Insights into the roles of recently developed coagulants as pretreatment to remove effluent organic matter for membrane fouling mitigation

© 2018 Elsevier B.V. Membrane fouling by dissolved organic matter (EfOM) in secondary treated effluent is a problematic and inevitable issue during wastewater reclamation using low pressure membrane filtration. This study evaluates the performance of coagulation/flocculation (C/F) using two recently developed coagulants (namely TiCl4 and ZrCl4) in comparison to conventional alum (i.e. Al2(SO4)3) as pretreatment to remove EfOM for subsequent ultrafiltration (UF) membrane fouling mitigation. At the optimal dosage, TiCl4-based C/F pretreatment showed the greatest performance in membrane fouling mitigation, followed by ZrCl4 and then alum. The underlying mechanisms were well explained by classical fouling models and the extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory, highlighting a dominant role of standard blocking in the fouling potential of the C/F treated EfOM. The interfacial free energy of cohesion and adhesion showed that C/F pretreatment using TiCl4 and ZrCl4 as coagulant can lower the binding affinity between EfOM molecules and between EfOM molecules and membrane surface, ultimately reduce membrane fouling. The results of size exclusion chromatography (SEC) and fluorescence excitation emission matrix- parallel factor analysis (EEM-PARAFAC) also supported the classical fouling mechanisms, providing additional insights into the potential roles of chemical interactions in the preferential removal of certain organic substances by C/F pretreatment and the chemical composition of subsequent membrane foulants. Protein-like components were highly associated with reversible fouling after the C/F, while the reversibility of humic-like substances was enhanced upon C/F pretreatment. After C/F pretreatment, small sized EfOM molecules became the dominant fraction responsible for UF membrane fouling

Synergistic effect of dual flocculation between inorganic salts and chitosan on harvesting microalgae Chlorella vulgaris

© 2020 Elsevier B.V. The flocculation efficiency of microalgae Chlorella vulgaris for subsequent harvesting was investigated using single flocculants of inorganic salts, synthetic polymer, chitosan and dual flocculants of inorganic salts and chitosan. Synthetic polymer (FlopamTM) could achieve over 90% optical density removal (OD680removal) at a low flocculant dose (20 to 40 mg polymer per litre of algal suspension) through the bridging mechanism and charge neutralisation. Inorganic salts (i.e. ferric chloride and aluminium sulphate) and chitosan individually resulted in low flocculation efficiency (80% flocculation efficiency, significantly higher than the sum of each individual flocculation. The improvement in flocculation efficiency was 57 and 24% respectively for ferric chloride/chitosan and aluminium sulphate/chitosan. Charge neutralisation of microalgal cells by ferric chloride or aluminium sulphate combined with bridging by chitosan produced the synergy

Relationship between the synergistic/antagonistic effect of anaerobic co-digestion and organic loading

© 2017 Results from this study reveal a notable relationship between the synergistic/antagonistic performance of sewage sludge – food waste anaerobic co-digestion (AcoD) and organic loading. At the same sewage sludge content, biomethane potential assays show an increasing specific methane yield as the content of food waste increased to the optimum organic loading of 15 kg VS/m3. Under these conditions, the specific methane yields experimentally measured in this study were considerably higher than those calculated by adding the specific methane individual co-substrates during mono-digestion. On the other hand, at above the optimum organic loading value, the antagonistic effect (i.e. lower specific methane yield compared to mono-digestion) was observed. The relationship between synergistic performance of AcoD and organic loading was also evidenced in the removal of volatile solids as well as chemical oxygen demand. Further analysis of the intermediate products show that methanogenesis was the rate limiting step during AcoD at a high organic loading value. As the organic loading increased, the digestion lag phase increased and the hydrolysis rate decreased

Performance of a seawater-driven forward osmosis process for pre-concentrating digested sludge centrate: Organic enrichment and membrane fouling

© 2018 The Royal Society of Chemistry. This study demonstrated the potential of seawater-driven forward osmosis for enriching organic matter in digested sludge centrate. The results indicated that the cellulose triacetate membrane offered better performance than the polyamide membrane in terms of organic materials enrichment, fouling resistance and membrane cleaning efficiency. Membrane fouling decreased the enrichment efficiency of organic matter since the deposition of suspended particulate matter on the membrane surface caused fouling and loss of organic matter from the concentrated sludge centrate. The results showed that increasing the draw solution concentration increased flux but did not aggravate membrane fouling, however, it could reduce the efficiency of physical flushing to recover the flux. Seawater showed comparable forward osmosis performance to that of analytical grade NaCl as draw solutes in terms of flux and organic enrichment. The results also showed that seawater as the draw solution resulted in more membrane fouling and lower flux recovery compared to NaCl

Anaerobic digestion of soft drink beverage waste and sewage sludge

© 2018 Soft drink beverage waste (BW) was evaluated as a potential substrate for anaerobic co-digestion with sewage sludge to increase biogas production. Results from this study show that the increase in biogas production is proportional to the increase in organic loading rate (OLR) rate due to BW addition. The OLR increase of 86 and 171% corresponding to 10 and 20% BW by volume in the feed resulted in 89 and 191% increase in biogas production, respectively. Under a stable condition, anaerobic co-digestion with BW did not lead to any significant impact on digestate quality (in terms of COD removal and biosolids odour) and biogas composition. The results suggest that existing nutrients in sewage sludge can support an increase in OLR by about 2 kg COD/m3/d from a carbon rich substrate such as soft drink BW without inhibition or excessive impact on subsequent handling of the digestate

Membrane scaling and prevention techniques during seawater desalination by air gap membrane distillation

Membrane scaling and mitigation techniques during air gap membrane distillation (AGMD) of seawater were investigated. The results showed a strong influence of AGMD operating temperature on not only the process water flux but also membrane scaling and subsequent cleaning efficiency. Elevating feed/coolant temperature from 35/25 to 60/50 °C increased water flux, but also exacerbated membrane scaling of the AGMD process. Membrane scaling was more severe, and occurred at a lower water recovery (68%) when operating at 60/50 °C compared to 35/25 °C (78%) due to increased concentration polarisation effect. Operating temperature also affected the efficiency of the subsequent membrane cleaning. Membrane scaling that occurred at low temperature (i.e. 35/25 °C) was more efficiently cleaned than at high temperature (i.e. 60/50 °C). In addition, membrane cleaning using vinegar was much more efficient than fresh water. Nevertheless, vinegar cleaning could not completely restore the membrane surface to the original condition. Traces of residual scalants on the membrane surface accelerated scaling in the next operation cycle. On the other hand, anti-scalant addition could effectively control scaling. Membrane scaling during AGMD of seawater at 70% water recovery and 60/50 °C was effectively controlled by anti-scalant addition

Enhanced biological phosphorus removal and its modeling for the activated sludge and membrane bioreactor processes

A modified activated sludge process (ASP) for enhanced biological phosphorus removal (EBPR) needs to sustain stable performance for wastewater treatment to avoid eutrophication in the aquatic environment. Unfortunately, the overall efficiency of the EBPR in ASPs and membrane bioreactors (MBRs) is frequently hindered by different operational/system constraints. Moreover, although phosphorus removal data from several wastewater treatment systems are available, a comprehensive mathematical model of the process is still lacking. This paper presents a critical review that highlights the core issues of the biological phosphorus removal in ASPs and MBRs while discussing the inhibitory process requirements for other nutrients' removal. This mini review also successfully provided an assessment of the available models for predicting phosphorus removal in both ASP and MBR systems. The advantages and limitations of the existing models were discussed together with the inclusion of few guidelines for their improvement. © 2013 Elsevier Ltd

Current status and perspectives on anaerobic co-digestion and associated downstream processes

© The Royal Society of Chemistry. Anaerobic co-digestion (AcoD) has the potential to utilise spare digestion capacity at existing wastewater treatment plants to simultaneously enhance biogas production by digesting organic rich industrial waste and achieve sustainable organic waste management. While the benefits of AcoD regarding biogas production and waste management are well established, the introduction of a new organic waste (i.e. co-substrate) with different chemical composition compared to residential sewage sludge is expected to impact on not only the anaerobic digestion process itself but also downstream processing of biogas and digestate. This work critically evaluates the potential impact (both positive and negative) of co-digestion on key downstream processes in the context of AcoD of sewage sludge and organic waste. AcoD can potentially lead to significant changes in biogas quality, digestate dewaterability, biosolids odour and the nutrient balance within the overall wastewater treatment process. The literature reviewed here suggests that effective management of these impacts can enhance the economic and environmental benefits of AcoD. Potential techniques to manage the impact of AcoD on downstream processing include co-substrate selection to minimise sulphur content, co-substrate pretreatment to improve dewaterability, process optimisation to minimize downstream impacts, biological desulphurisation of biogas, and side stream nutrient recovery. These techniques have been investigated and in some cases successfully applied for conventional anaerobic digestion. Nevertheless, further research is needed to adapt them for AcoD. In particular, the issue of nutrient accumulation due to AcoD can be seen as an opportunity to utilise recently commercialised technologies (e.g. Phosnix and Ostara) and currently emerging processes (e.g. forward osmosis and electrodialysis) for phosphorus recovery from food waste and wastewater

Effects of COD/N ratio on soluble microbial products in effluent from sequencing batch reactors and subsequent membrane fouling

© 2018 Elsevier Ltd The relative ratios of chemical oxygen demand (COD) to nitrogen (N) in wastewater are known to have profound effects on the characteristics of soluble microbial products (SMP) from activated sludge. In this study, the changes in the SMP characteristics upon different COD/N ratios and the subsequent effects on ultrafiltration (UF) membrane fouling potentials were examined in sequencing batch reactors (SBR) using excitation emission matrix-parallel factor analysis (EEM-PARAFAC) and size exclusion chromatography (SEC). Three unique fluorescent components were identified from the SMP samples in the bioreactors operated at the COD/N ratios of 100/10 (N rich), 100/5 (N medium), and 100/2 (N deficient). The tryptophan-like component (C1) was the most depleted at the N medium condition. Fulvic-like (C2) and humic-like (C3) components were more abundant with N rich wastewater. Greater abundances of large size biopolymer (BP) and low molecular weight neutrals (LMWN) were found under the N deficient and N rich conditions, respectively. SMPs from various COD/N exhibited a greater degree on membrane fouling following the order of 100/2 > 100/10 > 100/5. C1 and C2 had close associations with reversible and irreversible fouling, respectively, while the reversible fouling potential of C3 depended on the COD/N ratios. No significant impact of COD/N ratio was observed on the relative contributions of SMP size fractions to either reversible or irreversible fouling potential. However, the COD/N ratios likely altered the BP foulants' composition with greater contribution of proteinaceous substances to reversible fouling under the N deficient condition than at other N richer conditions. The opposite trend was observed for irreversible fouling. Our results provided further insight into changes in different SMP constitutes and their membrane fouling in response to microbial activities under different COD/N ratios

Physical cleaning techniques to control fouling during the pre-concentration of high suspended-solid content solutions for resource recovery by forward osmosis

© 2017 The fouling propensity of digested sludge centrate, and the effectiveness of membrane flushing, air-scouring, and ultrasonication for physical cleaning were systematically evaluated. Accelerated fouling conditions were applied to simulate the long-term and intensive pre-concentration scenario that is required for phosphorus recovery from digested sludge centrate. The results suggest that membrane fouling during forward osmosis operation to pre-concentrate digested sludge centrate is mostly due to the deposition of small mineral crystals and particulate matter on the membrane surface. Both high cross-flow velocity flushing and ultrasonication were effective at preventing membrane fouling under accelerated fouling conditions. Our results also highlight the potential of intermittent membrane cleaning for achieving a higher cumulative permeate volume and lower energy consumption in comparison to continuous application to prevent membrane fouling. Among several physical cleaning regimes investigated in this study, the combination of ultrasonication and high cross-flow velocity flushing was the most effective and could maintain stable FO operation over several consecutive cleaning cycles