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In document IWA Biofilms 2020 (Sider 115-137)

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Advancing Resource Recovery Using Hybrid Membrane Aerated Biofilm Reactor Processes

Carlson, A.*, He, H.*, and Wagner, B.*, Yang, C.* Daigger, G.*

*Department of Civil and Environmental Engineering, University of Michigan, 1351 Beal Avenue, Ann Arbor, MI 48109 USA

Keywords: MABR; Hybrid; Nutrients Summary of key findings

Modeling results indicate significant opportunities to accomplish greater TIN removal with the hybrid MABR process, and at significantly lower suspended growth SRT. This occurs because influent COD is largely available for denitrification, compared to the conventional MLE process where a relatively large aerobic zone is needed to allow nitrifiers to grow in the suspended growth system. Shorter SRT reduces the fraction of influent biodegradable organic matter oxidized, allowing a higher fraction to be captured for other uses, such as conversion to biogas. Significant energy savings are possible, both because the process oxygen requirement is reduced and the required oxygen is provided using the much more energy-efficient MABR. Mixed liquor recirculation is also not needed. Further opportunities to optimize the MABR component to accomplish partial nitritation and anammox also exist. Experimental results to date are consistent with model predictions.

Background and relevance

Hybrid biological systems incorporate biofilm and suspended growth processes, thereby combining the advantages of each system component. Coupled trickling filter/activated sludge processes, popular in the 1980’s and early 1990’s due to their energy-efficiency and ability to produce high-quality effluents, represent one example. Another is integrated fixed film activated sludge (IFAS) which couples moving bed biofilm reactor (MBBR) media into a suspended growth process. These processes used biofilm options (trickling filters, MBBR’s) available at the time.

Membrane Aerated Biofilm Reactors (MABR’s) represent a new biofilm reactor type that is currently becoming commercially available (Houweling and Daigger, 2019). MABR’s offer unique features compared to other available biofilm technologies in that oxygen is delivered directly to the interior of submerged biofilms, rather than first being transferred to the bulk liquid surrounding the biofilm. This is referred to as counter-current rather than co-current diffusion (Downing and Nerenberg, 2008).

Delivery of oxygen directly to the biofilm allows an aerobic biofilm to be created as a component of a largely anoxic/anaerobic process. Transfer of oxygen directly to the biofilm, rather than first to the bulk liquid, also results in a significant increase in oxygen transfer efficiency. A research group at the University of Michigan has been investigating how the unique features of MABR’s can be coupled with other suspended growth and biofilm process components to create hybrid processes that offer significant opportunities to increase resource recovery compared to conventionally available processes (Daigger, et al, 2019).

Results and discussion

Here we present three concepts illustrating the potential advantages of hybrid MABR processes based on modelling using the SUMO wastewater process simulator (Dynamita). Influent wastewater flows and loadings were developed using standard per-capita values. SUMO default wastewater

characteristics and stoichiometry and kinetics were used.

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Hybrid MABR/Anoxic Suspended Growth Processes Can Produce Effluent TIN ≈ 1 mg-N/L with Much Less Energy and Increased Carbon Capture Than Conventional Systems. Figure 1.1 compares the effluent total inorganic nitrogen (TIN) and biogas production from the anaerobic digestion of primary and waste activated sludge (WAS) (20 day digester SRT) for a hybrid

MABR/anoxic suspended growth process (hybrid MABR) with a conventional MLE process operating at 20 oC. The MABR process component is sized primarily for nitrification at an ammonia loading rof 3 g-N/m2-day. Some denitrification occurs in the MABR biofilm, largely using dissolved readily

biodegradable organic matter, with most denitrification occurring in the suspended growth using particulate and colloidal organic matter as carbon source. The results indicate that effluent TIN of approximately 1 mg-N/L can be achieved with the hybrid MABR at a suspended growth SRT of around 1.5 Days, compared to an effluent TIN of over 6 mg-N/L for the conventional MLE process.

Much lower SRT for the MABR/suspended growth process results in less oxidation of biodegradable organic matter and increased capture of

biodegradable organic matter for conversion into biogas in the anaerobic digester. Results indicate similar effluent TIN if primary treatment is not provided but with less conversion of influent COD to biogas (results not shown). The process oxygen requirement for the hybrid MABR process is less than for the conventional MLE process because of the much lower SRT.

The process energy requirement is further reduced because oxygen is transferred with the much more efficient MABR process

compared to conventional oxygen transfer systems, and because mixed

liquor recirculation is not needed, resulting in a total energy requirement less than one-third that of the

conventional system. Much

Figure 1.1 Effect of Suspended Growth SRT on Effluent TIN and Percent of Influent COD Converted to Biogas in Conventional Anaerobic Digester (20 Day HRT) for MLE and Hybrid MABR Process at oC Following Primary Treatment.

lower suspended growth SRT allows use of a bioreactor that is around one-third the size required for the conventional MLE process. Biogas production is approximately one-quarter greater. These advantages for the hybrid MABR process increase as wastewater temperature decreases because the temperature sensitivity for heterotrophs (which define the suspended growth SRT for the hybrid MABR process) is less than nitrifiers (which largely defines the suspended growth SRT for the conventional process).

MABR/Anaerobic and Anoxic Suspended Growth Processes Can Produce Effluent OP < 0.1 mg- P/L and TIN ≈ 1 mg/L with Much Less Energy Than Conventional Systems. Table 1.1 compares the effluent ortho-phosphorus (OP) and TIN for a hybrid MABR N&P removal process with a conventional A2/O process following primary treatment and operating at a temperature of 20 oC.

MLE (TIN) MLE (Biogas) 10

MABR (TIN) MABR (Biogas)

55%

50%

45%

40%

35%

0 2 4 6 8 30%

Suspended Growh SRT (Days)

Effluent TIN (mg-N/L) Biogas Production (Percent of Influent COD)

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Similar effluent OP concentrations are produced, while effluent TIN is much lower for the hybrid MABR process. Modestly lower suspended growth SRT for the hybrid MABR process (aerobic zone needed to nitrify is not needed) leads to a modest reduction in the process oxygen requirement.

Energy requirements are reduced substantially, however, due to much greater oxygen transfer efficiency for the MABR process compared to conventional oxygen transfer systems and elimination of the need for mixed liquor recirculation.

Hybrid MABR Anammox- Based Systems Can Accomplish Effective Mainstream Total Nitrogen Removal From Non- Nitrified Secondary Effluent.

We are identifying a number of additional interesting combinations coupling MABR with suspended growth processes and MBBR’s. MABR’s can be configured to accomplish partial nitritation and anammox. Figure1.2 illustrates such an approach and indicates that modeling suggests that the optimum ratio of oxygen transferred to nitrogen removed is lower than the strict stoichiometric ratio. This occurs because of nitrogen removal by heterotrophs that are always present in autotrophic enrichments. Thus, further opportunities to reduce carbon requirements and energy requirements for biologicalnitrogen removal exist. We are also investigating the use of MABR to accomplish partial nitritation, coupled with less expensive biofilm materials such as MBBR for anammox.

Naturally our modelling work is progressing much faster than the resulting experimental work. However, we have preliminary experimental results using real municipal wastewater (Ann Arbor, MI) that are consistent with the predictions of the hybrid MABR modelling work presented above. At short suspended growth SRTs (≈ 0.5 days) denitrification occurs largely using influent dissolved (readily biodegradable organic matter), Modest increases in SRT (1 – 2 days) results in increased denitrification due to

hydrolysis of particulate and colloidal organic matter. These results will be complete for presentation at the conference.

Table 1.1 Comparison of Hybrid MABR N&P Removal Process with Conventional Biological N&P Removal Process at 20 oC Following Primary Treatment.

Item Hybrid MABR

N&P Process Conventional A2/O

SRT (Days)

Anaerobic 0.55 0.85

Anoxic 3.35 2.45

Aerobic 0.1 3.50

Total 4.5 6.8

MLR (%) N/A 400

AOR (kg/day)

MABR 3,500 N/A

Conventional 1,566 6,904

Effluent OP (mg-P/L) 0.2 0.1

Effluent TIN (mg-N/L) 0.6 6.9

Biogas (% Influent COD) 36.7 36.1

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Figure 1.2 MABR Effluent Total Nitrogen as Function of the Nitrogen and Oxygen Flux. Black Lines Represents the Optimal Ratio of Oxygen to Nitrogen Required to Oxidize 1 g N Using the Partial Nitritation and Anammox Pathways (1.75 mg O2/mgN), Calculated from their Stoichiometric Equations (Rittmann & McCarty, 2020).

References

Daigger, G.T.,Carlson, Al. L., Chen, X., and B. R. Johnson, B.R. (2019) Coupled anoxic suspended growth and membrane aerated biofilm reactor process options in Proceedings of the 92nd Annual Water Environment Federation Technical Conference and Exhibition, WEFTEC, Chicago, IL

Downing, L. S. and Nerenberg, R. (2008) Effect of bulk liquid BOD concentration on activity and microbial community structure of a nitrifying, membrane-aerated biofilm. Applied Microbiology and Biotechnology, 81, 153–162.

Houweling, D. and Daigger, G.T. (2019) Intensification of the Activated Sludge Process Using Media Supported Biofilms. IWA Publishing, London.

Rittmann, B. E. and P. L. McCarthy, P.L. (2020) Environmental Biotechnology: Principles and Applications, Second Edition, McGraw-Hill, N.Y.

Tchobanoglous, G., Burton, F.L. and Stensel., H.D., (2013). Wastewater Engineering: Treatment and Reuse, Metcalf & Eddy, Inc., 4th Edition/Revised, McGraw Hill, N.Y., 2013.

Presenting Author

Dr. Glen T. Daigger, Ph.D., P.E., BCEE, NAE Professor of Engineering Practice

Department of Civil and Environmental Engineering, University of Michigan, USA Is the presenting author an IWA Young Water Professional? N (i.e. an IWA member under 35 years of age)

Bio:

Dr. Daigger is currently Professor of Engineering Practice at the University of Michigan and President and Founder of One Water Solutions, LLC, a water engineering and innovation firm. He previously served as Senior Vice President and Chief Technology Officer for CH2M HILL (now Jacobs) where he was employed for 35 years, as well as Professor and Chair of Environmental Systems Engineering at Clemson University. He has advised many of the major cites of the world, including New York, Los Angles, San Francisco, Detroit, Singapore, Hong Kong, Istanbul, and Beijing. He is currently a member of the Board of Directors of the Water Research Reuse Foundation (TWRF), and a Past President of the International Water Association (IWA). The recipient of numerous awards, including the Kappe, Freese, and Feng lectures and the Harrison Prescott Eddy, Morgan, and the Gascoigne Awards, and the Pohland Medal, he is a Distinguished Member of the American Society of Civil Engineers (ASCE), a Distinguished Fellow of IWA, and a Fellow of the Water Environment Federation (WEF). A member of a number of professional societies, Dr. Daigger is also a member of the U.S. National Academy of Engineers and the Chinese Academy of Engineers.

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Algal-Bacterial membrane-aerated biofilm reactor: synergistic metabolism and microbial community structure

Wang, Rongchang*, Ren, Yuli*, Cheng, Xia*, and Lewandowski, Zbigniew **

* Institute of Biofilm Technology, Key Laboratory of Yangtze Aquatic Environment (MOE), State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China (Email: wangrongchang@tongji.edu.cn)

** Center for Biofilm Engineering, Montana State University, Bozeman, MT 59717, USA

Keywords: algal-bacterial consortia; membrane-aerated biofilm reactor; microbial community structure

Summary of key findings

Nutrient removal and microbial community structure in an algal-bacterial membrane-aerated biofilm reactor were investigated for treating piggery biogas slurry. The NH +-N removal load was 2.22 ± 0.15 gN/(m2∙d) at 54.1 ± 3.2 mg/L of influent NH +-N. NH +-N removal unstabilized and effluent water quality significantly fluctuated at 113.4 ± 5.0 mg/L of influent NH +-N. NH +-N removal efficiency and removal load recovered to 65.7% and 1.72 ± 0.31 gN/(m 2∙d), respectively, after the influent NH +-N concentration switched back to 60.5 ± 7.3 mg/L. COD was negatively correlated with Nitrosomonas and Nitrobacter and positively correlated with Nitrospira, and Nitrosomonas and Nitrobacter were negatively correlated with effluent nitrite and nitrate. Effluent nitrite, nitrate, chemical oxygen demand, and extracellular polymeric substances significantly affect microbial community structure of the algal- bacterial membrane-aerated biofilm, and microalgae inoculation improves the survival and activity of nitrite-oxidizing bacteria in membrane-aerated biofilms.

Background and relevance

The algal-bacterial (AB) consortium shows great potential as a promising biotechnology for livestock wastewater treatment. The AB consortium is used to purify anaerobic digestion effluent, including high- concentration nutrients (Zhang et al., 2020). The AB photobioreactor completely removes nutrients in anaerobic digestion effluent, with the maximum lipid productivity of algae (Xie et al., 2018). Microalgae inoculation in the AB consortium doubles the ammonia removal capacity compared to a microalgal reactor (Rada-Ariza et al., 2017). In the algal-bacterial symbotic system, microalgae absorb nitrogen and phosphorus for self-proliferation through assimilation and produce O2 through photosynthesis as electron acceptor for bacteria breathe(Mahdavi et al., 2015; Munoz & Guieysse, 2006). Bacteria use this oxygen to oxidize organic matter and produce CO2 by respiration for algae photosynthesis (Unnithan et al., 2014;

Vasseur et al., 2012). About 61%–93% nitrogen removal is because of algal assimilation in the AB batch reactor treating livestock wastewater (Su et al., 2011). Many studies have been carried out to explore the synergetic relationships between microalgae and bacteria in algal-bacterial symbiotic system(Ramanan et al., 2016; Xia et al., 2020).

However, the algal-nitrifying bacterial consortium for nutrient removal often requires a long hydraulic retention time (HRT) of more than 10 days, narrowing its prospects for application (Hernández et al. , 2013;

Thi et al., 2019). In addition, traditional aeration technologies often consume 45%–75% of energy in a livestock wastewater treatment plant (Karya et al., 2013). Therefore, a more effective and efficient method of decreasing the HRT and aeration is required.

The membrane-aerated biofilm reactor (MABR) is a new type of sewage treatment system with a theoretical oxygen utilization rate of 100%. The MABR uses an oxygen-permeable membrane for bubbleless aeration(Lan et al., 2018) and as a microorganism carrier whose oxygen mass transfer carrier is much higher compared to traditional aeration systems (Tian et al., 2016). The algal-bacterial membrane-aerated biofilm reactor (AB-MABR), constructed by combining the MABR with algal biofilm technology, has many advantages. For example, the effluent water quality is not easily affected by suspended microorganisms, microalgal cells are easier to harvest for futher biofuel recovery, and bacteria and

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microalgae show good attachment growth on the microorganism carrier surface to form a photosynthetic biofilm that retains the dominant algal and bacterial genera (Hoh et al., 2016). Zhang et al. (2020) used the AB-MABR to investigate the effect of the chemical oxygen demand (COD) on algal growth (Zhang et al., 2020). They found that the AB-MABR has excellent organic carbon degradation ability and that algae selectively excite the abundance of bacteria that support algal growth. The effect of natural fluctuation in the COD, color, and community structure on real livestock wastewater treatment must be investigated in order to evaluate the practicality of the AB-MABR for livestock wastewater treatment. However, there are few studies on ammonia removal from piggery biogas slurry, which includes heavy metals, antibiotics, and other toxic and harmful substances that cause water quality to greatly fluctuate.

The aim of this study was to investigate whether the AB-MABR could be applied for treating high- NH + -N wastewater. An AB consortium with nitrifying bacteria and the green microalga Chlorella vulgris was constructed to treat piggery biogas slurry. We analyzed the nutrient content, including NH +- N, NO -N, and NO -N concentrations and the COD. In addition, we studied the effects of the AB- MABR on DOM reduction, soluble microbial products (SMPs), extracellular polymeric substances (EPSs), functional gene abundance, and microbial community structure under different influent NH +- N concentrations. We also discussed the mechanism of nitrogen conversion in the AB-MABR in order to provide theoretical guidance and technical support for improving livestock wastewater quality.

Results

Nitrogen and COD transformations

On days 1–4, the effluent NH4 -N concentration rapidly dropped to 29.4 mg/L along with NO2 -N accumulation. The nitrifying bacteria were sparsely attached to the microorganism carrier in the form of flocs, so free ammonia could diffuse into the floc layer. The initial free ammonia concentration was

~1.56 mg/L higher than 1 mg/L, which would inhibit nitrite-oxidizing bacteria (NOB), resulting in temporary NO2-N accumulation (Zhang et al., 2015). Subsequently, the dense biofilm limited the free ammonia mass transfer. Therefore, NOB inhibition near the aerated membrane gradually weakened, decreasing the NO2-N concentration. After 39 days, the effluent NH4+-N concentration was maintained at 23.9 mg/L, after which C. vulgaris inoculation was performed. The effluent NH4+-N concentration further decreased and stabilized at ~8 mg/L after 12 days of C. vulgaris exposure, with almost no NO2-N accumulation. NH4 -N is mainly converted by nitrification rather than C. vulgaris assimilation. C. vulgaris mainly forms a symbiotic microenvironment with nitrifying bacteria, producing oxygen by photosynthesis to promote the growth and metabolism of nitrifying bacteria in the MABR.

When actual piggery biogas slurry with an NH4+-N concentration of 54.1 ± 3.2 mg/L was supplied, the effluent NH4 -N concentration fluctuated within the range of 0–8 mg/L, the effluent NO3 -N concentration was 45.2 ± 1.8 mg/L, and the effluent NO 2 -N concentration was negligible, indicating that almost all the degraded NH4+-N is converted to NO3-N without NO2-N accumulation (Fig. 1a). The NH4 -N removal loading of actual piggery biogas slurry by the AB-MABR and average NH4 -N removal rate were 2.22 ± 0.15 gN/(m 2∙d) and 94.7%, respectively (Fig. 1b). These results showed that NH4+-N degradation in actual piggery biogas slurry by the AB-MABR is feasible. Complex components in the actual water matrix did not affect microorganisms. However, when the AB-MABR treated piggery biogas slurry with an NH4+-N concentration of 113.4 ± 5.0 mg/L, the system became extremely unstable (stage IV). In the early period (days 82–98), effluent NH4 -N and NO3 -N concentrations greatly fluctuated in the range of 21.9–73.7 and 21.4–63.9 mg/L, respectively. The NH4+-N removal rate was 57.3% ± 17.9%, decreasing to 37.4%

compared to stage III. NO2 -N slowly accumulated and peaked at 19.8 mg/L. In the late period (days 99–

106), the effluent NH4 -N concentration decreased to 11.3 ± 7.3 mg/L but did not completely stabilize with acclimatization of microorganisms. The NO2-N-to-NO3-N conversion also accelerated. Therefore, AOB and NOB activity and function are weakened by toxic substances in piggery biogas slurry with an NH4+-N concentration of 113.4 ± 5.0 mg/L. In addition, the biofilm in the aeration membrane slowly sloughs off in stage IV. When the influent NH4 -N concentration in the stage V piggery biogas slurry was restored to 60.5

± 7.3 mg/L, the effluent NH 4 -N concentration was 19.9 ± 9.0 mg/L, NH +-N removal loading was 1.72

± 0.31 gN/(m 2∙d), and the NH +-N removal rate was 65.7% ± 12.5% without NO -N accumulation (Fig.

1). Compared to stage III, the effluent NH +-N concentration increased, while NH +-N removal loading and the NH +-N removal rate decreased by 0.50 gN/(m2∙d) and 29.0%, respectively. These findings indicated

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that the MABR still maintains NH +-N removal capacity, although it is lower compared to stage III. The main reason is the decrease in the available biomass because of toxic actions in stage IV.

Fig. 1. Changes of (a) nitrogen content, (b) NH4+-N removal loading and (c) COD content under different influent NH4+-N concentration.

The changes of COD during the treatment are shown in Fig. 2(c). COD content decreased from 68.95±7.73 to 36.33±15.15 mg/L with a removal rate of 47.3% in stage III. In stage I and II, the nitrobacteria and algae was regarded as autotrophic biofilm in the system for non-COD in feedwater. Algae is facultative microorganism under the existing of COD and illumination meamwhile other heterotrophic bacteria appeared with the COD(Ferreira and Sant'Anna, 2017). Therefore, the biofilm was hybrid biofilm having the ability to removal NH4+-N and COD in stage III. In stage IV, large fluctuations in effluent COD concentration was observed with 129.39±8.50 mg/L influent COD. There still has COD removal in days 82 ~ 90 though removal efficiency was decreasing with time. Subsequently, the effluent COD was higher than influent COD concentration. It is speculated that the dead microorganism killed by toxic sustance released a lot organic matter resulting in the increasing of COD. In days 96~103, the effluent COD concentration decreased with the acclimatization of microorganisms which was accordance with the results of NH4+-N removal. In stage V (days 106~112), influent COD concentration adjusted to 67.60±9.25 mg/L and the changes of effluent COD concentration showed a downward trend and then stabilized at 24.49±6.14 mg/L. At the moment, the COD removal rate was around 63.8% higher than that in stage III, indicating the inhibition effect of pig biogas slurry on COD-available microorganisms is reversible and can be relieved by reducing the concentration of biogas slurry in the later stage. The above results indicate that once the concentration is feed appropriate, AB-MABR can effectively and rapidly remove NH4+-N and COD that are difficult to degradae in the real pig biogas slurry with high NH4 -N.

Microbial community structure

In the nitrifying bacterial MABR system, the main phyla included Proteobacteria (30.1%), Bacteroidetes (26.5%) at phylum level (Fig. 2) which were mainly responsible for COD and NH4+-N remova. Chloroflexi (16.8%) was verified as benificial bacterium providing fundamental structure to support biofilm formation(Zhang et al., 2018). Chloroflexi existed facilitating the formation of biofilm. In stage II, the dominat bactetia was also Proteobacteria, Bacteroidetes and Chloroflexi and the percentage of the three bacteria was not changed significantly as the inoculationg of Chlorella. This demonstrated that AB-MABR has an excellent nitrogen removal capabilities and the algae proliferation did not influent it seriously. The

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abundance of Acidobacteria increased from 4.3% to 12.2% meanwhile Cyanobacteria appeared in this period. Cyanobacteria, a photosynthetic bacterium, grow quickly undersufficient light and suitable nutrition conditions whose function is very similar to that of Chlorella that can assimilate ammonia nitrogen. This indicated the nitrogen converting bacteria enriched after the Chlorella was inoculated.

Cyanobacteria was the most dominant bacterial phylum in the stage III with a relative abundance ranging from 3.2% to 36.4% and Proteobacteria, Nitrospirae and Chloroflexi were the suboptimal bacterial phylum with the relative abundance of 12.9%, 10.6% and 9.7%, respectively. Cyanobacteria can indirectly reflect the growth of Chlorella, and the increase in the proportion of Cyanobacteria indicates that Chlorella grew well. This is consistent with the results of 18s rDNA that Chlorella grew faster under polyculture. When the influent NH +-N concentration increased to 113.4±5.0 mg/L in stage IV, the abundance of Cyanobacteria diminished to 16.2%, which partly explained its unsatisfactory nitrogen removal performance since Cyanobacteria and the Chlorella were inhibited. Bacteroidetes was the most dominant in this period with a abundance of 20.7% and Nitrospirae grew to 17.5%.

Fig.2. Composition and relative abundance of bacteria

communities at phylum level. Fig.3. Redundancy analysis for bacterial communities with respect to environmental variables and functional genes.

RDA was used to evaluate the effects of environmental factors (NH4+-N, NO3--N, NO2--N and COD) and function genes (amoA, nxrA and rbcL) on microbacteria communities (Fig. 6). The species in samples A and B are relatively close while samples C and E are close but significant diverge from D indicating the inhibitory effect of the AB-MABR system on the treatment of high concentration pig sludge is reversible, which is consistent with the result of water quality detection. NO3--N, NO2--N, COD, and EPS were the most influential factors responsible for bacterial population shift in the system. The correlation between NH4+ -N and effluent COD, -NO3--N was weak but has a strong positive correlation with NO2--N. COD correlated negatively with the bacteria associated with nitrogen conversion, such as Nitrosomonas and Nitrobacter, and positively with Nitrospira. Nitrosomonas and Nitrobacter accociated with amoA and nxrA relatived with nitrogen conversion but negatively correlated with NO3

--N, NO2--N, because accumulation of NO2--N affected the activity of AOB and NOB (Tora et al., 2010).

The functional gene nxrA positively correlated with the photosynthesis gene rbcL. NOB is sensitive to the environment. The microenvironment formed between bacteria and algae can increase the tolerance of NOB to the environment. Photosynthesis of chlorella also promoted the activity of NOB and increase the expression of nxrA gene.

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In document IWA Biofilms 2020 (Sider 115-137)