Effect Of Coagulation Pretreatment Research Articles

Performance of gravity-driven membrane filtration for treatment of low-temperature and low-turbidity water: Effect of coagulation pretreatment

As a promising technology for decentralized drinking water treatment, performance of gravity-driven membrane (GDM) filtration for low-temperature and low-turbidity water purification and effects of coagulation pretreatment were investigated in this study. Flux development and permeate water quality of three GDM systems, i.e., control GDM without pretreatment, GDM with in-line coagulation (IC) pretreatment (IC-GDM), GDM with coagulation-sedimentation (CS) pretreatment (CS-GDM), were compared. Moreover, composition and morphology of bio-cake layer were characterized to explain underlying mechanisms for performance improvement by coagulation pretreatment. Results suggested that permeate flux of control GDM stabilized at ∼4.0 L/(m2·h) for raw water with relatively low temperature (8.1– 10.8 °C) and turbidity (1.15– 2.47 NTU). IC pretreatment increased stable flux by 65 %, although the bio-cake layer thickness in IC-GDM was increased by 71.7 %, probably due to a significant reduction (57.1 %) in extracellular polymeric substances (EPS) in the bio-cake layer. Meanwhile, CS pretreatment increased stable flux by 87.5 %, which can be attributed to 63.3 % reduction in EPS and 47.0 % decrease in bio-cake layer thickness. As for permeate water quality, dissolved organic matter (DOM) was significantly reduced by both IC and CS pretreatments, mainly due to transformation from dissolved state to particulates. Compared with control GDM, both IC and CS pretreatments resulted in decrease in microbial abundance and activity and changes in microbial community, probably due to decrease in bioavailable phosphate and DOM. This study suggested that both IC and CS pretreatments can be integrated with GDM filtration for efficient purification of low-temperature and low-turbidity water.

Effect of coagulation pretreatment on the performance of gravity-driven membrane filtration with Yangtze River water

Gravity-driven membrane (GDM) filtration is promising for decentralized drinking water treatments. To improve the permeability and permeate quality of GDM process, three GDM systems (in-line coagulation system, pre-coagulation system, and system without coagulation as control) were operated in parallel at 65 mbar for 63 days to evaluate the effects of coagulation on flux development, permeate quality, composition and structure of the biofouling layer. The optimal dose of coagulant (polyaluminium chloride) was determined as 4 mg/L. The stable flux increased by 140% and 210% in the in-line coagulation and pre-coagulation systems compared to that of the control system, respectively. Dissolved organic compounds were removed by 31.9% and 29.4% in the in-line coagulation and pre-coagulation systems, while the control system showed a poor removal efficiency (5%). The effective removal of organics from raw water by coagulation resulted in less substrates available in the biofouling layers, thus affecting the production of extracellular polymeric substances (EPS) by limiting the biological processes. The EPS content in the biofouling layer of pre-coagulation system was the lowest, consistent with the highest flux. And such nutrient-poor biofouling layers modified the microbial community, changing the dominant bacteria from Firmicutes to Actinobacteria, thereby affecting the flux. Loose and heterogeneous biofouling layers with numerous holes and cracks were observed under coagulation conditions, which was conducive to the passage of water. All systems exhibited excellent ammonia removal efficiency (96%), but coagulation extended the start-up time for nitrification from 3 days to 5 days. Overall, these results demonstrate that coagulation can improve the GDM filtration performance, and the pre-coagulation system is globally optimal.

Reverse electrodialysis for power generation using seawater/municipal wastewater: Effect of coagulation pretreatment

Reverse electrodialysis (RED) converts salinity gradient energy (SGE) into electrical energy. Seawater and river water are the most promising feed solutions for RED, but municipal wastewater instead of river water could be acceptable if the wastewater treatment plant is close to the sea because river water is often used for human demands, i.e., for drinking water and agricultural irrigation. In addition, the environmental impact at the mixing point (the brackish water region) must be considered. Wastewater pretreatment is necessary to prevent performance reduction in the subsequent RED process. In this study, the effect of pretreatment using polyaluminum chloride (PAC) as a coagulant on the RED performance was investigated using model and real municipal wastewater and seawater samples. The results reveal that the presence of PAC residue after coagulation negatively affected the subsequent RED process, especially because of the increased cation exchange membrane resistance, which resulted in a 40% reduction in power generation. However, the optimized PAC dosage (and lower amount of PAC residue) resulted in a 20% improvement in RED power generation because 50% of the wastewater organic content had been removed. Thus, coagulation pretreatment with the optimized PAC dose is useful before RED power generation when using municipal wastewater.

Effect of coagulation pretreatment on microfiltration of paper mill wastewater using electrospun membranes

Effect of coagulation pretreatment on microfiltration of paper mill wastewater using electrospun membranes

Removal of perfluorooctane sulfonates from water by a hybrid coagulation–nanofiltration process

Perfluorooctane sulfonates (PFOS) as a persistent, bioaccumulative and toxic pollutant has been detected in surface water around the world and attracted great attention in recent years. It’s important and urgent to develop effective technology to remove PFOS. The effect of coagulation pretreatment on the removal efficiency of PFOS by nanofiltration (NF) was investigated. Two types of NF membranes were used to test the PFOS rejections in three different solutions including DI water, humic acid (HA) solution and coagulation solution. The characterizations such as HA and PFOS accumulation on the membranes, scanning electron microscopy (SEM) and surface ζ potential analysis of virgin and fouled membranes were conducted to study the separation mechanism. The experimental results showed that PFOS can be partly removed by coagulation. The NF270 membrane exhibited a higher PFOS rejection (>95%) than the HYDRACORE membrane (40–60%) in single NF process. The surface negative charge density of both NF membranes reached a minimum value after filtering coagulation effluent, which might be a result of floc adsorption on the membrane surface. Coagulation pretreatment was effective to enhance the PFOS rejection of the NF membranes. In particular, the rejection efficiency of the HYDRACORE membrane increased from 55% to 86% at a trans-membrane pressure of 0.4MPa. The SEM images showed that the fouling layer formed by flocs was loose and highly permeable.

Effect of coagulation mechanisms on the fouling and ultrasonic cleaning of PTFE membrane

In this study, the effect of coagulation pretreatment on membrane fouling and ultrasonic cleaning efficiency was investigated using a dead-end polytetrafluoroethylene (PTFE) microfiltration system. The extent of membrane fouling was examined under different coagulation mechanisms such as charge neutralization (CN), electrostatic patch effect (EPE) and sweep flocculation (SW). Fouling through EPE mechanism provided the greatest flux decline and least permeate flux recovery over CN and SW. EPE produces more stable, smaller and more compact flocs while CN and SW have large, easily degraded and highly-branched structured flocs. The predominant fouling mechanism of EPE, CN and SW is pore blocking, a combination of pore blocking and cake formation, and cake formation, respectively. Better permeate flux recovery is observed with SW over CN and EPE, which implies formation of less dense and more porous cake deposits. The morphology of fouled membranes was examined using scanning electron microscopy (SEM).

Effect of coagulation pretreatment on membrane distillation process for desalination of recirculating cooling water

The objective of this work was to study the performance of coagulation pretreatment and the effect of coagulation pretreatment on membrane distillation (MD) process for desalination of recirculating cooling water (RCW). In this study, RCW was desalinated by MD process after pretreated by coagulation, precision filtration, acidification and degassing. Pre-coagulation of RCW with poly-aluminum chloride (PACl) coagulant improves the elimination of total organic carbon (TOC), total phosphorus (TP) substances. The optimized pH value and PACl dosage were 7 and 15 mg/L, respectively. Compared with MD process without coagulation pretreatment, much slighter MD flux decrease was observed when coagulation pretreatment was employed for desalination of RCW using MD process. Compared with the membrane flux of MD process for the RCW without coagulation pretreatment, about 23% improvement of MD flux was obtained by employing coagulation pretreatment after 30 days operation. The variation of crystal morphology on membrane surface and hydrophobic character of PVDF hollow membranes were investigated. The results show that distorted rhombic magnesium-calcite scale formed on membrane surface when coagulation pretreatment was employed. However, the deposited magnesium-calcite on the membrane surface was loosely packed with particles of much smaller size when RCW without coagulation pretreatment was fed. It is because that the coagulation pretreatment removed most of the natural organic matter (NOM) and antiscalant additives in RCW, which act as an inhibitor to calcium carbonate crystal growth in water. After coagulation pretreatment, bigger crystal deposit formed on membrane surface with the increase in CF of RCW. It was rather difficult for bigger crystal to enter membrane pores and thus the formation of bigger crystal reduced partial wetting of membrane, which enhanced trans-membrane flux. The hydrophobic character of PVDF hollow membranes used in MD process can be easily recovered by chemical rinsing using acid–base solutions. The results indicate that the pretreatment with chemical coagulant, PACl, is effective for the desalination of RCW by MD process.

Effect of coagulation pretreatment on the fouling of ultrafiltration membrane

The purpose of this study is to understand the effect and mechanism of preventing membrane fouling, by coagulation pretreatment, in terms of fractional component and molecular weight of natural organic matter (NOM). A relatively higher molecular weight (MW) of hydrophobic compounds was responsible for a rapid decline in the ultrafiltration flux. Coagulation could effectively remove the hydrophobic organics, resulting in the increase of flux. It was found that a lower MW of neutral hydrophilic compounds, which could remove inadequately by coagulation, was responsible for the slow declining flux. The fluxes in the filtration of coagulated water and supernatant water were compared and the results showed that a lower MW of neutral hydrophilic compounds remained in the supernatant water after coagulation could be rejected by a membrane, resulting in fouling. It was also found that the coagulated flocs could absorb neutral hydrophilic compounds effectively. Therefore, with the coagulated flocs formed on the membrane surface, the flux decline could be improved.

Effect of coagulation pretreatment on fouling of an ultrafiltration membrane

The purpose of this study is to understand the effect and mechanism on preventing membrane fouling by coagulation pretreatment in terms of fractional component and molecular weight of NOM. A relatively high molecular weight (MW) of hydrophobic compounds was responsible for rapid decline in ultrafiltration flux. Coagulation could effectively remove hydrophobic organics, resulting in the increase of flux. It was found that lower MW of neutral hydrophilic compounds, which could be poorly removed by coagulation, was responsible for the slow flux decline. The flux in the filtration of coagulated and supernatant water was compared and the results show that lower MW of neutral hydrophilic compounds which remained in the supernatant water after coagulation could be rejected by the membrane, resulting in fouling. It was also found that coagulated flocs could effectively absorb neutral hydrophilic compounds. Therefore, with the coagulated flocs formed on the membrane surface, the flux decline could be improved.

Effect of coagulation pretreatment on membrane filtration performance

The effect of coagulation on microfiltration and ultrafiltration performance was evaluated in a recent laboratory study. The primary variables used in the study included source water, type of coagulant, coagulant dose, coagulation application conditions, and membrane material. Fouling was evaluated using flat sheet membranes in an unstirred filtration cell. As described in this article, a linear relationship between the removal of dissolved organic carbon (DOC), as measured by ultraviolet absorbance at 254 nanometres, and the reduction of fouling was observed. In addition, coagulation conditions appeared to influence the extent of fouling of the microfiltration membrane but had no effect on the ultrafiltration membranes. Low coagulant doses frequently caused greater fouling than no coagulation, but coagulant doses for enhanced coagulation always reduced fouling. Prefiltration was used to separate the effects of coagulation on particulate matter and DOC, and the results suggested that more than 80% of the fouling in these experiments was because of dissolved matter.