Alkalinity Supplementation Research Articles

Optimization of alkalinity supplementation in the AnSBBR using acidified cassava starch wastewater

In aiming to enhance performance and minimize costs, it is important to conduct research centered on the significance of alkalinity supplementation in anaerobic reactors. This study evaluated the impacts of alkalinity supplementation in the methane production, in a second-stage anaerobic sequencing batch biofilm reactor (AnSBBR) fed by acidified cassava wastewater. The AnSBBR was operated at 30 °C, cycle time (CT) of 8 and 6 h and sodium bicarbonate (gNaHCO3.gCOD[chemical oxygen demand]−1. L−1) of 1, 0.66, 0.33 and 0. The organic loading rate (OLR) were 17.1 and 22.8 gCOD.L−1d−1 for the CT of 8 and 6 h, in that order. The results showed that the reactor has been able to supply the alkalinity demand in the system, presenting removal of COD higher than 70%, even without alkalinity supplementation in the influent. The methane production rates (MPR) reached 3.42 ± 0.21 LCH4 L−1d−1 in the condition with 1 gNaHCO3.gCOD−1. L−1, with CT of 6 h. The main soluble metabolite was acetic acid and butyric acid. There was a decrease in methane production and greater instability in the reactor monitoring parameters without NaHCO3 supply. However, the degree of acidification was low, compared to the conversion of organic matter in all conditions. Thus, the AnSBBR maintained stable up to the ratio of 0.66 gNaHCO3.gCOD−1. L−1. Conditions with 1 gNaHCO3.gCOD−1. L−1. (CT 8 and 6 h) exhibited notable energy potential (≥90.2 KJ.d−1), underscoring their capacity for energy generation, even within shorter CT (6 h), thus rendering them suitable for application in industrial facilities.

Bottom ash from smouldered digestate and coconut coir as an alkalinity supplement for the anaerobic digestion of fruit waste

Anaerobic digestion (AD) of readily hydrolysed substrates such as fruit waste requires the addition of a pH buffering agent. This study evaluated the use of bottom ash from the combustion of spent coconut coir and the digestate produced from rejected berry fruit and plant waste as a buffering agent. The performance of the ash was compared with using an equivalent amount of NaHCO3 as a buffering agent. Digestions of berry fruit waste were performed in 160 mL serum vials using anaerobic wastewater sludge as an inoculum. The methane yield at the optimum levels of buffering with NaHCO3 as the buffering agent was 233 ± 12 NmL CH4/g VS. The methane yield at an equivalent addition of alkalinity as ash was significantly less, (124 ± 3 NmL CH4/g VS), but still beneficial compared to the methane yield obtained from experiments with no added alkalinity (40 ± 1 NmL CH4/g VS). Further dosages of ash in systems containing optimal level of NaHCO3 buffer also revealed a decrease in the methane yield proportionally to the added ash concentration, which is suggested to be caused by soluble concentrations of Fe in the ash supplemented systems that are higher than reported inhibitory levels.

Hydrogen and organic acid production from dark fermentation of sugarcane vinasse without buffers in mesophilic and thermophilic conditions

AbstractBACKGROUNDSugarcane vinasse has a low pH and several studies report that alkalinity supplementation is essential to the stability of its dark fermentation (DF). In the context of biorefineries, the reduction, or even elimination, of alkalinizing agents could be extremely environmentally and economically advantageous. The main objective of this study was to evaluate the valorisation of vinasse DF through bioH2 and value‐added metabolite production without buffers in two similar Upflow Anaerobic Sludge Blanket (UASB) under mesophilic (U30) and thermophilic (U55) conditions.RESULTSAverage organic acid yields of 376 ± 52 and 248 ± 122 mg‐CODOA.g−1CODtin, and productivities of 17 396 ± 5220 and 14 024 ± 1642 mg‐CODOA.L−1 day−1 were obtained for U30 and U55 reactors, respectively, with only vinasse as substrate (CODOA stands for organic acids chemical oxygen demand (COD) and CODin influent COD). BioH2 production was feasible with sucrose as substrate and pH below 3.0, but it ceased with substrate replacement by vinasse along with a natural pH increase. A change in microbial community was also observed: Ethanoligenens, Clostridium sensu stricto 12, and Liquorilactobacillus (using sucrose and with a pH <3.0) were replaced by Prevotella, Megasphaera, Pectinatus, Clostridium sensu stricto 11, and Lactobacillus using vinasse (with a pH > 4.0).CONCLUSIONResults showed that DF of sugarcane vinasse without any buffering addition might be a promising approach for the valorization of real wastewater through the production of organic acids. Nevertheless, bioH2 production was not favored and further strategies would be needed to maintain hydrogen‐producing bacteria when sucrose is replaced by real effluent with a natural pH increase. © 2021 Society of Chemical Industry (SCI).

Removal of Nitrate from Water Using TiO2/ PVDF Membrane Photobioreactor

Removing low-concentration nitrate from water is desirable because it may cause eutrophication when discharged, but it is challenging using current technologies. Membrane photobioreactor (MPBR) technology (which is the combination of membrane and microalgae cultivation) emerges as a suitable option to efficiently reduce the nutrient load from wastewater. This study developed a high-efficiency microalgal-bacterial Membrane PhotoBioReactor (MPBR) to biologically remove nitrate from water. In order to obtain low-fouling membranes, TiO2 nanoparticles were entrapped in PVDF membranes prepared by the phase inversion method. Successful fabrication of composite membranes (PVDF/TNT) was confirmed by using XRD, FTIR, DSC, TGA, SEM, and AFM. Membrane properties were studied using contact angle, tensile strength, pure water flux, Bovine Serum Albumin (BSA) rejection, antifouling properties, and porosity. Compared with the pristine PVDF films, the hydrophilicity, permeability, and antifouling performance of the proposed membrane were improved. A laboratory-scale MBR equipped with a synthesized membrane was used to evaluate the performance of mixotrophic denitrification under different Carbon Nitrogen (C/N) ratios, Hydraulic Retention Times (HRT), Nitrate loading, and Influent alkalinity. Methanol was supplied as a carbon source. Almost complete denitrification was achieved when the bioreactor was fed with 75 mg/L NO3−–N, 150 mg/L methanol at 4 h HRT without external alkalinity supplementation. The results as a whole indicated that the MPBR can be applied effectively to the removal of nitrate from real wastewater.

A combo technology of autotrophic and heterotrophic denitrification processes for groundwater treatment

Abstract In this study, a sequential process (heterotrophic up-flow column and completely mixed membrane bioreactors) was proposed combining advantages of the both processes. The system was operated for 249 days with simulated and real groundwater for nitrate removal at concentrations varying from 25 to 145 mg·L−1 NO3−-N. The contribution of heterotrophic process to total nitrate removal in the system was controlled by dozing the ethanol considering the nitrate concentration. By this way, sulfur based autotrophic denitrification rate was decreased and the effluent sulfate concentrations were controlled. The alkalinity requirement in the autotrophic process was produced in the heterotrophic reactor, and the system was operated without alkalinity supplementation. Throughout the study, the chemical oxygen demand in the heterotrophic reactor effluent was (23.7 ± 22) mg·L−1 and it was further decreased to (7.5 ± 7.2) mg·L−1 in the system effluent, corresponding to a 70% reduction. In the last period of the study, the real groundwater containing 145 mg·L−1 NO3−-N was completely removed. Membrane was operated without chemical washing in the first 114 days. Between days 115–249 weekly chemical washing was required.

Anaerobic co-digestion of industrial waste landfill leachate and glycerin in a continuous anaerobic bioreactor with a fixed-structured bed (ABFSB): Effects of volumetric organic loading rate and alkaline supplementation

Anaerobic co-digestion from industrial waste landfill (IWL) leachate and glycerin was evaluated in a continuous anaerobic bioreactor with fixed-structured bed (ABFSB) under mesophilic conditions (30 ± 1 °C). The ABFSB reactor had a useful volume of 4.77 L. The support medium consisted of polyurethane (PU) foam arranged in an orderly manner. The substrate used corresponded to a mixture of 95% IWL leachate and 5% glycerin (v/v) and the inoculum consisted of flocculent anaerobic sludge. The ABFSB reactor was operated in two phases – the 1st phase (between the 49th and the 439th days of operation): started after the inoculum adaptation and the effect of the increase in the organic loading rate (OLR) was evaluated (2; 3.5; 7.1 and 11.6 gCOD L−1 d−1); the 2nd phase (between the 440th and 471st days of operation): the effect of the reduction in alkalinity supplementation via sodium bicarbonate (NaHCO3) was evaluated (0.56; 0.42; 0.28; 0.14 gNaHCO3 gCODinfluent−1 and non-supplementation). The OLR of 7.1 gCOD L−1 d−1 was the condition that presented the best results in the 1st phase for the parameters: maximum methane flow rate (MFR) (7.61 LNCH4 d−1), methane yield (MY) (0.30 LNCH4 gCODremoved−1), volumetric methane production rate (VMPR) (2.79 LNCH4 L−1 d−1), COD removal efficiencies (ERTCOD) and soluble COD (ERSCOD) higher than 90%. In the 2nd phase, the reactor was operated with an OLR of 7.1 gCOD L−1 d−1 and a hydraulic retention time (HRT) of 35.22 h and CODinfluent of 10.68 g L−1. The minimum alkalinity supplementation required was 0.28 gNaHCO3gCODinfluent−1, a condition in which the reactor showed stability and satisfactory performance for the parameters: MFR (9.48 LNCH4 d−1), MY (0.33 LNCH4 gCODremoved−1) and VMPR (3.18 LNCH4 L−1 d−1). It was concluded that the ABFSB reactor proved to be stable in the IWL leachate co-digestion and glycerin, when the alkalinity supplementation, effectively necessary, was met.

A novel self-buffering membrane distillation-based thermophilic anaerobic bioreactor

The stability of an anaerobic bioreactor is a function of alkalinity that provides buffer capacity to maintain a suitable pH for anaerobic microorganisms. The greatest advantage of membrane distillation (MD) is the high rejection of most solutes, including alkaline salt, and it has a high potential to retain alkalinity for the anaerobic system. For the first time, a thermophilic anaerobic granular bioreactor integrated with an MD membrane to maintain the buffered pH was studied in an anaerobic bioreactor without alkalinity supplement. This study presented a comparison between a conventional thermophilic anaerobic bioreactor (TAn1) and a thermophilic anaerobic bioreactor integrated with a MD (TAn2-MD). The two TAns (TAn1 and TAn2) were operated in the same conditions except there was no alkalinity feeding for the TAn2-MD. The results showed that the pH in the TAn2-MD unit was always higher than 6.8 with the organic loading rates from 6 to 18 kg COD/m3/day. However, to maintain the pH of the TAn1 unit at the same level as the TAn2-MD unit, the minimum alkalinity requirements for TAn1 were 1.5, 1.56, 1.62, and 1.8 g/L as CaCO3 at an organic loading rate of 6, 10, 14, and 18 kg COD/m3/day, respectively. Although there was no alkalinity supplement for the TAn2-MD system, two stability indexes, FOS/TAC (volatile organic acids to buffer capacity) and IA/PA (intermediate alkalinity to partial alkalinity), indicated the system was operated at a stable condition. In summary, the designed TAn2-MD can accomplish high-efficiency organic carbon removal (higher than 99.4%) without alkalinity supplement, achieve zero-liquid-discharge, and reclaim high-quality wastewater in one step.

Intermittent micro-aeration: New strategy to control volatile fatty acid accumulation in high organic loading anaerobic digestion

This study developed an intermittent oxidation-reduction potential (ORP)-controlled micro-aeration system for high solids anaerobic digestion (AD) of lignocellulosic biomass without volatile fatty acids (VFA) accumulation at high organic loading rate (OLR). Traditional AD of Napier grass, a model lignocellulosic biomass, at an OLR of 5 g volatile solids (VS)/L/day resulted in an accumulation of total VFA concentration up to 9.2 g/L as acetic acid (HAc) equivalent, causing rapid drops in pH and methane yield, and driving the digester to the verge of failure. Once intermittent (every 24 h) ORP-controlled micro-aeration (at ORP of −470 mV) was initiated, the total VFA concentration rapidly decreased to 3.0 g HAc/L and the methane yield improved, resulting in stable digester performance without the need for alkalinity supplementation or OLR reduction. By combining reactor performance results, mass balance analyses, microbial community characterization data, and a bioenergetic evaluation, this study suggested that rapid VFA conversion and CH4 production were carried out by facultative anaerobes and hydrogenotrophic methanogens under micro-aerobic conditions. This novel operating approach can be applied as an effective control strategy for high OLR AD processes especially in the event of VFA accumulation.

Reduction of alkalinity supplementation for acid-based wastewater treatment using a thermophilic multi-feed upflow anaerobic sludge blanket reactor

ABSTRACT Generally, Shochu distillery wastewater treatment is required the addition of alkalinity agents for an increase of pH in the UASB reactor. However, to reduce the cost of alkalinity supplementation, cost-effective reactor operation method has been desired. This study aimed to reduce the alkalinity supplementation for a thermophilic (55°C) multi-feed up-flow anaerobic sludge blanket (MF UASB) reactor for the low-cost treatment of the wastewater from the production of the Japanese distilled alcohol called shochu. Shochu distillery wastewater contains high concentrations of organics (46,500–57,600 mgCOD L−1; COD: chemical oxygen demand) and volatile fatty acids (16,200–25,000 mgCOD L−1), and low pH (4.1–4.5). With alkalinity supplementation of 0.045 mgCaCO3 mgCOD−1 using 24% NaOH, the MF UASB reactor achieved an 87 ± 2% COD removal rate with an organic loading rate of 24 kgCOD m−3 day−1 for 554 days reactor operation (hydraulic retention time of 10 h and influent concentration of 10,000 mgCOD L−1). The organic removal rate decreased to 19 ± 3% in the MF UASB reactor when alkalinity supplementation was reduced to 0.015 mgCaCO3·mgCOD−1. In this study , the minimum alkalinity supplementation was 0.045 mgCaCO3 mgCOD−1 at an organic loading rate of 24 kgCOD m−3 day−1.

Integrated sulfur- and iron-based autotrophic denitrification process and microbial profiling in an anoxic fluidized-bed membrane bioreactor

The integrated sulfur- and Fe0-based autotrophic denitrification process in an anoxic fluidized-bed membrane bioreactor (AnFB-MBR) was developed for the nitrate-contaminated water treatment in order to control sulfate generation and avoid alkalinity supplement. The nitrate removal rate of the AnFB-MBR reached 1.22 g NO3−-N L−1d−1 with NO3−–N ranging 40–200 mg L−1 at hydraulic retention times of 1.0–5.0 h. The denitrification in the integrated system was simultaneously carried out by sulfur- and Fe0-oxidizing autotrophic denitrifiers. The effluent sulfate generation was decreased by 29.3–70.3% and 31.2–50.9% due to the functional role of Fe0-based denitrification in the integrated system. Alkalinity produced by Fe0-oxidizing autotrophic denitrification could compensate for the alkalinity consumption by sulfur-based autotrophic denitrification. The sulfur- and Fe0-oxidizing autotrophic denitrifying bacterial consortium was composed mainly of bacteria from Thiobacillus, Sulfurimonas, and Geothrix genera. The integrated modes leads to a harmonious co-existence of sulfur- and Fe0-oxidizing denitrifying microbes, which may make a difference to the functional performance of the bioreactor. Overall, the integrated sulfur- and Fe0-based autotrophic denitrification could overcome the shortcomings of excess sulfate generation and external alkalinity supplementation compared to the sole sulfur-based autotrophic denitrification, indicating further potential for the technology in practice.

Treatment of Septic Tank Effluent by Membrane Bioreactor: A Laboratory–scale Feasibility Study

A laboratory scale membrane bioreactor (MBR) fed on real septic tank effluent was studied at different levels of alkalinity (0, 250 and 500 mg NaHCO3/L addition) and sludge retention time (SRT, complete sludge retention, 10 and 20 days). A long–term operation of 267 days was divided into 5 stages to examine the SRT and alkalinity influences on parameters related to nitrification, chemical oxygen demand (COD) removal, extracellular polymeric substances (EPS) production and membrane cleaning. The results of the study showed that the removals of TCOD, SCOD and NH4+–N varied between 86–94%, 71–86%, and 70–94%, respectively. Appropriate alkalinity supplement and SRT control can enhance the COD removal and nitrification. Irreversible membrane fouling occurred fast and water cleaning for the improvement of filtration capacity was ineffective. The results also revealed that the rejection of EPS played a major role both in the enhancement of removal efficiency as well as the increase of filtration resistance during the operation.

Nitrate removal efficiency of a mixotrophic denitrification wall for nitrate-polluted groundwater in situ remediation

Sand tank experiments were conducted to evaluate a mixotrophic denitrification process for nitrate contaminated groundwater in situ remediation. Nitrate removal efficiency reached up to 86.6% when the nitrate loading rate was ≤522mgNd−1kg−1 WS (woodchip-sulfur), while 58.7% of the reduced nitrate was heterotrophically removed and 41.3% was autotrophically removed. Nitrite accumulation was only observed at the experimental phases with relatively low nitrate removal efficiency. Ammonium accumulation in the present mixotrophic bio-system was overwhelmingly less than that in heterotrophic denitrification wall (DW). The sulfate produced by the autotrophic bacteria could be effectively controlled under the mixotrophic environment, and a minimum water contact time of 5.64d was required for the growth of microorganisms which were closely related to sulfate-reducing bacteria. No alkalinity supplement was needed to adjust the pH. The mixotrophic DW could maintain more superior permeability than the heterotrophic DW, and the longevity of the present DW was estimated as 935d when it operated effectively. The present study revealed the operational mechanism of the mixotrophic DW and could provide reference significance for field applications.

Characteristics of a Combined Heterotrophic and Sulfur Autotrophic Denitrification Technology for Removal of High Nitrate in Water

A combined heterotrophic and sulfur autotrophic denitrification technology for NO3--N wastewater treatment was started up by adding elemental sulfur in the heterotrophic denitrifying reactor, and the characteristics of pH constant and sludge reduction were studied. The results showed that the sulfur autotrophic denitrification bacteria in heterotrophic denitrifying reactor could achieve rapid growth. After running for 65d, TOC/N was controlled between 0.65 and 0.75, and the combined denitrification process did not require external alkalinity supplementation as the alkalinity need of autotrophic denitrifiers was supplemented by the heterotrophic denitrifiers. After running for 116d, the total nitrogen removal rate reached above 85%, and the denitrification efficiency was kept steady at 2.5 kg·(m3·d)-1. Compared to heterotrophic denitrification, the sludge production was greatly reduced, which was only 60% of that produced in combined heterotrophic and sulfur autotrophic denitrification reactor. NO2--N accumulated by using collaborative denitrification treatment of high concentration of NO3--N wastewater, the concentration reached 20 mg·L-1 even at the eventual plateau stage, which would require deep processing.

Tidal benthic mesocosms simulating future climate change scenarios in the field of marine ecology

AbstractIncreasing human activities cause local to global changes in sea surface temperatures, ocean acidity, eutrophication, and rising sea levels. Many laboratory experiments investigate the effects of these regime shifts on single species and single stressors, showing variable responses within and among species, while different combinations of stressors can have synergistic, additive or antagonistic effects. Large‐scale multi‐species and multi‐stressor experiments can more reliably predict future ecosystem changes. A unique mesocosm facility was developed and set up at the AWI Wadden Sea Station – Sylt, Northern Germany to investigate the particular effects of future climate changes on predominant marine intertidal communities. Each of 12 benthic mesocosms serves as an independent experimental unit with novel techniques of tide and current simulations as well as multi parameter measurement systems to simulate multi‐factorial climate change scenarios including the combination of warming, acidification, nutrient enrichment, and sea level rise. Temperature, pH, oxygen, and salinity can be continuously monitored and logged, while discretely collected samples of total alkalinity, light availability, chlorophyll a (Chl a), nutrients and seston supplement these online datasets. Herein we demonstrate the functionality of the new benthic mesocosm system including first experimental results on the responses of Fucus vesiculosus forma mytili, and its associated community to the combination of warming, ocean acidification, and increased nutrient enrichment.

Comparison of single-stage and temperature-phased two-stage anaerobic digestion of oily food waste

Anaerobic digestion is an effective technology to recover energy from oily food waste. A single-stage system and temperature-phased two-stage systems with and without recycle for anaerobic digestion of oily food waste were constructed to compare the operation performances. The synchronous operation indicated the similar ability to produce methane in the three systems, with a methane yield of 0.44L/g VSadded. The pH drop to less than 4.0 in the first stage of two-stage system without recycle resulted in poor hydrolysis, and methane or hydrogen was not produced in this stage. Alkalinity supplement from the second stage of two-stage system with recycle improved pH in the first stage to 5.4. Consequently, 35.3% of the particulate COD in the influent was reduced in the first stage of two-stage system with recycle according to a COD mass balance, and hydrogen was produced with a percentage of 31.7%, accordingly. Similar solids and organic matter were removed in the single-stage system and two-stage system without recycle. More lipid degradation and the conversion of long-chain fatty acids were achieved in the single-stage system. Recycling was proved to be effective in promoting the conversion of unsaturated long-chain fatty acids into saturated fatty acids in the two-stage system.

Sulfate addition as an effective method to improve methane fermentation performance and propionate degradation in thermophilic anaerobic co-digestion of coffee grounds, milk and waste activated sludge with AnMBR

This study was conducted to investigate the effects of sulfate on propionate degradation and higher organic loading rate (OLR) achievement in a thermophilic AnMBR for 373days using coffee grounds, milk and waste activated sludge (WAS) as the co-substrate. Without the addition of sulfate, the anaerobic system failed at an OLR of 14.6g-COD/L/d, with propionate accumulating to above 2.23g-COD/L, and recovery by an alkalinity supplement was not successful. After sulfate was added into substrates at a COD/SO42− ratio of 200:1 to 350:1, biogas production increased proportionally with OLR increasing from 4.06 to 15.2g-COD/L/d. Propionic acid was maintained at less than 100mg-COD/L due to the effective conversion of propionic acid to methane after the sulfate supplement was added. The long-term stable performance of the AnMBR indicated that adding sulfate was beneficial for the degradation of propionate and achieving a higher OLR under the thermophilic condition.

High-load anaerobic co-digestion of cheese whey and liquid fraction of dairy manure in a one-stage UASB process: Limits in co-substrates ratio and organic loading rate

Cheese whey is considered an important pollutant in dairy wastewaters and an environmental problem to solve. This study aimed to develop a treatment process that combines the advantages of co-digesting cheese whey with manure and the short hydraulic retention time of a high-load anaerobic treatment system by using liquid fractions of dairy manure as a co-substrate. The proposed co-digestion process consists of a one-stage UASB reactor with an external settler and effluent recycling for alkalinity supplementation. Under a constant hydraulic retention time of 2.2days and increasing proportion of cheese whey in the feed, the system demonstrated stable operation up to a 75% cheese whey fraction in the feed, with an applied organic loading rate of 19.4kg COD m−3d−1, obtaining a 94.7% COD removal and a volumetric methane production rate of 6.4m3 CH4 m−3d−1. Critical biomass washout was experienced when the cheese whey fraction in the feed was 85%. Operation at a constant 60% cheese whey fraction in the feed mixture enabled stable operation under an organic loading rate of 28.7kg COD m−3d−1 and 1.3days HRT, with 95.1% COD removal and a volumetric methane production rate of 9.5m3 CH4 m−3d−1. This new high-load co-digestion method proposed is a promising solution for areas where cheese factories and intensive livestock farming are responsible for environmental pollution caused by unsuitable cheese whey and manure management practices.

Biodegradation of High-Strength Palm Oil Mill Effluent (POME) through Anaerobes Partitioning in an Integrated Baffled Reactor Inoculated with Anaerobic Pond Sludge

Performance of a laboratory-scale integrated baffled reactor for the treatment of raw palm oil mill effluent (POME) was investigated. Initially, the reactor was fed with diluted POME (COD = 1,830 mg/L and OLR = 0.46 g COD/L day) which was then increased gradually to actual concentration (COD = 45,500 mg/L and OLR = 11.38 g COD/L day). Reactor operation was studied in two different hydraulic retention times (HRTs) (4 and 6 days) using POME with no effluent-recycled feed and after alkalinity supplementation. Chemical oxygen demand (COD) removal of 79 and 83 % at an HRT of 4 and 6 days were attained at the highest organic loading rate (OLR = 11.38 g COD/L day). The presence of Arcella-like and Metopus-like species and pH profile in the bioreactor’s compartments imply that anaerobic system is active in the reactor throughout the study. Use of methanogen-enriched inocula, smooth OLR augmentation, and appropriate separation of acidogens and methanogens in the reactor were the reasons for satisfactory performances of the system.

Treatment of Domestic Sewage in an Anaerobic–Aerobic Fixed‐bed Reactor with Recirculation of the Liquid Phase

AbstractThis study reports the performance of a combined anaerobic–aerobic packed‐bed reactor that can be used to treat domestic sewage. Initially, a bench‐scale reactor was operated in three experimental phases. In the first phase, the anaerobic reactor was operated with an average organic matter removal efficiency of 77% for a hydraulic retention time (HRT) of 10 h. In the second phase, the reactor was operated with an anaerobic stage followed by an aerobic zone, resulting in a mean value of 91% efficiency. In the third and final phase, the anaerobic–aerobic reactor was operated with recirculation of the effluent of the reactor through the anaerobic zone. The system yielded mean total nitrogen removal percentages of 65 and 75% for recycle ratios (r) of 0.5 and 1.5, respectively, and the chemical oxygen demand (COD) removal efficiencies were higher than 90%. When the pilot‐scale reactor was operated with an HRT of 12 h and r values of 1.5 and 3.0, its performance was similar to that observed in the bench‐scale unit (92% COD removal for r = 3.0). However, the nitrogen removal was lower (55% N removal for r = 3.0) due to problems with the hydrodynamics in the aerobic zone. The anaerobic–aerobic fixed‐bed reactor with recirculation of the liquid phase allows for concomitant carbon and nitrogen removal without adding an exogenous source of electron donors and without requiring any additional alkalinity supplementation.

Sulfur-oxidizing autotrophic and mixotrophic denitrification processes for drinking water treatment: Elimination of excess sulfate production and alkalinity requirement

This study evaluated the elimination of alkalinity need and excess sulfate generation of sulfur-based autotrophic denitrification process by stimulating simultaneous autotrophic and heterotrophic (mixotrophic) denitrification process in a column bioreactor by methanol supplementation. Also, denitrification performances of sulfur-based autotrophic and mixotrophic processes were compared. In autotrophic process, acidity produced by denitrifying sulfur-oxidizing bacteria was neutralized by the external NaHCO3 supplementation. After stimulating mixotrophic denitrification process, the alkalinity need of the autotrophic process was satisfied by the alkalinity produced by heterotrophic denitrifiers. Decreasing and lastly eliminating the external alkalinity supplementation did not adversely affect the process performance. Complete denitrification of 75mgL−1 NO3–N under mixotrophic conditions at 4h hydraulic retention time was achieved without external alkalinity supplementation and with effluent sulfate concentration lower than the drinking water guideline value of 250mgL−1. The denitrification rate of mixotrophic process (0.45g NO3–NL−1d−1) was higher than that of autotrophic one (0.3g NO3–NL−1d−1). Batch studies showed that the sulfur-based autotrophic nitrate reduction rate increased with increasing initial nitrate concentration and transient accumulation of nitrite was observed.