Repurposing hydrolysis acidification tank in municipal wastewater treatment plants for sludge reduction and biological nutrient removal

The hydrolysis acidification tank (HAT) was usually selected as the pretreatment of combined industrial and domestic wastewater for municipal wastewater treatment plants (MWWTPs). Strict source control and urbanization substantially reduced industrial wastewater fed into MWWTPs, and thus HAT should be repurposed. Long-term survey of a full-scale MWWTP showed HAT had adverse effects on biological nutrient removal (BNR) of subsequent activated sludge systems because it increased difficulty of BNR owing to carbon source loss, and generated sulfide preferentially combining with ammonia monooxygenase in anaerobic phase of posterior biological treatment process to significantly inhibit activities of nitrifiers. Full-scale experiment of bypassing HAT achieved more efficient nitrogen and phosphorus removal, and thus three pilot-scale systems were designed to investigate process performance by upgrading HAT to anaerobic side-stream reactor (ASSR) and anaerobic main-stream reactor (AMSR). The three systems were equally effective in chemical oxygen demand (COD), total phosphorus and ammonium nitrogen removal. Upgrading HAT to ASSR and AMSR enhanced total nitrogen removal from 63.3 to 65.1 and 72.0%, and achieved sludge reduction of 15.4 and 40.5%. Intrasystem flow analysis confirmed that upgrading increased proportion of COD utilized for denitrification. MiSeq sequencing results showed that AMSR enriched hydrolytic and predatory bacteria for sludge reduction, while ASSR favored the growth of slow growers. The results confirmed that AMSR is a cost-effective and advantageous strategy for HAT upgrading with higher nitrogen removal, efficient sludge reduction, and improved sludge settleability and dewaterability.