Effect of Nitrogen Loading Rates on Nitrogen Removal Performance of Partial-denitrification Coupling With Anammox Biofilm System

Partial-denitrification has received wide attention as a new approach for nitrite (NO₂^--N) generation. The novel nitrogen removal process coupling partial-denitrification and anammox has great potential in application. However, considering the low substrate concentration and frequent fluctuation of municipal wastewater, efficient retention and enrichment of anammox bacteria appears to be a crucial issue for this technology. Taking this into consideration, this study established a process in biofilm systems using two types of carriers for partial denitrification coupling with anammox, and aimed to compare the long-term nitrogen removal performance, and mainly investigated the nitrogen transformation and microbial activities variation under decreasing nitrogen loading rates, as well as an in-depth analysis of extracellular polymeric substance (EPS). Result showed that, feeding with wastewater contained NH₄⁺-N and NO₃^--N, using acetate as organic carbon source, partial denitrification coupling with anammx systems with fixed biocarriers of polymeric sponges (R1) and polyethylene hollow rings (R2) were established successfully. When the influent of NH₄⁺-N and NO₃^--N gradually decreasing from about 150 mg/L to 50 mg/L, corresponding to the nitrogen loading rates (NLR) declining from 0.61 to 0.21 kg/(m³·d), both of the systems maintained the stably high nitrogen removal performance, and the average total nitrogen (TN) removal efficiencies in low NLR phase were 87.6% and 83.6%, respectively. Anammox were the dominant pathways of nitrogen removal, accounting for as high as 98.2% and 97.4%, respectively. The decreasing NLR resulted in the reduced rates of partial denitrification, while it had little impact on the NO₂^--N accumulation efficiency, which achieved 95.1% in R1 and was higher than R2 (89.8%), and it showed a less decrease in anammox activities than R2, indicating that the polymeric sponges was more suitable for long-term operation of this process under low NLR. More EPS were secreted and increasing protein would benefit for coping with varying NLR. Overall, the partial-denitrification coupling with anammox process holds significant advantage in treating low-strength wastewater, providing a new solution for bottleneck in applying anammox process, with great significance of research and values of practical application.