Specific Denitrification Rate Research Articles

The influence of electric current density on specific denitrification rate of and nitrogen removal rate in electrochemical and electrobiological rotating contactor

The influence of electric current density on specific denitrification rate of and nitrogen removal rate in electrochemical and electrobiological rotating contactor

Improving Nitrogen Removal Efficiency in SBR Reactors by Controlling Operational Phases

Wastewater treatment facilities using the activated sludge process are the most widely used solutions worldwide, regardless of the region. According to statistics, the most common option for Vietnam’s conditions is the sequenced batch reactor, which is used at a third of wastewater treatment plants (WWTP). Thus, studies of treatment processes in SBRs that seek high-quality treatment may have a wide range of further applications in practice. Another reason is that wastewater treated to meet Vietnamese standards has a significant negative impact on water resources. The research was carried out in relation to Hoaxuan wastewater treatment plants, where pilot models were installed that worked on influent wastewater. Studies of the nitrification process in SBRs (cycle duration 4 h) have shown that the highest efficiency of removal of pollutants (organic pollution by 90%, ammonium nitrogen by 80%) is achieved at a pH value close to 8.0. The increase in pH also had a positive effect on the increase in the specific rate of nitrification. At the second stage of the research, the process of denitrification in SBRs (cycle duration of 6.5 h) was considered. The results demonstrated that treatment quality corresponded to the standards for the Russian Federation, which are stricter than Vietnamese standards (3.0 mg/L vs. 30 mg/L of biochemical oxygen demand; 0.4 mg/L vs. 5 mg/L of ammonium nitrogen). For the second part of the study, a specific denitrification rate was determined in order to establish its dependence on the main parameters of the SBR model performance.

Enhancing biofilm growth in an integrated fixed-film activated sludge process through modification of polypropylene carriers

Biofilm is crucial in implementing integrated fixed-film activated sludge (IFAS) systems. However, current carriers face challenges in long-term stable operations. This study proposes using cationic polyacrylamide (PAM) as a positive-charged addictive and surface abrasion to modify polypropylene (PP) pipes to enhance biofilm growth and maintain long-time operation in IFAS applications. The modified PP carriers have significantly increased specific surface area compared to unmodified ones. The growth of biofilm on modified carriers is as much as over four times due to the improved biocompatibility of the material. The follow-up aerobic IFAS process achieves over 40% total nitrogen (TN) removal through simultaneous nitrification and denitrification with no specific anoxic stages or reactors. The sludge settling performance of the system is also improved with a stable 30-min sludge settling ratio of 18%. The maximum specific denitrification rate of the detached biofilm is 17.82 mg NO3−-N/g MLVSS/h, which is 33% higher than the pure activated sludge system. The high-throughput sequencing analysis results reveal that the abundance of biofilm-formation beneficial microbes increased from 0.02% to 16.47%.

Specific ammonium oxidation and denitrification rates in an MBR treating real textile wastewater for simultaneous carbon and nitrogen removal

AbstractBACKGROUNDDissolved oxygen (DO) and aeration on/off time are the determining parameters for simultaneous carbon and nitrogen removal in biological treatment processes. The most effective way to decide the optimum operational conditions, e.g. aeration on/off time or DO concentration during the aeration period, is to determine specific nitrification/denitrification rates. For this purpose, the effects of DO (6 and 3 mg L−1) and aeration on/off time (from 2/2 to 90/360 min) on the specific ammonium oxidation and denitritation/denitrification rates in a membrane bioreactor (MBR) treating real textile wastewater were deeply investigated.RESULTSThe highest specific ammonium oxidation, denitritation and denitrification rates were obtained as 5.4, 3.8, and 5.3 mg N g−1 volatile suspended solids h−1, respectively, at an aeration on/off time of 90/360 min, which corresponded to the increase in specific ammonium oxidation rates by 1.8 and 2.1 times compared to continuous aeration conditions where DO was 6 and 3 mg L−1, respectively.CONCLUSIONHigher specific ammonium oxidation and denitrification rates can be achieved with the intermittent aeration compared to continuous aeration. Hence existing treatment plants can be retrofitted for higher performance with reduced energy requirements. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Interrogating the performance and microbial ecology of an enhanced biological phosphorus removal/post-anoxic denitrification process at bench and pilot scales.

Research focused on interrogating post-anoxic enhanced biological phosphorus removal (EBPR) at bench and pilot scales. Average bench-scale effluent ranged from 0.33 to 1.4 mgP/L, 0.35 to 3.7 mgNH3 -N/L, and 1.1 to 3.9 mgNOx -N/L. Comparatively, the pilot achieved effluent (50th percentile/average) of 0.13/0.2 mgP/L, 9.7/8.2 mgNH3 -N/L, and 0.38/3.3 mgNOx -N/L under dynamic influent and environmental conditions. For EBPR process monitoring, P:C ratio data indicated that 0.2-0.4 molP/molC will result in stable EBPR; relatedly, a target design influent volatile fatty acid (VFA):P ratio would exceed 15 mgCOD/mgP. Post-anoxic EBPR was enriched for Nitrobacter spp. at 1.70%-20.27%, with Parcubacteria also dominating; the former is putatively associated with nitritation and the latter is a putative fermenting heterotrophic organism. Post-anoxic specific denitrification rates (SDNRs) (20°C) ranged from 0.70 to 3.10 mgN/gVSS/h; there was a strong correlation (R2 = 0.94) between the SDNR and %Parcubacteria for systems operated at a 20-day solids residence time (SRT). These results suggest that carbon substrate potentially generated by this putative fermenter may enhance post-anoxic EBPR. PRACTITIONER POINTS: Post-anoxic EBPR can achieve effluent of <0.2 mgP/L and <12 mgN/L. The P:C and VFA:P ratios can be predictive for EBPR process monitoring. Post-anoxic EBPR was enriched for Nitrobacter spp. over Nitrospira spp. and also for Parcubacteria, which is a putative fermenting heterotrophic organism. Post-anoxic specific denitrification rates (20°C) ranged from 0.70 to 3.10 mgN/gVSS/h. BLASTn analysis of 16S rDNA PAO primer set was shown to be improved to 93.8% for Ca. Accumulibacter phosphatis and 73.2%-94.0% for all potential PAOs.

Volatile fatty acids production from municipal waste streams and use as a carbon source for denitrification: The journey towards full-scale application and revealing key microbial players

Volatile fatty acids (VFAs) production is attracting interest as a sustainable approach to maximize resource recovery from organic wastes. This study explored the interlink between long-term system resilience of VFA production from primary sludge (PS) and external organic waste (OW) without pH control and the microbial community dynamics as well as the effect of substrate variability. The study elucidated the practicality of using VFA-rich effluent as a carbon source for wastewater denitrification. A 15 L bench-scale semi-continuous reactor was operated for 315 days with a feed of 70% v/v PS and 30% v/v OW and scaled up to a 2 m3 pilot-scale continuous reactor operated for 264 days. In the bench-scale study, the system was resilient with VFA production of up to 24,700 ± 400 mg COD/L and a yield of 506 ± 25 mg COD/g VSfed. The VFA composition was dominated by caproic acid up to 62% w/w. In the pilot-scale reactor, substrate variability influenced VFA production with a concentration of up to 21,500 ± 500 mg COD/L. The system was shown to be economically viable. The microbial community was dominated by Lachnospiraceae, Streptococcaceae and Comamonadaceae. The relative abundance of Lachnospiraceae gave a strong positive statistical correlation with caproic acid concentrations. The VFA-rich effluent exhibited a higher specific denitrification rate than methanol and acetate. Moreover, a continuous denitrification experiment with real nitrified wastewater resulted in a high nitrate removal efficiency with a maximum of 98%. The study demonstrates the production of bio-based products from organic wastes as alternatives to fossil-based products.

The key factor in High Pressure Hydrogenotrophic Denitrification: Hydrogen partial pressure

In the envisaged hydrogen economy, H2 could be an interesting alternative electron donor for the denitrification of drinking water or wastewater. The main obstacle to engineering the hydrogenotrophic denitrification process is the low solubility of H2 in water under atmospheric pressure, which limits denitrification rate and nitrogen removal efficiency. In this paper, we demonstrated a novel configuration of hydrogenotrophic denitrification, namely High Pressure Hydrogenotrophic Denitrification (HPHD). Elevated H2 partial pressure (pH2) was employed to increase dissolved H2 concentration and concomitantly enhance denitrification rate. Our results showed that the specific denitrification rate increased from 9.6 mg N/(gVSS·h) at 0.5 bars to 51.0 mg N/(gVSS·h) at 9 bars in HPHD. The denitrification effect could be retained with elevated pH2 at a low temperature. The specific denitrification rate at 3 bars and 15 °C was 20.5 mg N/(gVSS·h), approximately 1.5 times that at 1 bar and 30 °C, which was quite beneficial for hydrogenotrophic denitrification under cold conditions. Different from nitrite reduction, less impact was observed on nitrate reduction by low temperature, which explained high nitrite accumulation in HPHD at 15 °C. Overall, our investigations shed light on the role of pH2 in the promising solution for nitrogen removal in HPHD.

Simultaneous methanogenesis and denitrification coupled with nitrifying biofilm for high-strength wastewater treatment: Performance and microbial mechanisms

A combined system consisting of an upflow blanket filter (UBF) and a moving-bed biofilm reactor (MBBR) was developed for the simultaneous removal of organic matters and ammonia from high-strength wastewater. With a constant COD of approximately 2000 mg/L and ammonium nitrogen in a series of concentrations (e.g., 50, 200 and 400 mg/L in stages I to III) of the influent wastewater, the removal efficiencies of COD, ammonium nitrogen and total nitrogen reached 96.10%-98.19%, 100%, and 79.12%-82.15%, respectively. With the increase of influent ammonia nitrogen concentration, the specific methanogenic activity of the UBF granules decreased significantly, while the specific denitrification rates of the UBF granules and specific nitrification rates of the MBBR biofilms increased significantly. Microbial community analysis showed that Methanobacterium and Methanosaeta were the dominant methanogens in the UBF granules, while Candidatus Competibacter, Thauera and Acinetobacter were identified as dominant denitrifiers. In addition, nitrifiers were enriched in MBBR biofilms at 11.33% and 13.87% of the average abundance of Nitrosomonas and Nitrospira, respectively, at stage III (influent ammonium at 400 mg/L, COD/NH4+-N = 5). The ecological network analysis, including full-networks and sub-networks, indicated that the interactions between methanogens and denitrifiers in the UBF granules were strong when the influent ammonium concentration reached 400 mg/L. No intensive interactions were observed among the functional bacteria in the MBBR biofilms over the entire operation. Overall, this study provides a new strategy for the application and construction of efficient biological processes to achieve simultaneous removal of organic matter and nitrogen for high-strength wastewater treatment.

Evaluation on applying Condensate from Food Waste Dry Process as External Carbon Source to Biological Denitrification Process

Objectives:In this study, the applicability of dry condensate generated from food waste dry feed conversion process was evaluated as an external carbon source to increase the efficiency of the biological denitrification process.Methods:The experiment was carried out in the form of a batch test, and dry condensate was injected into each reactor according to the COD and T/N ratio (C/N ratio). Then, the reactor was operated under anoxic conditions for 5 hours, and nitrate nitrogen was analyzed at intervals of 1 hour.Results and Discussion:The dry condensate from food waste showed a low nitrogen content (T-N 220 mg/L) compared to a high organic matter content (TCOD 21,863 mg/L), and was analyzed to satisfy the conditions of an external carbon source in the denitrification process. And as a result of the experiment, it was found that the nitrate nitrogen removal efficiency was up to 76% after 5 hours at a C/N ratio 7.50:1, which was significantly increased compared to the control which was without condensed water (removal efficiency 4%). Moreover, the removal efficiency of 28% was exhibited even at the C/N ratio 0.95:1, which exhibited the lowest efficiency. In addition, as a result of calculating the specific denitrification rate (SDNR) for each C/N ratio, 7.50:1 was found to be as 0.0726 mg NO3-/mg VSS/d, which was significantly increased than that of control as 0.0038mg NO3-/mg VSS/d.Conclusion:Based on these results, it is judged that it is possible to utilize the dried condensate of food waste as an external carbon source in the denitrification process.

Pilot-scale hydrolysis of primary sludge for production of easily degradable carbon to treat biological wastewater or produce biogas

Organic compounds in wastewater are required for the biological removal of nitrogen, but they can also be used for biogas production. Distribution of the internal organic carbon at the plant is therefore critical to ensure high quality of the treated water, reduce greenhouse gas emissions, and optimize biogas production. We describe a wastewater treatment plant designed to focus equally on energy production, water quality, and reduced emissions of greenhouse gases. A disk filter was installed to remove as much carbon as possible during primary treatment. Primary sludge was then hydrolyzed and centrifuged. The hydrolysate centrate contained volatile fatty acids and was used either for the secondary wastewater treatment or to produce biogas. The yield during hydrolysis was 30–35 g volatile fatty acid per kg dry material or 40–65 g soluble COD per kg total solid. The specific denitrification rate was 20–40 g/(g·min), which is on the same order of magnitude as that for commonly used external carbon sources. Hydrolysis at around 35 °C and pH 7 gave the best results. The hydrolysate centrate can be stored and added to the biological treatment to improve water quality and reduce emissions of nitrous oxide or it can be used to produce biogas to optimize the operation of the plant.

Denitrification performance and characteristics of untreated corncob for enhanced nitrogen removal of municipal sewage with low C/N ratio

Unpretreated corncob was applied in denitrification bio-filter (DNBF) and anoxic tank of AAO system, respectively, to treat sewage with low C/N ratio, and both two approaches achieved good denitrification performance. Although shorter HRT could effectively decrease effluent chroma and COD of corncob-DNBF, nitrogen removal efficiency declined unexpectedly. Higher internal reflux ratio was beneficial for corncob-AAO without damage to anoxic environment for denitrification, while there was no risk of effluent chroma and excessive COD. Different supplement modes could realize same denitrification effect with distinct advantages, which were higher specific denitrification rate and biomass amount, respectively. The latter mode, applying corncob at secondary treatment, was preferable for its operational stability and convenience. Stoichiometry analysis indicated the unit COD demand of AAO decreased from 5.70 to 5.04 g COD/g N after adding corncob, and the oxygen demand (or energy consumption of aeration) decreased as well. The dominant substrates decomposer in corncob-AAO altered to Kouleothrix (affiliated to phylum Chloroflexi), and the main denitrifying bacteria were unclassified_f__Methylophilaceae and Azospira. Accordingly, functional enzymes for degrading glucan, xylan and lignin and processing denitrification showed satisfying abundance in the integrated system, especially in the newly formed biofilm.

Vacuum-enhanced anaerobic fermentation: Achieving process intensification, thickening and improved hydrolysis and VFA yields in a single treatment step

This study assessed the feasibility of a novel vacuum-enhanced anaerobic digestion technology, referred to as IntensiCarbTM (IC), under mild vacuum pressure (110 mbar), compared to a control (conventional fermenter), and evaluated the impact of the vacuum on the activities of various microbial groups. Both fermenters (test and control) were operated with mixed (50% v/v) municipal sludge at solids concentrations of 2–2.5%, pH of 7.8–8.1, 40–45 °C, a theoretical solids retention time (SRT) of 3 days with different hydraulic retention times (HRT). The intensification factor (IF) of the IC, defined as SRT/HRT, was controlled at 1.3 and 2.0. Simultaneous thickening and fermentation intensification were achieved. Compared with the control, the IC, despite the shorter HRTs, achieved 29.5 to 90.2% increase in the VFA yield (79 to 116 mg ΔVFA/ g VSS vs 61 mg ΔVFA/ g VSS), and 16.2% to 56.4% increase (280 to 377 mg ΔsCOD/ g VSS vs 241 mg ΔsCOD/ g VSS), in the hydrolysis yield. Fermentate from the IC exhibited comparable specific denitrification rates to acetate. Further, the solids-free condensate contained low nutrient concentrations, and thus was far superior to a typical centrates from dewatering as a carbon source. No adverse effects of vacuum on the activity of fermentative bacteria and methanogens were observed. This study demonstrated that the IC can be deployed as an intensification technology for both fermentation and anaerobic digestion of biosolids with the additional significant advantage, i.e. elimination of sidestream ammonia treatment requirements.

Investigating the use of anaerobically stored carbon in post‐anoxic denitrification

Significant methanol savings are hypothesized to result from anaerobic storage of internal carbon that is used for post‐anoxic denitrification. An investigation into this internal carbon‐driven denitrification was performed via a series of batch tests using biomass from Hampton Roads Sanitation District's (HRSD's) water resource recovery facilities (WRRFs): the Virginia Initiative Plant (VIP), Nansemond Plant (NP), and Army Base (AB) Treatment Plant. Internal carbon specific denitrification rates (SDNRs) increased during winter, by as much as 1 mg N/g MLVSS/h for VIP. Increasing the aeration time by 2–4 h lowered the SDNR by an average of 0.21–0.35 mg N/g MLVSS/h. No internal carbon denitrification was observed for biomass from non‐nitrifying/denitrifying, biological phosphorus removal (bio‐P) WRRFs. The increase in internal carbon SDNRs when the anaerobic acetate dose increased from 20 to 100 mg COD/L ranged from 0.06 to 0.28 mg N/g MLVSS/h. Higher phosphorus uptake rates were found to correlate to higher internal carbon SDNRs, but no significant post‐anoxic P uptake was observed. The first steps are taken towards developing a strategy for full‐scale implementation of this relatively novel type of denitrification by evaluating how some factors affect its occurrence.Practitioner Points Significant methanol savings at a full‐scale facility may result from use of internally stored carbon for post‐anoxic denitrification.Short aerobic HRTs and high anaerobic zone VFA loading increase the post‐anoxic internal carbon‐driven denitrification.Non‐nitrifying, bio‐P biomass is not capable of internal carbon‐driven denitrification.Internal carbon‐driven denitrification is correlated with the activity of polyphosphate accumulating organisms.

Influence of additional methanol on both pre- and post-denitrification processes in treating municipal wastewater.

Changes in functional properties of biological denitrification in the long-term use of methanol were explored in both pre- and post-denitrification processes. The two systems employed were sequencing batch reactor (SBR) using post-denitrification in temporal sequence, and Carrousel oxidation ditch, which was equipped with a separate pre-denitrification zone. In the SBR, stable nitrate reduction rates reached after 37 days elapsed with addition of methanol (TOC/N = 1.4-1.8) at the start of anoxic phase, and specific denitrification rate increased from 0.378 mgNOx-N·(gVSS·h)-1 to 2.406 mgNOx-N·(gVSS·h)-1. Besides, by means of nitrogen uptake rate (NUR) batch tests based on methanol-adapted sludge, the appropriate range of TOC/N ratios for complete denitrification was estimated to be 1.10-2.68. By comparison, the Carrousel oxidation ditch that was fed with methanol in the anaerobic zone took fewer days (29 days) to obtain a constant effluent nitrate. Moreover, the denitrification yield in ditch was elevated from an initial value of 0.082 mgTN/mgCOD to 0.123 mgTN/mgCOD, and the nitrogen removal efficiency reached up to a level of 68%. The focus on denitrification potential with external methanol is valuable to provide information for developing carbon dosage control, as well as predict the nitrate effluent quality of the plant.

A novel approach to estimate and control denitrification performance in activated sludge systems with respirogram technology

Respirogram technology has been widely applied for aerobic process, however, the response of respirogram to anoxic denitrification is still unclear. To reveal such response may help to design a new method for the evaluation of the performance of denitrification. The size distribution of flocs measured at different denitrification moments demonstrated a clear expansion of flocs triggered by denitrification, during which higher specific endogenous and quasi-endogenous respiration rates (SOURe and SOURq) were also observed. Furthermore, SOURq increases exponentially with the specific denitrification rate (SDNR), suggesting that there should be a maximum SDNR in conventional activated sludge systems. Based on these findings, an index Rq/t, defined as the ratio of quasi-endogenous (OURq) to maximum respiration rate (OURt), is proposed to estimate the denitrification capacity that higher Rq/t indicates higher denitrification potential, which can be readily obtained without complex measurement or analysis, and it offers a novel and promising respirogram-based approach for denitrification estimation and control by taking measures to extend anoxic time to maintain its value at a high level within a certain range.

Enhancement of denitrification efficiency using municipal and industrial waste fermentation liquids as external carbon sources

The addition of external carbon source for nitrogen removal from wastewater is an essential step in wastewater treatment. In this study, various external carbon sources from the fermentation of primary sludge (PS), thickened waste activated sludge (TWAS), food waste (FW), bakery processing & kitchen waste (BP + KW), fat, oil, & grease (FOG), and whey powder (WP) were successfully employed for wastewater denitrification. Methanol and acetate were also used as controls due to their common use as external carbon sources for wastewater denitrification. The denitrification performance and kinetics such as the specific denitrification rate (SDNR), denitrification potential (PDN), and the biomass yield were studied at a constant TVFA as COD/N ratio of 5 for all substrates. Complete denitrification was achieved with a NO3−-N removal efficiency of 98–99%, and no NO2− accumulation was observed at the end of the experiments for all substrates. The results revealed that the liquid fermentation filtrates exhibited higher SDNRs than methanol and acetate. This indicates the high organic matter utilization efficiency and better denitrification ability of fermentation filtrates over conventional carbon sources. WP exhibited the highest SDNR of 17.6 mg NOx − N/g VSS/h, which is approximately four times that of methanol (4.6 mg NOx − N/g VSS/h). The other carbon sources had SDNRs two to three times higher than that of methanol. However, the fermentation filtrates exhibited higher biomass yields of 0.26–0.37 mg VSS/mg COD compared to methanol of 0.21 mg VSS/mg COD, which could lead to higher sludge handling costs. Moreover, methanol exhibited higher PDN of 0.25 g N/g COD compared to all the fermentation filtrates.

Application of humin-immobilized biocathode in a continuous-flow bioelectrochemical system for nitrate removal at low temperature

Solid-phase humic substances (humin) can work as an additional electron donor to support the low temperature denitrification but the reducing capacity of its non-reduced form is limited. In this study, a continuous-flow denitrifying BES with a humin-immobilized biocathode (H-BioC) was established. Humin was expected to function as a redox mediator and be persistently reduced on the cathode to provide reducing power to a denitrifying biofilm. Results showed that the H-BioC maintained a stable denitrification capacity with low nitrite accumulation for more than 100 days at 5 °C, and the specific microbial denitrification rate and electron transfer rate were 3.97-fold and 1.75-fold higher than those of the unaltered cathode. The results of repeated cycles of humin reduction and oxidation experiments further suggested that the redox activity of humin was stable. Acidovorax was the most dominant genus in both H-BioC biofilm and unaltered cathodic biofilm, while Rhodocyclaceae (unclassified_f_) was more enriched in H-BioC biofilm. Further Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analyses indicated that biofilm formation, electron transfer, and nitrate reduction functions were more abundant in H-BioC, suggesting a possible enhancement mechanism by humin. The results of this study raise the possibility that immobilization of solid-phase humin may be a useful strategy for electrostimulated heterotrophic denitrification in groundwater where the indigenous bacteria have poor electroactivity.

Fe(II) enhances simultaneous phosphorus removal and denitrification in heterotrophic denitrification by chemical precipitation and stimulating denitrifiers activity

Using Fe(II) salt as the precipitant in heterotrophic denitrification achieves improved TP removal, and enhancement in denitrification was often observed. This study aimed to obtain a better understanding of Fe(II)-enhanced denitrification with sufficient carbon source supply. Laboratory-scale experiments were conducted in SBRs with or without Fe(II) addition. Remarkably improved TP removal was experienced. TP removal efficiency in Fe(II) adding reactor was 85.8 ± 3.4%; whereas, that in the reactor without Fe(II) addition was 31.1 ± 2.8%. Besides improved TP removal, better TN removal efficiency (94.1 ± 1.1%) were recorded when Fe(II) was added, and that in the reactor without Fe(II) addition was 89 ± 0.8%. The specific denitrification rate were observed increase by 12.6% when Fe(II) was added. Further microbial analyses revealed increases in the abundances of typical denitrifiers (i.e. Niastella, Opitutus, Dechloromonas, Ignavibacterium, Anaeromyxobacter, Pedosphaera, and Myxococcus). Their associated denitrifying genes, narG, nirS, norB, and nosZ, were observed had 14.2%, 19.4%, 21.6%, and 9.9% elevation, respectively. Such enhancement in denitrification shall not be due to nitrate-dependent ferrous oxidation, which prevails in organic-deficient environments. In an environment with a continuous supply of Fe(II) and plenty of carbon sources, a cycle of denitrifying enzyme activity enhancement in the presence of Fe(II) facilitating nitrogen substrate utilization, stimulating denitrifier metabolism and growth, elevating denitrifying genes abundance, and increasing denitrifying enzymes expression were thought to be responsible for the Fe(II)-enhanced heterotrophic denitrification. Fe(II) salt is often a less expensive precipitant and has recently become attractive for TP removal in wastewater. The findings of this study solidify previous observation of enhancement of both TP and TN removal by adding Fe(II) in denitrification, and would be helpful for developing cost-effective pollutant removal processes.

Field assessment of full-scale solar-powered floating biofilm reactors for improving water quality in a micro-polluted river near Lake Taihu

Small rivers in rural areas near Lake Taihu that receive non-point-source pollutant are often used by the rural population as sources of water for domestic use. Improving water quality in situ using a floating biofilm reactor is an economically way of decreasing the health risks posed by such water to humans and improving the aquatic environment. In this study, the feasibility of using solar-powered floating biofilm reactors to improve water quality was assessed by performing field tests. An aerated reactor alone decreased the ammonium concentration, chemical oxygen demand, total nitrogen concentration, and total phosphorus concentration by 31.94%, 10.25%, 10.36%, and 9.38%, respectively. An innovative system using alternating aerated and nonaerated biofilm reactors decreased the total nitrogen concentration by up to 21.38%. A higher specific denitrification rate was found for biofilm cultured in a non-aerated reactor than for biofilm culture in an aerated reactor. The dominant bacteria responsible for removing nitrogen in biofilms on carriers and natural suspended particles in the same area were different. The biofilm on the reactor carriers were enriched in Gammaproteobacteria, but biofilms on natural suspended particles in the same zone were enriched in Deltaproteobacteria. This, and the dominance of Halomonas in the zeolite biofilms, indicating that it is essential to use appropriate biofilm carriers to enhance denitrification of polluted surface water. The degree to which water quality is improved in practice will be determined by the number and sizes of reactors used, and the flow rate and pollutant loading of the polluted river being treated.

Effect of carbon-to-nitrogen ratio on high-rate nitrate removal in an upflow sludge blanket reactor for polluted raw water pre-treatment application

The drinking water treatment plants (DWTPs) in the developing countries urgently need an efficient pre-treatment for nitrate (NO3−) removal to cope with the increasing NO3− pollution in raw water. An upflow sludge blanket (USB) reactor applied for NO3− removal from domestic wastewater may be adopted by the DWTPs. However, studies on the optimal carbon-to-nitrogen ratio (C/N) and operation of USB reactor at short hydraulic retention times (HRT) for high-rate polluted raw water pre-treatment are lacking. In this study, we first investigated the optimal C/N for biological NO3− removal in a sequencing batch reactor (SBR). An USB reactor was then operated with the optimal C/N for pre-treating synthetic raw water contaminated with NO3− (40 mg N L− 1) to monitor the NO3− removal performance and to examine opportunities for reducing the HRT. After operating the SBR with designed C/N of 4, 3 and 2 g C g− 1 N, we selected C/N of 3 g C g− 1 N as the optimal ratio due to the lower carbon breakthrough and nitrite (NO2−) accumulation in the SBR. The USB reactor achieved complete NO3− and NO2− removal with a lower designed C/N of 2 g C g− 1 N due to the longer sludge retention time when compared with that of SBR (10 d). The high specific denitrification rate (18.7 ± 3.6 mg N g− 1 mixed liquor volatile suspended solids h− 1) suggested a possible HRT reduction to 36 min. We successfully demonstrated an USB reactor for high-rate NO3− removal, which could be a promising technology for DWTPs to pre-treat raw water sources polluted with NO3−.