Aerobic digestion of pharmaceutical and domestic wastewater sludges at ambient temperature

The feasibility of aerobic digestion for pharmaceutical and domestic sludges was studied in order to determine the best operational conditions for digestion and to assess the need for pH stabilisation. Sludge concentrations ranging from 1500 to 11 000 mg l−1 and from 4500 to 22 000 mg l−1 were tested with representative domestic and pharmaceutical waste activated sludges, respectively. Temperature, dissolved oxygen, pH, mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), settled sludge volume, sludge volume index (SVI), stirred sludge volume index (SSVI), specific oxygen uptake rate (SOUR), sludge capillary suction time (CST), supernatant biochemical oxygen demand (BOD5), supernatant chemical oxygen demand (COD), supernatant turbidity, sludge floc morphology and biodiversity were all monitored. The reductions achieved in settled sludge volume, MLSS and MLVSS were higher for domestic than for pharmaceutical sludges. A significant improvement in domestic sludge settleability (due to a change in floc morphology) appeared between 14 and 21 d of aerobic digestion, followed by an increase in supernatant turbidity and a decrease in sludge filterability. Eventually, pH stabilisation at 6.5 improved sludge characteristics, leading to a clearer recycled supernatant and a better sludge filterability. No odour or foam problems were recorded. The aerobic digestion performance achieved was a reduction of 42–53% MLSS, 53–64% MLVSS, 66–93% SOUR, 53–81% settled sludge volume and full stabilisation for domestic sludges; while for pharmaceutical sludges reductions of 4–18% MLSS, 6–25% MLVSS, and 64–79% SOUR were achieved with full stabilisation.