Balancing yield, kinetics and cost for three external carbon sources used for suspended growth post-denitrification

2-Hydroxy-1,4-Naphthoquinone: A Promising Redox Mediator for Minimizing Dissolved Organic Nitrogen and Eutrophication Effects of Wastewater Effluent

Researchers and engineers are committed to finding effective approaches to reduce dissolved organic nitrogen (DON) to meet more stringent effluent total nitrogen limits and minimize effluent eutrophication potential. Here, we provided a promising approach by adding specific doses of 2-hydroxy-1,4-naphthoquinone (HNQ) to postdenitrification bioreactors. This approach of adding a small dosage of 0.03-0.1 mM HNQ effectively reduced the concentrations of DON in the effluent (ANOVA, p < 0.05) by up to 63% reduction of effluent DON with a dosing of 0.1 mM HNQ when compared to the control bioreactors. Notably, an algal bioassay indicated that DON played a dominant role in stimulating phytoplankton growth, thus effluent eutrophication potential in bioreactors using 0.1 mM HNQ dramatically decreased compared to that in control bioreactors. The microbe-DON correlation analysis showed that HNQ dosing modified the microbial community composition to both weaken the production and promote the uptake of labile DON, thus minimizing the effluent DON concentration. The toxic assessment demonstrated the ecological safety of the effluent from the bioreactors using the strategy of HNQ addition. Overall, HNQ is a promising redox mediator to reduce the effluent DON concentration with the purpose of meeting low effluent total nitrogen levels and remarkably minimizing effluent eutrophication effects.

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.

Microbial bromate reduction following ozonation of bromide-rich wastewater in coastal areas

Ozonation of wastewater can reduce the release of organic micropollutants, but may result in the formation of undesirable by-products, such as bromate from bromide. Bromide is one of the most abundant ions in seawater, the primary precursor of bromate during ozonation, and the end product in microbial bromate reduction. Investigations were carried out to compare the concentration of bromide in wastewater in coastal and non-coastal catchment areas, to monitor bromate formation during ozonation, and to assess the potential for subsequent bromate reduction with denitrifying carriers. Higher bromide concentrations were systematically observed in wastewater from coastal catchment areas (0.2-2 mg Br^-/L) than in wastewater from non-coastal areas (0.06-0.2 mg Br^-/L), resulting in elevated formation of bromate during ozonation. Subsequent investigations of bromate reduction in contact with denitrifying carriers from two full-scale moving bed biofilm reactors (MBBRs) showed that 80 % of the bromate formed during ozonation could be reduced to bromide in 60 min with first-order rate constants of 0.3-0.8 L/(g_biomass·h). Flow-through experiments with denitrifying carriers also showed that combined reduction of bromate and nitrate could be achieved below a concentration of 2 mg NO_x^--N/L. These findings indicate that bromide-rich wastewater is more likely to be of concern when using ozonation in coastal than in non-coastal areas, and that bromate and nitrate reduction can be combined in a single biofilm reactor.

Mainstream short-cut N removal modelling: current status and perspectives

This work gives an overview of the state-of-the-art in modelling of short-cut processes for nitrogen removal in mainstream wastewater treatment and presents future perspectives for directing research efforts in line with the needs of practice. The modelling status for deammonification (i.e., anammox-based) and nitrite-shunt processes is presented with its challenges and limitations. The importance of mathematical models for considering N₂O emissions in the design and operation of short-cut nitrogen removal processes is considered as well. Modelling goals and potential benefits are presented and the needs for new and more advanced approaches are identified. Overall, this contribution presents how existing and future mathematical models can accelerate successful full-scale mainstream short-cut nitrogen removal applications.

The nitrification recovery capacity is the key to enhancing nitrogen removal in the AOA system at low temperatures

Anaerobic/aerobic/anoxic (AOA) is suitable for advanced nitrogen removal of low C/N wastewater as an energy-saving process. Investigations of the temperature impact on the AOA process are critical to its application in cold regions or seasons. In this study, the nitrogen removal performance in AOA at low and room temperatures was investigated. The nitrification capacity of the AOA process was recovered at low temperature and the endogenous denitrification performance was enhanced by converting the partial aerobic zone into anoxic. At 15 °C, treating real municipal sewage with a low C/N ratio (3.36), TIN and NH₄⁺-N removal efficiencies of 84.3 ± 6.6% and 97.4 ± 3.3% respectively, were achieved. The oxygen uptake rate test, quantitative PCR, and high-throughput sequencing results indicated that the activity and abundance of ammonia-oxidizing bacteria (AOB) increased at low temperature, which was the key for nitrification capacity recovery. Overall, the recoverability of nitrification capacity in the AOA system made advanced nitrogen removal possible at low temperatures.

Partial denitrification-anammox (PdNA) application in mainstream IFAS configuration using raw fermentate as carbon source

This research examined the feasibility of raw fermentate for mainstream partial denitrification-anammox (PdNA) in a pre-anoxic integrated fixed-film activated sludge (IFAS) process. Fermentate quality sampled from a full-scale facility was highly dynamic, with 360-940 mg VFA-COD/L and VFA/soluble COD ratios ranging from 24% to 48%. This study showed that PdNA selection could be achieved even when using low quality fermentate. Nitrate residual was identified as the main factor driving the PdN efficiency, while management of nitrate conversion rates was required to maximize overall PdNA rates. AnAOB limitation was never observed in the IFAS system. Overall, this study showed PdN efficiencies up to 38% and PdNA rates up to 1.2 ± 0.7 g TIN/m² /d with further potential for improvements. As a result of both PdNA and full denitrification, this concept showed the potential to save 48-89% methanol and decrease the carbon footprint of water resource recovery facilities (WRRF) by 9-15%. PRACTITIONER POINTS: Application of PdNA with variable quality fermentate is feasible when the nitrate residual concentration is increased to enhance PdN selection. To maximize nitrogen removed through PdNA, nitrate conversion rates need enhancement through optimization of upstream aeration and PdN control setpoints. The IFAS PdNA process was never anammox limited; success depended on the degree of PdN achieved to make nitrite available. Application of PdNA with fermentate can yield 48-89% savings in methanol or other carbon compared with conventional nitrification and denitrification. Integrating PdNA upstream from polishing aeration and anoxic zones guarantees that stringent limits can be met (<5 mg N/L).

Enrichment of a denitratating microbial community through kinetic limitation

Denitratation, or the intentionally engineered accumulation of nitrite (NO₂^-) from selective reduction of nitrate (NO₃^-), can be combined with downstream anammox to reduce chemical and energy use associated with conventional nitrification and denitrification. This study aimed to enrich a denitratating microbial community capable of significant NO₂^- accumulation by applying added kinetic limitation to an already stoichiometrically-limited, glycerol-driven denitratation process. Operation at solids residence time, SRT=3.0 d, resulted in optimal denitratation performance and a microbial community dominated by NO₃^--respirers, noted by one order of magnitude lower total copy numbers of nirS and nirK gene transcripts compared to longer SRTs. Selective NO₃^- reduction to NO₂^- was achieved at all SRTs although longer SRTs (less kinetic limitation) supported microbial communities more capable of full denitrification as described by a lower NO₂^- accumulation ratio (NAR=42±5%) and higher steady-state nitrous oxide (1.5 mg/L N₂O-N) accumulation. Shorter SRTs (more kinetic limitation) led to higher observed yields (Y=0.63 mg-COD/mg-COD) with more electrons dedicated for cell synthesis (f_s=0.56±0.10), which potentially contributed to the accumulation of NO₃^-. Enrichment of a denitratating-dominant microbial community by optimizing kinetic limitation operating parameters could support significant NO₂^- accumulation and reduce chemical and energy use for biological nitrogen removal when combined with downstream anammox.

Organic carbon determines nitrous oxide consumption activity of clade I and II nosZ bacteria: Genomic and biokinetic insights

Harnessing nitrous oxide (N2O)-reducing bacteria is a promising strategy to reduce the N2O footprint of engineered systems. Applying a preferred organic carbon source as an electron donor accelerates N2O consumption by these bacteria. However, their N2O consumption potential and activity when fed different organic carbon species remain unclear. Here, we systematically compared the effects of various organic carbon sources on the activity of N2O-reducing bacteria via investigation of their biokinetic properties and genomic potentials. Five organic carbon sources-acetate, succinate, glycerol, ethanol, and methanol-were fed to four N2O-reducing bacteria harboring either clade I or clade II nosZ gene. Respirometric analyses were performed with four N2O-reducing bacterial strains, identifying distinct shifts in DO- and N2O-consumption biokinetics in response to the different feeding schemes. Regardless of the N2O-reducing bacteria, higher N2O consumption rates, accompanied by higher biomass yields, were obtained with acetate and succinate. The biomass yield (15.45 ± 1.07 mg-biomass mmol-N2O-1) of Azospira sp. strain I13 (clade II nosZ) observed under acetate-fed condition was significantly higher than those of Paracoccus denitrificans and Pseudomonas stutzeri, exhibiting greater metabolic efficiency. However, the spectrum of the organic carbon species utilizable to Azospira sp. strain I13 was limited, as demonstrated by the highly variable N2O consumption rates observed with different substrates. The potential to metabolize the supplemented carbon sources was investigated by genomic analysis, the results of which corroborated the N2O consumption biokinetics results. Moreover, electron donor selection had a substantial impact on how N2O consumption activities were recovered after oxygen exposure. Collectively, our findings highlight the importance of choosing appropriate electron donor additives for increasing the N2O sink capability of biological nitrogen removal systems.

How to Produce an Alternative Carbon Source for Denitrification by Treating and Drastically Reducing Biological Sewage Sludge

Minimizing the biological sewage sludge (BSS) produced by wastewater treatment plants (WWTPs) represents an increasingly difficult challenge. With this goal, tests on a semi-full scale Thermophilic Alternate Membrane Biological Reactor (ThAlMBR) were carried out for 12 months. ThAlMBR was applied both on thickened (TBSS) and digested biological sewage sludge (DBSS) with alternating aeration conditions, and emerged: (i) high COD removal yields (up to 90%), (ii) a low specific sludge production (0.02-0.05 kgVS produced/kgCODremoved), (iii) the possibility of recovery the aqueous carbon residue (permeate) in denitrification processes, replacing purchased external carbon sources. Based on the respirometric tests, an excellent biological treatability of the permeate by the mesophilic biomass was observed and the denitrification kinetics reached with the diluted permeate ((4.0 mgN-NO3-/(gVSS h)) were found comparable to those of methanol (4.4 mgN-NO3-/(gVSS h)). Moreover, thanks to the similar results obtained on TBSS and DBSS, ThAlMBR proved to be compatible with diverse sludge line points, ensuring in both cases an important sludge minimization.

Primary sludge fermentate as carbon source for mainstream partial denitrification-anammox (PdNA)

Primary sludge fermentate, a concentrated hydrolyzed wastewater carbon, was evaluated for use as an alternative carbon source for mainstream partial denitrification-anammox (PdNA) in a suspended growth activated sludge process in terms of partial denitrification (PdN) efficiency, PdNA nitrogen removal contributions, and final effluent quality. Fermenter operation at a 2-day sludge retention time (SRT) resulted in the maximum achievable yield of 0.14 ± 0.05 g sCOD/g VSS without release of excessive ammonia and phosphorus to the system. Based on the results of batch experiments, fermentate addition led to PdN efficiency of 93 ± 14%, which was similar to acetate at a nitrate residual of 2-3 mg N/L. In the pilot-scale mainstream deammonification reactor, PdN efficiency using fermentate was 49 ± 24%, which was lower than acetate (66 ± 24% during acetate period I and 70 ± 21% during acetate period II), most probably due to lower nitrate and ammonium kinetics in the PdN zone. Methanol cost-saving potential for the application of PdNA as the main short-cut nitrogen pathway was estimated to be 30% to 55% depending on the PdN efficiency achieved. PRACTITIONER POINTS: Primary sludge fermentate was evaluated as an alternative carbon source for mainstream partial denitrification-anammox (PdNA). Fermenter operated at a 1 to 2 day SRT resulted in the maximum achievable yield without the release of excessive ammonia and phosphorus to the system. Although 93% partial denitrification efficiency was achieved with fermentate in batch experiments, around 49% PdN efficiency was achieved in pilot studies. Application of PdNA with fermentate can result in significant methanol cost savings.

Advanced nitrogen removal of low C/N ratio sewage in an anaerobic/aerobic/anoxic process through enhanced post-endogenous denitrification

Currently, it is a major challenge for waste water treatment plants (WWTPs) to achieve enhanced nitrogen removal economically and effectively from carbon-limited sewage to meet gradually stringent discharge quality standards. Enhanced nitrogen removal can be achieved by endogenous denitrification (ED) treatment of low C/N municipal sewage, but its application is limited by the slow reaction rate. In this study, a novel process of Sludge Double Recirculation-Anaerobic/Aerobic/Anoxic (SDR-AOA) was developed to improve nitrogen removal efficiency via ED. ED was successfully enhanced by an extra sludge recirculation to post-anoxic zone and the denitrification rate increased from 0.1 to 0.17 kgN/(m³·d). Moreover, the pre-anaerobic zone enhanced the intracellular carbon storage, which might also favor the ED process. Overall, under an influent C/N of 2.67, nitrogen removal efficiency of 97.7% was achieved with effluent total inorganic nitrogen (TIN) of 1.56 ± 1.77 mg/L and nitrogen removal rate (NRR) of 0.14 kgN/(m³·d). Therefore, this study provides a convenient approach to improve the nitrogen removal efficiency of municipal sewage with low C/N.

Modeling Denitrification as an Electric Circuit Accurately Captures Electron Competition between Individual Reductive Steps: The Activated Sludge Model-Electron Competition Model

Heterotrophic denitrification consists of the four-step sequential reduction of nitrate to dinitrogen gas over nitrite, nitric oxide, and nitrous oxide. Oxidation processes, commonly of organic compounds, provide the electrons needed for the sequential reaction steps. The intracellular electron distribution is a competitive process among the four reduction steps. In this study, a model describing organic carbon oxidation and four-step denitrification through electron competition is proposed [Activated Sludge Model-Electron Competition (ASM-EC)]. The model describes denitrification rates as an analogy to how current intensity varies through a parallel set of resistors in electric circuits. The ASM-EC model was calibrated with data from batch experiments with heterotrophic denitrifying communities, where reduction of mixtures of nitrogen oxides was monitored, while different carbon sources were supplied in excess. The carbon sources included methanol, ethanol, acetate, and their ternary mixture. The electron distribution preference and electron uptake rates varied between the carbon sources and were captured by the model structure for most of the experiments. The ASM-EC model uses fewer parameters compared to existing state-of-the-art denitrification models and performed equally well in the tested scenarios. We advocate the use of this model for denitrification in the activated sludge model, which can easily be integrated in existing model structures, because it provides a parsimonious description of electron competition during denitrification.

Characteristics of dissolved organic nitrogen in effluent from a biological nitrogen removal process using sludge alkaline fermentation liquid as an external carbon source

The addition of sludge alkaline fermentation liquid (SAFL) to biological nutrient removal (BNR) processes has been widely shown to reduce the dissolved inorganic nitrogen (DIN) in the effluent. However, knowledge regarding the effect of using SAFL in a BNR as an additional carbon source on the characteristics of dissolved organic nitrogen (DON) in the effluent is limited. This study investigated the effect of SAFL addition on effluent DON features and microbial community dynamics in a BNR process treating municipal wastewater. The performance of SAFL was compared with other two reactors (i.e., without an external carbon source and with the addition of sodium acetate). The results showed that the addition of SAFL can significantly reduce effluent DON (p < 0.05). The effluent DON was slightly higher with SAFL than with sodium acetate, but the bioavailable DON of the two reactors was similar (1.06 ± 0.11 vs 1.04 ± 0.12 mg/L, respectively, p > 0.05). The SAFL addition led to a decreased percentage of low molecular weight DON (p < 0.05) as well as an increased ratio of fulvic-like and humic-like substances to proteins-like substances. Moreover, the SAFL addition resulted in a lower percentage of substances resembling proteins/amino sugars and a higher percentage of lignin-like molecules than sodium acetate. These features accounted for the low DON bioavailability. The SAFL promoted the increased abundance of Bacteroidetes, Chloroflexi, Comamonadaceae and Rhodocyclaceae, which could be associated with the decreased effluent DON and its bioavailability. This study indicates that using SAFL as a BNR carbon source not only improves the removal of DIN but also reduces effluent DON and specifically DON bioavailability. These results facilitate the acquisition of comprehensive knowledge regarding the use of SAFL as an alternative external carbon source in the BNR process.

Mixed dye wastewater treatment in a bioelectrochemical system-centered process

The feasibility of mixed dye wastewater treatment was evaluated with a novel integrated bioprocess that consisted of a hybrid anaerobic reactor (HAR) with a built-in bioelectrochemical system, an aerobic biofilm reactor (ABFR) and a denitrification reactor (DR). The position of the DR significantly affected chemical oxygen demand (COD) and colority in effluent, and placing the DR after the ABFR improved effluent quality probably due to minimization of the undesired autoxidation of aromatic amine in dye wastewater. The optimal integrated process of HAR + ABFR + DR efficiently treated mixed dye wastewater, and concentrations of COD and TN were decreased down to 75 ± 18 mg/L and 12.91 ± 0.31 mg/L, respectively, along with colority 48 ± 4 times. Total phosphorus reduced to below 0.5 mg/L with coagulation using poly aluminum chloride, and the effluent quality fully met the discharge standard. This comprehensive study suggests the feasibility of the BES based process for practical application to mixed dye wastewater treatment.

Nitrate residual as a key parameter to efficiently control partial denitrification coupling with anammox

Despite the increased research efforts, full-scale implementation of shortcut nitrogen removal strategies has been challenged by the lack of consistent nitrite-oxidizing bacteria out-selection. This paper proposes an alternative path using partial denitrification (PdN) selection coupled with anaerobic ammonium-oxidizing bacteria (AnAOB). A nitrate residual concentration (>2 mg N/L) was identified as the crucial factor for metabolic PdN selection using acetate as a carbon source, unlike the COD/N ratio which was often suggested. Therefore, a novel and simple acetate dosing control strategy based on maintaining a nitrate concentration was tested in the absence and presence of AnAOB, achieving PdN efficiencies above 80%. The metabolic-based PdN selection allowed for flexibility to move between PdN and full denitrification when required to meet effluent nitrate levels. Due to the independence of this strategy on species selection and management of nitrite competition, this novel approach will guarantee nitrite availability for AnAOB under mainstream conditions unlike shortcut nitrogen removal approaches based on NOB out-selection. Overall, a COD addition of only 2.2 g COD/g TIN removed was needed for the PdN-AnAOB concept showing its potential for significant savings in external carbon source needs to meet low TIN effluent concentrations making this concept a competitive alternative. PRACTITIONER POINTS: Nitrate residual is the key control parameter for partial denitrification selection. Metabolic selection allowed for flexibility of moving from partial to full denitrification. 2.2 g COD/g TIN removed was needed for partial denitrification-anammox process.

pH control of an upflow pyrite-oxidizing denitrifying bioreactor via electrohydrogenesis

Maintenance of stable pH during pyrite-oxidizing denitrification process is important. Here, we demonstrated effective pH control (7.80 ± 0.20-8.40 ± 0.30) in an electrochemical-H₂ and pyrite-oxidizing denitrifying bioreactor (HPR) through in situ electrohydrogenesis. HPR achieved a higher nitrate removal activity (maximum:19.66 ± 0.63 mg NO₃^--N/(L·h)) with excellent resistance to high nitrate loading (up to 400 mg/L NO₃^--N) compared to that of the control groups. Nitrate removal rate of HPR fitted the Michaelis-Menten kinetic model (R² = 0.98, p < 0.01) well, and the denitrification followed the zero-order rate law. The results of the biofilm community analyses suggested that Thauera was the dominant bacteria in the cathode biofilm of HPR and may prefer hydrogen as an electron donor for autotrophic denitrification, while the relative abundance of Pseudomonas were similar in the cathode biofilm and pyrite biofilm. This study provides a new alternative for effective pH control in denitrifying bioreactors with pyrite as a packing material.

Removal Characteristics of Dissolved Organic Nitrogen and Its Bioavailable Portion in a Postdenitrifying Biofilter: Effect of the C/N Ratio

Addition of external carbon sources to postdenitrification biofilters (DNFs) is frequently used in municipal wastewater treatment plants to enhance dissolved inorganic nitrogen removal. However, little is known about its influence on the removal of dissolved organic nitrogen (DON). This study investigated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and bioavailable DON (ABDON) in the pilot-scale DNFs treating real secondary effluent. Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. There was no significant difference in effluent DON concentrations in DNFs at C/N ratios of 4, 5, and 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p < 0.05, t-test). According to the chemical composition analysis, effluent DON at high C/N ratios tends to contain less % molecular weight < 1 kDa nitrogenous organic compounds and proteins/amino sugars-like nitrogenous organic formulas, which is likely responsible for its low bioavailability. Overall, this study indicates the benefit of a high C/N ratio during the DNF process in terms of controlling the DON forms that readily stimulate algal growth.

Impact of external carbon dose on the removal of micropollutants using methanol and ethanol in post-denitrifying Moving Bed Biofilm Reactors

Addition of external carbon sources to post-denitrification systems is frequently used in wastewater treatment plants to enhance nitrate removal. However, little is known about the fate of micropollutants in post-denitrification systems and the influence of external carbon dosing on their removal. In this study, we assessed the effects of two different types and availability of commonly used carbon sources -methanol and ethanol- on the removal of micropollutants in biofilm systems. Two laboratory-scale moving bed biofilm reactors (MBBRs), containing AnoxKaldnes K1 carriers with acclimated biofilm from full-scale systems, were operated in continuous-flow using wastewater dosed with methanol and ethanol, respectively. Batch experiments with 22 spiked pharmaceuticals were performed to assess removal kinetics. Acetyl-sulfadiazine, atenolol, citalopram, propranolol and trimethoprim were easily biotransformed in both MBBRs (biotransformations rate constants k_bio between 1.2 and 12.9 L g_biomass^-1 d^-1), 13 compounds were moderately biotransformed (rate constants between 0.2 and 2 L g_biomass^-1 d^-1) and 4 compounds were recalcitrant. The methanol-dosed MBBR showed higher k_bio (e.g., 1.5-2.5-fold) than in the ethanol-dosed MBBR for 9 out of the 22 studied compounds, equal k_bio for 10 compounds, while 3 compounds (i.e., targeted sulfonamides) were biotransformed faster in the ethanol-dosed MBBR. While biotransformation of most of the targeted compounds followed first-order kinetics, removal of venlafaxine, carbamazepine, sulfamethoxazole and sulfamethizole could be described with a cometabolic model. Analyses of the microbial composition in the biofilms using 16S rRNA amplicon sequencing revealed that the methanol-dosed MBBR contained higher microbial richness than the one dosed with ethanol, suggesting that improved biotransformation of targeted compounds could be associated with higher microbial richness. During continuous-flow operation, at conditions representative of full-scale denitrification systems (hydraulic residence time = 2 h), the removal efficiencies of micropollutants were below 35% in both MBBRs, with the exception of atenolol and trimethoprim (>80%). Overall, this study demonstrated that MBBRs used for post-denitrification could be optimized to enhance the biotransformation of a number of micropollutants by accounting for optimal carbon sources and extended residence time.

Assessment of the endogenous respiration rate and the observed biomass yield for methanol-fed denitrifying bacteria under anoxic and aerobic conditions

In this study, the endogenous respiration rate and the observed biomass yield of denitrifying methylotrophic biomass were estimated through measuring changes in denitrification rates (DNR) as a result of maintaining the biomass under methanol deprived conditions. For this purpose, activated sludge biomass from a full-scale wastewater treatment plant was kept in 10-L batch reactors for 8 days under fully aerobic and anoxic conditions at 20 °C without methanol addition. To investigate temperature effects, another biomass sample was placed under starvation conditions over a period of 10 days under aerobic conditions at 25 °C. A series of secondary batch tests were conducted to measure DNR and observed biomass yields. The decline in DNR over the starvation period was used as a surrogate to biomass decay rate in order to infer the endogenous respiration rates of the methylotrophs. The regression analysis on the declining DNR data shows 95% confidence intervals of 0.130 ± 0.017 day^-1 for endogenous respiration rate under aerobic conditions at 20 °C, 0.102 ± 0.013 day^-1 under anoxic conditions at 20 °C, and 0.214 ± 0.044 day^-1 under aerobic conditions at 25 °C. Results indicated that the endogenous respiration rate of methylotrophs is 20% slower under anoxic conditions than under aerobic conditions, and there is a significant temperature dependency, with an Arrhenius coefficient of 1.10. The observed biomass yield value showed an increasing trend from approximately 0.2 to 0.6 when the starvation time increased from 0 to 10 days.

Model predictive control of two-step nitrification and its validation via short-cut nitrification tests

Short-cut nitrification (SCN) is shown to be an attractive technology due to its savings in aeration and external carbon source addition cost. However, the shortage of excluding nitrite nitrogen as a model state in an Activated Sludge Model limits the model predictive control of biological nitrogen removal via SCN. In this paper, a two-step kinetic model was developed based on the introduction of pH and temperature as process controller, and it was implemented in an SBR reactor. The simulation results for optimizing operating conditions showed that with increasing of dissolved oxygen (DO) the rate of ammonia oxidation and nitrite accumulation firstly increased in order to achieve a SCN process. By further increasing DO, the SCN process can be transformed into a complete nitrification process. In addition, within a certain range, increasing sludge retention time and aeration time are beneficial to the accumulation of nitrite. The implementation results in the SBR reactor showed that the data predicted by the kinetic model are in agreement with the data obtained, which indicate that the two-step kinetic model is appropriate to simulate the ammonia removal and nitrite production kinetics.

Influence of carbon sources on nutrient removal in A2/O-MBRs: Availability assessment of internal carbon source

Both internal carbon source and some external carbon sources were used to improve the nutrient removal in Anaerobic-Anoxic-Oxic-Membrane Bioreactor (A²/O-MBRs), and their technical and cost analysis was investigated. The experimental results showed that the nutrient removals were improved by all the carbon source additions. The total nitrogen and phosphorus removal efficiency were higher in the experiments with external carbon source additions than that with internal carbon source addition. It was found that pathways of nitrogen and phosphorus transform were different dependent on different carbon source additions by the mass balance analysis. With external carbon source addition, the simultaneous nitrification and denitrification occurred in aerobic zone, and the P-uptake in aerobic phase was evident. Therefore, with addition of C-MHP (internal carbon source produced from sludge pretreatment by microwave-H₂O₂ process), the denitrification and phosphorus-uptake in anoxic zone was notable. Cost analysis showed that the unit nitrogen removal costs were 57.13CNY/kg N of C-acetate addition and 54.48CNY/kgN of C-MHP addition, respectively. The results indicated that the C-MHP has a good technical and economic feasibility to substitute external carbon sources partially for nutrient removal.

Tertiary nitrogen removal for municipal wastewater using a solid-phase denitrifying biofilter with polycaprolactone as the carbon source and filtration medium

Tertiary nitrogen removal technologies are needed to reduce the excess nitrogen that is discharged into sensitive aquatic ecosystems. An integrated solid-phase denitrification biofilter (SDNF) was developed with dual media to remove nitrate and suspended solids (SS) from the secondary effluent of municipal wastewater treatment plants. Biodegradable polymer pellets of polycaprolactone (PCL) served as the biofiltration medium and carbon source for denitrification. Long-term continuous operation of the SDNF was conducted with real secondary effluent to evaluate the denitrification performance and effects of influent nitrate loading rates (NLR) and operating temperatures. The results indicated that both nitrate and SS were effectively removed. The SDNF had a strong tolerance for fluctuations in influent NLR, and a maximum denitrification rate of 3.80 g N/(L·d) was achieved. The low temperature had a significant impact on nitrogen removal, yet the denitrification rate was still maintained at a relative high level to as much as 1.23 g N/(L·d) even at approximately 8.0 °C in winter. Nitrite accumulation and excessive organics residue in the effluent were avoided throughout the whole experiment, except on occasional days in the lag phase. The observed biomass yield was calculated to be 0.44 kgVSS/kgPCL. The microbial diversity and community structure of the biofilm in the SDNF were revealed by Illumina high-throughput sequencing. The special carbon source led to an obvious succession of microbial community from the initial inoculum (activated sludge from aerobic tanks), and included a decrease in microbial diversity and a shift in the dominant groups, which were identified to be members of the family Comamonadaceae in the SDNF. The SDNF developed in this study was verified to be an efficient technology for tertiary nitrogen removal from secondary effluent.

Nitrite accumulation during denitrification depends on the carbon quality and quantity in wastewater treatment with biofilters

This study aims to understand the mechanisms of nitrite appearance during wastewater denitrification by biofilters, focusing on the role of the carbon source. Experiments were carried out at lab-scale (batch tests) and full-scale plant (Parisian plant, capacities of 240,000 m(3) day(-1)). Results showed that the nature of the carbon source affects nitrite accumulation rates. This accumulation is low, 0.05 to 0.10 g N-NO2(-) per g N-NO3(-) eliminated, for alcohols such as methanol, ethanol, or glycerol. The utilization of glycerol leads to fungal development causing clogging of the biofilters. This fungal growth and consequent clogging exclude this carbon source, with little nitrite accumulation, as carbon source for denitrification. Whatever the carbon source, the C/N ratio in the biofilter plays a major role in the appearance of residual nitrite; an optimal C/N ratio from 3.0 to 3.2 allows a complete denitrification without any nitrite accumulation.

Enhancing combined biological nitrogen and phosphorus removal from wastewater by applying mechanically disintegrated excess sludge

The goal of the study was to evaluate the possibility of applying disintegrated excess sludge as a source of organic carbon to enhance biological nitrogen and phosphorus removal. The experiment, performed in a sequencing batch reactor, consisted of two two-month series, without and with applying mechanically disintegrated excess sludge, respectively. The effects on carbon, nitrogen and phosphorus removal were observed. It was shown that the method allows enhancement of combined nitrogen and phosphorus removal. After using disintegrated sludge, denitrification effectiveness increased from 49.2 ± 6.8% to 76.2 ± 2.3%, which resulted in a decline in the NOx-N concentration in the effluent from the SBR by an average of 21.4 mg NOx-N/L. Effectiveness of biological phosphorus removal increased from 28.1 ± 11.3% to 96.2 ± 2.5%, thus resulting in a drop in the [Formula: see text] concentration in the effluent by, on average, 6.05 mg PO4(3-)-P/L. The application of disintegrated sludge did not deteriorate effluent quality in terms of COD and NH4(+)-N. The concentration of NH4(+)-N in both series averaged 0.16 ± 0.11 mg NH4(+)-N/L, and the concentration of COD was 15.36 ± 3.54 mg O2/L.

Internal carbon source from sludge pretreated by microwave-H2O2 for nutrient removal in A2/O-membrane bioreactors

To improve the nutrient removal, the feasibility was studied for the organics released from sludge pretreated by microwave-H2O2 process (MHP) to be used as internal carbon source in two A2/O-membrane bioreactors (MBRs). The experiments were conducted for the nutrient removal and the membrane fouling. The results showed that the removal efficiencies of TN and TP were improved by 11% and 28.34%, respectively, as C/N ratio was adjusted to 8 by adding the internal carbon source, and the ratio of soluble chemical oxygen demand (sCOD) consumed easily for denitrification was about 46% of the total sCOD in the internal carbon source. The addition of the internal carbon sources did not lead to severe membrane fouling in the experimental A2/O-MBR. It is implied that the organics released from sludge pretreated by MHP could be used as the internal carbon source to enhance the nutrient removal in A2/O-MBRs.

Effect of H2S on N2O reduction and accumulation during denitrification by methanol utilizing denitrifiers

Sulfide is produced in sewer networks, and previous studies suggest that sulfide in sewage could alter the activity of heterotrophic denitrification and lead to N2O accumulation during biological wastewater treatment. However, the details of this phenomenon are poorly understood. In this study, the potential inhibitory effects of sulfide on nitrate, nitrite, and N2O reduction were assessed with a methanol-utilizing denitrifying culture both prior to and after its exposure and adaptation to sulfide. Hydrogen sulfide was found to be strongly inhibitory to N2O reduction, with 50% inhibition observed at H2S concentrations of 0.04 mg H2S-S/L and 0.1 mg H2S-S/L for the unadapted and adapted cultures, respectively. In comparison, both nitrate and nitrite reduction was more tolerant to H2S. A 50% inhibition of nitrite reduction was observed at approximately 2.0 mg H2S-S/L for both unadapted and adapted cultures, while no inhibition of nitrate reduction occurred at the highest H2S concentrations applied (2.0 mg H2S-S/L) to either culture. N2O accumulation was observed during nitrate and nitrite reduction by the adapted culture when H2S concentrations were above 0.5 and 0.2 mg H2S-S/L, respectively. Additionally, we reveal that hydrogen sulfide (H2S), rather than sulfide, was likely the true inhibitor of N2O reduction, and the inhibitory effect was reversible. These findings suggest that sulfide management in sewers could potentially have a significant impact on N2O emission from wastewater treatment plants.

Effect of pH on N₂O reduction and accumulation during denitrification by methanol utilizing denitrifiers

Acidic pH has previously been found to increase nitrous oxide (N₂O) accumulation during heterotrophic denitrification in biological wastewater treatment. However, the mechanism of this phenomenon still needs to be clarified. By using an enriched methanol utilizing denitrifying culture as an example, this paper presents a comprehensive study on the effect of pH (6.0-9.0) on N₂O reduction kinetics with N₂O as the sole electron acceptor, as well as the effect of pH on N₂O accumulation with N₂O as an intermediate of nitrate reduction. The pH dependency of nitrate and nitrite reduction was also investigated. The maximum biomass-specificN₂O reduction rate is higher than the corresponding maximum nitrate and nitrite reduction rates in the entire pH range studied. However, the maximum biomass-specific N₂O reduction rate is much more sensitive to pH variation outside of the optimal range (pH 7.5 to pH 8.0) than the maximum biomass-specific nitrate and nitrite reduction rates. The half-saturation coefficient of the N₂O reductase increased from 0.10 mg N₂O-N/L to 0.92 mg N₂O-N/L as pH increased from pH 6.0 to 9.0. At pH 6.0, approximately 20% and 40% of the denitrified nitrate accumulated as N₂O in the presence and absence of methanol (as an exogenous carbon source), respectively. However, at pH 6.5, these fractions decreased to 0% and 30%, respectively. No N₂O accumulation occurred at pH 7.0 to 9.0 independent of the availability of methanol. These results suggest that the competition for electrons among different nitrogen oxides reductases likely plays a role in N₂O accumulation at low pH conditions.