Autotrophic biological nitrogen removal in a bacterial-algal symbiosis system: Formation of integrated algae/partial-nitrification/anammox biofilm and metagenomic analysis

Bacterial and algal symbiosis systems have been gradually applied in wastewater treatment. In this study, an integrated algal/partial nitrification/anammox biofilm system (IAPNABS) was constructed through light cultivation. The diverse biofilm was formed under the light, which was demonstrated by metagenomic analysis. The external biofilm at the steady stage was dominated by algae and nitrifying bacteria. Specifically, Geitlerinema and Chlorella were the two main species of algae, accounting for 6.0% and 1.9%, respectively; Nitrosomonas_europaea was the dominant species of nitrifying bacteria (2.8%). Anammox bacteria were mainly found in the inner biofilm, with Candidatus Kuenenia Stuttgartiensis and Candidatus Brocadiaceae Bacterium B188 representing the dominant species (1.0% and 0.3%, respectively). This layered biofilm performed efficient nitrogen removal. When initial NH4+-N in the system was 100 ± 20 mg/L, the IAPNABS achieved highest NH4+-N and TN removal rate (8.9 mg/(L·h) and 6.3 mg/(L·h), respectively) after 8 h of light and 2 h of dark. The oxygen produced by the algae was consumed by nitrifying bacteria, which provided the anaerobic environment and the nitrite substrate to maintain the activity of anammox bacteria. The abundance of the key enzymes involved in nitrogen removal (e.g. ammonia monooxygenase (Amo), hydrazine dehydrogenase (Hdh) and hydrazine synthase (Hzs)) further demonstrated main nitrogen metabolic processes in the layered biofilm. This study provides a new idea for autotrophic nitrogen removal from wastewater, which can greatly reduce the additional oxygen supply requirement.