Extracellular polymer substances and physicochemical properties of flocs in steady- and unsteady-state activated sludge systems

The content of protein, polysaccharide and DNA were quantified in four different industries and municipal sludge floes by continuous operation of five lab-scale activated sludge reactors under steady- and unsteady-state conditions. Physicochemical properties such as surface charge, bound water and contact angle in floe biomass were determined under both conditions. The effect of anaerobic conditions and nutrient deficiencies on the floc surface properties were investigated. Extracellular polymer substances (EPS) analysis showed that between 70 and 80% of the extracellular organic material could be attributed to protein. Lower amounts of protein and high DNA levels were found in the extracellular substances from flocs grown on more complex organics and substrates. Changing the wastewater from chemical, leather and dye to that of the wine industry and municipal treatment plants resulted in an increase in protein and a decrease in DNA level of floe EPSs. Good settling characteristics were observed in both wine industry and municipal activated sludge samples and this was associated with high negative charged floe surface and both lower bound water and contact angle containing floe EPS. Low protein and high DNA level in EPS is associated with both high contact angle, bound water and low negative charges in activated sludge samples. This is associated with high sludge volume index (SVI) value, indicating poor settleability. In all samples, low SVI values were associated with a high negatively charged floe surface and low contact angle and bound water containing floe EPS. Hydrophobicity (large contact angles) of floes is obtained with high SVI containing activated sludge samples. Increases in protein and polysaccharide levels in EPS floe were observed at sequential low/high F/M ratios and low dissolved oxygen concentrations, In contrast, decreases in protein and polysaccharide amounts were obtained under anaerobic conditions. Nitrogen deficiency increased the protein level in the EPS and was associated with good settling (low SVI) and high negatively charged floe surfaces while phosphorous deficiency decreased the protein content of EPS and surface charge of floes in activated sludge. (C) 2002 Elsevier Science Ltd. All rights reserved.