Enhanced hydrolysis of lignocellulose in corn cob by using food waste pretreatment to improve anaerobic digestion performance.

This study aims to enhance hydrolysis and anaerobic digestion of corn cob (CC) by using food waste (FW) pretreatment. FW, which tends to be acidification in fermentation, was applied in this process as an acid-like agent to accelerate lignocellulose hydrolysis, aiming to promote methane yield in further digestion process. The effect of FW pretreatment on pH, soluble chemical oxygen demand (SCOD), volatile fatty acids (VFAs), cellulose/hemicellulose contents and cellulose crystallinity are specially focused. FW:CC = 1:3 based on volatile solid (VS) was found to be the optimal mixing ratio in pretreatment and its hydrolysis efficiency was 28% higher than the control group. An increase of 13.2% in cellulose reduction and a decrease of 6.7% in cellulose crystallinity was achieved at this ratio. Supplementation of FW increased VFA concentrations in slurry mixture that directly change the activities of enzymes and microorganisms. In the stage of methane production, the digester A3 (FW:CC = 1:6 based on VS) with higher hydrolysis efficiency presented the best performance in methane production with a specific methane yield of 401.6 mL/g·VS, due to the recovery of the pH in this digester to the optimal pH range for methanogens' metabolism (pH 6.3-7.2). Kinetics studies of cellulose/hemicellulose degradation indicated that the pretreatment of FW could improve the degradation of cellulose. Three-dimensional excitation emission matrix (3DEEM) results further confirmed that FW play an important role in lignocellulose hydrolysis. In addition, variations of lignocellulosic textures during the pretreatment were also cleared by using field emission-scanning electron microscopy (FE-SEM) analysis.

Prompt nitrogen removal by controlling the oxygen concentration in sediment microbial fuel cell systems: the electrons allocation and its microbial mechanism.

It has been proved that the nitrogen can be removed from the sediment in a sediment microbial fuel cell system (SMFCs), but the competition between nitrate and oxygen for electrons would be a key factor that would affect the removal efficiency, and its mechanism is not clear. Based on organic sediment fuel, an SMFC was constructed, and the influence of dissolved oxygen (DO) on nitrogen transformation and cathodic microbial communities was investigated. The results showed that the best total nitrogen removal efficiency of 60.55% was achieved at DO level of 3 mg/L. High DO concentration affected the removal efficiency through the electrons' competition with nitrate, while low DO concentration suppressed the nitrification. Comamonas, Diaphorobacter and Brevundimonas were the three dominant genera responsible for denitrification at DO concentration of 3 mg/L in this study. The establishment of SMFCs for nitrogen removal by regulating DO level would offer a promising method for sediment treatment.

A conceptual method to simultaneously inhibit methane and hydrogen sulfide production in sewers: The carbon metabolic pathway and microbial community shift.

In this study, the impact of COD/SO42- ratio in sewage on methane and hydrogen sulfide production in sewer biofilms was investigated by using three identical lab-scale gravity sewer systems. The results showed that the COD/SO42- played a key role in the competition between methanogenic archaea (MA) and sulfate reducing bacteria (SRB). Both the lowest methane and hydrogen sulfide production were obtained at COD/SO42- ratio of 6. The carbon transformation revealed that the activity of both MA and SRB was inhibited at this COD/SO42- ratio. Methanosarcina and Methanobacterium were the two dominant MA, while Desulfonema, Desulfotomaculum and Desulfovibrio were the dominant SRB in this case. The specific SRB activity measured by batch tests proved that acetate was mainly degraded by the MA, while propionate was the preferred substrate for the SRB.

Thermal Structure-Induced Biochemical Parameters Stratification in a Subtropical Dam Reservoir.

Although stratification in deep lakes is well-discussed, few studies pay attention to thermal structure as well as its influences on stratification of biochemical parameters in subtropical lakes in mountainous cities. Here, we studied the depth profile of temperature and biochemical parameters in Longjing Lake, a subtropical reservoir in a mountainous city. Thermal stratification became strong during summer. Biochemical parameters were strongly associated with thermal structure. Stratification started at 2~6 m depth with a substantial decrease in dissolved oxygen, biochemical oxygen demand (BOD5), chlorophyll a, and pH, corresponding to an increase in total nitrogen, ammonium (), nitrite (), total inorganic nitrogen (TIN), total phosphorus, and soluble reactive phosphorus (SRP) with depth; the majority of biochemical parameters showed slight variations from 12 m downward. Our results indicated the stratification of Longjing Lake was stronger and more stable than the stratification of tropical and temperate lakes in lowland cities.

Effects of C/N ratio on nitrous oxide production from nitrification in a laboratory-scale biological aerated filter reactor.

Emission of nitrous oxide (N2O) during biological wastewater treatment is of growing concern. This paper reports findings of the effects of carbon/nitrogen (C/N) ratio on N2O production rates in a laboratory-scale biological aerated filter (BAF) reactor, focusing on the biofilm during nitrification. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and microelectrode technology were utilized to evaluate the mechanisms associated with N2O production during wastewater treatment using BAF. Results indicated that the ability of N2O emission in biofilm at C/N ratio of 2 was much stronger than at C/N ratios of 5 and 8. PCR-DGGE analysis showed that the microbial community structures differed completely after the acclimatization at tested C/N ratios (i.e., 2, 5, and 8). Measurements of critical parameters including dissolved oxygen, oxidation reduction potential, NH4+-N, NO3--N, and NO2--N also demonstrated that the internal micro-environment of the biofilm benefit N2O production. DNA analysis showed that Proteobacteria comprised the majority of the bacteria, which might mainly result in N2O emission. Based on these results, C/N ratio is one of the parameters that play an important role in the N2O emission from the BAF reactors during nitrification.

Simultaneous Microcystis Algicidal and Microcystin Degrading Capability by a Single Acinetobacter Bacterial Strain.

Measures for removal of toxic harmful algal blooms often cause lysis of algal cells and release of microcystins (MCs). In this study, Acinetobacter sp. CMDB-2 that exhibits distinct algal lysing activity and MCs degradation capability was isolated. The physiological response and morphological characteristics of toxin-producing Microcystis aeruginosa, the dynamics of intra- and extracellular MC-LR concentration were studied in an algal/bacterial cocultured system. The results demonstrated that Acinetobacter sp. CMDB-2 caused thorough decomposition of algal cells and impairment of photosynthesis within 24 h. Enhanced algal lysis and MC-LR release appeared with increasing bacterial density from 1 × 103 to 1 × 107 cells/mL; however, the MC-LR was reduced by nearly 94% within 14 h irrespective of bacterial density. Measurement of extracellular and intracellular MC-LR revealed that the toxin was decreased by 92% in bacterial cell incubated systems relative to control and bacterial cell-free filtrate systems. The results confirmed that the bacterial metabolite caused 92% lysis of Microcystis aeruginosa cells, whereas the bacterial cells were responsible for approximately 91% reduction of MC-LR. The joint efforts of the bacterium and its metabolite accomplished the sustainable removal of algae and MC-LR. This is the first report of a single bacterial strain that achieves these dual actions.

Mechanism and kinetics of biofilm growth process influenced by shear stress in sewers.

Sewer biofilms play an important role in the biotransformation of substances for methane and sulfide emission in sewer networks. The dynamic flows and the particular shear stress in sewers are the key factors determining the growth of the sewer biofilm. In this work, the development of sewer biofilm with varying shear stress is specifically investigated to gain a comprehensive understanding of the sewer biofilm dynamics. Sewer biofilms were cultivated in laboratory-scale gravity sewers under different hydraulic conditions with the corresponding shell stresses are 1.12 Pa, 1.29 Pa and 1.45 Pa, respectively. The evolution of the biofilm thickness were monitored using microelectrodes, and the variation in total solids (TS) and extracellular polymer substance (EPS) levels in the biofilm were also measured. The results showed that the steady-state biofilm thickness were highly related to the corresponding shear stresses with the biofilm thickness of 2.4 ± 0.1 mm, 2.7 ± 0.1 mm and 2.2 ± 0.1 mm at shear stresses of 1.12 Pa, 1.29 Pa and 1.45 Pa, respectively, which the chemical oxygen demand concentration is 400 mg/L approximately. Based on these observations, a kinetic model for describing the development of sewer biofilms was developed and demonstrated to be capable of reproducing all the experimental data.

Shortcut nitrification-denitrification in a sequencing batch reactor by controlling aeration duration based on hydrogen ion production rate online monitoring.

The hydrogen ion production rate (HPR) and the pH of the aeration phase in a sequencing batch reactor (SBR) were simultaneously measured by a novel respirometric-titrimetric instrument. The results showed that HPR could indicate the end of ammonia oxidation with a greater accuracy and sensitivity than pH. An SBR was used to treat synthetic wastewater containing 360 mg/L chemical oxygen demand (COD) and 40 mg/L NH(4+)-N at 20 degrees C with dissolved oxygen (DO) lower than 2.0 mg/L. Controlling the aeration duration based on HPR online monitoring, shortcut nitrification-denitrification was successfully performed for approximately two months with a stable nitrite accumulation rate (NAR) above 88%, and the COD and NH(4+)-N removal ratios were both higher than 90%. Based on the HPR online monitoring data, the estimated NH(4+)-N concentrations in nitrification were closely related to the measured concentrations, with a correlation coefficient of 0.9722, and the estimated values were lower than the measured values mainly because of the titration delay at the beginning of the aeration phase.

An EGSB-SBR based process for coupling methanogenesis and shortcut nitrogen removal.

An integrated process consisting of an anaerobic/anoxic expanded granular sludge bed (EGSB) reactor and an aerobic sequencing batch reactor (SBR) was developed by a mode of sequencing batch operation, in which methanogenesis, denitrification and anammox were coupled in EGSB with methanogenesis first, then denitrification and anammox simultaneously, and partial nitrification occurred in SBR for providing nitrite to EGSB. This process extended the application of the anammox process to the treatment of wastewater containing high concentrations of chemical oxygen demand (COD) and ammonium. When the volumetric exchange ratio between EGSB and SBR was controlled at 57% with the influent pH at 6-8, 74.38-83.65% of NH(4)(+)-N, 72.68-83.12% of total nitrogen (TN) and 88.34-98.86% of COD were removed in a range of 200-4,500 mg/L COD and 40-90 mg/L NH(4)(+)-N respectively. TN removal by anammox and shortcut denitrification was 26.35-58.64 and 0-32.80% of the removed nitrogen, respectively. The results showed that the contribution of anammox gradually decreased with an increase in the C/N ratio of influent, whereas the reverse was true for shortcut denitrification. The COD removal by methanogenesis was 70.89-98.79% of the removed COD, and increased with increasing C/N ratio.

Pollutant concentrations and pollution loads in stormwater runoff from different land uses in Chongqing.

To investigate the distribution of pollutant concentrations and pollution loads in stormwater runoff in Chongqing, six typical land use types were selected and studied from August 2009 to September 2011. Statistical analysis on the distribution of pollutant concentrations in all water samples shows that pollutant concentrations fluctuate greatly in rainfall-runoff, and the concentrations of the same pollutant also vary greatly in different rainfall events. In addition, it indicates that the event mean concentrations (EMCs) of total suspended solids (TSS) and chemical oxygen demand (COD) from urban traffic roads (UTR) are significantly higher than those from residential roads (RR), commercial areas (CA), concrete roofs (CR), tile roofs (TRoof), and campus catchment areas (CCA); and the EMCs of total phosphorus (TP) and NH3-N from UTR and CA are 2.35-5 and 3 times of the class-II standard values specified in the Environmental Quality Standards for Surface Water (GB 3838-2002). The EMCs of Fe, Pb and Cd are also much higher than the class-III standard values. The analysis of pollution load producing coefficients (PLPC) reveals that the main pollution source of TSS, COD and TP is UTR. The analysis of correlations between rainfall factors and EMCs/PLPC indicates that rainfall duration is correlated with EMCs/PLPC of TSS for TRoof and TP for UTR, while rainfall intensity is correlated with EMCs/PLPC of TP for both CR and CCA. The results of this study provide a reference for better management of non-point source pollution in urban regions.

Selection and synthesization of multi-carbon source composites to enhance simultaneous nitrification-denitrification in treating low C/N wastewater.

Low carbon/nitrogen ratio (C/N) wastewater is widespread and difficult to treat. To find a resolution to this issue, this study systematically evaluated the constituents of composite solid carbon (i.e., skeletons, carbon sources and crosslinking agents), and proposed a new multi-carbon source composite S1 (MCSC.S1). The effects on nitrogen removal were further determined through a sequencing batch moving bed biofilm reactor (SBMBBR). The results showed that MCSC.S1, which was composed of polyvinyl alcohol-sodium alginate (PVA-SA), corncob + poly (R-ß-hydroxybutyrate) (CC + PHB), and H3BO3-4% CaCl2+Na2SO4 had high stability and absorption. With MCSC.S1, total nitrification removal was enhanced by more than 48.56% through releasing carbon and absorbing the attached denitrifying bacteria. In addition, it was found that MCSC.S1 can simulate the simultaneous nitrification and denitrification (SND) process and contribute to 29.85% of the total nitrogen removal. 16S gene-based analysis attributed this supplementary nitrogen removal to the enrichment of nitrification (i.e., Proteobacteria, Actinobacteria and Chloroflexi), denitrification of associated bacteria (i.e., Nitrospirota) in MCSC.S1 added reactor, and the increase in nitrogen recycling associated genes. These findings collectively demonstrate that the new MCSC.S1 could effectively enhance nitrogen removal efficiency in low C/N ratio wastewater.

The potential of electrotrophic denitrification coupled with sulfur recycle in MFC and its responses to COD/SO42- ratios.

Electrotrophic denitrification is a promising novel nitrogen removal technique. In this study, the performance and the mechanism of electrotrophic denitrification coupled with sulfate-sulfide cycle were investigated under different anodic influent COD/SO42- ratios. The results showed that electrotrophic denitrification contributed to more than 22% total nitrogen removal in cathode chamber. Higher COD/SO42- ratios would deteriorate the sulfate reduction but enhance methane production. Further mass balance indicated that the electron flow utilized by methanogenic archaea (MA) increased while that utilized by sulfate-reducing bacteria (SRB) decreased as the COD/SO42- ratio increased from 0.44 to 1.11. However, higher COD/SO42- ratios would produce more electrons to strengthen electrotrophic denitrification. Microbial community analysis showed that the biocathode was predominantly covered by Thiobacillus that encoded with narG gene. These findings collectively suggest that electrotrophic denitrification could be a sustainable approach to simultaneously remove COD and nitrogen under suitable COD/SO42- ratio based on sulfur cycle in wastewater.

Improving the performance of coupled solid carbon source biofilm carriers through pore-forming methods.

Coupled solid carbon source biofilm carriers (CCBs) was usually utilized to enhance the treatment efficiency of low carbon/nitrogen (C/N) wastewater. However, current CCBs have low carbon release capacity because of its small inner mass transfer coefficient. Therefore, this study innovatively applied pore-forming methods to modify CCBs. After orthogonal selections, two porous CCBs, which were respectively prepared through circulating freezing pore-forming method (CCB2) and ammonium bicarbonate pore-forming method (CCB3), were proposed and further applied in sequencing batch moving bed biofilm reactors (SBMBBRs). The results indicated that circulating freezing pore-forming method could improve the mechanical strength and carbon source release rate of CCBs. In addition, CCB2 could significantly enhance the total nitrogen (TN) removal efficiency of SBMBBRs, when compared with the non-porous CCBs (i.e., CCB1). Further biofilm and simultaneous nitrification and denitrification (SND) rate calculation attributed this enhancement to the higher biofilm amount (i.e., 0.06 g g-1 CCB) and the higher SND rate (i.e., 33.60%). Microbial community analysis reiterated these observations that CCB2 and CCB3 could accumulate Proteobacteria, Actinobacteriota and Nitrospirota, and also stimulate nitrification and denitrification associated pathways. More importantly, the cost calculation indicated CCB2 cost only 47.37% of CCB1 and 31.34% of CCB3, showing highly economic applicability. Overall, our results collectively proved that CCBs manufactured by circulating freezing pore-forming method could provide more carbon releasing points and microorganisms attaching positions, exhibiting effectively nitrogen removal when treating low C/N wastewater.

A nine-point pH titration method to determine low-concentration VFA in municipal wastewater.

Characterization of volatile fatty acid (VFA) in wastewater is significant for understanding the wastewater nature and the wastewater treatment process optimization based on the usage of Activated Sludge Models (ASMs). In this study, a nine-point pH titration method was developed for the determination of low-concentration VFA in municipal wastewater. The method was evaluated using synthetic wastewater containing VFA with the concentration of 10-50 mg/l and the possible interfering buffer systems of carbonate, phosphate and ammonium similar to those in real municipal wastewater. In addition, the further evaluation was conducted through the assay of real wastewater using chromatography as reference. The results showed that the recovery of VFA in the synthetic wastewater was 92%-102 and the coefficient of variance (CV) of reduplicate measurements 1.68%-4.72%. The changing content of the buffering substances had little effect on the accuracy of the method. Moreover, the titration method was agreed with chromatography in the determination of VFA in real municipal wastewater with R(2)= 0.9987 and CV =1.3-1.7. The nine-point pH titration method is capable of satisfied determination of low-concentration VFA in municipal wastewater.

Addressing algal blooms by bio-pumps to reduce greenhouse gas production and emissions with multi-path.

The collapse of dense algal blooms is identified as a significant source of methane (CH4) emissions. When flocculation is used for algae removal, algal carbon is often turned into CH4 and carbon dioxide (CO2). Here, we established a "bio-pump" to control algal blooms and reduce greenhouse gas (GHG) emissions by the introduction of submerged macrophytes to the aquatic ecosystem and combination of flocculation and capping. The results suggested that this strategy contributed to an approximately 98% algae removal and sustainably improved dissolved oxygen (DO) in the water and sediment after the 40-day incubation. The aerobic condition at the sediment-water interface and deeper oxygen penetration in the sediment inhibited the abundance of microorganisms related to anaerobic CH4 production, then changed the metabolic pathway and fate of algal carbon. After the 40-day incubation, compared with flocculation-capping treatments, the bio-pump reduced 69.07% CH4 and 77.57% CO2 emissions, which was jointly contributed by the inhibition of anaerobic CH4 production, aerobic oxidation of CH4 and carbon sequestration of submerged macrophytes. This was also demonstrated from the finding of a decrease in methyl coenzyme M reductase (mcrA) gene, an increase in particulate methane monooxygenase (pmoA) gene and the absorption of 13C-labeled from algae biomass by submerged macrophytes at the end of incubation. Therefore, the bio-pump established in the present study can improve DO in algal blooms water and turn algal-derived organic matter into the plant biomass, which supplied a sustainable method for algae removal and GHG reduction.

Effects of green waste addition on waste activated sludge and fat, oil and grease co-digestion in mesophilic batch digester.

Anaerobic digestion (AD) is regarded as an effective method to treat waste activated sludge (WAS) and fat, oil and grease (FOG). Co-digestion of WAS/FOG could promote the methane yield but it will cause acid and salinity inhibition. Green waste (GW) was added into the digesters, and its effects on co-digestion of WAS and FOG in the mesophilic batch digester were investigated. Digestive performances (such as hydrolysis, acidogenesis and methanogenesis) were studied emphatically. The results showed that digester L6 (WAS:FOG:GW = 1:2:1, VS basis) presented the highest specific methane yield (SMY, 341.5 mL/g VS). The results of kinetics study verified that there was a slower hydrolysis rate when GW was applied as a co-substrate, which could reduce the potential of acid inhibition. Volatile fatty acid (VFA) and electrical conductivity analysis showed that GW addition could keep moderate VFA concentrations and alleviate the negative effects of high-salinity substrates on the digestive systems. The microbial community and diversity analysis proved that GW addition was beneficial to keep the balance of hydrolytic bacteria, acidogens and acetogens. The results of this study indicated that GW addition could enhance the energy recovery and system stability in the WAS/FOG co-digestive system.

Potential applications of endogenous sulfide for enhanced denitrification of low C/N domestic wastewater in anodic mixotrophic denitrification microbial fuel cell: The mechanism of electrons transfer and microbial community.

Anodic mixotrophic denitrification microbial fuel cell (MFC) was developed for pollutants removal and electricity generation in treatment of low C/N domestic wastewater. The experimental results show that the MFC achieved up to 100% of acetate, 100% of sulfide, and more than 91% of nitrate removal efficiency in all the MFCs. Particularly, thiosulfate was generated as the main intermediate of sulfide oxidation, and the sulfate generation ratio ranged from 66.93% to 73.76%. Those electrons produced during the acetate and sulfide oxidation were mainly used for denitrification and electricity generation. The microbial community analysis revealed that heterotrophic denitrifying bacteria (HDB) and sulfide-based autotrophic denitrifying bacteria (SADB) were the dominant bacteria for pollutants removal, and those facultative autotrophic bacterium (FAB) were key functional genera for high sulfate generation under both low and high sulfide concentrations. Meanwhile, the microbial functional prediction revealed that sulfide oxidation gene of Sqr and Sox were highly expressed. Moreover, a preliminary sulfide-based autotrophic denitrification (SAD) potential estimation indicated that the sulfide generated in the WWTPs had great potential for denitrification.

Chemical removal and selectivity reduction of nitrate from water by (nano) zero-valent iron/activated carbon micro-electrolysis.

The processes of (n)ZVI/AC（（nano）zero valent iron/activated carbon）micro-electrolysis were applied for nitrate removal from groundwater, aiming to reduce nitrate to N2, an environmentally friendly end product. (n)ZVI was utilized and combined with selected commercial AC to form the micro-electrolysis. Effect of different operational parameters, including reductant dosage, (n)ZVI/AC ratios, and reaction pH, on nitrate and TN removal were cleared. The results showed that nZVI presents higher reaction activities than ZVI in micro-electrolysis. With the increase of the (n)ZVI/AC mass ration from 1:2 to 2:1, the TN removal increased from 16.8% to 38.9%, then declined with the further increase of the ratio. The nitrate removal was negatively correlated with the initial pH of the solution. Compared to ZVI particles, even with a lower dosage, nZVI presented 100% nitrate removal at acidic and neutral pH conditions, and TN removal could maintain higher than 35% with pH lower than 9.0. A kinetic model was also established to explain the pathways of nitrate removal, and the results indicated that AC not only enriched nitrate as an adsorbent but also present highly potential in catalytic converting nitrate to N2. The technique presented great potentials in removing nitrate from water and a promising application prospect.

Enhanced nitrate adsorption by using cetyltrimethylammonium chloride pre-loaded activated carbon.

This paper used cetyltrimethylammonium chloride (CTAC) pre-loaded activated carbon (AC) to research nitrate adsorption. Effects of various parameters such as AC types, AC dosage as well as initial pH were studied. The results indicated that the ACs modified by CTAC can get higher nitrate removal. Even pH is neutral and basic, an accepted removal about 2.5 mg/g can be observed. The more CTAC pre-loaded on the AC surface, the higher nitrate adsorption capacity can be obtained. pH is regarded as a key factor affecting interactions between adsorbent and adsorbate, and the results confirmed that the nitrate adsorption on modified AC decreases gradually with the growth of initial pH. Besides, the acidic pH condition is much favoured for adsorption while the results gained a nitrate adsorption about 4.28 mg/g at pH = 3 condition. Sorption mechanism of nitrate on CTAC modified AC was investigated through two kinetic modellings including pseudo-second-order and Weber and Morris intra-particle diffusion model. The results imply that the generalized kinetic models tally well with experimental data. Additionally, interference of co-existing anions is examined, and the results showed that higher co-anions concentration would bring a heavier depression of the nitrate uptake due to its competing for adsorption sites.

Regulation of nitrogen dynamics at the sediment-water interface during HAB degradation and subsequent reoccurrence.

The effects of harmful algal blooms (HABs) on nutrient dynamics have been extensively studied; however, the response of nitrogen to continuous HAB degradation and subsequent reoccurrence is not well understood. Here, a small-scale experiment was conducted to assess how nitrogen in the sediment-water interface (SWI) responds to HAB degradation and subsequent reoccurrence at different initial algal densities. The results showed that during the algae decomposition stage, the NH4 +-N flux of the SWI remained positive but decreased with the increase in algal density from 3.5 × 107 to 2.3 × 108 cells per L, indicating that the sediment was the source of NH4 +-N. In contrast, the deposit was a sink of NO3 --N. However, during the reoccurrence of HAB, the distribution of NH4 +-N and NO3 --N fluxes was completely converted. Nitrogen flux analysis throughout algae decomposition and reoccurrence indicated that although the sediment acted as a sink of nitrogen, the flux was dependent on the initial algal density. Our results confirmed that algae decomposition and reoccurrence would greatly affect the nitrogen cycle of the SWI, during which dissolved oxygen (DO) and initial algal density dominated. This study is the first to show that the regulation of nitrogen flux and migration changes during continuous HAB decomposition and subsequent reoccurrence.