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Factors Affecting Floc Properties During Aerobic Digestion: Implications for Dewatering

Laboratory aerobic digestion studies were conducted to determine the effect of divalent cations on the characteristics of aerobically digested activated sludge. Separate reactors were operated at two divalent cation concentrations. Characteristics examined included dewatering properties (cake solids, specific resistance to filtration, and capillary suction time), polymer conditioning requirement, floc strength, supernatant chemical oxygen demand (COD), supernatant turbidity, soluble cations (sodium, potassium, calcium, magnesium, and ammonium ions), soluble anions (nitrate and nitrite), and soluble exocellular polymers (proteins and polysaccharides). The reactor containing higher amounts of calcium and magnesium (1.25 mM each) exhibited better dewatering properties, higher floc strength, lower polymer conditioning requirement, lower soluble COD, lower supernatant turbidity, and lower soluble exocellular polymers than the reactor containing lower calcium and magnesium concentrations (0.40 mM each). Floc deterioration was associated with a release of soluble proteins and polysaccharides and monovalent cations (sodium and potassium) to the bulk solution. Divalent cations were not released to the bulk solution, indicating they may participate in floc binding. Mineralization of nitrogen was not affected, suggesting that aerobic digestion of activated‐sludge solids (degradation rates) was not affected. These results imply that the addition of divalent cations improves aerobic digester performance with regard to floc properties, supernatant quality (implications for recycled COD), and dewatering performance.