Analysis of filtration characteristics in submerged microfiltration for drinking water treatment

Hollow fiber membranes have been widely employed for water and wastewater treatments. Nevertheless, understanding the filtration characteristics of hollow fiber membranes is complicated by the axial distributions of transmembrane pressure (TMP) and flux, which are key factors for both fouling control and module design. In this study, model equations to account for different fouling mechanisms were derived to analyze the performance of submerged hollow fiber systems with different conditions in terms of feed water characteristics and membrane material. A series of experiments with synthetic feed and raw water were carried out using hydrophilic and hydrophobic membrane modules. The model successfully fits the experimental results for synthetic feed as well as raw water. The major fouling mechanisms for filtration of raw water using hydrophilic and hydrophobic membranes are identified as cake formation and standard blocking, respectively. The model calculations indicate that the distributions of flux and cake (fouling) resistance are sensitive to the fiber length of the membrane.     

Oilfield wastewater treatment by combined microfiltration and biological processes.

This work deals with the treatment of offshore oilfield wastewater from the Campos Basin (Rio de Janeiro State, Brazil). After coarse filtration, this high saline wastewater was microfiltrated through mixed cellulose ester (MCE) membranes, resulting in average removals of COD, TOC, O&G and phenols of 35%, 25%, 92% and 35%, respectively. The permeate effluent was fed into a 1-L air-lift reactor containing polystyrene particles of 2mm diameter, used as support material. This reactor was operated for 210 days, at three hydraulic retention times (HRT): 48, 24 and 12h. Even when operated at the lowest HRT (12 h), removal efficiencies of 65% COD, 80% TOC, 65% phenols and 40% ammonium were attained. The final effluent presented COD and TOC values of 230 and 55 mg/L, respectively. Results obtained by gas chromatography analyses and toxicity tests with Artemia salina showed that a significant improvement in the effluent's quality was achieved after treatment by the combined (microfiltration/biological) process.     

A permeability-controlled microfiltration membrane for reduced fouling in drinking water treatment

A novel surface-modified polypropylene microfiltration membrane is investigated for its potential use in drinking water treatment. The flux decline rate of the modified membrane is substantially lower than the original polypropylene membrane for filtration of a soft, high-natural organic matter (NOM) surface water because a progressive adjustment in membrane permeability counteracts the flux decline due to fouling. In general, the prospects for reduced flux decline by membrane modification depend upon the characteristics of raw water such as hardness, particulate and NOM properties and concentration, and pretreatment strategies.     

Pre-coagulation for microfiltration of an upland surface water

The effect of different coagulants on cake formation and hydraulic resistance in membrane filtration of strongly coloured (SUVA> or =4.8) upland surface water has been studied at bench-scale under constant pressure conditions. Coagulants used were aluminium sulphate, polyaluminium chloride, ferric chloride and ferric sulphate. Optimisation of coagulation parameters was carried by conventional jar testing. The R'c (specific cake resistance in m(-2)) values were determined for all coagulants over a range of coagulant doses and slow mixing flocculation periods. Experiments indicated slight differences in cake formation trends between ferric- and aluminium-based coagulants and chloride and sulphate counterions, but that the range of measured R'c values was small (0.9 and 2.6 x 10(18) m(-2)) over the range of doses studied. Greater than 99% UV(254) removal was achieved with every coagulant, whereas dissolved organic carbon (DOC) removal ranged from 78% to 88%. Optimisation of the pre-coagulation-membrane filtration process suggests ferric chloride to be slightly superior for the feedwater matrix studied on the basis of DOC removal, whereas ferric sulphate gave slightly lower filter cake specific resistance values.     

Comparison of electrocoagulation and chemical coagulation pretreatment for enhanced virus removal using microfiltration membranes

This research studied virus removal by iron electrocoagulation (EC) followed by microfiltration (MF) in water treatment using the MS2 bacteriophage as a tracer virus. In the absence of EC, MF alone achieved less than a 0.5-log removal of MS2 virus, but, as the iron-coagulant dosage increased, the log virus removal increased dramatically. More than 4-log virus removal, as required by the Surface Water Treatment Rule, was achieved with 6-9 mg/L Fe³⁺. The experimental data indicated that at lower iron dosages and pH (<∼8 mg Fe/L and pH 6.3 and 7.3) negatively charged MS2 viruses first adsorbed onto the positively charged iron hydroxide floc particles before being removed by MF. At higher iron dosages and pH (>∼9 mg Fe/L and pH 8.3), virus removal was attributed predominantly to enmeshment and subsequent removal by MF. Additionally, the experimental data showed no obvious influence of ionic strength in the natural water range of 10⁻⁷-10⁻² M on MS2 virus removal by EC-MF. Finally, EC pretreatment significantly outperformed chemical coagulation pretreatment for virus removal. The proposed mechanism for this improved performance by EC is that locally higher iron and virus concentrations and locally lower pH near the anode improved MS2 enmeshment by iron flocs as well as adsorption of MS2 viruses onto the iron floc particles.     

Membrane coagulation bioreactor (MCBR) for drinking water treatment

In this paper, a novel submerged ultrafiltration (UF) membrane coagulation bioreactor (MCBR) process was evaluated for drinking water treatment at a hydraulic retention time (HRT) as short as 0.5h. The MCBR performed well not only in the elimination of particulates and microorganisms, but also in almost complete nitrification and phosphate removal. As compared to membrane bioreactor (MBR), MCBR achieved much higher removal efficiencies of organic matter in terms of total organic carbon (TOC), permanganate index (COD(Mn)), dissolved organic carbon (DOC) and UV absorbance at 254nm (UV(254)), as well as corresponding trihalomethanes formation potential (THMFP) and haloacetic acids formation potential (HAAFP), due to polyaluminium chloride (PACl) coagulation in the bioreactor. However, the reduction of biodegradable dissolved organic carbon (BDOC) and assimilable organic carbon (AOC) by MCBR was only 8.2% and 10.1% higher than that by MBR, indicating that biodegradable organic matter (BOM) was mainly removed through biodegradation. On the other hand, the trans-membrane pressure (TMP) of MCBR developed much lower than that of MBR, which implies that coagulation in the bioreactor could mitigate membrane fouling. It was also identified that the removal of organic matter was accomplished through the combination of three unit effects: rejection by UF, biodegradation by microorganism and coagulation by PACl. During filtration operation, a fouling layer was formed on the membranes surface of both MCBR and MBR, which functioned as a second membrane for further separating organic matter.     

Combination of one-dimensional TiO2 nanowire photocatalytic oxidation with microfiltration for water treatment

This paper proposed the fabrication of two different diameter one-dimensional TiO₂ nanowires, 10 nm TNW₁₀ and 20-100 nm TNW₂₀, via hydrothermal process using different alkaline sources. TNW₁₀ and TNW₂₀ were used as photocatalysts for the degradation of humic acid (HA), the major natural organic matters (NOMs) in surface and ground water, followed by microfiltration. The evaluation of photocatalytic activities of them showed that TNW₁₀ was superior to the commercial P25 TiO₂ while TNW₂₀ was as good as P25. The membrane filtration verified that the two types of nanowires could be completely reclaimed. The membrane fouling caused by TNW₁₀ and TNW₂₀ was much less than that of P25 due to more porous cake and less pore plugging. No apparent decrease on their photocatalytic activity was observed in repeated reuse experiments. These one-dimensional TiO₂ nanowires would provide a new route for the combination of photocatalytic oxidation and membrane filtration for water treatment.     

Effluent dissolved organic nitrogen and dissolved phosphorus removal by enhanced coagulation and microfiltration

Plants aiming to achieve very low effluent nutrient levels (<3 mg N/L for N, and <0.1 mg P/L for P) need to consider removal of effluent fractions hitherto not taken into account. Two of these fractions are dissolved organic nitrogen (DON) and dissolved non-reactive phosphorus (DNRP) (mainly composed of organic phosphorus). In this research, enhanced coagulation using alum (at doses commonly employed in tertiary phosphorus removal) followed by microfiltration (using 0.22 μm pore size filters) was investigated for simultaneous effluent DON and dissolved phosphorus (DP) fractions removal. At an approximate dose of 3.2 mg Al(III)/L, corresponding to 1.5 Al(III)/initial DON-N and 3.8 Al(III)/initial DP-P molar ratios, maximum simultaneous removal of DON and DP was achieved (69% for DON and 72% for DP). At this dose, residual DON and DP concentrations were found to be 0.3 mg N/L and 0.25 mg P/L, respectively. Analysis of the trends of removal revealed that the DNRP removal pattern was similar to that commonly reported for dissolved reactive phosphorus. Since this study involved intensive analytical work, a secondary objective was to develop a simple and accurate measurement protocol for determining dissolved N and P species at very low levels in wastewater effluents. The protocol developed in this study, involving simultaneous digestion for DON and DNRP species, was found to be very reliable and accurate based on the results.     

Enhanced coagulation in a typical North-China water treatment plant

The characteristics of typical source waters in northern China and their enhanced coagulation features were studied in this paper. Through bench scale tests, a composite coagulant (HPAC) was selected for this kind of high alkalinity and pH water. It can be 30% more efficient in organic matter (OM) removal than the traditional coagulants (AlCl(3), FeCl(3)), and polyaluminum chloride (PACl), especially more efficient in removing high SUVA, hydrophobic and high molecular weight dissolved organic matter (DOM). It is found that some DOM with low SUVA has precedence over that with high SUVA to be removed at conventional dosages in some seasons, and that the priority of DOM removal is in the same sequence for all the coagulants. DOM with high SUVA is not always more easily removed. When applying HPAC as coagulant, flotation process can remove hydrophobic OM more efficiently than sedimentation process in pilot scale tests.     

Effects of super-powdered activated carbon pretreatment on coagulation and trans-membrane pressure buildup during microfiltration

As a pretreatment for membrane microfiltration (MF), the use of powdered activated carbon (PAC) with a particle size much smaller than that of conventional PAC (super-powdered PAC, or S-PAC) has been proposed to enhance the removal of dissolved substances. In this paper, another advantage of S-PAC as a pretreatment for MF is described: the use of S-PAC attenuates trans-membrane pressure increases during the filtration operation. The floc particles that formed during coagulation preceded by S-PAC pretreatment were larger and more porous than the floc particles formed during coagulation preceded by PAC pretreatment and those formed during coagulation without pretreatment. This result was due to increased particle–particle collision frequency and better removal of natural organic matter, which inhibits coagulation by consuming coagulant, before the coagulation reaction. The caked fouling layer that built up on the membrane surface was thus more permeable with S-PAC than with normal PAC. Both physically reversible and irreversible membrane foulings were reduced, and more stable filtration was accomplished with S-PAC pretreatment.     

Continuous combined Fenton's oxidation and biodegradation for the treatment of pentachlorophenol-contaminated water

Pentachlorophenol (PCP) was studied as a model recalcitrant compound for a sequential chemical oxidation and biodegradation treatment, in a continuous laboratory-scale system that combined a Fenton’s chemical reactor and a packed-bed bioreactor.PCP degradation and dechlorination were observed in the Fenton’s reactor at a residence time of 1.5 h, although no reduction of total organic carbon (TOC) was observed. Both PCP degradation and dechlorination were strongly dependent on the H₂O₂ dose to the chemical reactor. The PCP degradation intermediates tetrachlorohydroquinone and dichloromaleic acid were identified in this reactor. Further treatment of the Fenton’s reactor effluent with a packed-bed bioreactor (operating at a residence time of 5.5 h) resulted in partial biodegradation of PCP degradation intermediates and reduction in TOC, although no further reduction of PCP or dechlorination was achieved in the bioreactor. Increased residence time in the bioreactor had no significant impact on degradation of TOC. Recycle of the effluent from the bioreactor to the chemical reactor increased the TOC degradation, but not the extent of the PCP degradation or dechlorination.A mathematical model of the combined Fenton’s oxidation and biodegradation system supported the experimental results. While the model over-predicted the PCP and TOC degradation in the combined system, it adequately predicted the sensitivity of these parameters to different H₂O₂ doses and recycle rates. The model indicated that high recycle rates would improve TOC degradation.     

强化混凝处理高藻水效果的研究

针对高藻期原水的特点,采用复配混凝剂并进行烧杯试验对复配混凝荆方案进行了优化,结果表明,无论聚合氯化铝(PAC)与FeCl3复配还是聚合氯化铝铁(PAFC)与FeCl3复配,混凝反应后的沉淀出水浊度都明显低于单独投加FeCl3,并且pH值能稳定在7.5以上.在总投加量相同的情况下,先投PAFC或PAC再投FeCl3的投药顺序最优;PAFC和FeCl3复配投加的最佳质量比为3:1,PAC和FeCl3复配投加的最佳质量比为1:2;投加间隔时间为5～20 s.采用复配混凝剂的水处理成本至少低于单独投加三氯化铁30%.     

Use of crossflow microfiltration in wastewater treatment

Implementation of crossflow microfiltration in the field of wastewater treatment was investigated. The membrane used throughout the research was made of multifilament polyester yarn woven in the form of a double interleave cloth with a pore size of 20–40 μm. Secondary effluent and primary settled sewage, from Blyth Sewage Treatment Plant, were used in the investigation. The study showed that the permeation rate (flux) was linearly affected by the crossflow velocity (CFV) in the case of treating secondary effluent. Permeate quality was also found to be affected by the crossflow velocity values. In addition, the effect of feed suspended solids concentration was found to proceed according to the concentration-polarization phenomena. Using the crossflow microfiltration process in treating primary settled sewage, without pretreatment, was found to be impractical due to the low flux values.     

Optimisation and significance of ATP analysis for measuring active biomass in granular activated carbon filters used in water treatment

A method for determining the concentration of active microbial biomass in granular activated carbon (GAC) filters used in water treatment was developed to facilitate studies on the interactions between adsorption processes and biological activity in such filters. High-energy sonication at a power input of 40 W was applied to GAC samples for the detachment of biomass which was measured as adenosine triphosphate (ATP). Modelling of biomass removal indicated that a series of six to eight sonication treatments of 2 min each yielded more than 90% of the attached active biomass. The ATP concentrations in 30 different GAC filters at nine treatment plants in The Netherlands ranged from 25 to 5000 ng ATP cm(-3) GAC, with the highest concentrations at long filter run times and pretreatment with ozone. A similar concentration range was observed in nine rapid sand (RS) filters. ATP concentrations correlated significantly (p<0.05) with total direct bacterial cell counts in each of these filter types, but the median value of the ATP content per cell in GAC filters (2.1 x 10(-8) ng ATP/cell) was much lower than in the RS filters (3.6 x 10(-7) ng ATP/cell). Average biofilm concentrations ranging from 500 to 10(5) pg ATP cm(-2) were calculated assuming spherical shapes for the GAC particles but values were about 20 times lower when the surface of pores >1 microm diameter is included in these calculations. The quantitative biomass analysis with ATP enables direct comparisons with biofilm concentrations reported for spiral wound membranes used in water treatment, for distribution system pipes and other aquatic environments.     

Chemical coagulation of combined sewer overflow: heavy metal removal and treatment optimization

The coagulation of combined sewer overflow (CSO) was investigated by jar-testing with two commercial coagulants, a ferric chloride solution (CLARFER) and a polyaluminium chloride (WAC HB). CSO samples were collected as a function of time during various wet-weather events from the inlet of Boudonville retention basin, Nancy, France. Jar-tests showed that an efficient turbidity removal can be achieved with both coagulants, though lower optimum dosages and higher re-stabilization concentrations were obtained with the aluminum-based coagulant. Optimum turbidity removal also yielded effective heavy metal elimination. However, the evolution with coagulant dosage of Cu, Zn, Pb, Cr, soluble and suspended solids contents followed various patterns. The removal behaviors can be explained by a selective aggregation of heavy metal carriers present in CSO and a specific interaction between hydrolyzed coagulant species and soluble metals. Stoichiometric relationships were established between optimal coagulant concentration, range of optimal dosing, and CSO conductivity, thus providing useful guidelines to adjust the coagulant demand during the course of CSO events.     

Transition in fouling mechanism in microfiltration of a surface water

The main disadvantage of membrane filtration is membrane fouling, which remains as the major obstacle for more efficient use of this technology. Information about the constituents that cause fouling is indispensable for more efficient operation. We examined the changes in both foulant characteristics and membrane morphology by performing the pilot-scale filtration test using one microfiltration membrane. During the operation, we cut the membrane fibers three times, and the components that caused irreversible fouling were extracted by acid or alkaline solution. We found that the characteristic of inorganic matter extracted by acid solution completely differed depending on the filtration period. A large amount of iron was extracted in the second chemical cleaning, while manganese was the dominant component of the extracted inorganic matter in the third chemical cleaning. The analysis of Fourier transform infrared (FTIR) and cross polarization magic angle spinning carbon-13 (CPMAS (13)C) nuclear magnetic resonance (NMR) demonstrated that the contribution of humic substances and carbohydrate in the organic foulant had increased as fouling developed. The changes in the major foulant have no relation with the fluctuation in feed water. The analysis of membrane morphology illustrated that the cake layer started to build up after the blockage of membrane pores. Based on the above results, we hypothesized the following fouling mechanism: the pores were covered or narrowed with relatively large particles such as iron, carbohydrate or protein; small particles such as manganese or humic substances blocked the narrowed pores; and finally an irreversible cake layer started to build up on the membrane surface.     

Submerged microfiltration membrane coupled with alum coagulation/powdered activated carbon adsorption for complete decolorization of reactive dyes

Even the presence of very low concentrations of dyes (1mgL(-1)) in the effluent is highly visible and is considered aesthetically undesirable. It must be removed from wastewater completely. This study systematically evaluates the performance of adsorption (three kinds of powdered activated carbons), coagulation (AlCl3.6H2O) and membrane (submerged hollow fiber microfiltration) processes individually in treating two kinds of reactive dyes (Orange 16 and Black 5) and then using a hybrid process with combined coagulation-adsorption-membrane treatment system. Adsorption capacity and kinetics of Orange 16 were much higher and faster than those of Black 5. The dye removal efficiency by coagulation was highly dependent on dye concentration and solution pH. The hybrid process performance was far more superior that individual process in removing both kinds of dyes. It was evident that the combined coagulation-adsorption-membrane process has a great potential application for complete reactive dye removal, production of high-quality treated water and allows the reduction in the use of coagulant and adsorbent.     

Biological black water treatment combined with membrane separation

Separate treatment of black (toilet) water offers the possibility to recover energy and nutrients. In this study three combinations of biological treatment and membrane filtration were compared for their biological and membrane performance and nutrient conservation: a UASB followed by effluent membrane filtration, an anaerobic MBR and an aerobic MBR. Methane production in the anaerobic systems was lower than expected. Sludge production was highest in the aerobic MBR, followed by the anaerobic MBR and the UASB-membrane system. The level of nutrient conservation in the effluent was high in all three treatment systems, which is beneficial for their recovery from the effluent. Membrane treatment guaranteed an effluent which is free of suspended and colloidal matter. However, the concentration of soluble COD in the effluent still was relatively high and this may seriously hamper subsequent nutrient recovery by physical-chemical processes. The membrane filtration behaviour of the three systems was very different, and seemed to be dominated by the concentration of colloidals in the membrane feed. In general, membrane fouling was the lowest in the aerobic MBR, followed by the membranes used for UASB effluent filtration and the anaerobic MBR.     

浸没式膜混凝吸附反应器处理北江水的试验研究

采用浸没式膜混凝反应器(MCR)和浸没式膜混凝吸附反应器(MCAR)处理北江流域地表水,考察了两种工艺对污染物的去除效果及膜污染情况.结果表明,MCR与MCAR的出水浊度均低于0.100 NTU;随着混凝剂投加量的增加,MCR对有机物的去除效果逐渐提高,跨膜压差(TMP)的增加速率逐渐降低;在相同混凝剂投加量条件下,MCAR对有机物的去除效果优于MCR,且膜污染程度会进一步减缓.     

The impact of rainstorm events on coagulation and clarifier performance in potable water treatment

This paper examines turbidity removal at a water treatment works in England that receives raw water which is difficult to treat during certain rainstorm events. Rainstorm events lead to elevated levels of turbidity and organic matter found in river waters. A robustness index based on settled turbidity was used to identify periods and events that adversely affected the treatment process. This coupled with raw water sampling data indicated that a change in nature and an increase in natural organic matter (NOM) concentrations occurred following rainstorm events. Experimental work investigated the effect of temperature and NOM on the coagulation process and the results show how increasing levels of NOM significantly impaired the coagulation of particulate material, leading to an increase in settled turbidity.