B.A. Bolto

THE FOULING OF MICROFILTRATION MEMBRANES BY NOM AFTER COAGULATION TREATMENT

Microfiltration membranes used in drinking-water treatment are fouled by both colloidal material and natural organic matter (NOM) present in the raw water. The relative importance of these contributions to fouling may depend on whether or not the water is pretreated before microfiltration, and on the type and extent of any pretreatment. In this study, the causes of fouling were determined for microfiltration of a surface water through a polypropylene hollow-fibre membrane. Fouling was caused by colloidal material when the raw water was filtered untreated, and by NOM when the raw water was coagulated before filtration. The components of NOM which cause fouling of microfiltration membranes are not yet well-established, and were also investigated in this study. NOM from the raw water was fractionated into four specific classes of compounds on the basis of hydrophobicity and charge. The rates of fouling by each NOM fraction were measured separately. The major contribution to fouling was attributed to the NOM fraction comprising small, neutral, hydrophilic compounds. The NOM fractions comprising humic and fulvic acids made only a minor contribution to fouling.

Removal of natural organic matter by ion exchange

Ion exchange is an effective method for removing humic substances from drinking water supplies. We have explored a range of anion exchangers for removal of natural organic matter (NOM), both as isolated from surface waters and after fractionation into four fractions based on hydrophobic and hydrophilic properties. Resins of open structure and high water content are confirmed as the better performers, being very efficient at removal of any charged material, especially that of smaller molecular size. Quaternary ammonium resins containing polar groups are especially effective. The presence of a neighbouring OH group close to the quaternary nitrogen, heteroatoms in the bridge between the exchange site and the polymer backbone, a secondary amino group as the exchange site, or a low ratio of carbon to quaternary nitrogen is beneficial. A suitable balance of polar and non-polar regions in the resin structure appears to be required. Weakly basic amino groups may have a greater affinity for hydrophilic counter ions than quaternary ammonium groups, but generally there are fewer charged sites in the resin at neutral pH. Nevertheless, weak base resins have NOM uptakes nearly as high as strong base resins of similar water content. Water content was found to be the most important parameter, though the effect was less pronounced for strong base resins. For weak base resins of low charge density a non-electrostatic mechanism involving hydrogen bonding of the undissociated acidic species in the NOM to the unprotonated amino groups on the resins is proposed.

Adsorption characteristics of arsenic(III) and arsenic(V) on iron(III)-loaded chelating resin having lysine-Nα,Nα-diacetic acid moiety

Abstract An iron(III)-loaded chelating resin (Fe-LDA) with lysine-N α ,N α -diacetic acid functional groups has been prepared and its adsorption characteristics for arsenic(III) and arsenic(V) have been examined. Arsenic(V) was strongly adsorbed to the resin in the pH range from 2 to 4, while arsenic(III) was moderately adsorbed between pH 8 and 10. The isotherm data for arsenic(V) at pH 3.5 fitted well to a Langmuir equation with a very large binding constant of 7.2 × 10 4 dm 3 mol −1 and a capacity constant of 0.74 mmol g −1 The data for arsenic(III) at pH 9 also fitted to a Langmuir equation, with a binding constant of 190 dm 3 mol −1 and a capacity constant of 0.84 mmol g −1 . Regeneration of the resin was successfully carried out with 0.1 mol dm −3 sodium hydroxide solution. Both arsenic compounds can be almost quantitatively recovered from the resin under these conditions. Only a small amount of ferric ions (less than 0.1 %) was observed to come off the resin during the regeneration with alkaline solutions. Since the Fe-LDA resin showed little affinity for arsenic(III) in acidic media, the present adsorption system can provide satisfactory separation of arsenic(V) from arsenic(III). Arsenic(V) was successfully concentrated in the column packed with Fe-LDA resin from its dilute solution.

Cationic polymer and clay or metal oxide combinations for natural organic matter removal.

The effect of adding suspended matter in the form of clay or metal oxide when a cationic polymer was employed as the primary coagulant was found to be beneficial. The solids provide both an adsorbent for natural organic matter (NOM) and a nucleating species for precipitating the NOM-polymer complex. Metal oxides in conjunction with a cationic polymer were more promising than clay, with effectiveness in the order Fe2O3 > Fe3O4 > Al2O3 > MnO2. Magnesium oxide at a much lower dose was nearly as effective as ferric oxide, but of course raised the pH level significantly. A simpler and more convenient way of having reactive solids present was to add alum to form flocs; for one of the waters studied the alum dose could be reduced by 67% by adding 1 mg/L of polymer, to give equal or better performance than alum alone at the optimum dose.

Removal of THM precursors by coagulation or ion exchange.

The removal of natural organic matter (NOM) from drinking water supplies can be achieved by different processes, among them coagulation and adsorption. Synthetic waters made from concentrates of humic substances from reservoir and river waters were tested in the laboratory for ease of removAl of NOM by coagulation with cationic organic polymers and with alum, and by adsorption on anion exchangers. For polymers such as high molecular weight polydiallyldimethylammonium chloride (polyDADMAC) and cationic polyacrylamides of high charge, performance was nearly as effective as alum, with colour removals 86-100% of those obtained for alum. Ion exchange using the best commercially available resins designed for this purpose, a gel polystyrene and a macroporous acrylic resin, was more effective than alum treatment for two of the natural waters studied, but inferior for a third. The resins were overall superior to cationic polymers. The NOM was separated into four fractions based on hydrophobic and hydrophilic properties. Alum was not as effective as ion exchange for the elimination of individual ionic NOM fractions. It was better than cationic polymers for removal of humic and fulvic acids, although polyDADMAC was as good for one water. For the removal of charged compounds alum then polyDADMAC were the best performers for that water. Unequivocal evidence was obtained that coagulants remove material that is not adsorbed by resins, and vice versa. A combination of coagulation with a cationic polymer and adsorption by an anion exchanger removed essentially all of the NOM. The preference of the coagulants was for the larger, more hydrophobic molecules, and of resins for smaller highly charged hydrophilic molecules. Each fraction had trihalomethane formation potentials in the range 11-24 microg/mg, except for one water that was more reactive. Hence, the actual amount of each fraction in the original water becomes a crucial factor.

Ammonia removal by sweep gas membrane distillation.

Wastewater containing low levels of ammonia (100 mg/L) has been simulated in experiments with sweep gas membrane distillation at pH 11.5. The effects of feed temperature, gas flow rate and feed flow rate on ammonia removal, permeate flux and selectivity were investigated. The feed temperature is a crucial operating factor, with increasing feed temperature increasing the permeate flux significantly, but reducing the selectivity. The best-performing conditions of highest temperature and fastest gas flow rate resulted in 97% removal of the ammonia, to give a treated water containing only 3.3 mg/L of ammonia.

A review of water recovery by vapour permeation through membranes.

In vapour permeation the feed is a vapour, not a liquid as in pervaporation. The process employs a polymeric membrane as a semi-permeable barrier between the feed side under high pressure and the permeate side under low pressure. Separation is achieved by the different degrees to which components are dissolved in and diffuse through the membrane, the system working according to a solution-diffusion mechanism. The materials used in the membrane depend upon the types of compounds being separated, so water transport is favoured by hydrophilic material, whether organic or inorganic. The process is used for the dehydration of natural gas and various organic solvents, notably alcohol as biofuel, as well as the removal of water from air and its recovery from waste steam. Waste steam can be found in almost every plant/factory where steam is used. It is frequently contaminated and cannot be reused. Discharging the spent steam to the atmosphere is a serious energy loss and environmental issue. Recycling the steam can significantly improve the overall energy efficiency of an industry, which is responsible for massive CO(2) emissions. Steam separation at high fluxes and temperatures has been accomplished with a composite poly(vinyl alcohol) membrane containing silica nanoparticles, and also, less efficiently, with an inorganic zeolite membrane.

The use of soluble organic polymers in waste treatment

Organic polymeric flocculants have been used in water purification for several decades as coagulant aids or floc builders, after the addition of inorganic coagulants like alum, iron salts or lime. The increased use of cationic polyelectrolytes as primary coagulants instead of inorganic salts, which has occurred in recent times, arises from their significant inherent advantages. The main ones are faster processing, a lower content of insoluble solids to handle, whether by sedimentation, filtration, flotation or in biological conversion, and a much smaller sludge volume. Polymers have often been used in chemically assisted sedimentation of sewage solids to enhance the removal of suspended matter. The concept is applicable as well to the primary coagulation of industrial wastewaters where the separation may be based on flotation, as in examples from the leather, steel, wool scouring, cosmetic, detergent, plastics, dyehouse, paper, food processing and brewing industries. A cationic polymer of particular charge density is optimal, and hydrophobically modified polymers have relevance in the case of oil and grease removal. The burden of solids which must be floated is much reduced relative to systems utilising inorganic coagulants, and the dosage of chemicals overall is lower. In some cases the addition of some inorganic coagulant is unavoidable, as in the case of highly coloured effluents; in others, an anionic surfactant is needed to facilitate flotation.

An ion-exchange process with thermal regeneration VII. The rates of neutralization of weak electrolyte resins

Abstract The rates of adsorption of hydrochloric acid by 16 weakly basic ion-exchange resins have been measured, together with the rates of adsorption of sodium hydroxide by 16 weakly acidic ion-exchange resins. The results obtained for the weakly basic resin “Amberlite IRA-93” confirm that the adsorption rates are inversely proportional to the square of the bead radius, and are independent of the salt concentration, as demanded by the diffusion model of Helfferich for the concentration range considered. The adsorption rates are related to the physical and chemical structures of the resins, with the porosity of the resin beads being the most important factor in resins which are suitable for the operation of a thermally regenerable process. Other parameters studied include the effect of crosslinking, and the introduction of hydrophilic groups into the resin network. It was found that, even with the most porous resins available, the rates obtained are inadequate for the practical operation of the process if resin beads of normal size are to be used. New methods of handling microbeads, or novel mixed resin systems of normal particle size but involving short diffusion paths, are therefore necessary.

Colour and turbidity removal with reusable magnetite particles—VI: Pilot plant operation

Abstract A novel process which utilizes fine magnetite particles for the removal of colour and turbidity from water has been tested in a 60 1 min −1 pilot plant on water from an unconfined aquifer in Perth, Western Australia. The performance of the pilot plant closely matched jar test results. The process design was optimized to achieve a high quality product water on a raw feed which was difficult to treat in the conventional alum coagulation and filtration plant. A comparison of the conventional and magnetite treatment systems is made, which shows that the magnetite process is viable as an alternative water treatment technique.