Utilization of three-layered polyvinyl alcohol gel cubes for treating low-strength ammonium wastewater in a single-stage autotrophic nitrogen removal process

Abstract

This study examined the feasibility of using a three-layered polyvinyl alcohol (PVA) gel cube for low-strength ammonium wastewater (<50 mg-NH4+-N/L) treatment in a single reactor. The outer layer of this three-layered PVA gel cube was composed of immobilized nitrifying bacteria, while the inner layers were composed of anaerobic ammonium oxidation (anammox) bacteria. A single-stage autotrophic nitrogen removal (SANR) reactor containing these three-layered PVA gel cubes was operated at 35 °C for 197 days, with intermittent aeration. Although the activity of nitrite-oxidizing bacteria (NOB) was incompletely inhibited under a narrow range of dissolved oxygen (DO) concentrations (0.1–1.1 mg-O2/L), a nitrogen removal efficiency (NRE) of 71.9% was achieved at 35 °C. At this temperature, the high NRE was achieved due to the coexistence of anammox and denitrifying bacteria. After decreasing the temperature to 20 °C, the NRE was maintained at a similar level (69.8%) for 53 days. The change in the structure of the bacterial community was insignificant with decreasing temperature. The 16S rRNA high-throughput sequencing analysis revealed that Planctomycetes (46.8–53.4%) was the most abundant phylum, wherein anammox bacteria accounted for 45.4–51.9% of the relative biodiversity abundance. Candidatus Jettenia asiatica disappeared for the operation of 187 days, while Candidatus Brocadia sinica was abundant. After 220 days, the relative abundances of ammonium-oxidizing bacteria (AOB) and NOB doubled and tripled, respectively. The abundant AOB were Nitrosomonas aestuarii and Nitrosomonas ureae, while the dominant NOB was Nitrospira moscoviensis. Nitrate produced by undesirable NOB activity was converted to nitrite by Denitratisoma oestradiolicum, which led to the abundance of anammox bacteria.