Mixed Liquor Characteristics Research Articles

A novel membrane bioreactor inoculated with algal-bacterial granular sludge for sewage reuse and membrane fouling mitigation: Performance and mechanism

In this study, a novel membrane bioreactor (MBR) inoculated with algal-bacterial granular sludge (ABGMBR) was established to improve pollutant removal and alleviate membrane fouling. The ABGMBR system showed higher pollutant removal rate and longer operation time (152 day) compared to the control MBR (AGMBR). Moreover, the contents of the pollutants such as granular sludges, extracellular polymeric substances (EPS), and soluble microbial products on the membrane were remarkably reduced, leading to the formation of a porous and loose cake layer on the membrane and a slow increase in transmembrane pressure. Standard blocking was the main mechanism of membrane fouling; however, the membrane pore blockage was significantly reduced in ABGMBR. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory suggested that the aggregation and adhesion of foulants on the membrane were greatly inhibited in ABGMBR. Furthermore, correlation analysis showed significant differences in membrane fouling characteristics between AGMBR and ABGMBR. The ABGMBR system effectively retarded sludge disintegration and increased the repulsion between the sludge and membrane owing to the favorable mixed liquor characteristics. This study showcases the superior operational efficiency and anti-fouling performance of ABGMBR, offering a novel perspective on sewage reuse and membrane fouling mitigation.

Low-cost ceramic membranes manufacture using INKJET technology for active layer deposition and validation on membrane bioreactors

The fabrication of eco-friendly ceramic membranes based on low-cost raw materials, using digital INKJET printing techniques for active layer deposition and its validation in a membrane bioreactor (MBR) has been studied. The raw materials used in the manufacture of the support layer were UA50/2 clay, chamotte, calcium carbonate and potato starch. A MBR laboratory plant was operated to treat municipal wastewater with three ceramic membranes with different grammages in the selective layer deposition. The membrane performance at laboratory scale was evaluated in terms of transmembrane pressure (TMP) evolution for a constant permeate flux, permeate quality and mixed liquor characteristics. From these experiments, it was selected the membrane which obtained the lowest TMP profile maintaining appropriate water quality parameters (chemical oxygen demand (COD) removal percentage around 90% and turbidity around 0.06 NTU). This membrane was also operated at pilot plant scale in order to validate it at higher scale. Results indicated that TMP values were in the range of 0.06 and 0.1 bar, COD removal percentage was around 98% and microbiology analysis demonstrated that the quality of the effluent, according to European regulation 2020/741, can be classified as Class A and it can be reused for non-potable purposes.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

Optimization of nutrient rich solution for direct fertigation using novel side stream anaerobic forward osmosis process to treat textile wastewater

The current study focused on the performance of a lab scale side stream anaerobic fertilizer drawn forward osmosis (An-FDFO) setup and optimization of nutrient rich solution to achieve sustainable water reuse from high strength synthetic textile wastewater. Three fertilizer draw solutes including Mono Ammonium Phosphate (MAP), Ammonium Sulphate (SOA) and Mono Potassium Phosphate (MKP) were blended in six different ratios with total molar concentration not exceeding 1 M. Among six blended draw solutions (DS), combination with high concentration of SOA have shown highest flux and combination with high concentration of MKP have shown highest reverse solute flux, while those with high concentration of MAP remain moderate both in flux and RSF. During long term runs, SOA: MKP (0.75: 0.25 M) showed longest filtration duration of 217 h in Run 1, with highest initial flux of 8.29 LMH and minimum dilution factor to achieve final nutrients concentration fit for direct fertigation, followed by Run 3 MAP: SOA: MKP (0.2: 0.6: 0.2 M) and then Run 2 MAP: MKP (0.75: 0.25). Moreover, deterioration of mixed liquor characteristics occurs in membrane tank due to high RSF. Similarly, the same inhibitory effect of reverse salt on biogas production was also assessed through Bio-Methane Potential experiments. However, Anaerobic Continuous Stirring Tank Reactor exhibited high performance efficacy, highlighting the importance of side stream submerged configuration in forward osmosis (FO) process.

Operation of Submerged Anaerobic Membrane Bioreactors at 20 °C: Effect of Solids Retention Time on Flux, Mixed Liquor Characteristics and Performance

Four flat-sheet submerged anaerobic membrane bioreactors ran for 242 days on a simulated domestic wastewater with low Chemical Oxygen Demand (COD) and high suspended solids. Organic loading was maintained around 1.0 g COD L−1 day−1, while solids retention time (SRT) was varied from 20–90 days. This was achieved at a constant membrane flux, maintained by adjusting transmembrane pressure (TMP) in the range 1.8–9.8 kPa. Membrane fouling was assessed based on the required TMP, with mixed liquors characterised using capillary suction time, frozen image centrifugation and quantification of extracellular polymeric substances (EPS). SRT had a significant effect on these parameters: fouling was least at an SRT of 30 days and highest at 60 days, with some reduction as this extended to 90 days. Operation at SRT < 30 days showed no further benefits. Although operation at a short SRT was optimal for membrane performance it led to lower specific methane productivity, higher biomass yields and higher effluent COD. Short SRT may also have accelerated the loss of essential trace elements, leading to reduced performance under these conditions. A COD-based mass balance was conducted, including both biomass and methane dissolved in the effluent.

The use of pure oxygen for aeration in aerobic wastewater treatment: A review of its potential and limitations

In aerobic wastewater treatment, aeration is the most critical element of the treatment system. It supplies microorganisms with the required dissolved oxygen, maintains solids in suspension and, in membrane bioreactors, it controls fouling. However, conventional activated sludge is limited to the treatment of low strength wastewaters, as higher loadings require both higher biomass and higher dissolved oxygen concentrations. By replacing air with pure oxygen, oxygen transfer rates increase at lower flowrates. In this work, the potential and limitations of pure oxygen aeration are reviewed. The effect of the system’s operational parameters and the mixed liquor characteristics on oxygen transfer, and vice versa, are determined. Pure oxygen treats higher loadings without compromising effluent quality. Fine bubbles are more efficient in oxygen transfer due to their increased contact area. However, pure oxygen is not always essential, so it is recommended to be restricted to applications where air is not adequate.

A Study on Membrane Filtration Characteristics of Methanogenic Mixed Liquor in Two Phase Anaerobic Digestion of Food Waste

Objectives:An experimental study was conducted to investigate the membrane filtration characteristics of mixed liquor in methanogenic reactor to extend solid retention time (SRT) in food waste anaerobic digestion system.Methods:On the basis of the particle size distribution (0.5~700 µm) of the methanogenic mixed liquor, three grade membranes (MF, UF, NF) were tested in a stirred cell filtration and a plate type module. Furthermore foulants of membrane, especially UF, was investigated by SEM-EDS, FTIR, SEC.Results and Discussion:As a result UF membrane was selected for stable filtration of the liquor in terms of flux (2.51 L/m<sup>2</sup>･h･bar) and the flux recovery (100%) as well as filtration resistance (Total 7.15.E+13 m<sup>-1</sup>). Average flux was 18 L/m<sup>2</sup>･h･bar for the selected UF membrane in cross flow filtration using a flat plate module. The filtration results showed that membrane fouling was caused by gel and cake layer formed on the membrane surface and 90% of the initial flux could be recovered by physical washing. It was identified that major fouling causing materials were byproducts of carbohydrate and protein decomposition, and small amount of inorganic substance detected on the membrane surface were salt and struvite like materials.Conclusions:Based on the membrane filtration characteristics analyzed from the study, the UF membrane coupled anaerobic digestion is feasible to be applied as a novel food waste treatment system for SRT extension of the methanogenic reactor.

Evaluating the performance of inorganic draw solution concentrations in an anaerobic forward osmosis membrane bioreactor for real municipal sewage treatment

Sewage can become a valuable source if its treatment is re-oriented for recovery. An anaerobic forward osmosis membrane bioreactor (AnOMBR) was developed for real municipal sewage treatment to investigate performance, biogas production, flux change and mixed liquor characteristics. The AnOMBR had a good treatment capacity with removal ratio of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorus more than 96%, 88%, 89% and almost 100%. Although high DS concentration increased the initial flux, it caused rapid decline and poor recoverability of FO membrane flux. Low DS concentration led to too long hydraulic retention time, thus resulting in a low reactor efficiency. Additionally, it was observed that salt, protein, polysaccharide and humic acid were all accumulated in the reactor, which was not conducive to stable long-term operation. Based on the characteristics of membrane fouling, salt accumulation and AnOMBR performance, the optimal DS of 1 M NaCl solution was selected.

Comparison of hybrid ultrafiltration-osmotic membrane bioreactor and conventional membrane bioreactor for oil refinery effluent treatment

Although membrane bioreactors (MBR) has become the preferred choice for oil refinery reclamation, the osmotic membrane bioreactor (OMBR) has gained interest in the last years because of the low membrane fouling propensity and the high rejection capacity to both inorganic and organic contaminants. However, progress of OMBR is hindered by the salinity build-up in the bioreactor. This study evaluated the hybrid ultrafiltration-osmotic membrane bioreactors (UF-OMBR) performance in the treatment of oil refinery effluent and compare its performance with conventional MBR focusing the mixed liquor characteristics, the membrane fouling and the overall performance. The results indicated that the salinity build-up in the biological reactor was successfully prevented by integrating ultrafiltration to OMBR. Dissolved organic carbon (DOC) removal efficiencies for the UF and FO in the UF-OMBR were 81 ± 15 and 99.6 ± 0.5%, respectively, while for UF in the conventional MBR was 66 ± 8%. The high removal efficiency for UF in UF-OMBR compared to UF in conventional MBR can be attributed to the longer retention time of recalcitrant organic compounds. This could occur due to the high rejection of FO membrane which is not attenuated by UF permeate as efficiently as it was observed for salinity. However, this organic matter build-up in the biological reactor contributed to the greater UF membrane fouling in UF-OMBR compared to in conventional MBR. Low fouling tendency was observed for FO membrane. Overall, the UF-OMBR process was able of simultaneously producing two streams (UF and FO permeate) with different opportunities for water reclamation.

Performance evaluation of fertilizer draw solutions for forward osmosis membrane bioreactor treating domestic wastewater

The reverse solute flux (RSF) of draw solute ions towards the bioreactor is one of the main issues for concern during the forward osmosis membrane bioreactor (FO-MBR). This RSF can be significantly reduced with fertilizer-based draw solutes with anions of relatively large hydrated diameter. The individual performance of three such fertilizer-based draw solutes, ammonium sulfate (SOA), potassium hydrogen phosphate monobasic (MKP) and mono ammonium phosphate (MAP) was investigated in a forward osmosis membrane bioreactor (FO-MBR) integrated with direct contact membrane distillation (DCMD) process. During FO operation, results demonstrated that use of MAP significantly reduced the RSF (0.113 g/m2/hr (gMH)) towards the bioreactor, in contrast with that of SOA (0.568 gMH) and MKP (1.17 gMH). MKP exhibited the shortest filtration run of 12 days because of increased buildup of draw solute inside the bioreactor. On the other hand, SOA showed relatively prolonged filtration runs of 17 days, followed by 15 days for MAP. It was also found that MKP and SOA exhibited inhibitory effects on the mixed liquor characteristics in contrast with MAP in terms of biomass growth, particle size distribution and sludge filterability. Based upon these findings, MAP was found to be the most viable draw solute for the FO-MBR.

Effects of electrochemical processes application on the modification of mixed liquor characteristics of an electro-membrane bioreactor (e-MBR).

This study evaluated the effects of electrochemical processes on the mixed liquor characteristics of an electro-membrane bioreactor (e-MBR) applied to municipal wastewater treatment. A laboratory-scale e-MBR was assessed under two experimental runs: without the electric field (run I) and with electric field, controlled by the application of an electric current set in 10.0 A m-2 under intermittent exposure mode of 6 minutes ON/18 minutes OFF (run II). The electric field caused approximately 55% removal of both soluble microbial products (SMP) and extracellular polymeric substances (EPS), whereas the proteins/carbohydrates ratio in EPS was increased from 1.9 in the run I to 2.9 in run II, leading to an increment of flocs' hydrophobicity. Additionally, the sludge floc size average value was reduced from 42.2 μm in run I to 24.6 μm in run II, which led to a significant enhancement in the sludge settleability. As a result, the membrane fouling rate was always less than 3.80 mbar d-1 in run II, whereas in run I these values reached up to 34.7 mbar d-1. These results demonstrated that the electrochemical processes enhanced the mixed liquor filterability. Therefore, their implementation represents a great alternative to improve the operational stability of membrane bioreactors.

The effects of biocarriers on the mixed liquor characteristics, extracellular polymeric substances, and fouling rates of a hybrid membrane bioreactor

In this research, the influence of biocarriers on the mixed liquor characteristics, extracellular polymeric substances (EPS), and fouling rates of a hybrid membrane bioreactor (HMBR) were examined during greywater reclamation. The addition of the biocarriers increased the relative abundance of filamentous bacteria within the HMBR, which decreased the settleability of the sludge, but did not affect its dewaterability or filterability. Settleability was found to be strongly correlated to the biocarrier filling ratio (R2 = 9.0). The biocarriers appeared to have no effect on the total concentration or protein-to-polysaccharide (PN/PS) ratio of the mixed liquor EPS. However, an analysis of the fouled membranes revealed that the PN/PS ratio of the cake layer EPS was significantly reduced in the presence of the biocarriers. It was also found that the biocarriers reduced the fouling rate of the HMBR and extended its operating time by up to 40% (or 11 days). This rate reduction could not be attributed to any mechanical scouring mechanism, since the biocarriers were prevented from contacting the membrane module during operation.

Insight into the effect of organic and inorganic draw solutes on the flux stability and sludge characteristics in the osmotic membrane bioreactor

In this study, chloride based (CaCl2 and MgCl2) and acetate based (NaOAc and MgOAc) salts in comparison with NaCl were investigated as draw solutions (DS) to evaluate their viability in the osmotic membrane bioreactor (OMBR). Membrane distillation was coupled with an OMBR setup to develop a hybrid OMBR-MD system, for the production of clean water and DS recovery. Results demonstrate that organic DS were able to mitigate the salinity buildup in the bioreactor as compared to inorganic salts. Prolonged filtration runs were observed with MgCl2 and MgOAc in contrast with other draw solutes at the same molar concentration. Significant membrane fouling was observed with NaOAc while rapid flux decline due to increased salinity build-up was witnessed with NaCl and CaCl2. Improved characteristics of mixed liquor in terms of sludge filterability, particle size, and biomass growth along with the degradation of soluble microbial products (SMP) were found with organic DS.

Evaluating oxygen mass transfer parameters for large-scale engineering application of membrane bioreactors

Abstract For lack of relevant investigation in the context of large-scale membrane bioreactors (MBRs), evaluation of the oxygen mass transfer parameters is important for the optimized design of the aeration system in an MBR process. In this study, we sampled the actual sludge from several large-scale MBRs (>10,000 m 3 /d) for the oxygen transfer tests. It was found that specific surface area of bubbles, aeration intensity, and diffuser submerged depth had positive effect on the oxygen transfer coefficient ( k L a ), while some mixed liquor characteristics such as the mixed liquor suspended solids (MLSS) concentration and apparent viscosity exerted negative function on it. We further proposed quantitative expressions to predict the alpha-factor ( α -factor) under typical aeration conditions. The α -factor was found to be above 0.6 when MLSS was roughly less than 12 g/L, which was generally higher than the α -factor predicted in the previous literature, such that overestimation of aeration demand in large-scale MBRs might be avoided.

A study of the osmotic membrane bioreactor process using a sodium chloride solution and an industrial effluent as draw solutions

Osmotic membrane bioreactor (OMBR) is an emerging membrane process which has gained interest in the recent years because of the low energy consumption and the high effluent quality. The osmotic membrane bioreactor combines a forward osmosis (FO) membrane and a biological treatment. However, salt reverse flux is the main problem because of the negative effect of the salt concentration increase in the reactor on the microbial activity. This is the reason why the study of a suitable draw solution (DS) is very important in the overall performance of the reactor. This study compares the process performance using two draw solutions: a 53gL−1 NaCl solution and a real waste water solution (waste water from an absorption column consisting mainly of SO42− and NH4-N with concentrations of 153gL−1 and 19gL−1, respectively). The comparison is focused on the salt reverse flux during the reactor operation, the mixed liquor characteristics, the membrane fouling and the overall performance. The results indicated that the industrial wastewater showed a higher salt reverse flux, but also a less severe fouling and a higher the osmotic pressure difference in comparison with the NaCl solution. In terms of chemical oxygen demand (COD) removal efficiencies, both draw solutions attained values higher than 80%, though the efficiency was slightly lower when the industrial effluent was used as DS. This was related to the higher conductivity reached in the bioreactor when the industrial effluent was used as draw solution. In spite of it, the use of this industrial effluent as draw solution is strongly recommended because of the high permeate fluxes yielded, the low membrane fouling and the lack of necessity of regenerating the draw solution.

Application of powdered activated carbon (PAC) for membrane fouling control in a pilot-scale MBR system.

Membrane fouling is considered to be the most serious drawback in wastewater treatment when using membrane bioreactors (MBRs), leading to membrane permeability decrease and efficiency deterioration. This work aims to develop an integrated methodology for membrane fouling control, using powdered activated carbon (PAC), which will enhance the adsorption of soluble microbial products (SMP) and improve membrane filterability, by altering the mixed liquor's characteristics. Reversible fouling was assessed in terms of sludge filterability measurements, according to the standard time-to-filter (TTF) method, while irreversible fouling was assessed in terms of SMP removal. Results showed that the addition of PAC at the concentration of 3 g/L in the mixed liquor reduced SMP concentration and enhanced substantially the sludge filterability. Furthermore, the TTFPAC/TTFno PAC ratios were lower, than the corresponding SMPPAC./SMPno PAC ratios, indicating that the batch-mode, short-term addition of PAC promotes the reversible, rather than the irreversible fouling mitigation.

Effect of macroporous adsorption resin-membrane bioreactor hybrid system against fouling for municipal wastewater treatment

Membrane bioreactor (MBR) displays significant advantages in effluent quality, sludge production, footprint, and operation. However, membrane fouling limits the application of MBR. This study investigated membrane fouling in a macroporous adsorption resin–membrane bioreactor hybrid system established by adding macroporous adsorption resin (MAR) into MBR. MAR addition increased the critical flux by 27.97%, indicating that membrane fouling was successfully mitigated. Consequently, comparative experiments were designed to analyze the pathway. MAR addition mitigated external fouling development and improved mixed liquor characteristics, thereby mitigating gel layer formation and sludge floc deposition on the membrane surface. MAR effectively reduced the supernatant viscosity and dissolved COD by adsorbing soluble microbial products. Sludge production decreased because the sludge activity in MAR–MBR was inhibited. The fouled MAR could be regenerated effectively by deionized water and chemical cleaning. This work demonstrated the feasibility of using MAR–MBR to mitigate fouling in municipal wastewater treatment.

Effect of mean cell residence time on transmembrane flux, mixed-liquor characteristics and overall performance of a submerged anaerobic membrane bioreactor

ABSTRACTKinetic control of Mean Cell Residence Time (MCRT) was shown to have a significant impact on membrane flux under steady-state conditions. Two laboratory-scale flat-plate submerged anaerobic membrane bioreactors were operated for 245 days on a low-to-intermediate strength substrate with high suspended solids. Transmembrane pressure was maintained at 2.2 kPa throughout four experimental phases, while MCRT in one reactor was progressively reduced. This allowed very accurate measurement of sustainable membrane flux rates at different MCRTs, and hence the degree of membrane fouling. Performance data were gathered on chemical oxygen demand (COD) removal efficiency, and a COD mass balance was constructed accounting for carbon converted into new biomass and that lost in the effluent as dissolved methane. Measurements of growth yield at each MCRT were made, with physical characterisation of each mixed liquor based on capillary suction time. The results showed membrane flux and MLSS filterability was highest at short MCRT, although specific methane production (SMP) was lower since a proportion of COD removal was accounted for by higher biomass yield. There was no advantage in operating at an MCRT <25 days. When considering the most suitable MCRT there is thus a trade-off between membrane performance, SMP and waste sludge yield.

Membrane bioreactor vs. oxidation ditch: full-scale long-term performance related with mixed liquor seasonal characteristics

The relative strengths of membrane bioreactors (MBR) versus conventional activated sludge (CAS) processes have long been debated, but never compared systematically at full scale. To this end, we monitored full-scale wastewater treatment performance and mixed-liquor characteristics (sludge biomass, extracellular polymeric substances (EPS), supernatant with molecular weight and hydrophilic/hydrophobic distribution) for parallel MBR and CAS (via oxidation ditch—OD), for a year. Ammonia and suspended solids were better removed by MBR; temperature affected ammonia removal in OD. At low temperatures, sludge settlement declined in both processes, but solid–liquid separation via membrane filtration continued, although the membrane fouled. EPS characteristics were similar, and organic components in EPS and supernatant varied seasonally, in both processes. MBR had a lower production rate, but higher concentration, of supernatant organics and accumulated large molecular-weight polysaccharides. Ammonia removal was correlated with tightly bound proteinaceous EPS. Polysaccharides, in loosely bound EPS and supernatant, were responsible for effective settlement (OD), and membrane fouling (MBR). These results demonstrate how mixed liquor intermediates process and performance, and that performance depends on effective solid–liquid separation. MBRs excel when stringent requirements for pollutant-removal and extreme environmental conditions exist. The much cheaper OD remains competitive under less demanding conditions.

Treatment of hypersaline produced water employing a moderately halophilic bacterial consortium in a membrane bioreactor: effect of salt concentration on organic removal performance, mixed liquor characteristics and membrane fouling.

In this study the organic pollutant removal performance and the mixed liquor characteristics of a membrane bioreactor (MBR), employing a moderately halophilic bacterial consortium, for the treatment of hypersaline synthetic produced water containing 100-250 g L(-1) NaCl were considered. The COD and oil and grease (O&G) removal efficiencies in the range 81.6-94.6% and 84.8-94.0% respectively and MBR effluent turbidity lower than 2NTU were achieved. There was no pronounced membrane fouling at any salt concentration. O&G accumulation (less than 11% of the influent O&G) occurred in the mixed liquor at all salt concentrations, but biodegradation was identified as the major organic removal mechanism. With increasing salt concentration, initially increase in SVI and later formation of oil/biomass bodies took place but due to the presence of the membrane biomass washout did not occur. The mixed liquor was pseudoplastic and the apparent viscosity and flow behavior index generally increased with salt concentration.