High Mixed Liquor Suspended Solids Research Articles

Enhanced total nitrogen removal and membrane fouling control by increasing biomass in MBR equipped with ceramic membrane

Simultaneous nitrification and denitrification (SND) is an efficient method to remove nitrogen in municipal wastewater treatment. However, low dissolved oxygen (DO) concentrations are generally required, leading to serious membrane fouling in membrane bioreactors (MBRs). This study aimed to clarify the synergistic effect of biomass and DO on nitrogen removal and membrane fouling. To achieve this goal, four submerged MBRs equipped with ceramic membranes were operated with different biomass (mixed liquor suspended solids (MLSS)) concentrations (3 000 mg/L, 5 000 mg/L, 7 500 mg/L, and 12 000 mg/L) under various DO concentrations (2.0 mg/L, 1.0 mg/L, and 0.5 mg/L). As a result, increasing biomass in the MBRs enhanced total nitrogen (TN) removal via SND, and excellent TN removal efficiencies of 60.7% and 75.8% were obtained using the MBR with an MLSS concentration of 12 000 mg/L and DO concentrations of 2.0 mg/L and 1.0 mg/L. However, a further decrease in DO deteriorated TN removal due to the inhibition of nitrification. Moreover, high MLSS concentrations were beneficial to membrane fouling control for ceramic membranes in MBRs. The lowest transmembrane pressure development rate was observed for the MBR with an MLSS concentration of 12 000 mg/L. High biomass offset the adverse effect of DO decrease on membrane fouling to some extent, and improved the stability of the reactor. Therefore, biomass might be an important parameter for membrane fouling reduction in ceramic MBRs. Overall, optimal biomass and DO concentrations for TN removal were identified, providing useful information for the successful operation of MBRs with efficient TN removal and membrane fouling control.

Pilot study on ceramic flat membrane bioreactor in treatment of coal chemical wastewater

Some toxic and refractory pollutants in coal chemical wastewater can penetrate the biochemical treatment systems and cause high concentrations of suspended solids in the effluent, which may obstruct the subsequent advanced treatment. In this project, a submerged ceramic plate membrane system was integrated to the last oxic corridor of an existing multistage anoxic/oxic tank. In the ceramic flat membrane bioreactor, the influent chemical oxygen demand (COD) was 102.24–178.88 mg/L, with a removal ratio of approximately 30%. The NH3–N concentration in the effluent was relatively stable with an average value of 1.76 mg/L. The turbidity of the effluent was in the range of 0.235–0.852 NTU and was stable below 1 NTU. A flux of 30 L m−2·h−1 could meet the requirements of the pilot test. A gas-water ratio of 50:1 was found optimal. When the concentration of mixed liquor suspended solids (MLSS) was >3769 mg/L, the extracellular polymeric substance in the mixed solution was utilized by microorganisms as a substrate. High MLSS decreased membrane fouling rate. NaClO backwashing can effectively remove pollutants without adversely affecting the treatment efficiency of membrane bioreactors.

Enhancement of nutrient removal in an activated sludge process using aerobic granular sludge augmentation strategy with ammonium-based aeration control

To enhance nutrient removal from low-strength municipal wastewater in a continuous-flow activated sludge (CFAS) process using aerobic granular sludge (AGS) augmentation strategy, a pilot-scale demonstration was configured with a mainstream reactor (anaerobic/aerobic process) and a sidestream sequencing batch reactor for AGS production. The aeration of the mainstream reactor was controlled based on dissolved oxygen (DO) and ammonium concentrations during Phases I and II–III, respectively. During Phase III, an anoxic zone was created in the mainstream aerobic tank. Throughout the demonstration period, excellent sludge settleability in the mainstream reactor (SVI30 ≤ 80 mL g−1) under long sludge retention time conditions (≥12 d) allowed the maintenance of a high mixed liquor suspended solids concentration (≥3000 mg L−1). The total nitrogen (TN) removal ratio improved significantly during Phases II and III (49.3 ± 4.1% and 50.1 ± 10.2%, respectively) compared to Phase I (43.2 ± 5.5%). Low DO concentration (< 0.5 mg L−1) by the ammonium-based aeration tended to increase the simultaneous nitrification and denitrification efficiency (> 40%), enhancing TN removal (> 50%). The reduction of DO and nitrate concentrations in the returning sludge liquor can stabilize phosphorus removal (approximately 80% of the 25th percentile). In addition, the aeration efficiency during Phase III decreased by 26–29% compared to Phase I. These results suggest that the introduction of ammonium-based aeration control to the CFAS using the AGS augmentation strategy could contribute to superior sewerage treatment, including nutrient removal and a low carbon footprint.

Enhanced sludge settlement of two stage PN/Anammox for reject water treatment with respective diatomite addition

The present study investigated the potential of diatomite addition in enhancing sludge settlement of two-stage PN/Anammox for real reject water treatment, with a focus on sludge settling velocity, nitrogen removal capacity, sludge morphological features, and microbial community changes. The study found that diatomite addition significantly improved the sludge settleability of the two-stage PN/A process, resulting in a decrease in sludge volume index (SVI) from 70 to 80 mL/g to about 20–30 mL/g for both PN and Anammox sludge, although the sludge-diatomite interaction differed between the two types of sludge. In the PN sludge, diatomite acted as a carrier, while in the Anammox sludge, it acted as micro-nuclei. The addition of diatomite also increased the biomass amounts in the PN reactor, with a 5–29 % improvement attributed to its role as a biofilm carrier. The effects of diatomite addition on sludge settleability were more prominent at high mixed liquor suspended solids (MLSS), where sludge characteristics were deteriorated. Furthermore, the settling rate of the experimental group consistently exceeded that of the blank group after diatomite addition, with a significant decrease in SV. The relative abundance of Anammox bacteria was improved, and sludge particle size decreased in the diatomite-added Anammox reactor. Diatomite was effectively retained in both reactors, with less loss observed for Anammox than PN due to its more tightly wrapped structure, resulting in a stronger sludge-diatomite interaction. Overall, the results of this study suggest that diatomite addition has potential in enhancing the settling properties and performance of two-stage PN/Anammox for real reject water treatment.

Comparison of Effluent Suspended Solid Concentrations from Two Types of Rectangular Secondary Clarifiers

Secondary clarifiers play a significant role in the successful operation of activated sludge systems. Because of the restriction of available land, South Korean domestic wastewater treatment plants tend to employ rectangular clarifiers to settle mixed liquor suspended solids (MLSS) for activated sludge systems. A high MLSS concentration must be maintained in the bioreactor to ensure nitrification during winters, and achieve stringent effluent quality. The effluent suspended solid (SS) concentrations of two clarifier types currently being used in South Korea, primary rectangular clarifier-type and Gould Type I secondary clarifiers, were compared using computerized fluid dynamic simulations and hourly secondary effluent suspended solid concentrations. In addition, operational data such as hourly influent flow, daily MLSS concentrations, and sludge volume index were obtained and reviewed. This comparison reveals that the Gould Type I secondary clarifier is resilient to loading variation and produces effluent with a consistently lower suspended solid concentration than the primary rectangular clarifier-type under similar loading conditions and higher loading variations. The results suggest that the existing primary rectangular clarifier-type secondary clarifiers must be converted to Gould Type I.

Impact of source-separation of urine on treatment capacity, process design, and capital expenditure of a decentralised wastewater treatment plant

In this study, the impact of urine diversion on the treatment capacity, treatment process, and capital costs of a decentralised wastewater treatment plant (WWTP) was simulated using BioWin. The data for simulation including for economic analysis were obtained from a real decentralised WWTP at Sydney. Simulation was conducted for two alternative process design scenarios of a WWTP: membrane bioreactor (MBR) without denitrification and anaerobic MBR in place of aerobic MBR and compared to existing process design. The simulation shows that with about 75% urine diversion (through source separation), the treatment capacity of the existing WWTP can be doubled although above 40% urine diversion, the impact appears less rapid. When the urine diversion exceeds 75%, it was found that the anoxic tank for biological denitrification becomes redundant and the current wastewater treatment process could be replaced with a simpler and much less aeration intensive membrane bioreactor (MBR) producing similar effluent quality with a 24% reduction in capital expenditure (footprint) cost. Anaerobic MBR can be a potential alternative to aerobic MBR although pre-treatment becomes essential before reverse osmosis treatment for water reuse applications. Sensitivity analysis has revealed that by operating the bioreactor at higher mixed liquor suspended solids concentrations (9 g/L instead of 5 g/L) could help increase the WWTP treatment capacity by about 3.5 times at 75% urine diversion. Hence, urine diversion (until nitrogen-limiting conditions occur above 75% urine diversion) can increase the treatment capacity of an existing WWTP and reduce the capital expenses due to reduced plant footprint.

Supersaturated-oxygen aeration effects on a high-loaded membrane bioreactor (HL-MBR): Biological performance and microbial population dynamics

Conventional diffused aeration systems (such as fine-bubble diffusers) exhibit a poor oxygen transfer in wastewater treatment plants (WWTPs), particularly when operating at sludge concentrations higher than 15 g L−1. The supersaturated dissolved oxygen (SDOX) system has been proposed as an alternative for supplying dissolved oxygen (DO) at high mixed liquor suspended solids (MLSS) concentrations. The advantages introduced by such technology include the possibility of operating WWTPs at much higher than usual MLSS concentrations, increasing the treatment capacity of WWTPs. Recent studies have demonstrated that the SDOX system has higher oxygen transfer rates (OTRs) and oxygen transfer efficiencies (OTEs) relative to fine-bubble diffusers. However, it is unknown if the high-pressure conditions introduced by SDOX may possibly impact the biological performance of WWTPs. In this study, the effects of SDOX technology on the biological performance of a membrane bioreactor (MBR) were evaluated. The MBR was operated at an MLSS concentration of approximately 15 g L−1 in four phases as follows: (P1) with bubble diffusers, (P2) with an SDOX unit, (P3) with the bubble diffusers, and (P4) with the SDOX unit. The performance of the MBR was assessed by monitoring the sludge concentration, as well as changes in the particle size distribution (PSD), sludge activity, organic matter removal and nitrification performance, and changes in the microbial community within the MBR. The operational conditions exerted by the SDOX technology did not affect the concentration of active biomass during the study period. The biological performance of the MBR was not affected by the introduction of the SDOX technology. Finally, the microbial community was relatively stable although some variations at the family and genus level were evident during each of the study phases. Therefore, the SDOX system can be proposed as an alternative technology for DO supply in WWTPs increasing the overall treatment capacity.

Oxygen transfer performance of a supersaturated oxygen aeration system (SDOX) evaluated at high biomass concentrations

Oxygen transfer in wastewater treatment is significantly influenced by the mixed liquor suspended solids (MLSS). The effect is more pronounced at MLSS concentrations higher than 20g L−1 when supplying air by conventional diffused aeration systems. The oxygen transfer performance of a supersaturated oxygenation technology (i.e., the supersaturated dissolved oxygen (SDOX) system) was evaluated in clean water and in activated sludge with MLSS concentrations from 4 to 40gL−1 as a promising technology for uncapping such limitation. The evaluation was carried out at the laboratory facilities of the faculty of food technology and biotechnology at the University of Zagreb. The sludge was collected from a full-scale conventional activated sludge (CAS) wastewater treatment plant (WWTP) operated at a solid retention time (SRT) of approximately 5 days. The evaluation was carried out using a laboratory-scale setup consisting of a bench-scale SDOX system (2.75L) supplying pure oxygen to a 5L biological reactor. The SDOX exhibited oxygen mass transfer rate coefficient (KLa) values (2.6h−1) in clean water lower than for fine bubble diffusers (11h−1). However, higher oxygen transfer rate (OTR) values and alpha factors (mass transfer ratio of process-water to clean-water) as a function of the MLSS concentration were observed. A standard oxygen transfer efficiency (SOTE) of approximately 100 % in clean water was reported. The SDOX technology can be presented as a promising alternative for supplying dissolved oxygen (DO) into mixed liquor solutions; particularly, at the high MLSS concentrations required by high-loaded membrane bioreactor (HL-MBR) systems and aerobic digesters.

Milk processing wastewater treatment in an MBR: A comparative study on the use of two synthetic anti-fouling PES-UF membranes

Recently, membrane bioreactors (MBRs) have received extensive attention as an innovative wastewater treatment technology. Nevertheless, membrane fouling as a serious barrier has avoided the widespread application of the MBRs. In the present study, two polyethersulfone (PES) antifouling ultrafiltration membranes entrapped by 0.1 wt. % of NH2 functionalized multiwalled carbon nanotubes (NH2-MWCNTs) and of O-carboxymethyl chitosan modified Fe3O4 (OCMCS- Fe3O4) nanoparticles were employed in an MBR. The effect of both modified membranes on the fouling characteristics and biological performance of bioreactor was investigated. Experimental design was conducted using central composite design (CCD) of response surface methodology (RSM). The process performance of the bioreactor treating milk processing wastewater (MPW) was analyzed at different hydraulic retention times (HRTs) and mixed liquor suspended solids (MLSS). Five dependent process and operating parameters; TKN, NO3−, TN, TP removal efficiency and flux, were measured as response for both membranes. Independent increment of the variables gave rise to an increase in the process parameters. While, simultaneous increase in the variables led to the increase in flux. The Fe3O4−OCMCS embedded membrane indicated slightly higher flux than that of NH2-MWCNTs modified membrane. The optimum region was enclosed by high MLSS (≥12,000 mg/l) at any HRT studied.

Exploring Nanosilver-Coated Hollow Fiber Microfiltration to Mitigate Biofouling for High Loading Membrane Bioreactor.

For the first time, a nanosilver-coated hollow fiber microfiltration (MF) was fabricated by a simple chemical reduction method, then tested for membrane biofouling mitigation study under extreme high mixed liquor suspended solid (MLSS) concentration for long term. This study presents a simple and novel technique to modify a commercially available MF membrane using silver nanoparticles (AgNPs) followed by an investigation of mitigating membrane biofouling potentials using this modified membrane to compare with an unmodified membrane for 60-day operation period. The modified membranes showed that AgNPs was attached to the MF-membrane successfully with a high density of 119.85 ± 5.42 mg/m2. After long-term testing of 60 days in membrane bioreactor with a MLSS concentration of 11,000 mg/L, specific flux of the AgNPs coated MF (AgNPs-MF) decreased 59.7%, while the specific flux of the unmodified membrane dropped 81.8%, resulted from the increase of transmembrane vacuum pressure for the AgNPs-MF was lower than that of the unmodified one. The resistance-in-series model was used to calculate the resistance coefficients of membrane modules, and the result showed that the cake layer resistance coefficient of the unmodified membrane was 2.7 times higher than that of the AgNPs-MF after the 60-day operation, confirming that AgNPs displayed great antimicrobial properties to mitigate membrane biofouling under such high MLSS.

Limitations imposed by conventional fine bubble diffusers on the design of a high-loaded membrane bioreactor (HL-MBR).

The operation of membrane bioreactors (MBRs) at higher than usual mixed liquor suspended solids (MLSS) concentrations may enhance the loading rate treatment capacity while minimizing even further the system's footprint. This requires operating the MBR at the highest possible MLSS concentration and biomass activity (e.g., at high loading rates and low solid retention times (SRTs)). Both a negative effect of the MLSS concentrations and a positive effect of the SRT on the oxygen transfer have been reported when using conventional fine bubble diffusers. However, most of the evaluations have been carried out either at extremely high SRTs or at low MLSS concentrations eventually underestimating the effects of the MLSS concentration on the oxygen transfer. This research evaluated the current limitations imposed by fine bubble diffusers in the context of the high-loaded MBR (HL-MBR) (i.e., high MLSS and short SRT-the latter emulated by concentrating municipal sludge from a wastewater treatment plant (WWTP) operated at a short SRT of approximately 5days). The high MLSS concentrations and the short SRT of the original municipal sludge induced a large fraction of mixed liquor volatile suspended solids (MLVSS) in the sludge, promoting a large amount of sludge flocs that eventually accumulated on the surface of the bubbles and reduced the free water content of the suspension. Moreover, the short SRTs at which the original municipal sludge was obtained eventually appear to have promoted the accumulation of surfactants in the sludge mixture. This combination exhibited a detrimental effect on the oxygen transfer. Fine bubble diffusers limit the maximum MLSS concentration for a HL-MBR at 30gL-1; beyond that point is either not technically or not economically feasible to operate; an optimum MLSS concentration of 20gL-1 is suggested to maximize the treatment capacity while minimizing the system's footprint.

Process performance and multi-kinetic modeling of a membrane bioreactor treating actual oil refinery wastewater

The performance of a membrane bioreactor treating oily wastewater generated by a petroleum refinery was investigated at mixed liquor suspended solids concentrations of 6.5 and 8.5 g/l with hydraulic retention times ranging from 12 to 24 h. An evolutionary polynomial regression technique and multi-kinetic models consisting of the first-order, Grau second-order, and modified Stover-Kincannon were applied for predicting the performance of the bioreactor. The results showed that a maximum chemical oxygen demand reduction of 97% is achievable at the mixed liquor suspended solids of 8.5 g/l and hydraulic retention time of 24 h. The results also revealed positive influence of higher mixed liquor suspended solids and hydraulic retention time values on the bioreactor performance. In addition, the high accuracy of the evolutionary polynomial regression evidenced its capability in investigating the membrane bioreactor. Based on the analysis performed by the resulted model from the evolutionary polynomial regression, the optimum operating conditions (hydraulic retention time of 21 h; mixed liquor suspended solids of 8.2 g/l) were recommended. According to the kinetic evaluation, the Grau second-order and modified Stover-Kincannon kinetics seemed to be the best approach in describing the chemical oxygen demand reduction in the membrane bioreactor.

Removals of pharmaceutical compounds at different sludge particle size fractions in membrane bioreactors operated under different solid retention times.

Removals of 10 pharmaceutical compounds by microbial sludge in membrane bioreactors (MBR) operated under infinite and limited solid retention time (SRT) were investigated. High removal (>80%) of DCF, TMP, NPX, IBP, and TCS were achieved but CBZ removals were low (<20%). The residual pharmaceutical compounds leftover from the biodegradation in different sludge particle size fractions was quantified through physical separation and filtration in series. The results revealed that hydrophobic compounds were mainly adsorbed onto the coarse particles (>0.45μm) where majority of adsorption site was available. Meanwhile, hydrophilic and moderate hydrophobic compounds were less associated with particles and they were retained in fine particles and gel-like substances (1 kDa-0.45μm). Most of the studied pharmaceutical compounds associated with fine particles and gel-like substances was subsequently rejected by membrane filtration in the MBRs. The operation of the MBR at high mixed liquor suspended solids concentration under long sludge age conditions could enhance the removals of pharmaceutical compounds through increased adsorption site on the sludge particles.

Ultrasound-induced settleability and membrane filterability of activated sludge treating milk processing wastewater

In practice, established operation of activated sludge treatment system with high biomass concentration is restricted because of difficulty in settling the sludge in clarifiers. Membrane bioreactor is a modified form of activated sludge, and despite the superior advantages, its widespread application is restricted by the membrane fouling. Therefore, in this research, to improve the sludge sedimentation at high biomass concentration in the activated sludge system, high-frequency ultrasound (1.7 MHz) was exerted in clarifier of activated sludge system and its performance was compared with a bioreactor followed by an antifouling ultrafiltration membrane. The antifouling ability of NH2-functionalized multiwall carbon nanotubes (NH2-MWCNTs)-modified nanocomposite UF membrane during filtration of mixed liquor-activated sludge was investigated. Hydraulic retention time (HRT) (8–44 h) and mixed liquor suspended solids (MLSS) (6000–14,000 mg/L) were chosen as the operating variables to analyze the process. The biological process showed high COD removal efficiency throughout the experiments (> 96%). The membrane and high-frequency ultrasound had no effect on the system performance in terms of COD, TKN, TN, TP removal. However, the HRT and MLSS indicated an increasing impact on flux. The effect of ultrasound on the sludge properties, i.e., sludge volume index, sludge settling velocity and height of sludge at high MLSS concentration (14,000 mg/L), was not considerable. It was concluded at MLSS of below 10,000 mg/L, about 40% water recovery was obtained. As a result, activated sludge equipped with ultrasound showed a promising performance; however, its industrial development needs further examinations to attain the design criteria.Graphical

Effect of mixed liquor suspended solids (MLSS) on simultaneous nitrification and denitrification in a sequencing batch reactor

This study investigated the effect of MLSS on simultaneous nitrification and denitrification in a sequencing batch reactor treating domestic wastewater. The average removal efficiencies of COD and TN increased from 93 and 68.21% to 97 and 74.20% with the increase in MLSS from 3.5-4.0 to 7.5-8.0 g/L, respectively. However, the average removal efficiency of TN decreased to 39.45% when MLSS increased to 9.5-10.0 g/L. Simultaneous nitrification and denitrification (SND) was carried out in high MLSS (7.5-8.0 and 9.5-10.0 g/L), and SND efficiencies were 43.4-52.0% and 72.1-80.7%, respectively. Batch tests suggested that there was a strong potential to apply high MLSS for treating wastewater containing high-strength ammonia nitrogen.

Assessment of the operational performances of two activated sludge systems in Kuwait

Activated sludge systems (ASSs) are the most globally used wastewater treatment systems. However, there is very little information about the operational practices of ASSs in developing countries. This paper presents how the Riqqa and Umm-Al-Haiman ASSs, in Kuwait, are operated and the methodology used to evaluate and compare their performances. Over a period of 9months, weekly samples of the influents, aeration tank mixed liquors, and secondary effluents were collected and analyzed according to standard methods. Accordingly, the operational variables of the systems were estimated. The obtained results indicate that the plants were not operated within their recommended design values. The Riqqa ASS was inadequately aerated and hydraulically and organically overloaded, while the Umm-Al-Haiman ASS was operated at a very high mixed liquor suspended solids (MLSS) value. The method used by this study proved to be useful for identifying the operational problems of the studied ASSs, and it can be used to evaluating the performances of other ASSs.

High rate treatment of hospital wastewater using activated sludge process induced by high-frequency ultrasound

The biomass concentration of conventional activated sludge (CAS) process due to low sludge sedimentation in clarifiers is limited to 3000 mg/L. In this study, high-frequency ultrasound wave (1.8 MHz) was applied to enhance the CAS process performance using high Mixed Liquor Suspended Solid (MLSS) concentration. The study conducted using a pilot scale CAS bioreactor (with and without ultrasound) and their performance for treating a hospital wastewater were compared. Experimental conditions were designed based on a Central Composite Design (CCD). The sets of data analyzed, modeled and optimized using Response Surface Methodology (RSM). The effect of MLSS concentration 3000–8000 mg/L and hydraulic retention time (HRT) 2–8 h are considered as operating variables to investigate on process responses. The obtained results showed that high-frequency ultrasound was significantly decreased the sludge volume index (SVI) 50% and effluent turbidity about 88.5% at high MLSS. Also, observed that COD removal of both systems was nearly similar, as the maximum COD removal for sonicated and non-sonicated systems were 92 and 92.5% respectively. However, this study demonstrates that the ultrasound irradiation has not had any negative effect on the microbial activity.

A novel symbiotic system combining algae and sludge membrane bioreactor technology for wastewater treatment and membrane fouling mitigation: Performance and mechanism

A novel Algal-Sludge Bacterial-Membrane bioreactor (ASB-MBR) system, which combined a sludge membrane bioreactor and an algal system in a single tank, was developed in this study. The combined system exhibited excellent wastewater treatment performance, sludge flocs properties and membrane permeability. Compared with conventional MBR (C-MBR), ASB-MBR showed 25% faster growth rate. Meantime, the removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN) and phosphate (PO43−-P) in ASB-MBR were increased by 4.6%, 6.7%, 10.1% and 8.2%, respectively. Amelioration of microbial activity and the increase of dissolved oxygen (DO) concentration were identified as the main reasons for better COD and nitrogen removal, while algal great assimilation and “luxury” uptake of phosphorus and high mixed liquor suspended solids (MLSS) removal contributed to higher PO43−-P removal in ASB-MBR. Moreover, the membrane fouling was mitigated by 50% after inoculating algae. Further mechanism investigation demonstrated that algae has a positive influence on the inhibition of the filamentous bacteria overgrowth and the decrease of zeta potential absolute value, thus improved the flocs flocculability and stability in ASB-MBR. ASB-MBR exhibited higher SMPpr/SMPps ratio, lower bound extracellular polymeric substances (bound EPS) concentrations and smaller Pr/Ps ratio in bound EPS, which contributed to improve the flocs filterability and decrease the fouling layer accumulation. Deposited by this modified sludge, the flocs morphology of the bio-cake in ASB-MBR was more regular and less easy to accumulate on membrane surface resulted in higher membrane permeability.

Osmotic membrane bioreactors assisted with microfiltration membrane for salinity control (MF-OMBR) operating at high sludge concentrations: Performance and implications

Osmotic membrane bioreactor (OMBR) is an emerging technology with great potentials for wastewater treatment and reclamation. However, the contradiction between the alleviation of salinity build-up and the achievement of a high biomass concentration remains a main obstacle. Here, we attempt to simultaneously achieve a low salinity environment and a high biomass concentration in OMBRs through integrating a microfiltration (MF) membrane system in the OMBR and operating it at a longer sludge retention time (SRT) for controlling the salt accumulation and extending the retention of activated sludge, respectively. The results indicated that the salinity in terms of conductivity in the OMBR assisted with the MF membrane (MF-OMBR) was successfully kept at a low range of 2.5–4.0 mS/cm. In the meanwhile, a high concentration of mixed liquor suspended solid (MLSS) in the range of 8.09 ± 1.02 g/L was achieved in the MF-OMBR through operating at a longer SRT of 30 d. A high MLSS concentration enhanced the removals of ammonia nitrogen (NH4+-N) and total organic carbon (TOC) in the mixed liquors, and consequently increased the water quality of MF and forward osmosis (FO) permeates. In addition, the high MLSS concentration strengthened the removal of some trace organic chemicals (TrOCs) such as Trimethoprim by the activated sludge in the MF-OMBR system. Moreover, a high MLSS concentration did not induce severe FO membrane fouling compared with a low MLSS concentration in OMBRs, which might be due to a low fouling tendency of FO membrane operating at a low water flux.

Experimental study on advanced treatment of refinery wastewater by ozone catalytic oxidation and biological aerated filter

The performance of a membrane bioreactor treating oily wastewater generated by a petroleum refinery was investigated at mixed liquor suspended solids concentrations of 6.5 and 8.5 g/l with hydraulic retention times ranging from 12 to 24 h. An evolutionary polynomial regression technique and multi-kinetic models consisting of the first-order, Grau second-order, and modified Stover-Kincannon were applied for predicting the performance of the bioreactor. The results showed that a maximum chemical oxygen demand reduction of 97% is achievable at the mixed liquor suspended solids of 8.5 g/l and hydraulic retention time of 24 h. The results also revealed positive influence of higher mixed liquor suspended solids and hydraulic retention time values on the bioreactor performance. In addition, the high accuracy of the evolutionary polynomial regression evidenced its capability in investigating the membrane bioreactor. Based on the analysis performed by the resulted model from the evolutionary polynomial regression, the optimum operating conditions (hydraulic retention time of 21 h; mixed liquor suspended solids of 8.2 g/l) were recommended. According to the kinetic evaluation, the Grau second-order and modified Stover-Kincannon kinetics seemed to be the best approach in describing the chemical oxygen demand reduction in the membrane bioreactor.