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 4th 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(16th day),and much higher than those by SABRs(5.08mgL-1 on7th day, 7.49mgL-1 on8th day, 6.31 mgL-1 on9th day,and8.27mgL-1on8th day, 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 NOx-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 PO43--P in the batch experiments was due to the fine wires(Fig. 4f; Fig. S3c).