Fouling in membrane bioreactors: An updated review.

The goal of the current article is to update new findings in membrane fouling and emerging fouling mitigation strategies reported in recent years (post 2010) as a follow-up to our previous review published in Water Research (2009). According to a systematic review of the literature, membrane bioreactors (MBRs) are still actively investigated in the field of wastewater treatment. Notably, membrane fouling remains the most challenging issue in MBR operation and attracts considerable attention in MBR studies. In this review, we summarized the updated information on foulants composition and characteristics in MBRs, which greatly improves our understanding of fouling mechanisms. Furthermore, the emerging fouling control strategies (e.g., mechanically assisted aeration scouring, in-situ chemical cleaning, enzymatic and bacterial degradation of foulants, electrically assisted fouling mitigation, and nanomaterial-based membranes) are comprehensively reviewed. As a result, it is found that the fundamental understanding of dynamic changes in membrane foulants during a long-term operation is essential for the development and implementation of fouling control methods. Recently developed strategies for membrane fouling control denoted that the improvement of membrane performance is not our ultimate and only goal, less energy consumption and more green/sustainable fouling control ways are more promising to be developed and thus applied in the future. Overall, such a literature review not only demonstrates current challenges and research needs for scientists working in the area of MBR technologies, but also can provide more useful recommendations for industrial communities based on the related application cases.

Cultivation of four microalgae species in the effluent of anaerobic digester for biodiesel production.

This study investigated if an effluent from anaerobic digestion (AD) system can be used as a nutrients source for the microalgae cultivation, and in so doing, if the effluent can be properly treated. Nitrogen and phosphorus in the AD effluent well supported microalgal growth, and their removal efficiency reached >97.9% and 99.2%, respectively. Among four different algal species tested, Micractinium inermum particularly stood out, showing the highest biomass and FAME productivity: 0.16gL-1d-1 with 3.23gL-1 of dry cell weight, and 0.04gL-1d-1 with 27.54% (w/w) of FAME contents, respectively. As the concentrations of the nutrients decreased over time, the FAME contents were increased and its quality as well, satisfying several biodiesel quality standards. This study supports that the AD effluent can indeed serve as a cheap and nutrient-rich medium for microalgae cultivation, and equally importantly, microalgae can be a workable treatment option for it.

Producing desulfurized biogas through removal of sulfate in the first-stage of a two-stage anaerobic digestion.

In the present work, a two-stage anaerobic digestion system (TSADS) was newly designed to produce biogas with a greatly reduced H2 S content. The role of first (sulfidogenic)-stage digester was not only acidogenesis but also sulfidogenesis (sulfate reduction to H2 S), which would minimize the input of H2 S-producing source in the followed second (methanogenic)-stage digester. For the coexistence of acidogens and sulfate reducing bacteria (SRB) in the sulfidogenic-stage digester, it was found that pH played a crucial role. The acidogenic activity was not affected within a pH range of 4.5-6.0, while it was important to maintain a pH at 5.5 to achieve a sulfate removal efficiency over 70%. The highest sulfate removal attained was 78% at a hydraulic retention time (HRT) of 5 h at pH 5.5 ± 0.1. The H2 S content in the biogas produced in the conventional single-stage digester (SSAD), used as a control, reached 1,650 ± 25 ppmv . In contrast, the biogas produced in the methanogenic-stage digester of the developed process had an H2 S content of 200 ± 15 ppmv . Microbial analysis, done by the next generation sequencing technique, clearly showed the changes in community under different operating conditions. Desulfovibrio bastinii (4.9%) played a key role in sulfate removal in the sulfidogenic-stage of the TSADS owing to its characteristics of a short doubling time and growth in an acidic environment. Biotechnol. Bioeng. 2017;114: 970-979. © 2016 Wiley Periodicals, Inc.

Mitigation of ammonia inhibition by internal dilution in high-rate anaerobic digestion of food waste leachate and evidences of microbial community response.

A high-rate anaerobic digestion of food waste leachate were tested using intermittent continuously stirred tank reactors (iCSTRs) to evaluate how severe ammonia inhibition could be mitigated with internal dilution strategy, and to identify how bacterial and archaeal community respond in genus and species level. Experimental results show that the digestion performance was well maintained up to hydraulic retention time (HRT) of 40 days but could not keep steady-state as HRT decreased to 30 days due to severe free ammonia (FA) inhibition. Coupling internal dilution was the key to relieve the inhibition since it reduced FA concentration as low as 62 mg/L even at HRT 30 days, which corresponds to organic loading rate of 5 g COD/L/d, demonstrating CH4 yield of 0.32 L CH4 /g CODadded . It was confirmed that the dilution offers iCTSRs manage severe ammonia inhibition with the balanced community structure between bacteria and archaea in this high-rate anaerobic digestion. Genus and species level pyrosequencing evidence that FA inhibition to community dynamics of Methanosarcina and Methanosaeta is strongly connected to methanogenesis, and Methanosarcina plays a key role in an iCSTR with the dilution. Biotechnol. Bioeng. 2016;113: 1892-1901. © 2016 Wiley Periodicals, Inc.

Inhibition of residual n-hexane in anaerobic digestion of lipid-extracted microalgal wastes and microbial community shift.

Converting lipid-extracted microalgal wastes to methane (CH4) via anaerobic digestion (AD) has the potential to make microalgae-based biodiesel platform more sustainable. However, it is apparent that remaining n-hexane (C6H14) from lipid extraction could inhibit metabolic pathway of methanogens. To test an inhibitory influence of residual n-hexane, this study conducted a series of batch AD by mixing lipid-extracted Chlorella vulgaris with a wide range of n-hexane concentration (∼10 g chemical oxygen demand (COD)/L). Experimental results show that the inhibition of n-hexane on CH4 yield was negligible up to 2 g COD/L and inhibition to methanogenesis became significant when it was higher than 4 g COD/L based on quantitative mass balance. Inhibition threshold was about 4 g COD/L of n-hexane. Analytical result of microbial community profile revealed that dominance of alkane-degrading sulfate-reducing bacteria (SRB) and syntrophic bacteria increased, while that of methanogens sharply dropped as n-hexane concentration increased. These findings offer a useful guideline of threshold n-hexane concentration and microbial community shift for the AD of lipid-extracted microalgal wastes.

Harnessing dark fermentative hydrogen from pretreated mixture of food waste and sewage sludge under sequencing batch mode.

Food waste and sewage sludge are the most abundant and problematic organic wastes in any society. Mixture of these two wastes may provide appropriate substrate condition for dark fermentative biohydrogen production based on synergistic mutual benefits. This work evaluates continuous hydrogen production from the cosubstrate of food waste and sewage sludge to verify mechanisms of performance improvement in anaerobic sequencing batch reactors. Volatile solid concentration and mixing ratio of food waste and sludge were adjusted to 5 % and 80:20, respectively. Five different hydraulic retention times (HRT) of 36, 42, 48, 72, and 108 h were tested using anaerobic sequencing batch reactors to find out optimal operating condition. Results show that the best performance was achieved at HRT 72 h, where the hydrogen yield, the hydrogen production rate, and hydrogen content were 62.0 mL H2/g VS, 1.0 L H2/L/day, and ~50 %, respectively. Sufficient solid retention time (143 h) and proper loading rate (8.2 g COD/L/day as carbohydrate) at HRT 72h led to the enhanced performance with better hydrogen production showing appropriate n-butyrate/acetate (B/A) ratio of 2.6. Analytical result of terminal-restriction fragment length polymorphism revealed that specific peaks associated with Clostridium sp. and Bacillus sp. were strongly related to enhanced hydrogen production from the cosubstrate of food waste and sewage sludge.

Scenedesmus-based treatment of nitrogen and phosphorus from effluent of anaerobic digester and bio-oil production.

In this study, a microalgae-based technology was employed to treat wastewater and produce biodiesel at the same time. A local isolate Scenedesmus sp. was found to be a well suited species, particularly for an effluent from anaerobic digester (AD) containing low carbon but high nutrients (NH3-N=273mgL(-1), total P=58.75mgL(-1)). This algae-based treatment was quite effective: nutrient removal efficiencies were over 99.19% for nitrogen and 98.01% for phosphorus. Regarding the biodiesel production, FAME contents of Scenedesmus sp. were found to be relatively low (8.74% (w/w)), but overall FAME productivity was comparatively high (0.03gL(-1)d(-1)) due to its high biomass productivity (0.37gL(-1)d(-1)). FAMEs were satisfactory to the several standards for the biodiesel quality. The Scenedesmus-based technology may serve as a promising option for the treatment of nutrient-rich wastewater and especially so for the AD effluent.

Inhibition of nitrate reduction by NaCl adsorption on a nano-zero-valent iron surface during a concentrate treatment for water reuse.

Nanoscale zero-valent iron (NZVI) has been considered as a possible material to treat water and wastewater. However, it is necessary to verify the effect of the matrix components in different types of target water. In this study, different effects depending on the sodium chloride (NaCl) concentration on reductions of nitrates and on the characteristics of NZVI were investigated. Although NaCl is known as a promoter of iron corrosion, a high concentration of NaCl (>3 g/L) has a significant inhibition effect on the degree of NZVI reactivity towards nitrate. The experimental results were interpreted by a Langmuir-Hinshelwood-Hougen-Watson reaction in terms of inhibition, and the decreased NZVI reactivity could be explained by the increase in the inhibition constant. As a result of a chloride concentration analysis, it was verified that 7.7-26.5% of chloride was adsorbed onto the surface of NZVI. Moreover, the change of the iron corrosion product under different NaCl concentrations was investigated by a surface analysis of spent NZVI. Magnetite was the main product, with a low NaCl concentration (0.5 g/L), whereas amorphous iron hydroxide was observed at a high concentration (12 g/L). Though the surface was changed to permeable iron hydroxide, the Fe(0) in the core was not completely oxidized. Therefore, the inhibition effect of NaCl could be explained as the competitive adsorption of chloride and nitrate.

Application of a novel enzymatic pretreatment using crude hydrolytic extracellular enzyme solution to microalgal biomass for dark fermentative hydrogen production.

In this study, a novel enzymatic pretreatment of Chlorella vulgaris for dark fermentative hydrogen production (DFHP) was performed using crude hydrolytic extracellular enzyme solution (CHEES) extracted from the H2 fermented effluent of food waste. It was found that the enzyme extracted at 52 h had the highest hydrolysis efficiency of microalgal biomass, resulting in the highest H2 yield of 43.1 mL H2/g dry cell weight along with shorter lag periods. Even though a high amount of VFAs was accumulated in CHEES, especially butyrate, the fermentative bacteria on the DFHP was not affected from product inhibition. It also appears that the presence of organic acids, especially lactate and acetate, contained in the CHEES facilitated enhancement of H2 production acted as a co-substrate. Therefore, all of the experimental results suggest that the enhancement of DFHP performance caused by CHEES has a dual role as the hydrolysis enhancer and the co-substrate supplier.

Different susceptibilities of bacterial community to silver nanoparticles in wastewater treatment systems.

The release of silver (Ag) nanoparticles (NPs) into sewage streams has heightened concerns about potential adverse impacts on wastewater treatment processes. Here, we show that the rate constants of both biological nitrification and organic oxidation decreased exponentially with an increase in the Ag NP concentration, but nitrification was more severely inhibited than the organic oxidation even at low Ag NP concentrations (<1 mg Ag L(-1)) in batch experiments. The long-term exposure effects of Ag NPs on activated sludge bacteria were evaluated in sequencing batch reactors (SBRs) fed with two different substrates favoring heterotrophic and autotrophic bacteria. From a continuous operation for 50 days, it was found that heterotrophic bacteria in the organic removal process have higher tolerance to Ag NPs than do nitrifying bacteria. The effects of Ag NPs on the microbial community in both SBRs were analyzed using 16S ribosomal ribonucleic acid (rRNA) gene sequences obtained from pyrosequencing. The results showed that the level of microbial susceptibility is different for each type of microorganism and that the microbial diversity decreased dramatically after continuous exposure to Ag NPs for 50 days, resulting in a decrease of wastewater treatment efficiency.

Deactivation of nanoscale zero-valent iron by humic acid and by retention in water.

The effects of the deactivation of nanoscale zero-valent iron (NZVI), induced by humic acid (HA) and by the retention of NZVI in water, on nitrate reduction were investigated using a kinetic study. Both the nitrate removal and generation of ammonia were significantly inhibited as the HA adsorption amount and retention time were increased. However, HA removal was greatly enhanced when the NZVI was used after 1 d or 25 d of retention in water. The results are caused by the formation of iron oxides/hydroxides, which increased the specific surface area and the degree of NZVI aggregation which was observed by transmission electron microscopy (TEM). However, the nitrate reduction was greater at the beginning of reaction in the presence of HA when fresh NZVI was used, because of the enhanced electron transfer by the HA in bulk phase and on NZVI surface as train sequences. The pseudo second order adsorption kinetic equation incorporating deactivation and a Langmuir-Hinshelwood (LH) type kinetic equation provided accurate descriptions of the nitrate removal and ammonia generation, respectively. The deactivation constant and the reaction rate constant of the LH type kinetic equation were strongly correlated with the HA amount accumulated on NZVI. These results suggest that the HA accumulation on the NZVI surface reactive sites plays the dominant role in the inhibition and the inhibition can be described successfully using the deactivation model. The HA accumulation on NZVI was verified using TEM.

Effect of temperature on continuous fermentative hydrogen production from Laminaria japonica by anaerobic mixed cultures.

The temperature effect on continuous dark fermentative hydrogen production from non-pretreated Laminaria japonica was investigated in the present study. In a preliminary step, the fermentors were continuously operated as an inoculation process at three different temperatures, 35, 50 and 65°C, to respectively represent mesophilic, thermophilic, and hyperthermophilic conditions. An optimization process was subsequently conducted with a range of organic loading rate (OLR) and cultivation pH. Among the various operation conditions, the maximum H2 yield, 61.3±2.0 mL H2/g TS, was observed under a mesophilic condition at OLR of 3.4 g COD/L/d and pH 5.5. From a PCR-DGGE analysis, it was found that an increase of temperature can reduce the microbial diversity and change the predominant species. Finally, total cellulase activity was measured, to investigate the effect of temperature on hydrolysis of L. japonica. The highest cellulase activity was 0.19±0.02 FPU/mL, observed at 35°C, coinciding with the maximum H2 yield.

Conversion of organic solid waste to hydrogen and methane by two-stage fermentation system with reuse of methane fermenter effluent as diluting water in hydrogen fermentation.

In this study, a two-stage system converting organic solid waste (food waste+sewage sludge) to H2 and CH4 was operated. In the first stage of dark fermentative hydrogen production (DFHP), a recently proposed method that does not require external inoculum, was applied. In the second stage, anaerobic sequencing batch reactor (ASBR) and an up-flow anaerobic sludge blanket reactor (UASBr) were followed to treat H2 fermenter effluent. (H2+CH4-ASBR) system showed better performance in terms of total biogas conversion (78.6%), while higher biogas production rate (2.03 L H2/Lsystem/d, 1.96 L CH4/Lsystem/d) was achieved in (H2+CH4-UASBr) system. To reduce the alkali addition requirement in DFHP process, CH4 fermenter effluent was tested as a diluting water. Both the ASBR and UASBr effluent was effective to keep the pH above 6 without CH4 production. In case of using ASBR effluent, H2 production dropped by 15%, but alkali addition requirement was reduced by 50%.

Application of an electric field for pretreatment of a seeding source for dark fermentative hydrogen production.

In present study, an electric field was newly adopted as a pretreatment method for inoculum preparation in dark fermentative hydrogen production. Various voltages (5-20 V for 10 min) were applied, and the feasibility and efficiency of this method were compared with those of heat pretreatment (90°C for 20 min). Both the highest H2 yield, 1.43 mol H2/mol hexoseadded, and the highest production rate, 101.4 mL H2/L/h, were observed at 10 V. While RNA concentration of above 100mg/L was maintained up to 10 V, it was decreased at an applied voltage of 20 V, where the worst performance was observed. Microbial analysis results confirmed that only H2 producing bacteria were detected with electric pretreatment, while non-H2 producing bacteria coexist with heat and electric (5 V) pretreatment. The results suggested that application of an electric field has reasonable potential as an alternative method for preparing inoculums for hydrogen production.

Optimization of dark fermentative H2 production from microalgal biomass by combined (acid + ultrasonic) pretreatment.

In this study, individual pretreatments (acid and ultrasonic) and a combination of these pretreatments were optimized to enhance the efficiency of dark fermentative hydrogen production (DFHP) from microalgal biomass. The experimental results show that the maximum H2 production performance of 42.1 mL H2/g dry cell weight (dcw) was predicted at 0.79% (v/w) HCl and at a specific energy input (SEI) of 49,600 kJ/kg dcw in the combined pretreatment, while it was limited in both individual pretreatments. Repeated batch testing of the predicted optimal conditions revealed that the combined pretreatment conditions for DFHP from microalgal biomass were successfully optimized by increasing the solubilization of the feedstock and by reducing the formation of the toxic 5-hydroxymethylfurfural (HMF).

Low strength ultrasonication positively affects the methanogenic granules toward higher AD performance. Part I: physico-chemical characteristics.

To elucidate the correlation between enhanced biogas production and changed physico-chemical properties of methanogenic granules after low strength ultrasonication, in this study, the effects of low strength ultrasonication on the settling velocity, permeability, porosity, and fluid collection efficiency of the methanogenic granules were investigated. In addition, their morphological changes were visualized using a scanning electron microscopic technique. The experimental results indicate that low strength ultrasonication increased both the permeability (37%) and specific surface area (230%) of the granules through the generation of greater craters and cracks on the granular surface compared to the control granules. The penetration of nutrients and substrate into the granules was thereby enhanced, and more favorable conditions for achieving higher anaerobic performance were provided to the ultrasonicated granules. The microbial community shift caused by the changed physico-chemical properties of the methanogenic granules will be further analyzed in part II of this study.

Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester.

In this study, the positive effects of low-strength ultrasonication (LS-ultrasonication) on the anaerobic digestion (AD) performance were investigated by continuously operating an upflow anaerobic sludge blanket reactor (UASBr) and a dry digester. In the ultrasonicated UASBr system (1 s per min, 0.05 W/mL), ultrasonication enhanced the CH4 production by 38% and 19% in an ambient and a mesophilic condition, respectively. In addition, a different sludge yield and a changed electron flow were observed after ultrasonication. In the ultrasonicated dry digestion system (2 s per 30 s, 0.0025 W/mL), a 40% increase in the production of CH4 was observed after lowering the total solid content of the reactor from 12% to 10%, implying that a high solid content diminished the ultrasonic stimulation effect. Moreover, the ultrasonication strength itself appeared to be a more crucial factor than the ultrasonication density during the application of LS-ultrasonication in the AD system.

Dry anaerobic digestion of food waste under mesophilic conditions: performance and methanogenic community analysis.

The performance of dry anaerobic digestion (AD) of food waste was investigated under mesophilic conditions and the methanogenic community was investigated using 454 pyrosequencing. Stable dry AD was achieved by hydraulic retention time (HRT) control without the addition of alkali agents. The average CH4 production rate, CH4 content, and volatile solid reduction rate were 2.51±0.17m(3)/m(3)/d, 66±2.1%, and 65.8±1.22%, respectively, at an HRT of 40d. The methanogenic community of the seed sludge experienced a significant reduction in genus diversity from 18 to 4 and a dominant methanogenic shift from hydrogenotrophic to acetoclastic groups after the acclimation under dry condition. Almost all sequences of the dry anaerobic digester were closely related with those of Methanosarcina thermophila with similarity of 96.4-99.1%. The experimental results would serve as useful information to understand the dry AD system.

Effects on nano zero-valent iron reactivity of interactions between hardness, alkalinity, and natural organic matter in reverse osmosis concentrate.

Nanoscale zero-valent iron (NZVI) is considered to have potential to reduce nitrate in the concentrate generated by high pressure membrane processes aimed at water reuse. However, it is necessary to verify the effect of the matrix components in the concentrates on NZVI reactivity. In this study, the influence of hardness, alkalinity, and organic matter on NZVI reactivity was evaluated by the response surface method (RSM). Hardness (Ca2+) had a positive effect on NZVI reactivity by accelerating iron corrosion. In contrast, alkalinity (bicarbonate; HCO-3) and organic matter (humic acid; HA) had negative effects on NZVI reactivity due to morphological change to carbonate green rust, and to competitive adsorption of HA, respectively. The validity of the statistical prediction model derived from RSM was confirmed by an additional confirmation experiment, and the experimental result was within the 95% confidential interval. Therefore, it can be indicated that the RSM model produced results that were statistically significant.

Waste activated sludge hydrolysis during ultrasonication: two-step disintegration.

In order to clearly describe the hydrolysis of waste activated sludge (WAS) during ultrasonication by a 2-step disintegration process, concentrations of ribonucleic acid (RNA) and bound extracellular polymeric substance (EPS) were measured. Apparently, different decreasing patterns of RNA and EPS concentrations during WAS hydrolysis made it possible to distinguish the floc disintegration (FD) and cell lysis (CL). Initially, FD and CL appear to take simultaneously, but the dominant hydrolytic process is shifted from FD to CL after 10 min of ultrasonication. Additional kinetic analysis of WAS hydrolysis was also conducted. A five-fold greater hydrolysis rate constant of FD relative to that of CL was observed, reflecting the different strengths of floc and cells. Therefore, different rates of increased solubilization during WAS hydrolysis appear to account for the initial disintegration of the rather loose part (sludge floc) and the subsequent disintegration of the rigid part (microbial cells).