Abstract
In this study, an approach using influent COD/N ratio reduction was employed to improve process performance and nitrification efficiency in a membrane bioreactor (MBR). Besides sludge reduction, membrane fouling alleviation was observed during 330 d operation, which was attributed to the decreased production of soluble microbial products (SMP) and efficient carbon metabolism in the autotrophic nitrifying community. 454 high-throughput 16S rRNA gene pyrosequencing revealed that the diversity of microbial sequences was mainly determined by the feed characteristics, and that microbes could derive energy by switching to a more autotrophic metabolism to resist the environmental stress. The enrichment of nitrifiers in an MBR with a low COD/N-ratio demonstrated that this condition stimulated nitrification, and that the community distribution of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) resulted in faster nitrite uptake rates. Further, ammonia oxidation was the rate-limiting step during the full nitrification.
Highlights
Membrane bioreactor (MBR) technology is a reliable and promising process in wastewater treatment and reclamation owing to its distinctive advantages over conventional activated sludge (CAS) systems
The nitrification pathway of ammonium removal in MBRs is a two-step reaction undertaken by ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB): AOB oxidize ammonium to nitrite in the first step and NOB oxidize nitrite to nitrate in the following step [2,3]
Verhagen and Laanbroek [11] found that under such conditions the nitrifying bacteria were strongly reduced above the critical carbon-to-nitrogen ratios and the numbers of Nitrosomonas europaea decreased more than those of Nitrobacter winogradskyi. In light of these findings, we explored a novel approach to improve the process performance and nitrification efficiency in an MBR
Summary
Membrane bioreactor (MBR) technology is a reliable and promising process in wastewater treatment and reclamation owing to its distinctive advantages over conventional activated sludge (CAS) systems. Of particular significance is that the MBR systems avoid cell washout by retaining complete biomass, which favors the growth of autotrophic nitrifying bacteria and increases the nitrification efficiency, as reported previously [1]. The nitrification pathway of ammonium removal in MBRs is a two-step reaction undertaken by ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB): AOB oxidize ammonium to nitrite in the first step and NOB oxidize nitrite to nitrate in the following step [2,3]. In conventional MBRs, nitrification is not a strictly independent pathway and carbon oxidation is inevitable during this autotrophic process, which results in a bloom of heterotrophs. Even in an anoxic/oxic MBR, a considerable fraction of the organic carbon is still oxidized aerobically due to endogenous respiration of biomass as well as the leakage of organic carbon to aerobic tanks caused by the high recirculation flow [4,5]
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