Continuous aerobic granular sludge activated by a contracted final settler and associated hydraulic measures

A contracted & baffled final settler, with a low H (height)/D (diameter) ratio and a high hydraulic selective pressure (HSP) imposed by elevating up-flow velocity by influent or effluent, was applied to promote in-situ granulation in a continuous biological nutrient removal (BNR) process. Under observational HSP_obs = 0.9-3.2 m³/(m²·h), baffles could retain potential granules, and wash out flocs via 4-5 times higher up-flow velocity through baffle gaps. Continuous granulation was efficient within 44 d (HSP_obs = 0.9-1.9 m³/m²·h): granulation increased to 42%, and SVI₃₀ decreased to 52 mL/g from 97 mL/g, with COD ≥ 85%, TN ≥ 90% and TP ≥ 90%. A higher HSP_obs (1.9-3.2 m³/m²·h) produced bigger granules (700 μm) and a higher granulation (95%). Extracellular polymeric substances (EPS), interfacial thermodynamics analysis indicated the settler selectively enriched granules with enhanced compactness. Microbial analysis revealed the enrichment of floc-forming bacteria and filamentous bacteria. This study provides an efficient and simple method for continuous granulation.