Characteristics and fouling behaviors of Dissolved Organic Matter fractions in a full-scale submerged membrane bioreactor for municipal wastewater treatment

Delineating the Effects of Molecular and Colloidal Interactions of Dissolved Organic Matter on Titania Photocatalysis

In the face of significant challenges to practical applications of photocatalysis for water treatment, recent reports revealed a potential route to overcome a problem posed by dissolved organic matter (DOM). These studies showed that inhibition of photocatalytic processes by DOM is driven largely by competition for active surface sites on TiO₂ or other catalysts, and controlling the type of DOM present in solution could significantly mitigate DOM fouling. Whether or not control of solution parameters could achieve the same preventative action is not known. Here, a series of DOM isolates, including humic acid (HA) and transphilic (TPI), hydrophobic (HPO), or colloidal fractions of organic matter from a membrane bioreactor mixed liquor supernatant, were tested for inhibitory activity under a range of pH values (3, 5, 7, and 9) and ionic compositions (NaCl, CaCl₂, and Al₂(SO₄)₃ with ionic strengths (IS) ranging from 0 to 3 M). The resulting TiO₂-DOM agglomerates were monitored for size and ζ-potential. Inhibitory profiles were generated using para-chlorobenzoic acid (pCBA) as probe with varying concentrations of inhibitory DOM for each solution condition to discern the extent of surface-phase quenching of radicals. Manipulation of pH clearly impacted inhibition, and the effect varied by DOM type; for example, interference occurred at all pHs for HA, at neutral or basic pHs for TPI, and only at pH 7 for HPO. Particle sizes did not correlate with inhibitory action of DOM. Increases in ionic strength induced growth of TiO₂ and TiO₂-DOM agglomerates, but again, particle sizes did not correlate to inhibition by DOM. The changes to IS, regardless of ion type, were not affected by the presence of TPI or HPO. Since particle stability did not correlate directly with photocatalytic activity, we suggest that surface-based quenching reactions arise from site-specific adsorption rather than generalized particle destabilization and aggregation.

Applications of submerged and staged membrane systems for pollutant removal from effluents and mechanism studies - a review

Membrane based filtration is one of the promising technologies for rehabilitation of wastewater streams for reuse and recycle. Many advancements have emerged with the use of novel materials and innovative integrated technologies. The present investigation focuses on the treatment methods based on submerged and stages systems of membranes for water purification. Ceramic, polymeric and mixed matrix type of membranes fabricated for specific type of effluents, their modification methods for accelerating the rejection efficiency, permeability, durability, stability and antifouling properties are detailed in this review. Graphene oxide is the most considered membrane material for adsorption purposes as it exhibits larger surface area, abundant functional groups contain oxygen and has good supply of ligands which is responsible in metal adsorption as it enhances electrostatic interaction by bonding metal ions with graphene oxide nanosheets. Energy derivation in terms of biogas production was also reported in some integrated methods. In many cases, embedded nanomaterial matrices yielded maximum efficiencies in both the submerged and staged operations. However, submerged type of membranes are reported more than the staged type due to simpler configuration with relatively lesser equipment, operational and maintenance issues. In treatment of a low strength wastewater, aluminum oxide based membrane in fluidized bed assembly was reported to yield promising results with reduced power requirement.

Impact of permeate flux and gas sparging rate on membrane performance and process economics of granular anaerobic membrane bioreactors

This research investigated the impact of permeate flux and gas sparging rate on membrane permeability, dissolved and colloidal organic matter (DCOM) rejection and process economics of granular anaerobic membrane bioreactors (AnMBRs). The goal of the study was to understand how membrane fouling control strategies influence granular AnMBR economics. To this end, short- and long-term filtration tests were performed under different permeate flux and specific gas demand (SGD) conditions. The results showed that flux and SGD conditions had a direct impact on membrane fouling. At normalised fluxes (J₂₀) of 4.4 and 8.7 L m^-2 h^-1 (LMH) the most favourable SGD condition was 0.5 m³ m^-2 h^-1, whereas at J₂₀ of 13.0 and 16.7 LMH the most favourable SGD condition was 1.0 m³ m^-2 h^-1. The flux and the SGD did not have a direct impact on DCOM rejection, with values ranging between 31 and 44%. The three-dimensional excitation-emission matrix fluorescence (3DEEM) spectra showed that protein-like fluorophores were predominant in mixed liquor and permeate samples (67-79%) and were retained by the membrane (39-50%). This suggests that protein-like fluorophores could be an important foulant for these systems. The economic analysis showed that operating the membranes at moderate fluxes (J₂₀ = 7.8 LMH) and SGD (0.5 m³ m^-2 h^-1) could be the most favourable alternative. Finally, a sensitivity analysis illustrated that electricity and membrane cost were the most sensitive economic parameters, which highlights the importance of reducing SGD requirements and improving membrane permeability to reduce costs of granular AnMBRs.

Influences of electrode distance and electrolysis time on phosphorus precipitation and composition during electrolysis of anaerobic digestion effluent

Global demand for phosphorus (P) is increasing, which has led to concerns over future drought and has driven efforts to recover P from wastewater streams for reuse. In this study, platinum-coated titanium electrodes were applied to the electrochemical precipitation of P from anaerobic digestion effluent that was collected from a domestic wastewater treatment plant. The influence of the electrode distance on P removal and precipitation was investigated. In addition, the influence of the electrolysis time on the chemical structure and composition of the P precipitate was evaluated from the viewpoint of utilising the P precipitate as fertiliser. Regardless of the electrode distance (10, 5 and 1 mm), PO₄^3-, Ca²⁺ and Mg²⁺ were removed. The bulk solution pH increased during electrolysis because of the consumption of generated H⁺ as HCO₃^- transitioned to H₂CO₃ near the anode. A greater increase in the bulk solution pH was observed when the electrode distance was narrowed because of the enhanced H⁺ consumption. Narrowing the electrode distance reduced the energy consumption for P precipitation. The increase in the bulk solution pH with the narrowing electrode distance changed the dominant P precipitation pathway from onto the cathode to in the bulk solution. X-ray diffraction spectra of the precipitates showed that increasing the electrolysis time transformed amorphous P to hydroxyapatite and struvite. Most P existed in a citric acid-soluble form, which is recommended for use as a slow P release fertiliser. There were no significant changes in the citric acid-soluble P content of the precipitates with increasing electrolysis time. Therefore, increasing the electrolysis time has little influence on the suitability of the precipitate as a slow P release fertiliser.

Fouling, performance and cost analysis of membrane-based water desalination technologies: A critical review

While water is a key resource required to sustain life, freshwater sources and aquifers are being depleted at an alarming rate. As a mitigation strategy, saline water desalination is commonly used to supplement the available water resources beyond direct water supply. This is achieved through effective advanced water purification processes enabled to handle complex matrix of saline wastewater. Membrane technology has been extensively evaluated for water desalination. This includes the use of reverse osmosis (RO) (the most mature membrane technology for desalination), pervaporation (PV), electrodialysis (ED), membrane distillation (MD), and membrane crystallization (MCr). Though nanofiltration (NF) is not mainly applied for desalination purposes, it is included in the reviewed processes because of its ability to reach 90% salt rejection efficiency for water softening. However, its comparison with other technologies is not provided since NF cannot be used for removal of NaCl during desalination. Remarkably, membrane processes remain critically affected by several challenges including membrane fouling. Moreover, capital expenditure (CAPEX) and operating expenditure (OPEX) are the key factors influencing the establishment of water desalination processes. Therefore, this paper provides a concise and yet comprehensive review of the membrane processes used to desalt saline water. Furthermore, the successes and failures of each process are critically reviewed. Finally, the CAPEX and OPEX of these water desalination processes are reviewed and compared. Based on the findings of this review, MD is relatively comparable to RO in terms of process performance achieving 99% salt rejections. Also, high salt rejections are reported on ED and PV. The operation and maintenance (O&M) costs remain lower in ED. Notably, the small-scale MD OPEX falls below that of RO. However, the large-scale O&M in MD is rarely reported due to its slow industrial growth, thus making RO the most preferred in the current water desalination markets.

Fate and role of fluorescence moieties in extracellular polymeric substances during biological wastewater treatment: A review

In biological wastewater treatment systems, extracellular polymeric substances (EPS) are continuously excreted as a response to environmental changes and substrate conditions. It could severely affect the treatment efficacy such as membrane fouling, dewaterability and the formation of carcinogenic disinfection by-products (DBPs). The heterogeneous dissolved organic matter (DOM) with varying size and chemical nature constitute a primary proportion of EPS. In the last few decades, fluorescence spectroscopy has received increasing attention for characterizing these organic substances due to the attractive features of this low-cost spectroscopic approach, including easy sample handling, rapid, non-destructive and highly sensitive nature. In this review, we summarize the application of fluorescence spectroscopy for characterizing EPS and provide the potential implications for online monitoring of water quality along with its limitations. We also link the dynamics of fluorescent dissolved organic matter (FDOM) in EPS with operational and environmental changes in wastewater treatment systems as well as their associations with metal binding, membrane fouling, adsorption, toxicity, and dewaterability. The multiple modes of exploration of fluorescence spectra, such as synchronous spectra with or without coupling with two-dimensional correlation spectroscopy (2D-COS), excitation-emission matrix (EEM) deconvoluted fluorescence regional integration (FRI), and parallel factor analysis (PARAFAC) are also discussed. The potential fluorescence indicators to depict the composition and bulk characteristics of EPS are also of interest. Further studies are highly recommended to expand the application of fluorescence spectroscopy paired with appropriate supplementary techniques to fully unravel the underlying mechanisms associated with EPS.

Utilisation of polarity inversion for phosphorus recovery in electrochemical precipitation with anaerobic digestion effluent

Phosphorus is a non-replaceable and limited resource, whose future drought is a matter of concern. Its recovery from wastewater has gained attention as a method of making the recovered phosphorus available for beneficial use. This study applied electrolysis with a platinum-coated titanium electrode to an actual anaerobic digestion effluent and investigated the phosphorus removal and precipitation characteristics with various current values. The separation of the phosphorus precipitate from the sludge residue and the usefulness of the polarity inversion to recover the phosphorus precipitate were evaluated. The availability of the recovered precipitate as a fertiliser was also demonstrated. The anaerobic digestion effluent after the centrifugal dehydration process with a coagulant was collected from a wastewater treatment plant. Phosphorus was precipitated as a form of calcium phosphate, including hydroxyapatite, which was confirmed by X-ray diffraction. The generated gases from the electrodes during the electrolysis floated and condensed the sludge residue, leading to successful separation of the phosphorus precipitate from the sludge residue. The polarity inversion successfully detached the phosphorus precipitate from the electrode, and at least 3 min polarity inversion was enough to detach the whole phosphorus precipitate. The submerging phosphorus precipitate contained abundant phosphorus (12.6%), in which citric-acid-soluble phosphorus accounted for 94.4%, and the utilisation of the precipitate as a slow-release phosphorus fertiliser was suggested.

Identification of Foulants on Polyethersulfone Membranes Used to Remove Colloids and Dissolved Matter from Paper Mill Treated Effluent

In this study, membrane fouling caused by paperboard mill treated effluent (PMTE) was investigated based on a dead-end ultrafiltration (UF) pilot-scale study. The membranes employed were commercial hydrophobic UF membranes made of polyethersulfone (PES) with a molecular weight cut-off of 10 kDa, 50 kDa, and 100 kDa. Membrane fouling mechanism during dead-end filtration, chemical analysis, field emission scanning electron microscopy (FESEM), energy-dispersive spectrophotometry (EDS), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and 3D fluorescence excitation–emission matrix (3DEEM) analysis were applied to understand which fraction of the dissolved and colloidal substances (DCS) caused the membrane fouling. The results indicated that the phenomenon controlling fouling mechanism tended to be cake layer formation (R2 ≥ 0.98) for all membranes tested. The 3DEEM results indicate that the majority of the organic foulants with fluorescence characteristics on the membrane were colloidal proteins (protein-like substances I+II) and macromolecular proteins (soluble microbial products, SMP-like substances). In addition, polysaccharide (cellulosic species), fatty and resin acid substances were identified on the fouled membrane by the ATR–FTIR analysis and play an important role in membrane fouling. In addition, the FESEM and EDS analyses indicate that the presence of inorganic foulants on the membrane surfaces, such as metal ions and especially Ca2+, can accelerate membrane fouling, whereas Mg and Si are linked to reversible fouling.

A full-scale survey of sludge landfill: sludge properties, leachate characteristics and microbial community structure

In this study, a full-scale survey was conducted of a sludge landfill that had been sealed for 10 years to investigate sludge properties, leachate characteristics and microbial community structure. Vertical distribution of sludge and leachate pollutants in the landfill site showed that the sludge and soluble pollutants in the leachate were both distributed almost evenly even after long-term anaerobic digestion, and higher concentrations of soluble pollutants and richness of microbial community were observed at the middle layer. Compared to dewatered excess sludge generated from the activated sludge process before landfill, landfill sludge had a much lower organic content (28.1%), smaller particle size and worse dewaterability. Compared to municipal waste landfill, sludge landfill generated leachate with a lower concentration of organic substances, and comparable concentrations of nitrogenous and phosphorus pollutants. Bacterial community analysis by Illumina MiSeq sequencing showed that Proteobacteria, Firmicutes, Chloroflexi, Actinobacteria and Bacteroidetes were the major phyla, and some new genera (Methylocystaceae, Mariniphaga and Aminicenantes) were enriched in the sludge landfill. Archaeal community analysis showed that aceticlastic methanogenesis by Methanosaeta and Methanosarcina was the main pathway for methane production in the sludge landfill, in contrast to waste landfill with hydrogenotrophic methanogenesis as the main pathway.

Co-Deposition of Stimuli-Responsive Microgels with Foulants During Ultrafiltration as a Fouling Removal Strategy

In this study, we show that codeposition of temperature responsive microgels in the foulant cake layer and cleaning of the cake upon stimuli-induced size change of the microgels is an effective method of fouling removal. Humic acid in CaCl₂ solution was used as a model foulant and poly( n-isopropylacrylamide) (p(NIPAm)) and poly( n-isopropylacrylamide- co-sulfobetainemethacrylate) (p(NIPAm- co-SBMA)) were used as temperature responsive microgels. Filtrations were done below the lower critical solution temperature (LCST) and temperature was increased to above the LCST for cleaning. As an extra cleaning a temperature swing of above, below and then again above the LCST was applied. P(NIPAm) was found to be ineffective in cleaning the foulant deposit despite the 20-fold change in its volume with temperature change at LCST. P(NIPAm- co-SBMA) microgels, on the other hand, provided high fouling reversibility on hydrophilic poly(ether sulfone)(PES)/poly(vinylpyrrolidone) (PVP) and hydrophobic PES membranes. Better fouling reversibility with these microgels was observed at low and high solution ionic strength. While the use of microgels alone increased fouling reversibility to some extent, even in the absence of temperature stimulus, 100% reversibility could only be obtained when a temperature switch was applied in the presence of microgels, showing the effect of microgels' volume change in cleaning.