Investigation of extracellular polymer substances (EPS) and physicochemical properties of different activated sludge flocs under steady-state conditions

The influence of different wastewater compositions on the extracellular polymer substances (EPS) (protein, polysaccharide and DNA) and physicochemical properties (surface charge, bound water and contact angle) were studied in continuous fed completely stirred tank reactor (CSTR). Five laboratory-scale activated sludge reactors were operated under steady-state conditions. Protein was the predominant polymer followed by polysaccharide and DNA. Lower amounts of protein and higher DNA levels were found in the extracellular substances from flocs grown on more complex organics and substrates. A high level of protein (nearly 70 mg g−1 VSS) and a low DNA content (nearly 6 mg g−1 VSS) were measured in the EPS of the winery industry and municipal activated sludge while lower levels of protein (approximately 38–42 mg g−1 VSS) and higher levels of DNA content (approximately 11–17 mg g−1 VSS) were measured in the pulp-paper, textile (cotton knit fabrics) and petrochemical floc EPSs under steady-state conditions. Flocs with small (36 μm) and large (nearly 130 μm) diameters were observed in the pulp-paper, textile and petrochemical industries, respectively, associating with high SVI values. Larger contact angles (between 37 and 42°), low negatively charged surface values (between −0.11 and −0.25 meq g−1 VSS) and high amounts of bound water (between 20 and 24 g g−1 TSS) were determined in the pulp-paper, textile and petrochemical industry activated sludge samples. However, floc samples from the municipal and winery industry activated sludge had small contact angles (between 7 and 9°) and low bound water content (7–9 g g−1 TSS) together with high surface charge values (between −0.55 and −0.61 meq g−1 VSS), indicating good settleability. Since the reactor design of the five laboratory-scale reactor were similar and different wastewater were compared, it can be said that the EPS composition depends more on wastewater type rather than operating conditions.