Investigation of stable nitritation in a membrane bioreactor treating reject water

• Main Author: Jiang, Yishuai
• Other Authors: Ng Wun Jern
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering::Environmental pollution; Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/137692
• Institution: Nanyang Technological University

Employment of nitritation process in membrane bioreactors (MBRs) has attracted much research and practical application interest. Stable nitritation needs not only to favor growth of ammonia oxidizing bacteria (AOB), but also to inhibit that of nitrite oxidizing bacteria (NOB) in long-term operations. However, both AOBs and NOBs are retained in the MBR due to the membrane separation and particularly so when operated with long solids retention time (SRT). Membrane fouling is a significant challenge in MBR. The unique characteristic of nitritation MBR is that nitrite acclimation in the reactor can lead to the increase in free nitrous acid (FNA) concentration. The latter would impact on extracellular polymeric substances (EPS) production. Thus, this project had aimed to investigate feasibility of long-term stable nitritation in a MBR with long SRT, and the consequent membrane fouling under the influence of elevated FNA level. Such membrane fouling would then require a fouling mitigation strategy. The strategy developed in this study verified that spiked FNA shock together with DO limitation can be used for maintaining nitritation in MBRs with long SRTs at relatively low capital and operating expenditure (CAPEX and OPEX) when treating ammonium rich wastewater. This study conducted long-term analyses on changes in the trans-membrane pressure (TMP), EPS composition and microbial community change to understand the membrane fouling mechanism of a nitritation MBR.Protein (PN) and polysaccharide (PS) were found to increase with higher FNA concentrations. However, in this study, PN and PS concentrations were reduced by 23.4% and 19.0%, respectively when FNA concentration reached 1.47×10-2 mg HNO2-N/L. Reduction of EPS consequently led to reduced fouling rate. This suggested that FNA could be regulated to mitigate membrane fouling in the nitritation MBR. High throughput sequencing analysis showed that Nitrosomonas was enriched at high FNA concentrations in the nitritation MBR. This project further systematically investigated the effectiveness of FNA on the membrane fouling mitigation in the nitritation MBR. It was found that FNA up to 2.7×10-2 mg HNO2-N/L reduced LB-EPS and TB-EPS in both suspended sludge and biofilm, and thus maintained the TMP in the low range compared to the control nitritation MBRs. Moreover, EPS sub-fraction analysis by LC-OCD-OND explicated that FNA caused the significant reduction in hydrophobic DOC and LMW neutrals, which played a critical role in the PVDF membrane fouling. The EEM-FRI technique illustrated the oxidative effect of FNA triggered slightly more tyrosine like protein but much less SMP and humic acid like substances in the TB-EPS. The XPS analysis also revealed the inhibitory effect of FNA to the composition ratio of functionalities among the bound EPS. As a result, the in-situ generated FNA inhibited the biofilm formation in terms of thickness and mass, in a way to mitigate the membrane fouling in the nitritation MBRs.The information obtained in this project was of fundamental significance for understanding membrane fouling mechanism in the nitritation MBR and provided valid evidence for membrane fouling mitigation by FNA when treating high ammonium concentration wastewater.