Simultaneous removal of carbon and nitrate in an airlift bioreactor

Weak magnetic carriers reduce nitrite accumulation and boost denitrification at high nitrate concentrations by enriching functional bacteria and enhancing electron transfer

Biological denitrification is the dominant method for NO3- removal from wastewater, while high NO3- leads to NO2- accumulation and inhibits denitrification performance. In this study, different weak magnetic carriers (0, 0.3, 0.6, 0.9 mT) were used to enhance biological denitrification at NO3- of 50-2400 mg/L. The effect of magnetic carriers on the removal and mechanism of denitrification of high NO3- was investigated. The results showed that 0.6 and 0.9 mT carriers significantly enhanced the TN removal efficiency (>99%) and reduced the accumulation of NO2- (by > 97%) at NO3- of 1200-2400 mg/L 0.6 and 0.9 mT carriers stimulated microbial electron transport by improving the abundances of coenzyme Q-cytochrome C reductase (by 4.44-23.30%) and cytochrome C (by 2.90-16.77%), which contributed to the enhanced elimination of NO3- and NO2-. 0.6 and 0.9 mT carriers increased the activities of NAR (by 3.74-37.59%) and NIR (by 5.01-8.24%). The abundance of narG genes in 0.6 and 0.9 mT was 1.47-2.35 and 1.38-1.75 times that of R1, respectively, and the abundance of nirS genes was 1.49-2.83 and 1.55-2.39 times that of R1, respectively. Denitrifying microorganisms, e.g., Halomonas, Thauera and Pseudomonas were enriched at 0.6 and 0.9 mT carriers, which benefited to the advanced denitrification performance. This study suggests that weak magnetic carriers can help to enhance the biological denitrification of high NO3- wastewater.

Biochar Addition in Membrane Bioreactor Enables Membrane Fouling Alleviation and Nitrogen Removal Improvement for Low C/N Municipal Wastewater Treatment

Membrane bioreactors (MBRs) are frequently used to treat municipal wastewater, but membrane fouling is still the main weakness of this technology. Additionally, the low carbon-nitrogen (C/N) ratio influent has been shown to not only increase the membrane fouling, but also introduce challenges to meet the effluent discharge standard for nitrogen removal. Herein, the authors addressed the challenges by adding cost-effective biochar. The results suggested that the biochar addition can enable membrane fouling alleviation and nitrogen removal improvement. The reduced membrane fouling can be ascribed to the biochar adsorption capacity, which facilitates to form bigger flocs with carbon skeleton in biochar as a core. As a result, the biochar addition significantly altered the mixed liquor suspension with soluble microbial product (SMP) concentration reduction of approximately 14%, lower SMP protein/polysaccharide ratio from 0.28 ± 0.02 to 0.22 ± 0.03, smaller SMP molecular weight and bigger sludge particle size from 67.68 ± 6.9 μm to 113.47 ± 4.8 μm. The nitrogen removal is also dramatically improved after biochar addition, which can be due to the initial carbon source release from biochar, and formation of aerobic-anaerobic microstructures. Microbial diversity analysis results suggested more accumulation of denitrification microbes including norank_f__JG30-KF-CM45 and Plasticicumulans. Less relative abundance of Aeromonas after biochar addition suggested less extracellular polymer substance (EPS) secretion and lower membrane fouling rate.

Enhanced denitrification of dispersed swine wastewater using Ca(OH)2-pretreated rice straw as a solid carbon source

In a pilot-scale packed bed reactor, the denitrification performance and microbial community structure of the dispersed swine wastewater treatment using calcium hydroxide (Ca(OH)₂) pretreated rice straw as a carbon source were investigated. In a Ca(OH)₂-pretreated rice straw supported denitrification system (Ca(OH)₂-RS), the removal efficiency of NO₃^--N was 96.39% at an influent NO₃^--N load of 0.04 kg/(m³•d). Meanwhile, there was no obvious accumulation of NO₂^--N or chemical oxygen demand (COD) in the effluent of Ca(OH)₂-RS. The contents of soluble microbial byproduct-like substances and tryptophan-like substances in the effluent of Ca(OH)₂-RS were reduced by 46.2% and 43.4%, respectively, compared with the influent. Overall, the Ca(OH)₂-pretreated rice straw system had a strong resistance to fluctuations in water quality conditions, such as influent NO₃^--N and COD concentrations. According to the microbial assay results, the Ca(OH)₂ pretreatment enriched more denitrifying bacteria. Among them, Proteobacteria (42.33%) and Bacteroidetes (35.28%) were the dominant bacteria. Moreover, the main denitrifying functional bacteria, generanorank_f_Saprospiraceae (13.32%), norank_f_Porphyromonadaceae (4.22%), and Flavobacterium (3.25%), were enriched in Ca(OH)₂-RS. This suggested that using Ca(OH)₂-pretreated rice straw as a carbon source was a stable and efficient technology to enhance the denitrification performance of dispersed swine wastewater.

Biofilm characterization in removal of total chemical oxygen demand and nitrate from wastewater using draft tube spouted bed reactor

The present paper investigates the effect of dilution rate on the removal of total chemical oxygen demand and nitrate in the draft tube spouted bed reactor and morphological characteristics of biofilms formed by microorganisms of mixed culture on granular activated carbon (GAC). The nitrate and total chemical oxygen demand (COD) decreased from 97 to 81% and 95% to 87% respectively with increase in dilution rate from 0.6/h to 1.5/h showing that residence time in the reactor governs the nitrate and total COD reduction efficiency. Lower dilution rates favor higher production of biomass and extracellular polymeric substances (EPS). It was observed that the nitrate and total COD reduction rate increased with time along with simultaneous increase in EPS production. Thus, the performance of a reactor in terms of dynamic and steady-state biofilm characteristics associated with nitrate and organic reduction is a strong function of dilution rate. Hence these findings indicate that a draft tube spouted bed reactor is capable of simultaneously reducing total organics and nitrogen in industrial/municipal wastewater, as this reactor possesses two distinct regions aerobic and anoxic conditions which can prevail in different parts of a reactor.

Co-biodegradation of naphthenic acids in anoxic denitrifying biofilm reactors

Anoxic co-biodegradation of linear and cyclic naphthenic acids (NAs) namely octanoic acid, trans-4-methyl-1-cyclohexane carboxylic acid (trans-4MCHCA), cis- and trans-4-methyl-1-cyclohexane-acetic acids (cis-4MCHAA and trans-4MCHAA) was investigated in denitrifying biofilm reactors. In all evaluated compositions, co-biodegradation of NAs was coupled to denitrification, with octanoic acid showing the fastest biodegradation rate (1180.4 mg L^-1 h^-1 at loading rate of 1180.4 mg L^-1 h^-1), followed by trans-4MCHCA (398.1 mg L^-1 h^-1 at loading rate of 435.8 mg L^-1 h^-1), trans-4MCHAA (25.7 mg L^-1 h^-1 at loading rate of 221.7 mg L^-1 h^-1), and cis-4MCHAA (5.3 mg L^-1 h^-1 at loading rate of 16.9 mg L^-1 h^-1). Biodegradation of octanoic acid and trans-4MCHCA were not influenced by the presence of recalcitrant NAs (cis- and trans-4MCHAA). Co-biodegradation of cis- and trans-4MCHAA with octanoic acid, trans-4MCHCA, or their combination enhanced the biodegradability of these recalcitrant NAs, with the positive impact being more pronounced for trans-4MCHCA. Finally anoxic co-biodegradation of NAs under denitrifying conditions proceeded at rates that were faster than the aerobic rates obtained in similar mixtures. Anoxic biodegradation, therefore, is an effective alternative for in situ treatment of oil sands process water in anoxic stabilization ponds amended with nitrate, or as an ex situ treatment approach in denitrifying bioreactors whereby the cost and technical challenges of aeration are eliminated.

Intensified heterotrophic denitrification in constructed wetlands using four solid carbon sources: Denitrification efficiency and bacterial community structure

Biodenitrification using solid carbon sources is a cost-effective way for nitrate removal. In the study, wheat straw, cotton, poly(butylene succinate), and newspaper was chosen as the carbon source to compare the denitrification efficiency and bacterial communities in constructed wetlands. Parameters including COD, NO₃^--N, NO₂^--N and total nitrogen (TN) were analyzed. Results indicated that newspaper provided significantly higher NO₃^--N and TN removal efficiency than the other three solid carbon sources in low-temperature condition. Moreover, both newspaper and wheat straw allowed high NO₃^--N and TN removal efficiency in high-temperature condition. According to pyrosequencing analysis, denitrifying bacteria Dechloromonas and Thauera were the predominant genus in the anaerobic zone of CO- (3.92 and 2.35%, respectively), WS- (1.97 and 1.02%, respectively) and NP-CWs (1.71 and 1.31%, respectively). Genus of Levilinea was enriched in NP- (1.02%) and WS-CWs (0.91%). Furthermore, genus Paludibacter (2.69%) and Saccharofermentans (3.14%) showed high relative abundance in WS-CWs.

Evaluating the process performance and potential of a high-rate single airlift bioreactor for simultaneous carbon and nitrogen removal through coupling different pathways from a nitrogen-rich wastewater

The feasibility of a continuous feed and intermittent discharge airlift bioreactor for simultaneous carbon and nitrogen removal from a low COD/N wastewater was evaluated. The effect of two independent variables, HRT (10-20 h) and NH₄⁺/(NH₄⁺+NO₃^-) ratio (0.25-0.75), on the bioreactor performance was studied. The relatively high anaerobic to aerobic time ratio made an effective contribution to NH₄⁺, NO₃^-, and TN removal. TN removal was enhanced with increase in HRT and decrease in NH₄⁺/NH₄⁺+NO₃^- and at the optimum condition, 616 mg/L (88%) and 213 mg/L (76%) of sCOD and TN were removed, respectively. The results suggested that the nitrogen removal process was based on a combination of anaerobic ammonium oxidation (Anammox), simultaneous nitrification-denitrification (SND), and presumable dissimilatory nitrate reduction to ammonium (DNRA) mechanisms.

Removal of nitrate from water by acid-washed zero-valent iron/ferrous ion/hydrogen peroxide: influencing factors and reaction mechanism

In this paper, a system consisting of acid-washed zero-valent iron (ZVI), ferrous ion (Fe²⁺), and hydrogen peroxide (H₂O₂) was employed for the removal of nitrate (NO₃^-) from water, and the reaction mechanism for this is discussed. The effects of acid-washed ZVI, Fe²⁺, H₂O₂, and initial NO₃^- concentration on nitrate removal were investigated. Acid-washed ZVI before and after reaction with nitrate were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results reveal that the combined system can enhance the corrosion of ZVI and facilitate aqueous nitrate reduction. The products of nitrate reduction are mainly ammonium, with some N₂. The ZVI particles after reaction may have a core of ZVI with an oxidation layer mainly consisting of Fe₃O₄.

Slowly released molasses barrier system for controlling nitrate plumes in groundwater: a pilot-scale tank study

A well-type barrier system containing solidified molasses as a reactive medium was developed to promote the indigenous denitrifying activity and to treat nitrate plumes in groundwater. Three slowly released molasses (SRM) barrier systems harboring 60, 120, and 120 SRM rods, which were named System A, B, and C, respectively, were operated to examine nitrate removal efficiency in a pilot-scale sandy tank. These SRM systems induced a consistent removal of nitrate without pore clogging and hydraulic disturbance during the test period. The initial nitrate concentration was 142mgL(-1), and the concentrations decreased by 80%, 84%, and 79% in System A, B, and C, respectively. In particular, System C was inoculated with heterotrophic denitrifiers, but the nitrate removal efficiency was not enhanced compared to System B, probably due to the prior existence of indigenous denitrifiers in the sandy tank. The presence of nitrite reductase-encoding gene (i.e. nirK) at the site was confirmed by denatured gradient gel electrophoresis analysis.

A new developed airlift reactor integrated settling process and its application for simultaneous nitrification and denitrification nitrogen removal

This study presented the performance of simultaneous nitrification and denitrification (SND) process using a new developed hybrid airlift reactor which integrated the activated sludge reaction process in the airlift reactor and the sludge settling separation process in the clarifier. The proposed reactor was started up successfully after 76 days within which the COD and total nitrogen removal rate can reach over 90% and 76.3%, respectively. The effects of different COD/N and DO concentrations on the performance of reactor were investigated. It was found that the influent COD/N maintained at 10 was sufficient for SND and the optimum DO concentration for SND was in the range of 0.5 to 0.8 mg L⁻¹. Batch test demonstrated that both macroscopic environment caused by the spatial DO concentration difference and microscopic environment caused by the stratification of activated sludge may be responsible for the SND process in the reactor. The hybrid airlift reactor can accomplish SND process in a single reactor and in situ automatic separation of sludge; therefore, it may serve as a promising reactor in COD and nitrogen removal fields.

Simultaneous carbon and nutrient removal in an airlift loop reactor under a limited filamentous bulking state

Airlift loop reactors (ALRs) are important bioreactors for wastewater treatment. However, few studies have investigated the application of an ALR for simultaneous carbon and nutrient removal, especially for activated sludge systems. This study evaluated the performance of integrated nitrogen, phosphorus and COD removal in an ALR with a low height-to-diameter ratio in a limited filamentous bulking (LFB) state (SVI of 180-220mL/g). The average removal efficiencies for COD, NH(4)(+)-N, TN and TP were 91%, 92%, 86% and 94%, respectively. Additional research showed that only under the LFB state, the appropriate distribution of dissolved oxygen inside the ALR was established to promote a well-balanced aerobic and anoxic/anaerobic state. In addition, the macro-gradient of the substrate concentration at the inlet and the heavier bio-P sludge density compensated for the proliferation of filaments. Hence, the stable LFB state was achieved by balancing the floc-forming bacteria and the filamentous bacteria in the ALR.

Denitrification of high nitrate wastewater in a cloth strip bioreactor with immobilized sludge

Denitrification of synthetic high nitrate wastewater containing 40,000 ppm NO(3) (9,032 ppm NO(3)-N) was achieved using immobilized activated sludge in a column reactor. Active anoxic sludge adsorbed onto Terry cloth was used in the denitrification of high nitrate wastewater. The operational stability of the immobilized sludge system was studied both in a batch reactor and in a continuous reactor. The immobilized sludge showed complete degradation of different concentrations of NO(3)-N (1,129, 1,693, 3,387, 6,774, and 9,032 ppm) in a batch process. The reactors were successfully run for 90 days without any loss in activity. The immobilized cell process has yielded promising results in attaining high denitrifying efficiency.

Design and monitoring of horizontal subsurface-flow constructed wetlands for treating nursery leachates

Nursery leachates usually contain high concentrations of nitrates, phosphorus and potassium, so discharging them into the environment often causes pollution. Single-stage or two-stage horizontal subsurface flow constructed wetlands (HSSCW) filled with different substrates were designed to evaluate the effect and evolution over time of the removal of nitrogen and other nutrients contained in nursery leachates. The addition of sodium acetate to achieve a C:NO(3)(-)-N ratio of 3:1 was sufficient to reach complete denitrification in all HSSCW. The removal rate of nitrate was high throughout the operation period (over 98%). Nevertheless, the removal rate of ammonium decreased about halfway through the operation. Removal of the COD was enhanced by the use of two-stage HSSCW. In general, the substrates and the number of stages of the wetlands did not affect the removal of nitrogen, total phosphorus and potassium.

Treatment of nitrate contaminated water using an electrochemical method

Treatment of nitrate contaminated water which is unsuitable for biological removal using an electrochemical method with Fe as a cathode and Ti/IrO(2)-Pt as an anode in an undivided cell was studied. In the absence and presence of 0.50 g/L NaCl, the nitrate-N decreased from 100.0 to 7.2 and 12.9 mg/L in 180 min, respectively, and no ammonia and nitrite by-products were detected in the presence of NaCl. The nitrate reduction rate increased with increasing current density, with the nitrate reduction rate constant k(1) increasing from 0.008 min(-1) (10 mA/cm(2)) to 0.016 min(-1) (60 mA/cm(2)) but decreasing slightly with increasing NaCl concentration. High temperature favoured nitrate reduction and the reaction followed first order kinetics. The combination of the Fe cathode and Ti/IrO(2)-Pt anode was suitable for nitrate reduction between initial pH values 3.0 and 11.0. e.g. k(1)=0.010 min(-1) (initial pH 3.0) and k(1)=0.013 min(-1) (initial pH 11.0). Moreover, the surface of all used cathodes appeared rougher than unused electrodes, which may have increased the nitrate reduction rate (4-6%).