In situ observation of the growth of biofouling layer in osmotic membrane bioreactors by multiple fluorescence labeling and confocal laser scanning microscopy

Alleviation of RO membrane fouling in wastewater reclamation plants using an enhanced acid-base chemical cleaning method

Membrane fouling has always been a critical constraint in the operation of the reverse osmosis (RO) process, and chemical cleaning is essential for mitigating membrane fouling and ensuring smooth operation of the membrane system. This paper presents an optimized chemical cleaning method for the efficient cleaning of RO membranes in full-scale applications. Compared to the regular cleaning method (cleaning with 0.1 % NaOH + 1 % ethylenediaminetetraacetic acid + 0.025 % sodium dodecyl benzene sulfonate followed by 0.2 % HCl), the optimized cleaning method improves the cleaning efficiency by adding sodium chloride to the alkaline cleaning solution and citric acid to the acid cleaning solution. Notably, the membrane flux recovery rate with the optimized cleaning method is 45.74 %, and it improves the cleaning efficiency by 1.65 times compared to the regular cleaning method. Additionally, the optimized cleaning method removes 30.46 % of total foulants (organic and inorganic), which is 2.11 times higher than the regular cleaning method. The removal of inorganic ions such as Fe, Ca, and Mg is significantly improved with the optimized cleaning method. For organic matter removal, the optimized cleaning method effectively removes more polysaccharides, proteins, and microbial metabolites by disrupting the complex structures of organic matter. Furthermore, it also changes the microbial community structure on the RO membrane surface by eliminating microorganisms that cannot withstand strong acids, bases, and high salt environments. However, Mycobacterium can adapt to these harsh conditions, showing a relative abundance of up to 84.13 % after cleaning. Overall, our results provide a new chemical cleaning method for RO membranes in full-scale applications. This method effectively removes membrane foulants and enhances the understanding of the removal characteristics of foulants on RO membrane surfaces by chemical cleaning.

Improved permeability in ceramsite@powdered activated carbon (PAC)-MnOx coupled gravity-driven ceramic membrane (GDCM) for manganese and ammonia nitrogen removal with intermittent short-term vertical aeration

In our work, a gravity-driven ceramic membrane bioreactor (GDCMBR) was developed to remove Mn2+ and NH3-N simultaneously through the birnessite water purification layer in-situ construction on the ceramic membrane due to chemical pre-oxidation (powdered activated carbon (PAC)-MnOx). Considering the trade-off of biofouling and water production, the daily intermittent short-term vertical aeration mode was involving to balance this contradiction with the excellent water purification and improved membrane permeability. And the GDCMBR permeability of operation flux was improved for 5-7 LHM with intermittent short-term vertical aeration. Furthermore, only ∼7 % irreversible membrane resistance (Rir) also confirmed the improved membrane permeability with intermittent short-term vertical aeration. And some manganese oxidizing bacteria (MnOB) and ammonia oxidizing bacteria (AOB) species at genus level were identified during long-term operation with the contact circulating flowing raw water, resulting in the better Mn2+ and NH3-N removal efficiency. Additionally, the nano-flower-like birnessite water purification layer was verified in ceramsite@PAC-MnOx coupled GDCMBR, which evolute into a porous flake-like structure with the increasing intermittent short-term aeration duration. Therefore, the sustainable and effective intermittent short-term aeration mode in ceramsite@PAC-MnOx coupled GDCMBR could improve the membrane permeability with the satisfactory groundwater purification efficiency, as well as providing an energy-efficient strategy for membrane technologies applications in water supply safety.

Anaerobic acidification membrane bioreactor operating at acidic condition for treating concentrated municipal wastewater: Performance and implication

To address the low-carbon treatment requirements for municipal wastewater, a novel anaerobic acidification membrane bioreactor (AAMBR) was developed for recovering organic matter in terms of volatile fatty acids (VFAs). While the AAMBR successfully generated VFAs from municipal wastewater through forward osmosis (FO) membrane concentration, its operation was limited to a single pH value of 10.0. Here, performance of the AAMBR operating at acidic condition was evaluated and compared with that at alkaline condition. The findings revealed that the AAMBR with pH 5.0 efficiently transformed organic matter into acetic acid, propionic acid, and butyric acid, resulting in a VFAs yield of 0.48 g/g-CODfeed. In comparison with the AAMBR at pH 10.0, this study achieved a similar VFAs yield, a lower fouling tendency, a lower loss of nutrients and a lower controlling cost. In conclusion, this study demonstrated that a pH of 5.0 is optimal for the AAMBR treating municipal wastewater.

Integrating anaerobic acidification with two-stage forward osmosis concentration for simultaneously recovering organic matter, nitrogen and phosphorus from municipal wastewater

In order to meet the demand of municipal wastewater for low-carbon treatment and resource recovery, a novel process of anaerobic acidification membrane bioreactor (AAMBR) assisted with a two-stage forward osmosis (FO) (FO-AAMBR-FO) was developed for simultaneously recovering organic matter and nutrients from municipal wastewater. The results indicated that the first FO process concentrated the municipal wastewater to one tenth of the initial volume. The corresponding chemical oxygen demand (COD), ammonia nitrogen (NH4+-N) and total phosphorus (TP) concentration reached up to 2800, 200 and 33 mg/L, respectively. Subsequently, the AAMBR was operated at pH value of 10 for treating the concentration of municipal wastewater, in which the organic matter was successfully converted to acetic acid and propionic acid with a total volatile fatty acids (VFAs) concentration of 1787 mg COD/L and a VFAs production efficiency of 62.36 % during 47 days of stable operation. After that, the NH4+-N and TP concentration in the effluent of the AAMBR were further concentrated to 175 and 36.7 mg/L, respectively, by the second FO process. The struvite was successfully recovered with NH4+-N and TP recovery rate of 94.53 % and 98.59 %, respectively. Correspondingly, the VFAs, NH4+-N and TP concentrations in the residual solution were 2905 mg COD/L, 11.8 and 7.92 mg/L, respectively, which could be used as the raw material for the synthesis of polyhydroxyalkanoate (PHA). Results reported here demonstrated that the FO-AAMBR-FO is a promising wastewater treatment technology for simultaneous recovery of organic matter (in form of VFAs) and nutrients (in form of struvite).

Mechanistic study on the ferric chloride-based rapid cultivation and enhancement of aerobic granular sludge

Aerobic granular sludge (AGS) can achieve simultaneous carbon, nitrogen and phosphorus removal owing to its three-dimensional oxygen gradient structure. However, long start-up period and poor operational stability restrict its application and promotion. A novel rapid granulation strategy, viz., the short-term (7 days) addition of ferric chloride at the commissioning stage, was developed and verified in this study. The granulation period was shortened by 9 days, and the formed granules were compact and dense with an Fe3+ concentration of 250 mg L-1. The addition of flocculant not only maintained a high sludge concentration during the initial stages of granulation (5.3 g L-1), but also stimulated the secretion of TB-EPS and increased protein and polysaccharide contents, thereby expediting granule formation. Additionally, ferric chloride induced a diverse microbial community in granules, resulting in the emergence of new genera, such as Thaurea, Brevundimonas and Kinneretia, which improved pollutant removal performance and flocculent aggregation. The removal efficiencies of COD, PO43--P, and NH4+-N stabilized at 94.2, 62.4, and 71.3%, respectively. Therefore, it has been demonstrated that short-term ferric chloride dosing has a synergistic effect on aerobic granulation.

In situ surface modification of forward osmosis membrane by polydopamine/polyethyleneimine-silver nanoparticle for anti-fouling improvement in municipal wastewater treatment

In this work, we demonstrate the surface functionalization of cellulose triacetate membrane with co-deposition of polydopamine (PDA)/polyethyleneimine (PEI) and silver nanoparticles (AgNPs) for antifouling property in municipal wastewater treatment. PDA/PEI was first coated on the membrane surface by single-step co-deposition, while AgNPs were formed in situ through catechol groups of PDA immobilizing silver ions and subsequently reducing. The successful surface modification was verified by different membrane characterization techniques. The modified PDA/PEI-nAg CTA membrane exhibits enhanced hydrophilicity and improved antiadhesion and antimicrobial activity. Furthermore, the functional layer had an indistinctive effect on the membrane transport parameters. In addition, dynamic forward osmosis (FO) fouling experiment with raw municipal wastewater as feed solution indicated that the PDA/PEI-nAg CTA membrane exhibited notably lower water flux decrease compared to the nascent CTA membrane. The results of confocal laser scanning microscopy (CLSM) showed that PDA/PEI-nAg CTA membranes effectively reduced the adsorption of organic foulants (proteins and polysaccharides) and inhibited the formation and development of the fouling layer. The membrane surface modification of the CTA membrane with PDA/PEI and AgNPs efficiently mitigated membrane fouling in municipal wastewater treatment.

New insights into membrane fouling during direct membrane filtration of municipal wastewater and fouling control with mechanical strategies

Direct membrane filtration (DMF) technology achieves energy self-sufficiency through carbon recovery and utilization from municipal wastewater. To control its severe membrane fouling and improve DMF technology, targeted research on fouling behaviour and mechanisms is essential. In this study, a DMF reactor equipped with a flat-sheet ceramic membrane was conducted under three scenarios: without control, with intermittent aeration, and with periodic backwash. This system achieved efficient carbon concentration with chemical oxygen demand below 50 mg/L in permeate. Membrane fouling was dominated by intermediate blocking and cake filtration. And reversible external resistance accounted for over 85 % of total resistance. Predominant membrane foulants were free proteins, whose deposition underlies the attachment of cells and biopolymers. Backwash decreased the fouling rate and increased fouling layer porosity by indiscriminately detaching foulants from the membrane surface. While aeration enhanced the back transport of large particles and microbial activity, causing a relatively thin and dense fouling layer containing more microorganisms and β-d-glucopyranose polysaccharides, which implies a higher biofouling potential during long-term operation. In addition, aeration combined with backwash enhanced fouling control fivefold over either one alone. Therefore, simultaneous operation of backwash and other mechanical methods that can provide shear without stimulating aerobic microbial activity is a preferred strategy for minimizing membrane fouling during DMF of municipal wastewater.

Isolation and Staining Reveal the Presence of Extracellular DNA in Marine Gel Particles

Marine gel particles (MGP) are amorphous hydrogel exudates from bacteria and microalgae that are ubiquitous in the oceans, but their biochemical composition and function are poorly understood. While dynamic ecological interactions between marine microorganisms and MGPs may result in the secretion and mixing of bacterial extracellular polymeric substances (EPS) such as nucleic acids, compositional studies currently are limited to the identification of acidic polysaccharides and proteins in transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP). Previous studies targeted MGPs isolated by filtration. We developed a new way of isolating MGPs from seawater in liquid suspension and applied it to identify extracellular DNA (eDNA) in North Sea surface seawater. Seawater was filtered onto polycarbonate (PC) filters with gentle vacuum filtration, and then the filtered particles were gently resuspended in a smaller volume of sterile seawater. The resulting MGPs ranged in size from 0.4 to 100 µm in diameter. eDNA was detected by fluorescent microscopy using YOYO-1 (for eDNA), with Nile red (targeting cell membranes) as a counterstain. TOTO-3 was also used to stain eDNA, with ConA to localise glycoproteins and SYTO-9 for the live/dead staining of cells. Confocal laser scanning microscopy (CLSM) revealed the presence of proteins and polysaccharides. We found eDNA to be universally associated with MGPs. To further elucidate the role of eDNA, we established a model experimental MGP system using bacterial EPS from Pseudoalteromonas atlantica that also contained eDNA. Our results clearly demonstrate the occurrence of eDNA in MGPs, and should aid furthering our understanding of the micro-scale dynamics and fate of MGPs that underly the large-scale processes of carbon cycling and sedimentation in the ocean.

Effects of voltage and pressure on sludge electro-dewatering process and the dewatering mechanisms investigation

Electro-dewatering technology shows a good application prospect because of its high efficiency in removing water from sludge and low energy consumption, but the potential mechanisms of sludge electro-dewatering have not been investigated in depth, which seriously limits the further development and application of electro-dewatering technology. In this study, the effects of voltage and pressure on sludge electro-dewatering performance, physicochemical characteristics and extracellular polymeric substances (EPS) compositions and distributions were investigated. The spatial distributions of EPS main components, including polysaccharide (PS) and protein (PN), were characterized by a confocal laser scanning microscopy (CLSM). The experimental results showed that under the conditions of a voltage of 40 V and a pressure of 90 kPa, the moisture content of sludge was reduced from 83.15% to 53.12%, and the bound water content of sludge in the anode layer, middle layer and cathode layer were decreased significantly from 1.16 g/g dry solid (DS) to 0.20, 0.47 and 0.35 g/g DS, respectively. The PN content of EPS in anode layer was significantly lower than that in cathode layer due to the electrochemical oxidation, while the variation of PS content showed the opposite trend, which agreed with the results visualized by CLSM. Pearson's correlation coefficient and hierarchical cluster analysis revealed that PN in TB-EPS was the major factor influencing the effect of sludge electro-dewatering. This work can be helpful to understand the potential mechanisms of electro-dewatering and provide theoretical support for the further popularization and application of electro-dewatering technology.

Correlation of structural extracellular polymeric substances in the mesh biofilms with solids retention time and biofilm hydraulic resistance in dynamic membrane bioreactors

Dynamic membrane bioreactor (DMBR), which mainly relied on the in-situ formed biofilms on support materials with large aperture (e.g., nylon mesh) to separate fine particles in wastewater, has attracted a lot of attentions due to low cost. The filtration performance of DMBR is mainly determined by the structure and hydraulic resistance of biofilms formed on the mesh. Therefore, understanding the correlation of operation conditions with mesh biofilm compositions and permeability are critically important for optimizing DMBR operation. In present study, how structural extracellular polymeric substances, including alginate-like extracellular polysaccharide (ALE) and amyloid-like protein (AP), in mesh biofilms correlate to solids retention time (SRT) and biofilm structures was explored in DMBRs. At 5d-SRT, compact and gel-like mesh biofilms were formed with a high specific filtration resistance (SFR) of 459 × 10⁹ m/g, while at 40d-SRT porous mesh biofilms were developed with a low SFR of 24 × 10⁹ m/g. Consequently, the 5d-SRT MBR experienced more rapid rise in transmembrane pressure. Further studies found that the 5d-SRT mesh biofilms had a higher AP content, which was positively correlated to biofilm hydraulic resistance. On the contrary, the 40d-SRT mesh biofilms contained a higher content of ALE, suggesting that ALE was negatively correlated to biofilm hydraulic resistance. Therefore, AP instead of ALE likely played a more important role in the formation of compact and gel-like mesh biofilms.

Engineering pressure retarded osmosis membrane bioreactor (PRO-MBR) for simultaneous water and energy recovery from municipal wastewater

Osmotic membrane bioreactors (OMBR) have gained increasing interest in wastewater treatment and reclamation due to their high product water quality and fouling resistance. However, high energy consumption (mostly by draw solution recovery) restricted the wider application of OMBR. Herein, we propose a novel pressure retarded osmosis membrane bioreactor (PRO-MBR) for improving the economic feasibility. In comparison with conventional FO-MBR, PRO-MBR exhibited similar excellent contaminants removal performance and comparable water flux. More importantly, a considerable amount of energy can be recovered by PRO-MBR (4.1 kWh/100 m²·d), as a result of which, 10.02% of the specific energy consumption (SEC) for water recovery was reduced as compared with FO-MBR (from 1.42 kWh/m³ to 1.28 kWh/m³). Membrane orientation largely determined the performance of PRO-MBR, higher power density was achieved in AL-DS orientation (peak value of 3.4 W/m²) than that in AL-FS orientation (peak value of 1.4 W/m²). However, PRO-MBR suffered more severe and complex membrane fouling when operated in AL-DS orientation, because the porous support layer was facing sludge mixed liquor. Further investigation revealed fouling was mostly reversible for PRO-MBR, it exhibited similar flux recoverability (92.4%) to that in FO-MBR (95.1%) after osmotic backwash. Nevertheless, flux decline due to membrane fouling is still a restricting factor to power generation of PRO-MBR, its power density was decreased by 38.2% in the first 60 min due to the formation of fouling. Overall, in perspective of technoeconomic feasibility, the PRO-MBR demonstrates better potential than FO-MBR in wastewater treatment and reclamation and deserves more research attention in the future.

The functions of potential intermediates and fungal communities involved in the humus formation of different materials at the thermophilic phase

Humus is the final product of humus precursors (HPS) during the humification process, while the associated mechanisms of humus formation have not been clarified. Here, the HPS degradation intermediate and core fungal function for wheat straw and chicken manure compost (SCM), cow dung compost (CD), Chinese traditional medicine residue compost (CTM) and mushroom dreg and chicken manure compost (MCM) was investigated during the thermophilic phase. The results showed SCM and MCM were rich in proteins, lipids, cellulose, low-molecular-weight organic acids, while CD and CTM contained abundant carbohydrates, aliphatic compounds, easily biodegradable aromatic structures, and intermediates from the lignocellulose degradation. In particular, the HPS degrading intermediates including O-alkyl-C and aromatic C compounds were the critical factors, and Scedosporium, Hypsizygus and Remersonia were the core fungal genera for the humification. Furthermore, the potential fungal functional genes involved in carbohydrate and lignin degradation might be the key factors to drive the humification process.

A Novel Hybrid Reactor of Pressure-Retarded Osmosis Coupling with Activated Sludge Process for Simultaneously Treating Concentrated Seawater Brine and Wastewater and Recovering Energy

As an attractive way to deal with fresh water shortage, membrane-based desalination technologies are receiving increased interest. However, concentrated seawater brine, in needing further treatment, remains a main obstacle for desalination via membrane technology. Here, a hybrid technology integrating pressure-retarded osmosis with activated sludge process (PRO-MBR) was applied for simultaneously treating concentrated seawater brine and municipal wastewater. Performance of the PRO-MBR, including water flux, power density, contaminants removal, and membrane fouling was evaluated and compared at two different membrane orientations (i.e., active layer facing feed solution (AL-FS) mode and active layer facing draw solution (AL-DS) mode). During the PRO-MBR process, the municipal wastewater was completely treated regardless of the membrane orientation, which means that there was no concentrated sewage needing further treatment, owing to the biodegradation of microorganisms in the bioreactor. In the meantime, the concentrated brine of seawater desalination was diluted into the salinity level of seawater, which met the standard of seawater discharge. Owing to the high rejection of forward osmosis (FO) membrane, the removal efficiency of total organic carbon (TOC), total phosphorus (TP), ammonia nitrogen (NH4+-N), and total nitrogen (TN) was higher than 90% at both modes in the PRO-MBR. In addition, the PRO-MBR can simultaneously recover the existing osmotic energy between the municipal wastewater and the seawater brine at both modes. Compared with the AL-DS mode, the AL-FS mode took a shorter time and achieved a bigger power density to reach the same terminal point of the PRO-MBR owing to a better water flux performance. Furthermore, the membrane fouling was much more severe in the AL-DS mode. In conclusion, the current study demonstrated that the PRO-MBR at the AL-FS mode can be a promising and sustainable brine concentrate and municipal wastewater treatment technology for its simultaneous energy and water recovery.

Long-term performance, membrane fouling behaviors and microbial community in a hollow fiber anaerobic membrane bioreactor (HF-AnMBR) treating synthetic terephthalic acid-containing wastewater

Purified terephthalic acid (PTA) wastewater with properties of poor biodegradation and high toxicity is produced from refining and synthesis of petrochemical products. In this study, a lab-scale hollow fiber membrane bioreactor (HF-AnMBR) fed with synthetic PTA wastewater was operated over 200 days with stepwise decreased hydraulic retention time (HRT) to investigate the long-term performance, membrane fouling mechanism and microbial community evolution. Results showed that a stable chemical oxygen demand (COD) removal rate of 65.8 ± 4.1% was achieved at organic loading rate of 3.1 ± 0.3 g-COD/L-reactor/d and HRT 24 h, under which the methane production rate reached 0.33 ± 0.02 L/L-reactor/d. Further shortening HRT, however, led to the decreased COD removal efficiency and low methane bioconversion. A mild membrane fouling occurred due to the production of colloidal biopolymers and the interaction between increased colloidal substances secreted/cracked by microorganisms and membrane interface. Further 16S rRNA analysis indicated that microbial diversity and richness had changed with the variation of HRT while Methanosaeta, and Methanolinea species were always the dominant methanogens responsible for methane production. The results verify that HF-AnMBR is an alternative technology for PTA wastewater treatment along with energy harvesting, and provide a new avenue toward sustainable petrochemical wastewater management.

A green, hybrid cleaning strategy for the mitigation of biofouling deposition in the elevated salinity forward osmosis membrane bioreactor (FOMBR) operation

Forward osmosis membrane bioreactors (FOMBRs) are currently gaining attention from the wastewater treatment industry, for their potential to produce high effluent quality and a relatively better flux stability against fouling. However, only using physical cleaning methods is not sufficient to recover the water flux performance satisfactorily under a long-term operation. This study comprehensively investigated the efficiency of a hybrid, environmentally-friendly cleaning strategy involving a combination of physical and free nitrous acid (FNA) cleanings under a long-term FOMBR operation. During 92 days of FOMBR operation, physical cleaning recovered the water flux by 85%, whilst FNA cleaning contributed to an additional 5% of the recovery. In addition, FNA cleaning also offered a retardation of fouling deposition by maintaining the water flux 18-30% more than that obtained by only the physical cleaning. A possible mechanism for FNA's role as the cleaning reagent was proposed for the first time in this study based on the water flux performance and membrane autopsy analysis. The results showed FNA cleaning broke down the residual fouling layer, preferencing protein-based substances. A lower ratio of protein to polysaccharides of the residual fouling layer contributed to a more negatively charged membrane surface (- 42.34 ± 0.30 mV) compared to the virgin one (- 17.54 ± 0.81 mV). This resulted in a stronger electrostatic repulsion between the foulants and the membrane surface, and thus slowed down the biofouling deposition process. This study suggested FNA solution has the great potential not only to recover the membrane performance, also as a strategy to slow down fouling deposition.

Polymer Surface Dissection for Correlated Microscopic and Compositional Analysis of Bacterial Aggregates during Membrane Biofouling

Multispecies biofilms are a common limitation in membrane bioreactors, causing membrane clogging, degradation, and failure. There is a poor understanding of biological fouling mechanisms in these systems due to the limited number of experimental techniques useful for probing microbial interactions at the membrane interface. Here, we develop a new experimental method, termed polymer surface dissection (PSD), to investigate multispecies assembly processes over membrane surfaces. The PSD method uses photodegradable polyethylene glycol hydrogels functionalized with bioaffinity ligands to bind and detach microscale, microbial aggregates from the membrane for microscopic observation. Subsequent exposure of the hydrogel to high resolution, patterned UV light allows for controlled release of any selected aggregate of desired size at high purity for DNA extraction. Follow-up 16S community analysis reveals aggregate composition, correlating microscopic images with the bacterial community structure. The optimized approach can isolate aggregates with microscale spatial precision and yields genomic DNA at sufficient quantity and quality for sequencing from aggregates with areas as low as 2000 μm², without the need of culturing for sample enrichment. To demonstrate the value of the approach, PSD was used to reveal the composition of microscale aggregates of different sizes during early-stage biofouling of aerobic wastewater communities over PVDF membranes. Larger aggregates exhibited lower diversity of bacterial communities, and a shift in the community structure was found as aggregate size increased to areas between 25,000 and 45,000 μm², below which aggregates were more enriched in Bacteroidetes and above which aggregates were more enriched with Proteobacteria. The findings demonstrate that community succession can be observed within microscale aggregates and that the PSD method is useful for identification and characterization of early colonizing bacteria that drive biofouling on membrane surfaces.

Membrane cleaning and performance insight of osmotic microbial fuel cell

Osmotic microbial fuel cell (OsMFC) integrating forward osmosis into microbial fuel cell (MFC) favors the merits of organic removal, bioenergy generation, and high-quality water extraction from wastewater. This study demonstrated an 18.7% power density enhancement over a conventional MFC due to the water-flux-facilitated proton advection and net positive charge (NPC)-flux-promoted countercurrent proton exchange. Among the three examined membrane cleaning methods, chemical cleaning using 0.2% NaClO was found to be especially effective in removing organic foulants composed of proteins and polysaccharides, resulting in a water flux recovery of up to 91.6% with minimal impact on average maximum power density and internal resistance. The effects of operating parameters including anode HRT and draw solution concentration were studied. Shortening HRT from 6.0 to 3.0 h increased power density by 78.0% due to a high organic loading rate and a slightly reduced polarization concentration. Increasing draw solution concentration from 0.2 to 1.0 M NaCl enhanced power density by approximately 2.7-fold due to enhanced proton advection. Water-flux-facilitated proton advection played a more important role in determining the electricity generation performance of OsMFC than the NPC-flux-promoted countercurrent proton exchange under varied operating conditions.

Mechanistic insights into promoted dewaterability, drying behaviors and methane-producing potential of waste activated sludge by Fe2+-activated persulfate oxidation

Sludge management represents a critical challenge because of complex compositions and poor dewaterability. Fe²⁺-activated persulfate oxidation (Fe²⁺/S₂O₈^2-) is an effective, and widely investigated method for enhancing sludge dewatering. However, the potential effects of Fe²⁺/S₂O₈^2- on sludge drying efficiency, anaerobic biodegradation behaviors and potential recycling of sludge residua are not yet well-known. In this study, a new sludge disposal route (step i: enhanced dewatering via Fe²⁺/S₂O₈^2-, and step ii: drying-incineration or anaerobic digestion) was proposed and appraised comprehensively. Results showed that Fe²⁺/S₂O₈^2- oxidation destroyed extracellular polymeric substances, lysed sludge cells and enhanced the dewaterability greatly. Capillary suction time and mechanical filtration time at 2.0/1.6 mmol-Fe²⁺/S₂O₈^2-/g-VS decreased by 88.0% and 79.6%, respectively. Moreover, 89.8% of micro-pollutants (e.g., methylbenzene, ethylbenzene, p-m-xylene and o-xylene) in sludge were removed. Besides, the pretreatment was able to alter sludge drying behaviors and methane-producing potential. Pretreated sludge exhibited faster drying rate and shorter lag-time for methane production. Incineration residua of dewatered sludge could be re-coupled with S₂O₈^2- as the conditioner to enhance sludge dewaterability, thereby reducing the chemical input and disposal cost. This study provides a novel, self-sustainable strategy for sludge management, reutilization and final safe disposal.

Effects of inorganic particles and their interactions with biofilms on dynamic membrane structure and long-term filtration performance

In present study, the effects of inorganic particles and their interaction with biofilms on the filtration behavior of dynamic membrane bioreactor (DMBR) were investigated. When no inorganic particles were included in the simulated domestic wastewater, a porous biofilm DM was formed on support materials. As a result, the transmembrane pressure (TMP) did not increase (< 10 Pa) during the 97 days' experiment and the effluent turbidity was consistently lower than 1.0 NTU. When sands (1.3-69.2 μm; 50 mg/L) were the only inorganic particles contained in wastewater, the effluent turbidity became instable and ranged from 0.31 to 3.88 NTU, probably because the DM structures were disturbed by sand scouring. The natural clays (0.5-2.7 μm) in wastewater were very liable to deposit on the support materials of DMBRs to form thick and compact DMs with greater contents of biomass and EPS, especially co-existing with sands. Due to the existence of natural clays, the DM porosity decreased significantly and rapid rising in TMP occurred frequently. This study demonstrated that pure biofilms without containing inorganic particles were ideal materials for DMs, which could achieve long-term stable operation with low effluent turbidity (< 1 NTU) and low TMP (< 10 Pa), while inorganic particles with any size could deteriorate the filtration performance. Therefore, removing the inorganic particles in wastewater as many as possible prior DMBR is critically important for achieving long-term stable operation.

A novel forward osmosis reactor assisted with microfiltration for deep thickening waste activated sludge: performance and implication

Waste activated sludge (WAS) treatment has gained growing interests for its increasingly capacity and high process cost. Sludge thickening is generally the first process of the WAS treatment. However, traditional sludge thickening approach was restrained by large footprint, low thickening efficiency, and tendency of releasing phosphorus. Here, we reported a novel microfiltration (MF) membrane assisting forward osmosis (FO) process (MF-FO) for sludge thickening. The MF-FO reactor achieved a sludge thickening of the mixed liquor suspended solids (MLSS) concentration from approximately 7 to 50 g/L after 10-day operation. More importantly, the effluent quality after FO filtration was superior with total organic carbon (TOC), ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and total phosphorus (TP) of 1.94 ± 0.46, 0.02 ± 0.07, 4.55 ± 1.59 and 0.24 ± 0.26 mg/L, respectively. Additionally, the integration of MF membrane successfully controlled the salinity of the MF-FO reactor in a low range of 1.6-3.1 mS/cm, which mitigated the flux decline of FO membrane and thus prolonged the operating time. In this case, the flux decline of FO membrane in the MF-FO reactor was mainly due to the membrane fouling. Furthermore, the fouling layer on the FO membrane surface was a gel layer mainly composed of biofoulants and organic foulants when the MLSS concentration was less than 30 g/L, while it turned to a cake layer when the MLSS concentration exceeded 30 g/L. Results reported here demonstrated that the MF-FO reactor is a promising WAS thickening technology for its excellent thickening performance and high effluent quality of FO membrane.

Forward osmosis membrane bioreactor using Bacillus and membrane distillation hybrid system for treating dairy wastewater

Wastewater recycling is one of the best ways to alleviate water scarcity and water/wastewater pollution. The dairy industry is the largest industrial food wastewater source in many countries. In this study, we used a forward osmosis membrane bioreactor (FOMBR) and membrane distillation (MD) hybrid system to recycle dairy wastewater. And we developed a new Bacillus-FOMBR inoculated with salt-tolerant Bacillus sludge to protect against the negative effects of accumulated salt on sludge characteristics, microbial community and treatment effectiveness, and to alleviate membrane biofouling. A laboratory-scale FOMBR-MD experiment was operated for 40 days and water flux, salinity change, sludge characteristics, microbial community, nutrient removal efficiency, and FO membrane fouling were investigated. The Bacillus-FOMBR showed a small decrease in biomass concentration, and the hybrid system removed almost 100% of the contaminants. High-throughput sequencing analysis indicated that Pirellula and Hyphomicrobium species dominated the Bacillus-FOMBR, which are obliged to perform heterotrophic nitrification and aerobic denitrification. These nitrogen-removing bacteria ensured high nitrogen removal efficiency of the bioreactor. The total nitrogen (TN) concentration in the bioreactor increased and then decreased, which did not continuously increase as occurred in conventional FOMBRs. The TN removal efficiency of the bioreactor was mostly above 40% and the highest reached 79%. Besides, the Bacillus-FOMBR suffered little membrane biofouling because of the quorum quenching effect of the Bacillus species. We speculate that the Bacillus-FOMBR has potential to treat high-salt wastewater and high strength ammonia-nitrogen wastewater.

Quantification and modeling of the response of surface biofilm growth to continuous low intensity UVC irradiation

Though germicidal UV radiation is widely applied for disinfection of water and food, it may also be used to prevent bacterial growth and colonization on surfaces within engineered systems. Emerging UV source technologies, such as ultraviolet-C (UVC) LEDs, present new opportunities for deterring biofilms within certain devices, including medical equipment, food equipment, and potentially in plumbing fixtures for prevention of opportunistic respiratory pathogen infections. Rational design for incorporation of UVC sources into devices with complex internal geometries is currently hampered by the lack of an engineering framework for predicting reductions in biofilm growth rates in response to continuous low-intensity irradiation. Herein we have developed an experimental apparatus and method for growing biofilms under concurrent UV irradiation and quantifying the resulting suppression of surface growth. Under accelerated growth conditions over 48 h, E. coli surface biovolume was reduced by 95% compared to control biofilms (grown in the dark) by a UV intensity of 50.5 µW/cm² (254 nm). The required intensity for biofilm prevention was higher than expected, given the UV dose response of the bacteria employed and the cumulative doses delivered to the test surfaces. The results indicate that biofilms can establish even under irradiation conditions that would result in complete inactivation of planktonic cells, likely due to the shielding effects of colloidal material and microbial exudates. A pseudo-mechanistic model was also developed which correlated UV intensity to the resultant reduction in specific surface biovolume.

Effect of Initial Water Flux on the Performance of Anaerobic Membrane Bioreactor: Constant Flux Mode versus Varying Flux Mode

Anaerobic membrane bioreactors (AnMBRs) have aroused growing interest in wastewater treatment and energy recovery. However, serious membrane fouling remains a critical hindrance to AnMBRs. Here, a novel membrane fouling mitigation via optimizing initial water flux is proposed, and its feasibility was evaluated by comparing the membrane performance in AnMBRs between constant flux and varying flux modes. Results indicated that, compared with the constant flux mode, varying flux mode significantly prolonged the membrane operating time by mitigating membrane fouling. Through the analyses of fouled membranes under two operating modes, the mechanism of membrane fouling mitigation was revealed as follows: A low water flux was applied in stage 1 which slowed down the interaction between foulants and membrane surface, especially reduced the deposition of proteins on the membrane surface and formed a thin and loose fouling layer. Correspondingly, the interaction between foulants was weakened in the following stage 2 with a high water flux and, subsequently, the foulants absorbed on the membrane surface was further reduced. In addition, flux operating mode had no impact on the contaminant removal in an AnMBR. This study provides a new way of improving membrane performance in AnMBRs via a varying flux operating mode.

Ammonium ultra-selective membranes for wastewater treatment and nutrient enrichment: Interplay of surface charge and hydrophilicity on fouling propensity and ammonium rejection

Membrane fouling and ammonium transmembrane diffusion simultaneously pose great challenges in membrane-based pre-concentration of domestic wastewater for efficient subsequent resources recovery (i.e., energy and nutrients). Herein, amine-functionalized osmotic membranes were fabricated by optimizing the grafting pathway of polyamidoamine (PAMAM) dendrimer to mitigate fouling and ammonium transmembrane diffusion. Compared to the control membrane, the PAMAM-grafted membranes with abundant primary amine groups possessed substantially increased hydrophilicity and positive charges (i.e., protonated primary amines) and thus exhibited superior anti-fouling capability and ammonium selectivity. With further increasing the PAMAM grafting ratio, the membrane exhibited a steady enhancement in ammonium selectivity and eventually achieved an ultra-high ammonium rejection of 99.4%. Nevertheless, the anti-fouling capability of such ammonium ultra-selective membrane was weakened due to the suppression of the adverse impact of excessive positive charges over the beneficial effect of increased surface hydrophilicity. This in turn leads to a drop of ammonium rejection below 90% during domestic wastewater concentration. This study demonstrates that the membrane with a moderate primary amine loading could achieve the highest anti-fouling capability with only less than 10% flux decline and meanwhile maintain an excellent ammonium rejection above 94% during raw domestic wastewater concentration. This work provides theoretical guidance for fabricating simultaneously enhanced anti-fouling and ammonia-rejecting membranes.

Formation of disinfection by-products during sodium hypochlorite cleaning of fouled membranes from membrane bioreactors

Periodic chemical cleaning with sodium hypochlorite (NaClO) is essential to restore the membrane permeability in a membrane bioreactor (MBR). However, the chlorination of membrane foulants results in the formation of disinfection by-products (DBPs), which will cause the deterioration of the MBR effluent and increase the antibiotic resistance in bacteria in the MBR tank. In this study, the formation of 14 DBPs during chemical cleaning offouled MBR membrane modules was investigated. Together with the effects of biofilm extracellular polymeric substances (EPS), influences of reaction time, NaClO dosage, initial pH, and cleaning temperature on the DBP formation were investigated. Haloacetic acids (HAAs) and trichloromethane (TCM), composed over 90% of the DBPs, were increasingly accumulated as the NaClO cleaning time extended. By increasing the chlorine dosage, temperature, and pH, the yield of TCM and dichloroacetic acid (DCAA) was increased by up to a factor of 1-14, whereas the yields of haloacetonitriles (HANs) and haloketones (HKs) were decreased. Either decreasing in the chlorine dosage and cleaning temperature or adjusting the pH of cleaning reagents toward acidic or alkaline could effectively reduce the toxic risks caused by DBPs. After the EPS extraction pretreatment, the formation of DBPs was accelerated in the first 12 h due to the damage of biofilm structure. Confocal laser scanning microscopy (CLSM) images showed that EPS, particularly polysaccharides, were highly resistant to chlorine and might be able to protect the cells exposed to chlorination.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available in the online version of this article at 10.1007/s11783-021-1389-3 and is accessible for authorized users.

Treatment of coking wastewater in biofilm-based bioaugmentation process: Biofilm formation and microbial community analysis

Coking wastewater (CWW) containing complicated organic compositions and strong toxicity cause potential hazards to natural water bodies as well as human health. The aim of this study was integrating newly isolated Comamonas sp. ZF-3, biofilm-based bioaugmentation and fluidized bed reactor into an anoxic filter-fluidized bed reactor (AF-FBR) system to treat actual CWW. The results showed that 93 % of chemical oxygen demand (COD) and 97 % of ammonia nitrogen (NH₄⁺-N) removal efficiency were achieved with hydraulic retention time of 70 h. The main pollutants including phenolic compounds, heterocyclic compounds and polycyclic aromatic hydrocarbons could be removed via biofilm-based process in AF-FBR. The formation of carrier biofilm was consistent with the system performance as well as the biofilm community evolution, during which the microbial community was gradually dominated by some functional genus (e.g., Comamonas, Thiobacillus, Pseudomonas and Thauera), meanwhile, ammonium-oxidizing bacteria Nitrosomonas, nitrite-oxidizing bacteria Nitrospira and denitrifiers (e.g., Pseudomonas, Thiobacillus and Bacillus) coexisted in biofilm to form a microbial community for biological nitrogen removal. Such microbial community structure explained the observed simultaneous removal of COD and NH₄⁺-N in the AF-FBR.

Achieving stable operation and shortcut nitrogen removal in a long-term operated aerobic forward osmosis membrane bioreactor (FOMBR) for treating municipal wastewater

Forward osmosis membrane bioreactor (FOMBR) is an integrated physical-biological treatment process that has received increased awareness in treating municipal wastewater for its potential to produce high effluent quality coupled with its low propensity for fouling formation. However, reverse salt diffusion (RSD) is a major issue and so far limited studies have reported long-term FOMBR operation under the elevated salinity conditions induced by RSD. This study investigated the performance of a FOMBR in treating municipal wastewater under a controlled saline environment (6-8 g L^-1 NaCl) using two separate sodium chloride draw solution (NaCl DS) concentrations (35 and 70 g L^-1) over 243 days. At 35 g L^-1 NaCl DS, the water flux performance dropped from 6.75 L m^-2 h^-1 (LMH) to 2.07 LMH after 72 days of operation in the first experimental stage, when no cleaning procedure was implemented. In the subsequent stage, the DS concentration was increased to 70 g L^-1 and a weekly physical cleaning regime introduced. Under stable operation, the water flux performance recovery was 67% after 21 cycles of physical cleaning. For the first time in FOMBR studies, a shortcut nitrogen removal via the nitrite pathway was also achieved under the elevated salinity conditions. At the end of operation (day 243), the ammonia-oxidising bacteria (Nitrosomonas sp.) was the only nitrifier species in the system and no nitrite oxidising bacteria was detected. The above study proves that a FOMBR system is a feasible process for treating municipal wastewater.

Biofouling Mitigation Approaches during Water Recovery from Fermented Broth via Forward Osmosis

Forward Osmosis (FO) is a promising technology that can offer sustainable solutions in the biorefinery wastewater and desalination fields, via low energy water recovery. However, microbial biomass and organic matter accumulation on membrane surfaces can hinder the water recovery and potentially lead to total membrane blockage. Biofouling development is a rather complex process and can be affected by several factors such as nutrient availability, chemical composition of the solutions, and hydrodynamic conditions. Therefore, operational parameters like cross-flow velocity and pH of the filtration solution have been proposed as effective biofouling mitigation strategies. Nevertheless, most of the studies have been conducted with the use of rather simple solutions. As a result, biofouling mitigation practices based on such studies might not be as effective when applying complex industrial mixtures. In the present study, the effect of cross-flow velocity, pH, and cell concentration of the feed solution was investigated, with the use of complex solutions during FO separation. Specifically, fermentation effluent and crude glycerol were used as a feed and draw solution, respectively, with the purpose of recirculating water by using FO alone. The effect of the abovementioned parameters on (i) ATP accumulation, (ii) organic foulant deposition, (iii) total water recovery, (iv) reverse glycerol flux, and (v) process butanol rejection has been studied. The main findings of the present study suggest that significant reduction of biofouling can be achieved as a combined effect of high-cross flow velocity and low feed solution pH. Furthermore, cell removal from the feed solution prior filtration may further assist the reduction of membrane blockage. These results may shed light on the challenging, but promising field of FO process dealing with complex industrial solutions.

Customized thin and loose cake layer to mitigate membrane fouling in an electro-assisted anaerobic forward osmosis membrane bioreactor (AnOMEBR)

Anaerobic forward osmosis membrane bioreactor (AnOMBR) is a potential wastewater treatment technology, due to its low energy consumption and high effluent quality. However, membrane fouling is still a considerable problem which causes dwindling of water flux and shortening the membrane lifetime. In this study, electro-assisted anaerobic forward osmosis membrane bioreactor (AnOMEBR) was developed to treat wastewater and mitigate membrane fouling, in which the conductive FO membrane was used both as the separation unit and cathode. The formation, development and alleviation of membrane fouling in AnOMEBR were investigated. The results showed that the soluble microbial products (SMP) content and the proteins/polysaccharides (PN/PS) value in AnOMEBR were 26% and 15% lower than that in AnOMBR, respectively. The absolute value of Zeta of sludge mixture in AnOMEBR was 1.2 times that of the AnOMBR. The increase in the interaction between the membrane surface and the negatively charged foulants could inhibit the adsorption of foulants on membrane surface in the initial stage of membrane fouling. The strong interaction among foulants further affected the composition, structure and thickness of the cake layer on the FO membrane surface. AnOMEBR with a shorter hydraulic retention time, a higher organic loading rate and a lower osmotic pressure difference, could still obtain a lower flux decline rate of 0.063 LMH/h, which was 35.7% lower than AnOMBR. The wastewater treatment capacity of AnOMEBR was nearly 1.5 times that of the AnOMBR. This work provides an efficient strategy for mitigating membrane fouling and improving wastewater treatment capacity.

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant were investigated to elucidate fouling mechanisms and develop cleaning strategy. The performance decline of diffusers was observed with dynamic wet pressure increased by 3.2 times and standard oxygen transfer efficiency dropped to 73%, which contributed to 15.0% increase in total energy consumption. Oxygen-affinity, filamentous and extracellular polymeric substances secreting bacteria tended to accumulate on the diffuser surface. External (mainly biofilm growth), internal (organic and inorganic matters) and irrecoverable (mainly material aging) foulants accounted for 34.1%, 45.4% and 20.1% of total fouling, respectively. HCl cleaning failed to restore aeration efficiency because it eliminated structural support formed by inorganics, leaving organic foulants broken into smaller fragments and distributed more dispersed. NaClO showed better cleaning efficiency by effectively removing organic foulants. Sequential cleaning by NaClO and HCl, which achieved the best recovery, was recommended.

Pressure retarded osmosis coupled with activated sludge process for wastewater treatment: Performance and fouling behaviors

A novel hybrid technology integrating pressure retarded osmosis with activated sludge process (denoted as PRO-MBR) was proposed in this study for wastewater treatment. Here, performance and fouling behaviors of PRO-MBR were investigated. Excellent contaminants removal and power production were simultaneously achieved in the PRO-MBR. A significant drop of water flux in the PRO-MBR was mainly due to the severe fouling of the support layer in forward osmosis (FO) membrane including internal fouling and external fouling. Although the external fouling was identified to be the major type of fouling, the internal fouling dominated the overall decline of water flux. In addition, organic foulants and biofoulants were the dominant foulants for the external fouling while inorganic foulants were equal to organic foulants and biofoulants for the internal fouling. According to the variations of water flux in the PRO-MBR, the development of support layer fouling was divided into three stages.

Reduction of sludge by a biodrying process: Parameter optimization and mechanism

Sewage sludge generated from chemical fiber plants is a hazardous waste with high disposal costs, and it is important to reduce the amount of sludge produced. In this study, a biodrying technology was controlled by thermophilic microorganisms to reduce and detoxify hazardous waste sludge. Ten batches of experiments were conducted under optimal parameters (i.e., under the conditions of a one-time feeding with the equipment temperature controlled at 60 °C, where the dosage of the microbial agent accounted for 4% of the dry weight of sludge) to evaluate the performance of the biodrying system reducing and detoxifying hazardous waste sludge under a continuous reaction. The results showed that the degree and rate of dehydration during the continuous reaction were 65.97% and 2.51%, respectively. The degrees of reduction for dry matter and toxic substances were 22% and 9%, respectively. The calorific value of sludge after treatment increased from 1678.53 J (later abbreviated as J) to 12256.17 J. The reduction effect in the pilot-scale experiment was 65%. Spectral analysis and microbial sequencing of the freeze-dried sludge advance our understanding of biodrying mechanisms and provide a foundation for the application of key technologies for the reduction and detoxification of hazardous waste sludge.

Quorum sensing inhibition and tobramycin acceleration in Chromobacterium violaceum by two natural cinnamic acid derivatives

Chromobacterium violaceum, one free-living Gram-negative bacterium, is abundantly presented in tropics and sub-tropics soil and aquatic environment; it is also an opportunistic human pathogen. Here, two cinnamic acid derivatives, i.e., 4-dimethylaminocinnamic acid (DCA) and 4-methoxycinnamic acid (MCA), were identified as potential quorum sensing (QS) and biofilm inhibitors in C. violaceum ATCC12472. Both DCA (100 μg/mL) and MCA (200 μg/mL) inhibited the levels of N-decanoyl-homoserine lactone (C10-HSL) and reduced the production of certain virulence factors in C. violaceum, including violacein, hemolysin, and chitinase. Metabolomics analysis indicated that QS-related metabolites, such as ethanolamine and L-methionine, were down-regulated after treatment with DCA and MCA. Quantitative real-time polymerase chain reaction (qRT-PCR) demonstrated that DCA and MCA markedly suppressed the expression of two QS-related genes (cviI and cviR). In addition, DCA and MCA also inhibited biofilm formation and enhanced the susceptibility of biofilms to tobramycin, which was evidenced by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Our results indicated that DCA and MCA can serve as QS-based agent for controlling pathogens.Key Points • DCA and MCA inhibited QS and biofilm formation in C. violaceum.• The combination of DCA or MCA and tobramycin removed the preformed biofilm of C. violaceum. • DCA or MCA inhibited virulence factors and expressions of cviI and cviR of C. violaceum.• DCA or MCA are potential antibiotic accelerants for treating C. violaceum infection.

The growth process of the cake layer and membrane fouling alleviation mechanism in a MBR assisted with the self-generated electric field

The electric field assisted membrane bioreactor (MBR) is an effective technique to alleviate membrane fouling. In this study, the spontaneous electric field was introduced into the MBR to observe the growth process of cake layer on the membrane surface. The external resistance for spontaneous electric field MBR (S-50) and S-500 were 50 Ω and 500 Ω respectively. During the experiments, S-50 maintained the highest electric field intensity of 11.83 mV/cm. The reduction of extracellular polymeric substances (EPS) content in activated sludge, transmembrane pressure (TMP) growth rate reached 52.8% and 51.7% respectively. After 28 days operation, S-50 obtained the minimum contaminant specific biovolume (23.316 μm³/μm²), which was 68.2% lower than that of it in Control-MBR. The metal oxide or metal hydroxide were distributed in the cake layer. EPS played a significant role in the formation and growth of the cake layer. Based on the results obtained in this study, the growth of the biofouling layer on the membrane surface could be divided into three stages. EPS first deposited on the membrane surface, and then microorganisms embedded in the cake layer to form clusters. After that, EPS and total cells further increased and led to a faster biovolume growth rate. Subsequently, the biovolume growth rate decreased in the cake layer. The spontaneous electric field delayed the deposition of EPS on the membrane surface. The produced H₂O₂ and •OH were beneficial to the degradation of organics, causing the smaller contaminant biovolume on the membrane surface. This work aims to provide a theoretical basis for the practical application of the electric field to control membrane fouling.

Integrated effects of temperature and COD/N on an up-flow anaerobic filter-biological aerated filter: Performance, biofilm characteristics and microbial community

The integrated effects of temperature and COD/N ratio on performance, biofilm characteristics and microbial community in up-flow anaerobic filter-biological aerated filters (UAF-BAFs) were investigated. Results indicated that the UAF-BAF system could achieve excellent COD, NH₄⁺-N and TN removal, in which effluent quality well met the Class 1A standard. Biofilm physicochemical characteristics showed that the biomass, biofilm thickness and extracellular polymeric substance (EPS) content in the UAF-BAFs reduced with the decrease in COD/N ratio, but were enhanced under low temperature. The biofilm structure characterized by CLSM in the UAF-BAFs significantly shifted, which was closely correlated with operational conditions. Sequencing analysis revealed that Proteobacteria, Epsilonbacteraeota, Bacteroidetes and Firmicutes were dominant in the UAFs and the abundance of ammonium oxidizing bacteria (AOB) was responsible for nitrification performance in the BAFs. Functions analysis indicated that amino acid metabolism, carbohydrate metabolism, energy metabolism and lipid metabolism were clearly regulated by parameters changes.

Characteristics and influencing factors of organic fouling in forward osmosis operation for wastewater applications: A comprehensive review

Wastewater reuse is considered one of the most promising practices for the achievement of sustainable water management on a global scale. In the context of the safe reuse of water, membrane filtration is a competitive technique due to its superior efficiency in several processes. However, membrane fouling by organics is an inevitable challenge that is encountered during the practical application of membrane processes. The resolution of the membrane fouling challenge requires an in-depth understanding of many complex interactions between organic foulants and the membrane. In the last few decades, the forward osmosis (FO) membrane process, which exploits osmosis as a driving force, has emerged as an effective technology for water production with low energy consumption, thus leveraging the water-energy nexus. However, their successful application is severely hampered by membrane fouling, which is caused by such complex fouling mechanisms as cake enhanced osmotic pressure (CEOP), reverse salt diffusion (RSD), internal, and external concentration polarization as well as by the traditional fouling processes encompassing colloids, microbial (biofouling), inorganic, and organic fouling. Of these fouling types, the fouling potential of organic matter in FO has not been given sufficient attention, in particular, when FO is applied to wastewater treatment. This paper aims to provide a comprehensive overview of FO membrane fouling for wastewater applications with a special focus on the identification of the major factors that lead to the unique properties of organic fouling in this filtration process. Based on the critical assessment of organic fouling formation and the governing mechanisms, proposals were advanced for future research aimed at the mitigation of FO membrane fouling to enhance process efficiency in wastewater applications.

Entrapped-cells-based anaerobic forward osmosis membrane bioreactor treating medium-strength domestic wastewater: Fouling characterization and performance evaluation

A novel entrapped cells-based-anaerobic forward osmosis membrane bioreactor (E-FOMBR) was developed. Its performance and fouling were investigated in comparison with suspended cells-based-anaerobic forward osmosis membrane bioreactor (S-FOMBR). E-FOMBR and S-FOMBR were operated under the same conditions with two widely used draw solutions (NaCl and (NH₄)₂SO₄). The membrane fouling especially irreversible fouling in S-FOMBR was more severe than that in E-FOMBR regardless of the type of draw solution. The permeate flux of E-FOMBR were 1.79 and 1.85 LMH while those of S-FOMBR were 1.49 and 1.14 LMH with NaCl and (NH₄)₂SO₄ as draw solutions, respectively. More deterioration of biological activity (suggested by lower organic removal) due to accumulation of salt was observed in S-FOMBR compared to E-FOMBR. Proteobacteria dominated in both FOMBRs but was more abundant in E-FOMBR than S-FOMBR. The superiority of E-FOMBR over S-FOMBR included higher and stable system performance, higher flux, and longer operation time.

In situ extracting organic-bound calcium: A novel approach to mitigating organic fouling in forward osmosis treating wastewater via gradient diffusion thin-films

Forward osmosis (FO) has gained increasing interests in wastewater treatment and reclamation. However, membrane fouling has become one major obstacle hindering FO application. A novel mitigation approach for FO membrane fouling via in situ extracting Ca²⁺ binding with the organic foulants using the gradient diffusion thin-films (DGT) was proposed in this study. The DGT could effectively adsorb the Ca²⁺ binding with the sodium alginate via the chelation of the Chelex functional groups, and its adsorption amount of Ca²⁺ correspondingly increased as a function of the Ca²⁺ concentration in the feed solution. Owing to the extraction of Ca²⁺ from the fouling layer by the DGT, the FO membrane fouling was effectively mitigated evident by significant enhancement of water flux, and at the same time, foulants became easily removed by physical cleaning. The alleviation of FO membrane fouling by the DGT could be attributed to the fact that the structure of the fouling layer became more porous and looser after in situ removing Ca²⁺ from the alginate-Ca²⁺ gel networks. The feasibility of fouling control strategy via in situ removing Ca²⁺ binding with the foulants in the fouling layer was demonstrated, which provides new insights into fouling control mechanisms during FO treating wastewater.