Engineering viability of attached growth reactors for mainstream anammox: Superior stability and sludge minimization at low temperatures

Applying the anaerobic ammonium oxidation (anammox) process to mainstream wastewater treatment is often hindered by the difficulty of retaining slow-growing biomass at low temperatures and under hydraulic fluctuations. Herein, a suspended growth reactor (SGR) and an attached growth reactor (AGR) were comparatively evaluated to determine their process stability and engineering viability for mainstream nitrogen removal. Both reactors were operated for over 200 days at different temperatures (dropping from 35°C to 17°C) and hydraulic loading rates. The SGR, which operated with an “infinite SRT” strategy that relied solely on gravity settling, exhibited severe destabilization at low temperatures. In SGR, the deterioration of sludge settling properties led to irreversible biomass washout, reducing the nitrogen removal efficiency to 54% at 17°C. In contrast, the AGR demonstrated superior stability, maintaining an average removal efficiency of 95.8% even at 17.1°C with no external heating. This resilience was attributed to its “hybrid retention” mechanism, where the media packing (filling ratio = 70%) effectively retained both the attached biofilm and suspended solids, naturally decoupling the hydraulic retention time from the solids retention time (SRT). A mass balance analysis was conducted, revealing that the AGR achieved an exceptionally low observed sludge yield (0.010 g volatile suspended solids (g N)⁻¹), confirming that the extended SRT promoted endogenous decay and minimized excess sludge production. Therefore, the AGR configuration overcomes the biomass retention limitations of suspended systems, offering a reliable and cost-effective solution for applying the anammox process to mainstream wastewater treatment.