Downflow sponge biofilm reactors for polluted raw water treatment: Performance optimisation, kinetics, and microbial...

Water outages caused by elevated ammonium (NH₄⁺-N) levels are a prevalent problem faced by conventional raw water treatment plants in developing countries. A treatment solution requires a short hydraulic retention time (HRT) to overcome nitrification rate limitation in oligotrophic conditions. In this study, the performance of polluted raw water treatment using a green downflow sponge biofilm (DSB) technology was evaluated. We operated two DSB reactors, DSB-1 and DSB-2 under different NH₄⁺-N concentration ranges (DSB-1: 3.2-5.0 mg L^-1; DSB-2: 1.7-2.6 mg L^-1) over 360 days and monitored their performance under short HRT (60 min, 30 min, 20 min, and 15 min). The experimental results revealed vertical segregation of organic removal in the upper reactor depths and nitrification in the lower depths. Under the shortest HRT of 15 min, both DSB reactors achieved stable NH₄⁺-N and chemical oxygen demand removal (≥95%) and produced minimal effluent nitrite (NO₂^--N). DSB system could facilitate complete NH₄⁺-N oxidation to nitrate (NO₃^--N) without external aeration energy requirement. The 16S rRNA sequencing data revealed that nitrifying bacteria Nitrosomonas and Nitrospira in the reactor were stratified. Putative comammox bacteria with high ammonia affinity was successfully enriched in DSB-2 operating at a lower NH₄⁺-N loading rate, which is advantageous in oligotrophic treatment. This study suggests that a high hydraulic rate DSB system with efficient ammonia removal could incorporate ammonia treatment capability into polluted raw water treatment process and ensure safe water supply in many developing countries.