Influence of Influent Load on Nitrification/Denitrification with MBBR for Oil Shale Retorting Wastewater Treatment: Performance and Microbial Community Structure

Oil shale retorting wastewater is currently recognized as one of the most complex industrial wastewater due to its ultra-low chemical oxygen demand to total nitrogen (C/N) ratio, high organic load, and complex chemical composition. In this paper, the effect of influent load on the performance and microbial community changes of moving bed biofilm reactor(MBBR) in the treatment of highly polluted retorting wastewater was investigated. The MBBR was proposed and demonstrated to be efficient in treating the retorting wastewater, containing COD and NH₄⁺-N of 5000-10000mg/L and 3000-5000mg/L. The removal efficiency of COD and NH₄⁺-N increased steadily in 1-79d, and the effect was the best when the influent load was 0.0242–0.0312kg COD/kg MLSS·d and 0.0173–0.0208kg NH₄⁺-N/kg MLSS·d. The removal efficiency of COD was up to 83.46%, and the removal efficiency of NH₄⁺-N was 82.08%. The long-term operation showed that the biofilm could be rapidly formed after 40 days. Scanning electron microscope observation showed that cocci and short bacilli appeared in the pores of the fillers. With the increase of influent load, the total amount of extracellular polymeric substances increased significantly, in which the content of protein increased sharply in stage III, while the polysaccharide was in a state of continuous and stable growth. High-throughput sequencing analysis showed that the nitrifying functional genus Nitrosomonas was highly enriched in the MBBR system (0 → 17.55%) and played a key role in the high accumulation of NO₃⁻-N. The newly added denitrifying bacteria Brevundimonas (5.85%) and Reyranella (4.57%) enhanced the degradation of COD and maintained the nitrogen cycle in the system. The enrichment of Acinetobacter, Rugosibacter, and unclassified_Syntrophobacteraceae in the system played an important role in the degradation of complex organic compounds such as polycyclic aromatic hydrocarbons and aromatic compounds.