Model-Aided Transition from Lab to Full-Scale VFAs Recovery for Enhanced Nitrogen Removal in a WWTP

In the context of global scarcity of carbon and energy resources, resource recovery from wastewater treatment plants (WWTPs) has become increasingly important. This study focuses on recovering and reusing short-chain volatile fatty acids (VFAs) as sustainable bioproducts, particularly to be used as substrates in biological nutrient removal (BNR) activated sludge systems. We utilized Temperature Phased Anaerobic Digestion (TPAD) to produce VFAs from primary sludge at the largest Italian WWTP. The TPAD process employed three reactors: two smaller reactors operated sequentially under thermophilic (55 ℃, 2 days HRT) and mesophilic (38 ℃, 20 days HRT) conditions, and a larger reactor for scaled-up mesophilic processing (38 ℃, 20 days HRT). This configuration allowed for a comprehensive assessment of VFAs production and sludge biodegradability across different thermal regimes. The digestate from these reactors was subjected to extensive physicochemical and respirometric analyses to identify their organic fractions. A process simulation model, calibrated and validated with extensive data collection, was used to assess the impact of VFAs extraction and utilisation as an external carbon source on WWTP operations. Scenario analysis, considering variations in solid retention times (SRT) and digestate flow rates, indicated that optimal denitrification in anoxic units is achievable with an SRT exceeding 18 days and digestate flow rates between 100–160 m3/d. This study underscores the feasibility and key operational considerations for integrating VFAs recovery into existing WWTP frameworks to enhance nitrogen removal.