaerobic digestion. Production and destruction varied in the ranges of 25 % to 40 %, and 60 % to 70 %, respectively. The contents of trace metals in capsular biopolymer from wastewater were higher than the metal contents at other locations in the plant. Trace metals associated with capsule represented less than 10 % of the metals measured in samples; in samples with low SS concentration, trace metals associated with slime reached 45 % of the metal contents in samples.