Performance of aerobic digestion at different sludge solid levels and operation patterns

Laboratory batch and semi-continuous experiments were carried out at 20°C on the aerobic digestion of primary sludges and waste secondary activated sludges. Solids concentrations up to 60.000 mg l⁻¹ (6%) for primary sludges and 20.000 mg l⁻¹ (2%) for waste secondary activated sludges, were employed. It was found that aerobic digestion of more concentrated sludges was associated with slower solids degradation rates, but in the studied range of concentrations, more concentrated sludges yielded a higher mass of volatile suspended solids decomposed per unit digester volume per day. For the primary and secondary sludges with the same volatile suspended solids concentration, the amounts of volatile suspended solids decomposed per unit digester volume per day were closely comparable.The results from batch digestion experiments proved not to be directly applicable to the design of continuous flow aerobic digesters. It was also demonstrated that the treatment of a sludge with a higher solids concentration could be an effective means of utilizing the digester volume while obtaining a reasonably well stabilized product sludge.