Maximum COD Loading Capacity in UASB Reactors at 37°C

Abstract

The maximum capacity chemical oxygen demand (COD) loading in upflow anaerobic sludge blanket (UASB) reactors is evaluated using three 8.5 L reactors and high‐strength synthetic wastewaters composed of milk and sucrose at 37°C. The study was conducted over a wide‐range COD loading rate (18–260 g·L⁻¹·day⁻¹), by varying hydraulic retention time (HRT) (1.8–10 hr) and COD levels in wastewater (6,000–20,000 mg/L). At COD loading rates of up to 160 g·L⁻¹·day⁻¹, which corresponded to HRT of 1.8 h and COD of 12,000 mg/L in the wastewater, the process removed 94–98% of soluble COD and 75–90% of total COD. At rates higher than 160 g·L⁻¹·day⁻¹, the efficiency of COD removal deteriorated because of sludge disintegration and washout resulting from vigorous mixing by the biogas produced. The capability of volatile suspended solids (VSS) to convert COD to methane increased with COD loading; it reached the maximum of 1.7 g methane COD per gram VSS per day at the COD loading of 100 g·L⁻¹·day⁻¹, corresponding to the food‐to‐microorganism (F/M) ratio of 3 g COD per gram VSS per day. Characteristics of the sludge granules, effluent, and biogas are also discussed.