High-rate Anaerobic Treatment Research Articles

Full-scale study on high-rate low-temperature anaerobic digestion of agro-food wastewater: process performances and microbial community

ABSTRACT The fast-growing global population has led to a substantial increase in food production, which generates large volumes of wastewater during the process. Despite most industrial wastewater being discharged at lower ambient temperatures (<20 °C), majority of the high-rate anaerobic reactors are operated at mesophilic temperatures (>30 °C). High-rate low-temperature anaerobic digestion (LtAD) has proven successful in treating industrial wastewater both at laboratory and pilot scales, boasting efficient organic removal and biogas production. In this study, we demonstrated the feasibility of two full-scale high-rate LtAD bioreactors treating meat processing and dairy wastewater, and the microbial communities in both reactors were examined. Both reactors exhibited rapid start-up, achieving considerable chemical oxygen demand (COD) removal efficiencies (total COD removal >80%) and generating high-quality biogas (CH4% in biogas >75%). Long-term operations (6–12 months) underscored the robustness of LtAD bioreactors even during winter periods (average temperature <12 °C), as evidenced by sustained high COD removal rates (total COD removal >80%). The stable performance was underpinned by a resilient microbial community comprising active acetoclastic methanogens, hydrolytic, and fermentative bacteria. These findings underscore the feasibility of high-rate low-temperature anaerobic wastewater treatment, offering promising solutions to the zero-emission wastewater treatment challenge.

Inorganic carbon as a key factor governing competition between methanogens and acetogens in high-rate anaerobic treatment of methanol wastewater

The anaerobic treatment of methanolic wastewater has become a crucial issue. Sodium bicarbonate (NaHCO3), as a typical representative of inorganic carbon substances, not only provides alkalinity in the anaerobic treatment of methanol but also governs the competition between methanogens and acetogens. However, a suitable dosage has rarely been mentioned. In this study, we demonstrate the crucial role of NaHCO3 in the anaerobic treatment of methanol using a long-term operated up-flow anaerobic sludge blanket (UASB) reactor and batch tests. The UASB results revealed that by adding 3 to 5 g/L of NaHCO3 to the influent at a high organic loading rate of 50 gCOD/L/d, a significant accumulation of acetic acid was detected, with a maximum value of up to 1400 mg/L. In the context of microbial community evolution, this study profiled substrate competition between the acetogens Acetobacterium and Methanomethylovorans, which directly utilize methanol, as well as the synergistic effects of Acetobacterium and acetoclastic Methanosaeta. Abundance analysis of genes encoding relevant pathway enzymes also revealed that the acetic acid production pathway was significantly enhanced when more NaHCO3 was added. According to the dose–response mechanism in the batch test based on kinetic analysis, an acceptable threshold prediction scheme was beneficial in reasonably adding NaHCO3 to improve acid production capacity and rapid methanol degradation, and ensuring that the generated acetate could be rapidly degraded without inhibition, in the high-rate anaerobic treatment of methanolic wastewater.

Long-term comparison of pilot UASB and AnMBR systems treating domestic sewage at ambient temperatures

The upflow anaerobic sludge blanket (UASB) reactor and the anaerobic membrane bioreactor (AnMBR) are leading candidates for high-rate anaerobic treatment of domestic wastewater. The objective of this study was to perform a long-term comparison on the treatment of domestic sewage by the UASB and AnMBR systems at pilot scale at ambient temperature conditions and relate this to full-scale performance from a range of domestic strength UASB reactors. Analysis of overall operation indicated that the pilot reactor was more likely to be effective in removing solids (and less effective in soluble COD) than full-scale reactors, but that both had comparable performance ranges (i.e., from very effective, to very ineffective). Analyzing both UASB and AnMBR pilot plants treating the same wastewater, it can be concluded that both systems could achieve comparable performance (the UASB for very short periods), with COD removal efficiencies of more than 90%. However, the UASB could only achieve this performance for weeks at a time with good solids retention and long (>15 h) hydraulic retention time (HRT). This performance in general was not sustainable. For the UASB, the average COD removal at 15-h HRT was approximately 50%, with a significant reduction in performance as the hydraulic loading increased (which effectively reduced the HRT). Conversely, the AnMBR system produced exceptional COD removal at an HRT of 9 h. Considering this, the sustained volumetric methane production (per volume of reactor) was approximately 4 times that in the AnMBR than in the UASB system.

Development of a high-rate anaerobic thermophilic upflow packed bed reactor for efficient bioconversion of diluted three-phase olive mill wastewater into methane

Olive mill wastewater is the main by-product derived from olive mills using the three-phase extraction process, displaying a serious environmental risk due to its notable content in organics and phenolics. This work focused on a high-rate thermophilic anaerobic digester development, able to remove high organic load amounts, by converting it to biogas. The proposed system is an upflow packed bed reactor with recycling stream and biomass carriers appropriate for inoculum colonization. An anaerobic inoculum was acclimatized under thermophilic conditions and subsequently added into the reactor. Its performance examination against twice-centrifuged olive mill wastewater of 40 g COD/L followed, testing several runs of gradually decreased hydraulic retention time, reaching steady-state conditions until the minimum of 4.2 days, while optimum performance was achieved at 5.6 days. The production of 9.51 ± 0.37 NLCH4/LFeed is considered quite promising for the high-rate anaerobic treatment plants development for olive mill wastewater valorization, without the need of co-digestion.

Degradation of p-cresol, resorcinol, and phenol in anaerobic membrane bioreactors under saline conditions

Treating petrochemical wastewater is a challenge for conventional anaerobic reactors. One example is coal gasification wastewater that, besides its salinity, is rich in toxic and inhibitory aromatics, such as phenol, cresols, and resorcinol. Studies have shown that phenol and p-cresol share the same degradation intermediates, whereas resorcinol is degraded via another route. This study investigated the simultaneous degradation of p-cresol or resorcinol with phenol under anaerobic saline conditions. Batch experiments with anaerobic phenol-degrading biomass were conducted to assess the feasibility of the degradation of p-cresol and resorcinol. Volumetric uptake rates of 11.4 ± 2.4 mgp-cresol·L–1d–1 and 4.2 ± 1.9 mgresorcinol·L–1d–1 were determined. The effect of p-cresol and resorcinol on the specific methanogenic activity and the cell viability in phenol-degrading and non-adapted biomass was assessed. Half maximal inhibitory concentration (IC50) values of 0.73 gp-cresol·L-1 and 3.00 gresorcinol·L-1 were estimated for phenol-degrading biomass, whereas IC50 values of 0.60 gp-cresol·L-1 and 0.25 gresorcinol·L-1 were estimated for the non-adapted biomass. p-Cresol caused a higher decrease in the non-damaged cell counts in comparison to resorcinol. Two anaerobic membrane bioreactors under saline conditions [8 g Na+·L–1] were fed with mixtures of either phenol-p-cresol or phenol-resorcinol. At an influent phenol concentration of 2 g·L-1, maximum conversion rates of 22 mgp-cresol·gVSS-1d–1 and 16 mgresorcinol·gVSS–1d–1 were found. In both AnMBRs, Syntrophorhabdus sp. and Methanosaeta sp. were the most abundant bacteria and methanogen, respectively. The feasibility of simultaneous conversion of phenolic compounds under saline conditions in AnMBRs opens novel perspectives for the high-rate anaerobic treatment of chemical wastewater.

Performance evaluation and intensification of novel buoyant filter bioreactor with associated secondary treatment process for rice processing industries

Rice being the staple food of a larger population in general and to the India in particular and most of the industries are adapting the parboiled process. Rice production by parboiled process required large amount of water and discharging huge amount of wastewater to environment. Thus, most of the rice industries were falling under red or orange category of highly polluted industry. In this context the present study was undertaken to evaluate the performance of an ETP at rice processing industry. The ETP employed a primary high rate anaerobic treatment system called Buoyant filter bioreactor (BFBR) followed by secondary chemical treatments, to meet the effluent standards. The overall percentage removal efficiency of ETP observed is COD (90%), Colour (93%), Iron (100%), phosphate (100%), turbidity (94%), TSS (50%) and BOD (99%). A sharp reduction of BOD from 1500 to 10 mg/l was observed upon BFBR treatment indicating the excellent working condition of BFBR. The final effluent coming out from ETP meets the criteria for Permissible Limit as per ISI,1977 (On land of irrigation) and Permissible Limit as per ISI 1974 (In land surface water) which indicates the performance of the developed ETP is satisfactory.

Development of UASB–DHS System for Anaerobically-Treated Tofu Processing Wastewater Treatment under Ambient Temperature

Tofu is widely processed in East and Southeast Asian countries. During the production, highly polluted wastewater is discharged. This wastewater is commonly treated using a high-rate anaerobic wastewater treatment process; however, several organic compounds and nitrogen remain in the anaerobic effluent. The aim of this study was to develop a combined upflow anaerobic sludge blanket (UASB) and downflow hanging sponge (DHS) biosystem that that serves as a post-treatment for an expanded granular sludge blanket reactor used for treating tofu-processing wastewater in Japan for 699 days. The UASB reactor played a role in treating of COD, with 58 ± 16% and 74 ± 20% of total COD and soluble COD removed anaerobically. Besides, methane was recovered from removed soluble COD were 63 ± 28% and 87 ± 64% at winter and summer. Meanwhile, the DHS reactor showed its potential in treatment of BOD and TSS. The final effluents were recorded as 67 ± 38 mg·L-1, 50 ± 26 mg·L-1, and 22 ± 16 mg·L-1 of total COD, BOD and total suspended solids, respectively. This indicates that the proposed UASB-DHS system has proven its suitability as post-treatment system for anaerobically treated tofu-processing wastewater

Performance evaluation and kinetic modeling of down-flow high-rate anaerobic bioreactors for poultry slaughterhouse wastewater treatment.

In this study, the treatment of poultry slaughterhouse wastewater (PSW) was evaluated using two new down-flow high-rate anaerobic bioreactor systems (HRABS), including the down-flow expanded granular bed reactor (DEGBR) and the static granular bed reactor (SGBR). These two bioreactors have demonstrated a good performance for the treatment of PSW with removal percentages of the biochemical oxygen demand (BOD5), the chemical oxygen demand (COD), and fats, oil, and grease (FOG) exceeding 95% during peak performance days. This performance of down-flow HRABS appears as a breakthrough in the field of anaerobic treatment of medium to high-strength wastewater because down-flow anaerobic bioreactors have been neglected for the high-rate anaerobic treatment of such wastewater due to the success of up-flow anaerobic reactors such as the UASB and the EGSB as a result of the granulation of a consortium of anaerobic bacteria required for efficient anaerobic digestion and biogas production. Hence, to promote the recourse to such technologies and provide further explanation to their performance, this study approached the kinetic analysis of these two down-flow HRABS using the modified Stover-Kincannon and the Grau second-order multi-component substrate models. From a comparison between the two models investigated, the modified Stover-Kincannon model provided the best prediction for the concentration of the substrate in the effluent from the two HRABS. This analysis led to the determination of the kinetic parameters of the two models that can be used for the design of the two HRABS and the prediction of the performance of the SGBR and DEGBR. The kinetic parameters determined using the Modified Stover-Kincannon were Umax = 40.5 gCOD/L.day and KB = 47.3 gCOD/L.day for the DEGBR and Umax = 33.6 gCOD/L.day and KB = 44.9 gCOD/L.day for the SGBR; while, using the Grau second-order model, the kinetic models determined were a = 0.058 and b = 1.112 for the DEGBR and a = 0.135 and b = 1.33 for the SGBR.

Enhanced Methanization of Long-Chain Fatty Acid Wastewater at 20°C in the Novel Dynamic Sludge Chamber–Fixed Film Bioreactor

Lipid-containing wastewaters, such as those arising from dairy processing, are frequently discharged at temperatures ≤20°C. Their valorization at low ambient temperatures offers opportunities to expand the application of high-rate anaerobic wastewater treatment towards achieving energy neutrality by minimizing the energy demand for heating. Lipid hydrolysis generates long chain fatty acids (LCFA), which incur operational challenges and hinder stable bioreactor operation by inducing sludge flotation and washout, coupled with the added challenge of treatment at lower temperature (20°C). These challenges are tackled together uniquely during the treatment of LCFA-rich synthetic dairy wastewater (SDW) (33% COD-LCFA) through de-novo formed microbial granular sludge within the dynamic sludge chamber fixed film (DSC-FF) reactor. The novel reactor design facilitated sludge retention for the entire operational period of 150 days by containing settled, floating and LCFA-encapsulated granular sludge and biofilm within a single module. High COD removal efficiencies (87-98%) were achieved in the three replicated DSC-FF reactors, along with complete LCFA removal at 18-72 h HRT (LCFA loading rate of 220-890 mgCOD-LCFA/L·d) and partial LCFA removal at 12 h HRT (LCFA loading rate of 1333 mgCOD-LCFA/L·d). The high removal efficiencies of unsaturated and saturated LCFAs achieved are reported for the first time during continuous anaerobic wastewater treatment at low temperatures (20°C). Moreover, de novo granulation was achieved within 8 d from a combination of inoculum mixtures at a high LCFA concentration (33% COD-LCFA) in SDW. The results demonstrate the feasibility of the DSC-FF reactor for treating LCFA-rich wastewaters at discharge temperatures and offer potential for expanded and more energetically productive anaerobic valorization of lipid-rich wastewater.

Effects of encapsulation on the chemical inhibition of anaerobic hydrogen- and methane-producing microbial cells

Encapsulating microbial cells in an alginate matrix enables high-rate anaerobic biological treatment and resource recovery by increasing cell concentration and retention time. To determine whether encapsulation protected microbial cells from inhibitors, copper(II), chromium(VI) (as dichromate), ammonium, and chloroform were amended to hydrogen-producing anaerobic microbial cells that were either suspended or encapsulated in a composite-coated alginate matrix. Encapsulation mitigated dichromate inhibition by 36% based on the dose-response curve slope but increased ammonium inhibition by 210% and had no effect on inhibition by chloroform. This was explained by effective concentrations in the beads after measuring partition coefficients. Encapsulation rendered copper(II) unavailable through apparent chelation because it protected both the hydrogen-producing and a methane-producing community from copper(II) despite its high effective concentration in encapsulation. Understanding the effects that the encapsulation of microbial cells has on chemical inhibition can help expand the application conditions of wastewater treatment or resource recovery and avoid unanticipated toxicity.

Microbial Community Redundancy and Resilience Underpins High-Rate Anaerobic Treatment of Dairy-Processing Wastewater at Ambient Temperatures.

High-rate anaerobic digestion (AD) is a reliable, efficient process to treat wastewaters and is often operated at temperatures exceeding 30°C, involving energy consumption of biogas in temperate regions, where wastewaters are often discharged at variable temperatures generally below 20°C. High-rate ambient temperature AD, without temperature control, is an economically attractive alternative that has been proven to be feasible at laboratory-scale. In this study, an ambient temperature pilot scale anaerobic reactor (2 m3) was employed to treat real dairy wastewater in situ at a milk processing plant, at organic loading rates of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m3/day and hydraulic retention times (HRT) ranging from 36 to 6 h. Consistent high levels of COD removal efficiencies, ranging from 50 to 70% for total COD removal and 70 to 84% for soluble COD removal, were achieved during the trial. Within the reactor biomass, stable active archaeal populations were observed, consisting mainly of Methanothrix (previously Methanosaeta) species, which represented up to 47% of the relative abundant active species in the reactor. The decrease in HRT, combined with increases in the loading rate had a clear effect on shaping the structure and composition of the bacterial fraction of the microbial community, however, without affecting reactor performance. On the other hand, perturbances in influent pH had a strong impact, especially when pH went higher than 8.5, inducing shifts in the microbial community composition and, in some cases, affecting negatively the performance of the reactor in terms of COD removal and biogas methane content. For example, the main pH shock led to a drop in the methane content to 15%, COD removals decreased to 0%, while the archaeal population decreased to ~11% both at DNA and cDNA levels. Functional redundancy in the microbial community underpinned stable reactor performance and rapid reactor recovery after perturbations.

Application of in-situ H2-assisted biogas upgrading in high-rate anaerobic wastewater treatment

The H2-assisted biogas upgrading approach has recently attracted much interest as a low-cost and environmentally friendly alternative to commonly used ex-situ/ physiochemical biogas upgrading techniques. However, most studies conducted to date have been limited to anaerobic solid-waste treatment characterized by flocculant sludge and low organic loading rates (OLR). In an attempt to expand its application to high-rate anaerobic wastewater treatment, an innovative two-stage up-flow anaerobic sludge blanket reactor system was employed using anaerobic granular sludge. We found that the CH4 content of product gas was consistently >90% and that H2 and CO2 concentrations stayed below 5%, even when OLR was increased from 1 to 5 g L−1 d−1 and H2 feeding rates were increased from 0.13 to 0.63 g L−1 d−1. We were also able to show that CO (5–10%) in H2-rich syngas didn’t inhibit methanogenesis or had significant impact on microbial community structure, suggesting that H2-assisted biogas upgrading with H2-rich syngas is feasible.

High-rate anaerobic treatment of digestate using fixed film reactors

The effluent stream of the anaerobic digestion processes, the digestate, accommodates high residual organic content that needs to be further treated before discharge. Anaerobic treatment of digestate would not only reduce the residual organic compounds in digestate but also has a potential to capture the associated biogas. High-rate anaerobic reactor configurations can treat the waste streams using lower hydraulic retention times which requires less footprint opposed to the conventional completely stirred tank reactors. This study investigated the high-rate anaerobic treatment performance and the associated biogas capture from the digestate of a manure mixture composed of 90% laying hen and 10% cattle manures in fixed-film reactors. The results indicated that it was possible to reduce total chemical oxygen demand content of the digestate by 57–62% in 1.3–1.4 days of hydraulic retention time. The corresponding biogas yields obtained were in the range of 0.395–0.430 Lbiogas/g VSadded which were found to be comparable to many raw feedstocks. Moreover, significant total phosphorus reduction (36–47%) and greenhouse gas capture (over 14.5–18.1 tCO2e/d per m3 digestate) were also recorded in the anaerobic fixed-film reactors.

Performance of an ECSB reactor for high-rate anaerobic treatment of cheese industry wastewater: effect of pre-acidification on process efficiency and calcium precipitation.

An external circulation sludge bed (ECSB) reactor was studied at full-scale (1,000 m3) during anaerobic treatment of cheese and other dairy products industry wastewater (CWW). The latter was characterized by a high calcium content, therefore the study focused on the potential negative impact that calcium may have in the long-term. The degree of CWW acidification (25 and 40%) on ECSB reactor performance was evaluated over a wide range of organic loading rates from 5 up to 18 kg m3 d-1, while process efficiency and calcium precipitation were examined in detail. Independently of the operating conditions, the volatile suspended solids content of the anaerobic granular sludge, as well as its calcium content, remained stable along the ECSB reactor operation, indicating that there was no calcium build up in the biomass. The results of this study demonstrate that the ECSB design seems to be particularly suitable to treat calcium-rich wastewater that is probably due to the fact that in this system CaCO3 precipitates in the bulk liquid of the external circulation tank and not the biomass present in the main reactor, and that the CaCO3 crystals are washed-out from it due to the high upflow velocity applied to the system (5 m h-1).

Resolving stability issue of thermophilic high-rate anaerobic palm oil mill effluent treatment via adaptive neuro-fuzzy inference system predictive model

Abstract Palm oil mill effluent is a highly polluting and corrosive liquid waste generated from crude palm oil processing. Its varying characteristics depending on the crop season and loading rate of the milling process complicate the handling of anaerobic digestion of palm oil mill effluent. To ease monitoring and aid decision making to handle operation anomalies, this study adopts Adaptive Neuro-Fuzzy Inference System with hybrid learning algorithm and subtractive clustering to predict the output from a thermophilic high-rate anaerobic digester treating palm oil mill effluent. Influent pH, chemical oxygen demand, organic loading rate and historical data of effluent pH, chemical oxygen demand and total suspended solids were utilized to build the model for output prediction of treated effluent (pH, chemical oxygen demand and total suspended solids). Trained models were validated with experimental data. The models were able to predict the effluent pH, chemical oxygen demand and total suspended solids satisfactorily with average error of less than 8.32% and standard deviation of less than 7.65% with closely followed pattern between the actual values and simulated results. The viability of adaptive neuro-fuzzy inference system in modelling palm oil mill effluent using thermophilic high-rate anaerobic digester is further demonstrated by coefficient of determination, R2 of 0.82, Root Mean Square Error of 0.0377, Index of Agreement of 0.95 and d-factor of 0.0496.

High-rate anaerobic treatment of wastewater from soft drink industry: Methods, performance and experiences

At an Austrian soft drink company, an expanded granular sludge bed reactor for anaerobic wastewater treatment was inoculated with sludge from paper and food industries. Detailed online monitoring and laboratory examinations were carried out during startup and subsequent phases, which included a period of inhibition after ca. 80 days during which reactor degradative performance diminished suddenly, following a period of increased effluent VFA. After dosing iron chloride (FeCl2) and micronutrients and reducing organic loading to startup levels, the reactor eventually reached efficient operation (>85% COD degradation) after a gradual recovery phase. In this work performance data both at lab and full scale are elaborated along startup, adaptation, pre-inhibition, recovery and stable phases, and correlated between scales. High rate anaerobic treatment of soft drink industry wastewater was successful in terms of COD removal efficiency and final effluent COD (∼300 mg l−1), with a startup period (including inhibition) of ca. 5 months.

High-rate anaerobic treatment system for solid/lipid-rich wastewater using anaerobic baffled reactor with scum recovery

A laboratory scale experiment was conducted to investigate the treatment of solid/lipid-rich wastewater with an anaerobic baffled reactor (ABR) and a down-flow hanging sponge (DHS) reactor. In this study, experimental periods were divided into three phases to explore efficient treatment of solids and lipids in wastewater. In ABR, >90% of the influent chemical oxygen demand (COD) was removed and >70% of the removed COD was converted to methane under steady-state conditions during each phase. During this period, >4.5 kg COD m−3 d−1 was achieved on an average in Phases 1 and 3. Biogas contributed to scum formation, and the scum was categorized into lipid-rich and sludge-containing types, which have energy potentials of 53.4 and 212 kcal/kg-wet weight, respectively. Therefore, by recovering solids and lipids, which formed persistent scum, ABR can be applied as a high-rate treatment for solid/lipid-rich wastewater.

CORRELATION BETWEEN BIOMASS CONCENTRATION AND EXTENT OF SULPHATE REDUCTION IN AN ACIDOGENIC REACTOR

Two stage high rate anaerobic treatment systems comprising of an acidogenic reactor (or equalisation/ buffer tank) followed by a methanogenic reactor are becoming increasingly popular to treat high strength wastewater from industries. In these systems, sulphate present in the wastewater is reduced to sulphide either partially or completely in the acidogenic reactor and completely in the methanogenic reactor. The effect of fermentation products on the extent of sulphate reduction in the acidogenic reactor was investigated in a continuously-fed, well mixed laboratory-scale 3 L fermenter operating at a temperature of 35oC and pH of 6. The feed was based on either glucose or molasses as the carbon source. It was observed that as the carbohydrate concentration in feed was increased sulphate reduction was suppressed. It was confirmed that volatile organic acids like acetic, propionic and butyric acids, hydrogen and residual glucose did not cause inhibition of sulphate reduction. However, biomass concentration correlated negatively with extent of sulphate reduction. This correlation was expressed mathematically and the same expression with the same parameters adequately predicted the effect of biomass concentration on extent of sulphate removal for both steady state and transient data irrespective of glucose or molasses feed. It was seen from the best fit of this expression that a biomass concentration of 3300 mg-COD.L-1 would completely repress sulphate reduction in the acidogenic reactor. Even when sulphate removal was suppressed the presence of sulphate reducing bacteria (SRB) in the reactor was confirmed through Fluorescent In Situ Hybridisation (FISH) visualisation. Moreover, the numbers of SRB seemed to increase with carbohydrate concentration in feed. It was speculated the suppression of sulphate removal might be due to a switch in function of SRB from sulphate reducing to fermentation or acetogenesis. Keywords: sulphate reducing bacteria, anaerobic digestion, inhibition, sulphate reduction, acidogenic reactor, high rate anaerobic treatment

WERF Project ENER5R12: Mainstream Anaerobic Treatment Process Using the UASB Process

The upflow anaerobic sludge blanket (UASB) reactor has been recognized as an important wastewater treatment technology among high-rate anaerobic treatment methods. The objective of this study was to evaluate the performance of a UASB pilot plant treating domestic sewage at ambient temperature conditions and identifying future areas of research to better assess the implementation of this technology to treat low strength sewage. The merits of anaerobic bioreactors are highlighted on the basis of performance, and resource recovery potential. While UASB has the advantage of low capital, operation, and maintenance costs, the performance of the UASB pilot at low residence time is significantly diminished. UASB technology has the ability to achieve high removal efficiencies of COD at a range of temperature conditions. However, its efficiency is directly related to the operating HRT of the system.

Elemental sulfur as an electron acceptor for organic matter removal in a new high-rate anaerobic biological wastewater treatment process

Elemental sulfur can serve as an economical external sulfur source replacing sulfate as an electron acceptor in sulfidogenic processes for wastewater treatment with sludge minimization. However, the biological sulfur reduction has not yet been considered in wastewater treatment due to the insolubility of sulfur. In this study, a laboratory-scale sulfur-reducing anaerobic fluidized bed (SRAFB) reactor was built to investigate the feasibility of the high-rate sulfur-reducing anaerobic wastewater treatment process. After 273d of operation, the organics removal and sulfide production rates were determined as 1.71kgCODc/m3-d (CODc: carbonaceous COD) and 2.71kgS/m3-d, respectively, at a high organic loading rate of 2.14kgCODc/m3-d with a hydraulic retention time of only 3h. Biofilm growth on sulfur particles resulted in high sludge settleability (23±3mL/g of SVI30) (SVI30: sludge volume index in 30min) and thus high sludge concentration. As expected, a low sludge yield of 0.16kgVSS/kgCODc (VSS: volatile suspended solids) was also obtained. Although sulfur reducers could not be identified in this study, Geobacter and Desulfomicrobium were the most likely sulfur reducers. This work suggests that elemental sulfur can serve as an electron acceptor in the new high-rate anaerobic sulfidogenic process for sludge-minimized wastewater treatment.