Treatment of Polluted Urban Surface Waters by Sponge Based Aerobic Biofilm Reactor: Purification Performances and Resilience

doi:10.21203/rs.3.rs-837749/v1

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Treatment of Polluted Urban Surface Waters by Sponge Based Aerobic Biofilm Reactor: Purification Performances and Resilience

https://doi.org/10.21203/rs.3.rs-837749/v1

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Surface waters are suffering continuous discharging of pollutions, and low DO and black-odorous were easily formed, especially in those slow-flowing urban lakes and enclosed small ponds. In-situ treatment by artificial aeration or water cycling with a combination of polyurethane sponge as biofilm carriers can disentangle this situation without any land occupation. Long-term continuous experiments (187 days) showed that indigenous microorganisms in urban surface waters could form biofilms in the novel style of sponge-based aerobic biofilm reactors (SABRs). In urban lake waters treatment, the purification performances of SABRs were stable and resilient as the NH₄⁺-N and NO₂⁻-N removals were steady, even facing the abrupt increase of NH₄⁺-N and NO₂⁻-N concentrations in influent. Enhancing the polyurethane sponge filling ratios in SABRs can reduce DO but did not affect NH₄⁺-N removal. SABRs were also able to treat enclosed surface waters or black-odorous waterbodies. Combined SABRs with water cycling, NH₄⁺-N removal time was shorter than the time needed by water cycling when biodegradable organic matter was not present. The massive biodegradable organic matter could inhibit nitrification and prolong the purification time. Further results showed that organic matter could be used as carbon sources to eliminate the produced NO₃⁻-N in SABRs. Therefore, the developed new bioreactor could act as one effective way for treating N-polluted urban surface waters.

Environmental Chemistry

Toxicology

Surface waters

Polyurethane sponge

Filling ratios

Ammonium

Function resilience

Due to rapid economic development and population increase, surface waters are suffering excess nutrition discharged from municipal and agricultural drainage (Nsenga Kumwimba et al. 2018). Continuous nutrition inflow to the surface waters can potentially damage the ecosystems, and lead to low dissolved oxygen (DO), submerged aquatic plant disappearance (Yan et al. 2019), algae bloom (Wu et al. 2016), especially in those slow-flowing surface waters like urban lakes and/or enclosed small ponds (Hobbie et al. 2017).

Phosphorus in surface waters can be removed by chemical adsorption and precipitation (Funes et al. 2018; Min et al. 2020; Wang et al. 2018a; Wu et al. 2019b; Xiong and Peng 2008). Nitrogen removal from actual surface waters was difficult as it needs to combine nitrification and denitrification (Xia et al. 2017). Abiotic environmental conditions will affect nitrification and denitrification (Austin et al. 2019; Yang et al. 2019; Zhang et al. 2014), such as DO, temperature, pH, and ammonium concentration (NH₄⁺-N). DO in surface waters can be easily consumed by organic matter, then favoring denitrification and restraining the critical ammonium nitrification. Even more, stormwater runoff suddenly increases contaminant loading to surface water (Zhao et al. 2015), and the effluent of wastewater water treatments in cold seasons often contains relatively higher NH₄⁺-N than that in warm seasons (He et al. 2018), which may affect the surface water qualities and hydrobios.

Many physicochemical and biological approaches have been used to remove nutrients from polluted water before flowing to the surface waters. Wetlands (Austin et al. 2019; Ballantine et al. 2014; Saeed et al. 2019), vegetated drainage ditches (Nsenga Kumwimba et al. 2018), riparian buffer strips, and integrated buffer zones (Zak et al. 2018) can mitigate the adverse impacts of high nutrient loading with performance being better during the warm season. But these methods generally need a large scale of land (Liu et al. 2016) and limit their use in urban areas. Hence, in-situ treatment by floating beds was acceptable, and the combination of the plants with biofilm bacterial communities can enhance the removal rate without occupying land areas (Liu et al. 2016; Wang et al. 2018b; Wu et al. 2016; Zhang et al. 2015). However, floating beds were vulnerable to the season change (Liu et al. 2016), and most plants would die and the residues should be removed to avoid the second pollution during the cold season.

Polyurethane sponge is a synthetic polymer hydrophilic biological carrier and an ideal growth medium for biofilm formation (Feng et al. 2012; Guo et al. 2010; Zhang et al. 2016). It has been used in wastewater treatment to enhance nitrogen removal due to the high DO gradient in the cubic biofilms (Feng et al. 2012), and different filling ratios can affect the removal effectivity (Feng et al. 2012; Guo et al. 2010; Zhang et al. 2016; Zhao et al. 2019). Anaerobic biofilms on polyurethane also can enhance the biodegradation of refractory compounds (Shi et al. 2019). When cultured on polyurethane sponges, the mix-aerobic denitrifiers can effectively remove nitrogen in actual reservoir source water (Zhou et al. 2016). But little is known about the use of polyurethane sponges in urban surface waters.

Functional resilience of a reactor is critical in practice use, as the environmental conditions often change and may affect the treatment abilities. Thus it is needed to determine how bioreactors are able to cope with disturbance and recover after disturbance (Boog et al. 2018; Cabrol et al. 2012; Cho et al. 2016; Sukias et al. 2018). Previous research found that it took 14 days to completely recover nitrification under ammonia shock loading for steel wastewater treatment (Cho et al. 2016). However, there were seldom reports focusing on the resilience of a reactor in treating urban surface waters.

In this study, a novel style of biofilm reactor was developed to purify urban surface waters under various conditions. Polyurethane sponges were used as carriers to enrich the indigenous microorganisms to form biofilms, and the micro-polluted urban lakes and enclosed surface waters such as those small ponds in sub-districts were taken as study objects. We prospected that cultured biofilms can benefit contaminant removal in urban surface waters. The objectives of this study were: 1) to develop aerobic biofilm reactors with polyurethane sponge as carriers; 2) to check the resilience of function by suddenly increasing NH₄⁺-N or NO₂-N concentrations in influent; 3) to investigate the purification performances of biofilm reactors for enclosed-surface water treatment.

Aerobic biofilm reactor set-up

Cubic-shaped polyurethane sponges (10×10×10 mm) served as biofilm carriersin this study, purchased from AiQin Environmental Technology Co., Ltd., Jiangsu,China. The average specific surface area of a sponge cube was 91,000m³m^-2, and the porosity was 95%.

Sponge-based aerobic biofilm reactors (SABRs) were set in 100L plastic buckets and simulated as treat urban surface waters(Fig.1a).The effective volume of the plastic bucket was 80L.Each SABR consisted of anup-flow zoneand threebiofilm zones, all made by acrylics cylinder with 100 mm inner diameter. The up-flow zone was 600 mm in height, and biofilm zones were 280mm in length (consisting of a 100mm cylinder and a 180mm cone) (Fig.1b). Three biofilm zones were connected with the up-flow zoneand arranged 120°between themselves (Fig.1c).The top of the up-flow zone was sealed with a conical acrylic plate to prevent water from flowing out of the up-flow zone and flow the surface wateraveragely into the three biofilm zones.Fine iron wires were arranged in the connection area to prevent the carriers from dropping off when introduced into biofilm zones.Small holes were made at the top of the biofilm area to circulate the surface water and prevent the internal carriers from being washed away.This special design considered that in practical application, the reactorswill be placed in the middle of the surface water and reduce the interference of the surrounding people.

Polyurethane spongeswere filledonly in the biofilm zones, and the filling ratiosin biofilm zones were the same in one SABR. Filling ratiosof 20%, 40%, 60% and 80% were adopted andreferred to as FR20, FR40, FR60, and FR80, respectively.The filling ratios in this study werebased on the volume of the biofilm zone, which is different from previous studies as those filling rates were based on the whole reactor(Zhang et al. 2016; Zhao et al. 2019).

Reactor operating conditions

Purification performances and resilienceintreatment of urban lake waters

The urban lakes are the receiving watersfor urban pollution, such as the effluents from the wastewater treatment plants. Thus the experiment was fed with continuous flow.Surface waters were collected to laboratory every week from Lake Xuanwu, located in Nanjing City (China).The operation was dividedinto three stages (Table1).Influents were continuously fed to reactors by peristaltic pumps(6.95 mlmin^-1),and hydraulic retention time (HRT) was 8 daysasthe HRT of Lake Xuanwu was about 7 days.Ammonium chloride (NH₄Cl) was added to the lake waters(collected from Xuanwu Lake) to prepare the influent in stage Ⅱ and stage Ⅲ, and simulate the sudden increase ofNH₄⁺-Nconcentration in influent.No sediments were added into the buckets as dredgingis often adopted in urban lakes, such as Lake Xuanwu.After dredging,the organic carbon contents in the surface layer of sediment were low and denitrification was inhibited(Zhong et al. 2009).

Stage I:This stage was used to enrich the indigenous microorganisms to form biofilms on bio-carriersand study the purificationperformances of SABRs forurban lakes. Actual surface water wascontinuously fed to SABRs without any nutrition addition. DO supply by controlled airflow with 60mLmin^-1 and lasted for 70 days (StageⅠ-a).Submersible waterpumps (Tian Xia Yu Jia, TA8800, 2000Lh^-1) were used in StageⅠ-b for enhancing the hydraulic condition and lasted for 37 days.

Stage Ⅱ:This stage wasused to study the function resilience of SABRs as rainfallmay lead to non-point pollution flow into urban lakes,which often occurred in warm seasons.Nitrogen fertilizer is often used for urban landscaping.During the warm and rainy season, the nitrogen fertilizer will flow into the urban lake.Previousstudies found thatlow organic loading was favorable fornitrification(Luo and Meng 2020). However, most organic matters in surface runoff arehumic-like compoundsand are difficult to biodegrade(Zhao et al. 2015).Moreover,the dissolved organic carbon in surface water was only 2.03-7.67 mgL^-1,and the bioavailable organic carbon only accounting for 6%-11%(Wu et al. 2019a; Xu et al. 2020).Thus only NH₄⁺-Nconcentrationsin influentwereintermittentlyimproved to 2.00-4.00mgL^-1for 7 times.

StageⅢ:This stage was alsoused to study the functionalresilienceof SABR when facing a suddenincrease of pollution in influent as NH₄⁺-Nconcentrationin the effluent of industrial wastewater treatment plantscan reach over 10 mgL^-1(He et al. 2018). The organic mattersin wastewater treatment plantscan beremoved stably and effectively, and the organic matters in effluent were often refractory. Thus only NH₄⁺-Nconcentrationincreased suddenlyto15.00mgL^-1in influent and smoothly declined for 16 days, followed by influent as in stage Ⅱ(3.26mgL^-1NH₄⁺-N)for 7 days.

Purification performancesfor treatment of enclosed surface waters

Synthetic waters were usedto simulate the enclosed surface waters,which often contain high concentrations of organic matters and NH₄⁺-N with black-odorous appearances (such as those small ponds in subdistricts and parks).Duo to no continuous discharge into those enclosed surface waters, batch experiments wereadopted and to examine whether the biofilms cultured byurban lake waters can directlybe used in those stagnant waters. Control and only hydraulic circulation were also set up(same plastic buckets with 80L effective volume).As organic matter may not be present in some enclosed surface waters, two operating conditions were applied in the experiments.

No organic matterspresent: NH₄⁺-Nconcentrations wereimproved to about 14 mgL^-1throughNH₄Cl addition.NO₃⁻-Nconcentration was improved to 1.85 ± 0.10 mgL^-1to test whether denitrification can occur naturally in the control treatment.Before experiments, pH was adjusted to 7.68 ± 0.03by NaHCO₃addition (Table 1).

Organic matters present:Acetic acid and glucose (at a ratio of 1：1by chemical oxygen demand) were added into water to simulatethose black-odorous waterbodies (TOC 59.56±3.09mgL^-1)before experiments. Other characters of surface waters wereslightly higher than no organic matters present (Table 1).

Due to low DO and high organic matters in surrounding untreated surface waters, it is beneficial for denitrificationandremoving the produced NO₃⁻-N by SABRs. Thus batch experiments were used to removeNO₃⁻-N by mixing the effluent from SABR with the untreated surface waters. The mix ratios were 10%,20%, and 50%.Twocontrols were also adopted (effluent of SABR and untreated surface waters).The mixed waters were not purged with N₂and were open to air during the experiments. Each treatment was repeatedtwo times.

Analytical methods

Samples were taken from the outflowand immediatelyfiltered through 0.45μmpore sizefiltersthen stored in -20℃ until analysis.The concentrations of NH₄⁺-N, NO₂⁻-N, NO₃⁻-N,PO₄^3--P, TN, and TP were determined in accordance with standard methods(SEPA 2002).The DO, water temperature, and pH were in situ measured by a dissolved oxygen analyzer(YSI 5000,USA) and a digital pH meter (PB-10, Sartorius,Germany), respectively.Dissolved organic carbon concentrations(DOC) were measuredbya TOC analyzer (Torch- Teledyne Tekmar).

Statistical analyses

To compare the removal efficienciesofdifferenttreatments, one way-ANOVA followed by Tukey HSD post-hoc test was conducted by using SPSS software with significance considered atP< 0.05 (SPSS Statistics version 20.0).

Performance of SABRsintreating urban lake waters

During stageⅠ, the surface of polyurethane sponges in four SABRs changed from white to yellow and then to brown(Fig. S1), indicating thatbiofilmswereformed by the indigenous microorganismsin urban surface waters.Artificial aeration and water cycling were often used in surface water purification, as these methods can improve the DO and change the hydraulic condition(Cao et al. 2020). DO values in four buckets were improved to over 5.00mgL^-1(Fig. 2a), whichbenefitted NH₄⁺-N removal. With artificial aeration,DO concentrations in reactors did not show significant difference among the different filling ratios. Wherever, DO concentrationsdeclined with the filling ratios when water cycling was applied to change hydraulic conditions(Fig.2a).

Within 20 days of experiments, NH₄⁺-N and NO₂⁻-Nconcentrations in effluent of four reactors increased to 0.40-0.60mgL^-1and 0.05-0.10mgL^-1, respectively, andthen declined close to 0 mgL^-1(Fig. 3a, b). Filling fraction had only a slight influence on NH₄⁺-N removal, which consisted with usingpolyurethane spongesin MBBR for treating synthetic domestic wastewater(Zhang et al. 2016), but contradictedwiththeeffect of packing rates on NH₄⁺-N removal(Feng et al. 2012),due to the characters of synthetic wastewater with high COD in that study.

NO₃⁻-Nconcentrations of the four SABRs increased from 0.79mgL^-1to 0.95-1.13mgL^-1(Fig. S2a), and no TN removalswere observed (Fig. S2b), which means denitrificationswereinhibitedduring the experiment. Recently, some studies suggestedthatdenitrification canoccur in oxic waters,which ascribed to theheterogeneity microstructure of suspended sediments(Jia et al. 2016; Liu et al. 2013; Xia et al. 2017) and the aboundingappearanceof periphytic biofilms (Wu et al. 2014).However, denitrificationwas not observed in thisstudy because of the high DO and low nutrient level inlake waters.Firstly, denitrification could occurwhen DO concentration was up to 4.00-5.00mgL^-1, and the denitrification rate decreased with an increase of DO levels(Wang and Chu 2016). In the continuous flow stages, DO values in the bulk liquid were higher than 5.00mgL^-1, which could restraindenitrification. Secondly, traditional denitrification used organic compounds as carbon sources (Wang and Chu 2016),and simultaneous nitrification/denitrification can be achieved in wastewater treatment under continuous aerationwhen carbon source was enough(Macedo et al. 2019). But TOC in the water of Lake Xuanwuwas only 2.03mgL^-1(Xu et al. 2020), and most of the TOC in lake waters were recalcitrant to biodegradation(Koehler et al. 2012). Thirdly, thoughaerobic denitrifying microbeshave been isolated and identified from lakes, reservoir sediments(Huang et al. 2013; Wen et al. 2019; Yao et al. 2020; Zhou et al. 2016; Zou et al. 2014), and aerobic denitrificationwas observed in a drinking waterreservoir by indigenous aerobic denitrifiers via in situ oxygen enhancement(Zhou et al. 2016; Zhou et al. 2020). But aerobic denitrifiers also need carbon sourcesfordenitrification (Wen et al. 2019; Xia et al. 2020). Our previous studies have shown that Penicilliumtropicum, an aerobic denitrifying fungus, requires 5.80mgL^-1TOC to remove 1.00mgL^-1NO₃⁻-N in aerobic condition(Yao et al. 2020). Lastly, phosphorus deficiency would also inhibit denitrification (Fan et al. 2018; Zhou et al. 2016), as the phosphorus concentrations in influent were only about 0.05mgL^-1(Fig. S2c).

Resilience ofSABRswith suddenincrease of NH₄⁺-Nconcentrations in influent of urban lake waters

With a slight increase of NH₄⁺-N concentrations in influent in stageⅡ, DO in the four SABRs was lower than that in stageⅠand declined with the filling ratios (Fig. 2a). With the first sudden increase of NH₄⁺-Nconcentrationin influent(in stageⅡ), NH₄⁺-N and NO₂⁻-Nconcentrationsin effluentincreased slightly then declined close to 0 within 4-6 days and 4-8 days (Table 2), which were similar to the result under shorting HRT(Chang et al. 2019), aeration interruption(Boog et al. 2018; Murphy et al. 2016), and shock loading of influent (Cabrol et al. 2012; Cho et al. 2016; Sukias et al. 2018).

Subsequently, the concentration of NH₄⁺-N and NO₂⁻-N concentrations in the influent discontinuouslyincreased abruptly for six times,also the concentrations of NH₄⁺-N and NO₂⁻-N in the effluent firstlyincreased abruptly and then decreased.The periodsfor NH₄⁺-N and NO₂⁻-Nconcentrations to return to the previous stable state were shortened from 4-6 days to 0-4 daysand from 4-8 daysto 0-3 days,respectively(Table 2). One exceptionwas that it took 8 days to reduceNO₂⁻-Nconcentrationto below 0.01mgL^-1in FR20 becauseNO₂⁻-Nconcentrations in the 4^th shock loading weremuch higher than previousshocking loadings(Fig. 3b; Table 2). These resultswere similar tothe nitrification reactor treating steel wastewater.After the first shock loading, it took 14 days to recover nitrification,but only needed1 day after the second and third NH₄⁺-N shock loading(Cho et al. 2016).When NH₄⁺-N concentrationsin influent suddenly increased to 15.00mgL^-1in stage Ⅲ, it took 7-8 days to recover thefunction of nitrification(Table 2). This result further supported that the SABRsfunctionwas robust tovarious disturbances,which could be related to adaptation to fluctuating environmental conditions (Berga et al. 2017).In addition, the influent nitrifiermay be related to the resilience of SABRs, as the response of bacterial communities to disturbances can be affected by the dispersal(Yu et al. 2018), and the long HRT may also play an important roleas it can dilute the NH₄⁺-N concentration in surface water.

Similar to stageⅠ, the NO₃⁻-Nconcentrations in effluent were higher than those in influents (Fig. S2a; Fig. 2b), and noTN removal was observed (Fig. S2), indicating thatdenitrificationwas still inhibited, regardless of increasing NH₄⁺-N concentrations.

Performances ofSABRsin treatingenclosed surface waters

Condition withoutbiodegradable organic matter

Water cycling within SABRs can improve the DO in enclosed surface waters (Fig. 4a). DO in control declined slightlyfrom 6.00mgL^-1to 3.50mgL^-1duringthe initial10days, then decreased to 0.50mgL^-1within 3 days.However,DO values by water cycling were above 9.00mgL^-1during the whole stage, and DO values in four SABRs all declining from 7.90-8.90mgL^-1to 3.50-5.50mgL^-1within 8days, then slowly increased to 10.00mgL^-1.

In the absence of organic matter, NH₄⁺-N can be effectively removed by water cycling. It took 16 days to remove NH₄⁺-N by water cycling. When combined with the SABRs, the NH₄⁺-Nremoval time can be reduced to 9 days (Fig. 4c).NH₄⁺-Nconcentrationsincontrol declined slightly from 14.22 mgL^-1to 8.14mgL^-1withoutNO₂⁻-N accumulations. On the contrary,both water cycling and SABRstreatments had NO₂⁻-N accumulations, andNO₂⁻-N concentrationsincreased firstly andthen declinedrapidly(Fig.4d).The highest NO₂⁻-N concentration by water cyclingwas8.56 mgL^-1(16^th day),and much higher than those by SABRs(5.08mgL^-1 on7^thday, 7.49mgL^-1 on8^thday, 6.31 mgL^-1 on9^thday,and8.27mgL^-1on8^thday, respectively).These results revealed that biofilms in SABRspromoted nitrification and reducedpurification time.

Although the cultured biofilms had nitrification ability, it still needed2 days to activate nitrification to treat surface water containinga high concentration of NH₄⁺-N(Fig.4c, d, e).This result consistsof that nitrificationstarted after 46 hours when the initial NH₄⁺-N concentrationincreased to 8.10 mg L^-1 in TayNinh River water(Le et al. 2019). In other words, the biofilms cultured by urban lake waters can be used in the stagnant waters treatment, and it needed time to activate the microbial activity as the pollutant concentration in enclosed surface water was much higher than urban lake waters,even though the biofilms had surfersuddenly increase of NH₄⁺-N concentrations. Thus the limited nitrificationcapacity of biofilms may be the reason for high DO values at the beginning of the experiment (Fig. 4a).

Conditionwithbiodegradable organic matterpresent

In control, the organic matter ledto a decline of DO to 0mgL^-1during the initial5 days, while DO in four SABRs increased slightlyandthen declined to 2.00-3.10mgL^-1 within 2 days, followed byan increase to 8mgL^-1(Fig.5a).Within the same time, DOC concentrations in controldecreased from 59.56mgL^-1to 8.57mgL^-1within 11 days, while it took only 5-6 days to removethe biodegradable organic matters bywater cycling andSABRs(Fig.5b).

In the presence of organic matter, the time forNH₄⁺-N removal by SABRs needed more than 22 days, which was much longer than that without organic matter(Fig. 5c).By FR20, FR40, FR60 filling ratios,NH₄⁺-Nconcentrationsincreased slightlywithin initial2days, then declined and became stable at 10.04±0.41 mgL^-1(laggedfor 8-11 days),ultimatelydecreasedto 0.46-2.59 mgL^-1.However,NH₄⁺-Nconcentrationin FR80 was stabled and lagged for 7 days before decliningto 0.54 mgL^-1. In control,NH₄⁺-Nconcentrationstabled at 11.13±0.88mgL^-1at the end of the experiment,and only 21.73% was removed (Fig.5c). Surprisingly, NH₄⁺-Nwas also removed by water cyclingunder organic conditions, which may be becausethe organic mattersbenefitedmicrobe growth, and attached biofilms were formed on the wave zone.In summary, the organic matters in enclosed surface waters inhibitedNH₄⁺-N removal and prolonged the NH₄⁺-Nremoval time.

In control, NO₂⁻-N concentrationstraightly declined to 0 within 6 days. By water cycling, NO₂⁻-Nconcentrationincreasedto 2.29 mgL^-1 within 17 days then declined to 0 in 5 days (Fig. 5d).While the changes of NO₂⁻-N concentrations in SABRs showeda bimodal pattern, with accumulationsduring initial 6 days and then declined, followed by a second increase and decline tendency, which were different from the results in the conditionwithout organic matters(Fig. 4d). Previous studies showed that low organic loading was favorable fornitrification(Luo and Meng 2020). But NH₄⁺-NandNO₂⁻-Nconcentrations in FR20, FR40, and FR60increasedduring initial4 days, which was in accordance withdissimilatory nitrate reductionto ammonium (DNRA)(Carlson et al. 2020; Srinandan et al. 2012).Thus, DNRA was suspected to occur when there were abundant biodegradable organic matters in this study. When organic matterswere used up after 6 days, the subsequent decline ofNO₂⁻-Nconcentrationwas due tothe transformation ofNO₂⁻-Ninto NO₃⁻-N(Fig. 5d). The secondincrease and decline of NO₂⁻-N concentrationsin four SABRs were similartothe result of no organic mattercondition (Fig. 4d) and ascribed to the nitrification.

In control, biodegradable organic matters led to low DO,accompaniedby100% removal of NO₃⁻-Nin 5 days.While NO₃⁻-Nconcentrationsin SABRs only declined in the initial few days when DO were low (Fig. 5a, e).TN in controlwas removed by 37.56 %, and much higher thanby using water cycling and SABRs (removed by 8.62 %, 11.79 %, 7.78 %, 8.02 %,9.94 %in FR20, FR40, FR60, FR80, and water cycling, respectively) (Fig.S3b). These results revealedthatdenitrifying microbeswere abundant in actual surface water, and denitrification can be easily achieved when DO was low. It also implied that the addition of nitrate can induce denitrification in black-odorous waterbodies without any other treatment.

The results in Fig.6 showed that mixing the effluent of SABR with the untreated surface waters could eliminate the produced NO₃⁻-N (Fig. 6b, c, d). But different mix ratios have different results. By low mix ratios (10% and 20%), NO₃⁻-N were 100% removedwithin 1 day(Fig. 6b, c).However, by the mix ratio of 50% treatment, NO₃⁻-Nconcentrationdecreased rapidlyfrom 5.94 mg/L to 3.66 mgL^-1within 1 day andstabled at 3.20 mgL^-1(Fig. 6d),simultaneouslythe NO₂⁻-Nconcentration increased from 0.48 mgL^-1to 4.07 mgL^-1, asthe organic matter was not enough fordenitrification.Thus TN removal by 50% mix ratio (9.9%)was lower than 20% mix ratio (19%) and 10% mix ratio (15%) (Fig.6).Therefore, in order to remove TN to the maximum extent, it is necessary to quantify the biodegradable organic matter in closed surface waters.

As stated above, denitrification could occur naturallyby mixing the effluent of SABR with the surrounding-untreated surface waters.Thuspartition the enclosed-surface waters into two parts,with one part treated by SABRto removeorganic matters and NH₄⁺-N, then transfer the produced NO₃⁻-N to another part without aeration, andthis sequencing treatment may be an economicalway to tackle those black-odorous waterbodies when it contains high organic matters.Moreover, the produced NO₃⁻-Ncan promote the biodegradation ofhazardous organic chemicals(Wang et al. 2019) and improve the oxidation-reduction potential of sediment (Li et al. 2019)in another part of surface waters.

Other water quality parameters

Biological conversation of NH₄⁺-Nto NO_x-N often led to pH decline, but in the continue experiment in this study, pH in effluents was higher than influent (Fig.2b). It may due to the low flow influent or high HRT, and buffer effect of surface waters. Moreover, the fine wires used in this study can lead to a pH increase due to Fe corrosion(Di Capua et al. 2019). While in enclosed surface waters, nitrificationled to the decrease of pH (Fig. 4b;Fig. S3a),promoted the releaseof Fe²⁺and Fe³⁺fromfine wires then precipitates with phosphorus.Thus the removal of PO₄^3--P in the batch experiments was due to the fine wires(Fig. 4f; Fig. S3c).

A novel biofilm reactor (SABR) was constructed for urban surface waterspurification. Biofilms were formed on polyurethane sponges by the indigenous microorganisms in urban surface waters. SABRs could improve DO and efficiently remove NH₄⁺-Ninlake waters,but NH₄⁺-Nwasonly transformed into NO₃⁻-N. Increasing the filling ratiosof bio-carriersreduced DO but did not affectNH₄⁺-N and TN removal. SABRswere stable to the continued influent and showed resilience when facing various intermittent increases of NH₄⁺-Nand NO₂-N concentrationsin influent. While biodegradable organic matters in enclosed-surface waterscan inhibit NH₄⁺-N removal by SABRs and prolong the purificationtime.The produced NO₃⁻-N inSABRs wasrapidly removed through mixing with the untreated surface waters whenbiodegradableorganic matterswere present abundantly. Therefore, the developed new bioreactor provided one practical and efficient way for treatment of urban surface waters.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

All data generated or analyzed duringthis study are included in this published article.

Competing interests

The authors declare no competing interests.

Funding

This study was supported by grants from the National SpecialProgram of Water Environment (2017ZX07204005).

Author contributions

Shangwei He: Methodology, Investigation, Formal analysis, Writing -Original Draft. Na Song: Writing - Review &Editing. Zongbao Yao: Writing - Review &Editing. Helong Jiang: Conceptualization, Resources, Writing - Review &Editing, Funding acquisition.

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Table1 Operation conditions at different stages

	Influent water characters					DO supplied by	Mode
	NH₄⁺-N (mg/L)	NO₂^--N(mg/L)	NO₃^--N(mg/L)	PO₄^3--P(mg/L)	T(℃)	DO supplied by	Mode
StageⅠa ( 70 days)	0.05-0.49	0.00-0.13	0.09-1.50	0-0.10	14.4-25.8	Air	Continue
StageⅠb ( 37 days)	0.05-0.56	0.01-0.19	0.34-1.79	0.01-0.13	22.4-25.8	Water cycling	Continue
Stage Ⅱ ( 55 days)	0.34-4.69	0.00-1.08	0.74-5.49	0.01-0.12	20.5-26.2	Water cycling	Continue
Stage Ⅲ* ( 23 days)	15.00	0.15	1.53	0.06	20.4-24.6	Water cycling	Continue
Stage Ⅳ* ( 20 days)	14.22±1.15	0.05±0.00	1.85±0.10	0.04±0.00	11.4-18.2	Water cycling	Batch
Stage Ⅴ* ( 22 days)	14.22±0.30	0.43±0.10	3.57±0.10	0.43±0.06	6.5-14.6	Water cycling	Batch

*Influent water charactersat the beginning of the experiments.

Table2 Function resilience of four SABRs (Days needed to recover by increased suddenly of NH₄⁺-N and NO₂^--N in influent)

	NH₄⁺-N (days needed to recover to below 0.1 mg/L)								NO_2--N (days neededto recover to below 0.05 mg/L)
	Stage Ⅱ							Stage Ⅲ	Stage Ⅱ							Stage Ⅲ
	1st	2nd	3rd	4th	5th	6th	7th		1st	2nd	3rd	4th	5th	6th	7th
FR20	6	2	1	0	4	1	2	8	8	3	0	8	3	0	2	4
FR40	4	2	1	0	4	1	1	8	7	2	0	1	1	1	0	4
FR60	6	4	1	0	4	1	2	7	6	2	0	3	0	0	0	4
FR80	4	3	1	0	4	1	1	7	4	0	0	2	0	0	0	6

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Treatment of Polluted Urban Surface Waters by Sponge Based Aerobic Biofilm Reactor: Purification Performances and Resilience

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