Hybrid chitosan/FeCl3 coagulation–membrane processes: Performance evaluation and membrane fouling study in removing natural organic matter

A novel chitosan and polyferric sulfate composite coagulant for biogas slurry pretreatment by simultaneous flocculation and floatation: Performance and underlying mechanisms

Biogas slurry from anaerobic digestion is rich in nutrients but has not been fully utilized due to a high content of suspended solids (SS) causing clogging during agricultural irrigation. This study aimed to evaluate the performance of a novel chitosan and polyferric sulfate (CTS-PFS) composite coagulant for simultaneous flocculation and floatation to enhance SS removal while preserving nutrients in biogas slurry. Orthogonal method was used for experimental design to determine the optimal synthesis and operational conditions of CTS-PFS. Results show that CTS-PFS outperformed individual CTS and PFS coagulant in terms of SS removal and nutrient (nitrogen, phosphorus, and potassium) preservation. Compared to individual CTS and PFS coagulation, the combination of CTS and PFS at the mass ratio of 1:6 showed significantly higher performance by 41.5 % increase in SS removal and 5.2 % reduction in nutrient loss. The improved performance of CTS-PFS was attributed to its formation of polynuclear hydroxyl complexes with ferric oxide groups (e.g. Fe-OH, Fe-O-Fe, Fe-OH-Fe and COO-Fe) to strengthen charge neutralization and adsorption bridging. Data from this study further confirm that CTS-PFS enhanced the removal of small suspended particles and dissolved organic matter in the molecular weight range of 0.4-2.0 kDa and preserved ammonia and potassium better in biogas slurry. Bubbles were generated as hydrogen ions from coagulant hydrolysis interacted with bicarbonate and carbonate in biogas slurry for removing the produced flocs by floatation. Floc flotation was more effective in CTS-PFS coagulation due to the significant production of uniform bubbles, evidenced by the reduction in the viscosity of biogas slurry.

Adsorptive chito-beads for control of membrane fouling

Chitosan has excellent antimicrobial, adsorption, heavy metal removal, and adhesion properties, making it a good substitute for microplastic-based cleaners. Here, chitosan microbeads (chito-beads) of various sizes ranging from 32 μm to 283 μm were prepared via emulsion using a liquid on oil method and the feasibility of using them as an essential constituent in a chemical cleaning solution for a reverse-osmosis (RO) membrane-fouling-control process was assessed. Prior to the assessment the cleaning efficiency of a solution containing chito-beads, the interaction energy between chitosan and a representative organic foulant (humic acid (HA)) in a RO membrane fouling was analyzed using colloidal atomic force microscopy, and the strongest attraction between chitosan and HA was observed in an aqueous solution. When comparing the membrane cleaning efficiency of cleaning solutions with and without chito-beads, smaller chito-beads (32 μm and 70 μm) were found to have higher cleaning efficiency. Applications of chito-beads to the membrane cleaning process can enhance the cleaning efficiency through the physicochemical interaction with organic foulants. This study can widen the use of chito-beads as an additive to membrane chemical cleaning solutions to control membrane fouling in other membrane processes as well.

Response surface methodology to investigate the effects of operational parameters on membrane fouling and organic matter rejection in hard-shell encased hollow-fiber membrane

The response surface methodology (RSM) was found useful statistical tool for understanding combined effect of filtration, backwashing time and dosage of sodium hypochlorite (NaOCl) added into backwash water as three operational parameters on transient behavior of transmembrane pressure (TMP) and organic rejection efficiency. Here, encased, hollow-fiber ultrafiltration (UF) system was developed for treating synthetic humic acid (HA) solution. To carry out RSM analysis, experimental matrix was designed by Box-Behnken model. Results showed that the NaOCl dosage for the chemical enhanced backwashing (CEB) as single parameter played most dominant role in fouling mitigation. Effect of hydraulic cleaning by applying the permeate backwashing only without performing the CEB was almost negligible to flush the fouling layer from membrane. Considering two correlated parameters, backwashing time combined with NaOCl dosage was found much more important than combination of other parameter such as filtration time to reduce fouling rate. Regardless of the operational parameters, the organic rejection efficiency was maintained 30% only. The RSM analysis also found that 40.1 min of filtration, 1.1 min of backwashing and 834 mg/L of NaOCl were optimum operating parameters to enhance both permeate recovery and fouling mitigation.

A decade development in the application of chitosan-based materials for dye adsorption: A short review

The presence of dyes in the aquatic environment as a result of anthropogenic activities, especially textile industries, is a critical environmental challenge that hinders the availability of potable water. Different wastewater treatment approaches have been used to remediate dyes in aquatic environments; however, most of these approaches are limited by factors ranging from high cost to the incomplete removal of the dyes and contaminants. Thus, the use of adsorption as a water treatment technology to remove dyes and other contaminants has been widely investigated using different adsorbents. This study evaluated the significance of chitosan as a viable adsorbent for removing dyes from water treatment. We summarised the literature and research results obtained between 2009 and 2020 regarding the adsorption of dyes onto chitosan and modified chitosan-based adsorbents prepared through physical and chemical processing, including crosslinking impregnation, grafting, and membrane preparation. Furthermore, we demonstrated the effects of various chitosan-based materials and modifications; they all improve the properties of chitosan by promoting the adsorption of dyes. Hence, the application of chitosan-based materials with various modifications should be considered a cutting-edge approach for the remediation of dyes and other contaminants in aquatic environments toward the global aim of making potable water globally available.

Application of coagulation-ultrafiltration-nanofiltration in a pilot study for Tai Lake water treatment

In this study, the inline coagulation was combined with ultrafiltration and nanofiltration (UF-NF) in a pilot study for Tai Lake water treatment. The results showed that the combination process was very effective for Tai Lake water treatment in terms of organic removal and membrane fouling control. With inline coagulation, no irreversible membrane fouling was observed for either UF or NF at fluxes of 65-90 and 22-26 L/(m²  hr), respectively. The membrane foulants were analyzed, and the results indicated that the low molecular weight fractions in the feed were main membrane fouling contributors for both UF and NF, where hydrophilic substances and proteins, as well as neutral substances and humic acids with polycarboxyl groups, contributed significantly to UF and NF membrane fouling, respectively. Compared with direct UF-NF filtration without coagulation, the coagulants could aggregate organic micromolecules for cake formation. With inline coagulation, the moving flocs could generate shear stress to scrub the membrane surface for fouling control of UF. Moreover, with inline coagulation, the organics removal efficiency could be further increased by 10%-20%. With NF, the permeate had a TOC concentration of less than 0.5 mg/L, satisfying the drinking water quality. Therefore, the coagulation-UF-NF is very useful for Tai Lake water treatment. PRACTITIONER POINTS: Inline coagulation-UF-NF for Tai Lake Water treatment is implemented. Inline coagulation can aggregate hydrophilic substances to reduce membrane fouling. Moving flocs produce shear stress for fouling control of UF-NF. Superior quality of permeate is achieved with the combined coagulation-UF-NF process.

PAC-PDMDAAC pretreatment of typical natural organic matter mixtures: Ultrafiltration membrane fouling control and mechanisms

Precoagulation by polyaluminum chloride-poly dimethyl diallyl ammonium chloride (PAC-PDMDAAC) prior to ultrafiltration (UF) was conducted to evaluate the influence of PAC-PDMDAAC on controlling membrane fouling from typical natural organic matter (NOM) mixtures of humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA). Membrane flux decline and flux recovery after backwashing were investigated to evaluate the membrane fouling. The fouling mechanisms were determined from the floc size, floc structure and membrane resistance. PAC-PDMDAAC effectively alleviated membrane fouling caused by the HA, HA-BSA, HA-SA and HA-BSA-SA mixtures; furthermore, membrane fouling was better mitigated in the HA-SA and HA-BSA-SA mixtures. The untreated HA-SA and HA-BSA-SA mixtures caused much more serious total membrane resistance and fouling due to blocking and adsorption in the membrane pores by particles with sizes similar to those of the pores. The increased membrane flux and decreased irreversible resistance after the PAC-PDMDAAC pretreatment were attributed to the formation of flocs with a large size and small fractal dimension, which mainly formed a cake layer on the membrane surface. However, PAC-PDMDAAC was not particularly effective in reducing the irreversible membrane fouling originating from the HA and HA-BSA mixtures due to the formation of aggregates and pore blocking by microflocs.

Synthesis and characteristics of a novel FeNi3/SiO2/TiO2 magnetic nanocomposites and its application in adsorption of humic acid from simulated wastewater: study of isotherms and kinetics

The presence of natural organic matter such as humic acid in water creates various problems in water purification. Humic acid can react with chlorine in the disinfection step and lead to the production of trihalomethanes and haloacetic acids that these compounds have carcinogenic and mutagenic properties; therefore, they must be removed before arriving to the disinfection stage. The purpose of this research was adsorption of humic acid from simulated wastewater by synthesized FeNi₃/SiO₂/TiO₂ magnetic nanocomposites. FeNi₃/SiO₂/TiO₂ magnetic nanocomposites were synthesized by sol-gel procedure and its characteristics were determined by TEM, VSM, BET, FESEM, and XRD techniques. Then, the effects of such pH (3-11), FeNi₃/SiO₂/TiO₂ dosage (0.005-0.1 g/L), contact time (0-200 min), and initial concentration (2-15 mg/L) were studied on humic acid adsorption using FeNi₃/SiO₂/TiO₂. The results of adsorption experiments revealed that the highest percentage of humic acid removal (94.4%) was achieved at pH 3, initial concentration of 5 ppm, FeNi₃/SiO₂/TiO₂ dose of 0.1 g/L, and contact time of 90 min. The analyses of experimental isotherm data showed that the humic acid adsorption was described by Langmuir model and also the kinetic studies represented that the process of adsorption of humic acid on FeNi₃/SiO₂/TiO₂ was followed by the pseudo-second kinetic. According to the results, it can be concluded that FeNi₃/SiO₂/TiO₂ magnetic nanocomposites have a high ability to absorb humic acid from simulated wastewater.

Rendering plant wastewater reclamation by coagulation, sand filtration, and ultrafiltration

The rendering plant secondary effluent (SE) was reclaimed with coagulation, sand filtration and ultrafiltration for reuse in the plant and for potential reuse in irrigation. The best coagulant was selected and the pH and coagulant dosage were optimized with response surface methodology (RSM) to achieve low turbidity, conductivity, and content of carbon at a higher pH. Residual flocs from the coagulation were separated with sand filtration, and afterward, the effluent was treated with six ultrafiltration membranes. The pretreatment (coagulation and sand filtration) drastically reduced fouling (50-95%). The main water parameters (turbidity, conductivity, pH, content of carbon, chemical oxygen demand, and content of cations and anions) were determined in each treatment step. The physico-chemical parameters and microbiological analysis of the resulting permeate showed that it could be reused in the rendering plant for washing purposes, and it satisfies the main regulations and guidelines for wastewater reuse, i.e. US EPA and FAO.

Multiple dynamic Al-based floc layers on ultrafiltration membrane surfaces for humic acid and reservoir water fouling reduction

The integration of adsorbents with ultrafiltration (UF) membranes is a promising method for alleviating membrane fouling and reducing land use. However, adsorbents typically are only injected into the membrane tank once, resulting in a single dynamic protection layer and low removal efficiency over long-term operation. In addition, the granular adsorbents used can cause membrane surface damage. To overcome these disadvantages, we injected inexpensive and loose aluminum (Al)-based flocs directly into a membrane tank with bottom aeration in the presence of humic acid (HA) or raw water taken from the Miyun Reservoir (Beijing, China). Results showed that the flocs were well suspended in the membrane tank, and multiple dynamic floc protection layers were formed (sandwich-like) on the membrane surface with multiple batch injections. Higher frequency floc injections resulted in better floc utilization efficiency and less severe membrane fouling. With continuous injection, acid solutions demonstrated better performance in removing HA molecules, especially those with small molecular weight, and in alleviating membrane fouling compared with the use of high aeration rate or polyacrylamide injection. This was attributed to the small particle size, large specific surface area, and high zeta potential of the flocs. Additionally, excellent UF membrane performance was exhibited by reservoir water with continuous injection and acid solution. Based on the outstanding UF membrane performance, this innovative integrated filtration with loose Al-based flocs has great application potential for water treatment.

Optimization of coagulation pre-treatment for alleviating ultrafiltration membrane fouling: The role of floc properties on Al species

This study investigated membrane fouling in a coagulation/ultrafiltration (C-UF) process by comparing the floc properties and humic acid (HA) removal efficiency of three hydrous Al(III) species (Al_a, Al_b, and Al_c). The results indicated that the coagulation and membrane mechanisms were different for all three Al species because of the differences in floc properties. The HA removal efficiency increased with increasing Al dosage until an equilibrium was reached at the optimal dosage of 6 mg L^-1. In addition, membrane fouling gradually decreased as the Al dosages increased. Regardless of coagulant type, the OH and COOH functional groups of HA reacted with the Al species. Both external and internal membrane fouling were strongly dependent on the porosity of the cake layer and on the size distribution of the floc particulates, respectively. The pore area of the cake layer formed by the Al_a-coagulated effluent was large because of the strong charge neutralization. Moreover, Al_a generated large and loose flocs with a porous cake layer that mitigated external fouling. However, the internal fouling with the Al_c coagulant was significant because the concentration of residual aggregates in the membrane pores was high.

Optimization of coagulation with ferric chloride as a pretreatment for fouling reduction during nanofiltration of rendering plant secondary effluent

The treatment and reuse of rendering plant wastewater with membrane processes is a poorly investigated area that could result in substantial water savings. Membrane fouling is still the main obstacle when treating secondary effluents (SEs) with high content of effluent organic matter (EfOM). Thus, the optimization of coagulation with ferric(III) chloride (FeCl₃) as a pretreatment for nanofiltration was performed to reduce membrane fouling and achieve higher permeate quality. Coagulation was modeled (total carbon, inorganic carbon, dissolved organic carbon (DOC), turbidity, conductivity, and resulting pH) and optimized with response surface methodology (RSM) to remove DOC and turbidity with a pH close to neutral. The effluent after coagulation at optimal conditions (5.58 pH and 26.38 mg L^-1 of Fe³⁺) and sand filtration (SF) was subjected to nanofiltration (NF270, NF, and NF90 membranes). The fouling was compared to evaluate the efficiency of each pretreatment. Coagulation with FeCl₃ reduced the flux decline of nanofiltration membranes 4.2 to 19.3 times while SF barely reduced the fouling. Coagulation increased the flux recovery and chemical cleanliness after the membrane washing. In addition to fouling reduction, higher permeate quality was achieved.

Coagulation of humic waters for diffused pollution control and the influence of coagulant type on DOC fractions removed

This study examined the suitability of organic coagulants for treatment of typically humic peat extraction runoff water by comparing their performance with that of ferric sulphate (FS). The influence of coagulant type on dissolved organic carbon (DOC) fractions removed was analysed in detail using LC-OCD-OND (size exclusion liquid chromatography coupled with organic carbon and organic nitrogen detection) fractionation techniques. In general, lower coagulant dosage was needed under acidic (pH 4.5) than neutral (pH 6.5) conditions. Chitosan (Chit) and poly (diallyldimethyl) ammonium chloride (pDMAC) required significantly lower dosage (40-55%) than FS for acceptable purification, while a tannin-based coagulant (Tan2) required substantially higher dosage (55-75%) independent of water pH. FS demonstrated the best removal of DOC (<81%) and phosphorus (<93%) followed by pDMAC, while Chit and Tan2 achieved the highest removal of suspended solids (SS) (<58%), with flocs formed by Tan2 presenting the best settling properties. Higher molecular weight (MW) DOC fractions were more efficiently removed by all coagulants, with FS being the most efficient (biopolymers 69% and humic substances 91%), followed by Tan2. FS also displayed satisfactory removal of lower MW fractions (building blocks ∼46% and low MW neutrals 62%). Overall, FS was the best performing coagulant. Nevertheless, the organic polymers demonstrated satisfactory overall performance, achieving purification rates mostly inside the requirements set by Finnish environmental authorities.