Treatment Intensification of Digester Dewatering Sidestreams via Pilot- and Demonstration-Scale Revolving Algal Biofilms at Municipal Water Resource Recovery Facilities

Abstract

As water resource recovery facilities (WRRFs) pursue decarbonization, the treatment of concentrated sidestreams from anaerobic digestion offers a critical opportunity to reduce energy use and greenhouse gas emissions. This study evaluated the intensification potential of revolving algal biofilms (RAB) for sidestream nutrient removal via partial nitrification at pilot- and demonstration-scales. RAB systems were deployed at municipal WRRFs in Boulder and Iowa City, treating real dewatering sidestreams across a range of ammonium (NH4+–N) surface area loading rates (SALR) reaching up to 196 g N m–2d–2. The results demonstrated that RAB systems can achieve partial nitrification while treating such sidestreams. However, sustaining high nitrite accumulation ratios became challenging at NH4+–N SALRs exceeding 30 g m–2d–1, a threshold well above typical sector norms. In Boulder, increased loading promoted nitrite accumulation, whereas in Iowa City, extreme SALRs inhibited both ammonia- and nitrite-oxidizing bacteria (AOB and NOB), limiting overall nitrification. Nonetheless, limited complete nitrification was observed in Iowa City, likely driven by Candidatus Nitrotoga, an NOB genus tolerant of free ammonia (FA) and free nitrous acid. Although influent NH4+–N concentrations were similar at both sites, Iowa City’s higher pH and loading intensified FA toxicity. Nitrification, rather than biomass uptake, was the dominant NH4+–N removal pathway under high loading. These findings demonstrate that RAB systems can support partial nitrification under elevated loading conditions, highlighting their potential for oxygen-efficient NH4+–N treatment in phototrophic biofilm-based sidestream treatment.