Effect of membrane character and solution chemistry on microfiltration performance

To help understand and predict the role of natural organic matter (NOM) in the fouling of low-pressure membranes, experiments were carried out with an apparatus that incorporates automatic backwashing and long filtration runs. Three hollow fibre membranes of varying character were included in the study, and the filtration of two different surface waters was compared. The hydrophilic membrane had greater flux recovery after backwashing than the hydrophobic membranes, but the efficiency of backwashing decreased at extended filtration times. NOM concentration of these waters (7.9 and 9.1 mg/L) had little effect on the flux of the membranes at extended filtration times, as backwashing of the membrane restored the flux to similar values regardless of the NOM concentration. The solution pH also had little effect at extended filtration times. The backwashing efficiency of the hydrophilic membrane was dramatically different for the two waters, and the presence of colloid NOM alone could not explain these differences. It is proposed that colloidal NOM forms a filter cake on the surface of the membranes and that small molecular weight organics that have an adsorption peak at 220 nm but not 254 nm were responsible for “gluing” the colloids to the membrane surface. Alum coagulation improved membrane performance in all instances, and this was suggested to be because coagulation reduced the concentration of “glue” that holds the organic colloids to the membrane surface

Size exclusion chromatography as a tool for natural organic matter characterisation in drinking water treatment

Natural organic matter (NOM), ubiquitous in natural water sources, is generated by biogeochemical processes in both the water body and in the surrounding watershed, as well as from the contribution of organic compounds that enter the water as a result of human activity. NOM significantly affects the properties of the water source, including the ability to transport metals, influence the aggregation kinetics of colloidal particles, serve as a food source for microorganisms and act as a precursor in the formation of disinfection by-products (DBPs), as well as imparting a brown colour to the water. The reactivity of NOM is closely tied to its physicochemical properties, such as aromaticity, elemental composition, functional group content and molecular weight (MW) distribution. The MW distribution is an important consideration from a water treatment perspective for several reasons. For example, low MW NOM decreases the efficiency of treatment with activated carbon, and this fraction is thought to be the portion most difficult to remove using coagulation. The efficiency of membranes in the treatment of drinking water is also influenced by the MW distribution of NOM, while some studies have shown that the low MW fraction contributes disproportionately to the formation of bioavailable organic matter, therefore promoting the formation of biofilms in the distribution system. For these reasons, understanding the MW distribution of NOM is important for the treatment of natural waters for use as drinking waters. Optimisation of a high pressure size exclusion chromatography (HPSEC) method for analysis of the MW distribution of NOM in natural waters is described (Chapter 2). Several parameters influencing the performance of HPSEC are tested and an optimised set of conditions illustrated.These parameters included eluent composition, ionic strength of the sample, flow rate and injection volume. Firstly, it was found that increasing the ionic strength of the HPSEC eluent resulted in less exclusion of NOM from the stationary phase. Stationary phases used in HPSEC contain a residual negative charge that can repel the negatively charged regions of NOM, effectively reducing the accessible pore volume. By increasing the ionic strength, interactions between the stationary phase and eluent enabled a larger effective pore size for the NOM analytes. However, increasing ionic strength of the eluent also resulted in a loss of peak resolution for the NOM portion able to access the pore volume of the stationary phase. Determining the ideal eluent composition required the balancing of these two outcomes. Matching of the ionic strength of the sample with the eluent was also an important consideration. Retention times were slightly lower when the sample ionic strength was not matched with the eluent, especially for the lowest MW material, although the effect on chromatography was minimal. Flow rate had no effect on the resolution of the HPSEC chromatogram for the portion of material able to permeate the pore space of the stationary phase. Changes in the volume of sample injected had a marked effect on the elution profile of the NOM sample. Besides the obvious limitation of detection limit, only minor changes in elution profile were obtained up to an injection volume of 100 µL. Volumes above this value, however, resulted in significant peak broadening issues, as well as an undesirable effect on the low MW portion of detected DOC.In Chapter 3, high pressure size exclusion chromatography with UV254 [subscript] and on-line detection of organic carbon (HPSEC-UV254[subscript]-OCD) was used to compare the removal of different apparent MW fractions of DOC by two process streams operating in parallel at the local Wanneroo groundwater treatment plant (GWTP). One of these two process streams included alum coagulation (operating in an enhanced coagulation mode (EC) for increased DOC removal) and the other stream included a magnetic ion exchange (MIEX®) process followed by alum coagulation (MIEX®-C). The MIEX® process is based on a micro-sized, macroporous, strong base anion exchange resin with magnetic properties, which has been designed to remove NOM through ion exchange of the anionic sites in NOM. Water was sampled from five key locations within these process streams, and the DOC at each location was characterised in terms of its MW distribution. HPSEC was carried out using three different on-line detector systems, namely OCD, UV absorbance detection at 254 nm, and fluorescence detection (λex[subscript]= 282 nm; λim[subscript] = 353 nm). This approach provided significant information on the chemical nature of the DOC in the various MW fractions. The MIEX®-C process was found to outperform the EC process: these two processes removed similar amounts of high and low MW DOC, but the MIEX®-C process showed greater removal of DOC from the intermediate MW fractions. The two coagulation processes (EC and coagulation following MIEX®) showed good removal of the fractions of highest MW, while the MIEX® process alone was found to remove DOC across all MW fractions.These results seem to indicate that anionic groups, particularly susceptible to removal with MIEX® treatment, are well distributed across all MW fractions of NOM. In agreement with previous studies, MIEX®-C outperformed EC in the overall removal of DOC (MIEX®-C removed 25 % more DOC than EC). However, 70% of the additional DOC removed by MIEX®-C was comprised of a surprisingly narrow range of medium-high MW fractions. The development of a novel online organic carbon detector (OCD) for use with HPSEC for determining the MW distribution of NOM is described in Chapter 4. With UV absorbance detection, the magnitude of the signal is based on the extinction coefficient of the chromophores in the analytes being investigated; whereas the signal from an OCD is proportional to the actual organic carbon concentrations, providing significantly more information. The development of an online OCD involved the separation of analytes using HPSEC, removal of inorganic carbon species which may interfere with organic carbon determination, oxidation of the organic carbon to carbon dioxide, separation of the produced carbon dioxide from the aqueous phase and subsequent detection of the gaseous carbon dioxide. In the new instrument, following separation of components by HPSEC, the sample stream was acidified with orthophosphoric acid to a concentration of 20 mmol L-1[superscript], resulting in a pH of ≤ 2, in order to convert inorganic carbon to carbon dioxide. This acid dose was found to remove greater than 99 % of inorganic carbon once the acidified sample was passed through a hydrophobic polytetrafluoroethylene (PTFE) membrane allowing the passage of dissolved gases (under negative pressure from a vacuum pump) but restricting the flow of the mobile phase.Several factors influenced the oxidation of the organic carbon in the next step, including the dose of persulfate, the type and intensity of UV radiation and the composition of the capillary through which the sample stream passes. Through optimisation of this process, it was found that a persulfate dose of 0.84 mmol L-1[superscript] in the sample stream was required for optimum oxidation efficiency. A medium pressure UV lamp was compared to a vacuum UV lamp for its efficiency in oxidation of organic carbon to carbon dioxide. While the medium pressure lamp produced a far smaller percentage of its total radiation at the optimum wavelength for oxidation of organic compounds, the greater overall intensity of the medium pressure lamp was shown to be superior for this application. The composition of the capillary was shown to have a considerable effect on the oxidation efficiency. A quartz capillary, internal diameter 0.6 mm, was compared with a PTFE capillary, internal diameter 0.5 mm, for the oxidation of organic carbon by external UV treatment. While peak width, an important consideration in chromatographic resolution, was greater for the larger internal diameter quartz capillary, the lower UV transparency of PTFE combined with the shorter contact time, due to the reduced internal diameter of the capillary, resulted in a less efficient oxidation step using the PTFE capillary. The quartz capillary was therefore chosen for use in the UV/persulfate oxidation step for oxidation of organic carbon to carbon dioxide. Separation of the produced carbon dioxide from the sample stream was achieved by sparging with nitrogen and contacting the gas/liquid mixture with a hydrophobic PTFE membrane, restricting the passage of the liquid while allowing the nitrogen and carbon dioxide gases to pass to the detection system.The only factor influencing this separation was the flow of the nitrogen sparge gas, with a flow of 2 mL min-1[superscript] found to be optimum. Detection of produced carbon dioxide was via a Fourier transform infrared (FTIR) spectrometer with a Iightpipe accessory. The Iightpipe accessory was designed for use as a detector for gas chromatography and the small size of the detector cell was ideal for use with this application. Using the new system described, concentrations of a single peak could be determined with a detection limit of 31 ng and a determination limit of 68 ng. The development of the new OCD allowed characterisation of NOM in terms of its MW distribution and the UV and fluorescence spectral properties of each MW fraction. Further characterisation of MW fractions of NOM from a local groundwater bore was carried out by separation of the fractions by preparative HPSEC, followed by off-line analysis. Preparative HPSEC involved the injection of a pre-concentrated groundwater sample multiple times, using a large scale HPSEC column, then collecting and combining material of identical MW. This allowed each MW fraction of the sample to be further characterised as described in Chapter 5. Preparative HPSEC has only previously been applied to a small number of samples for the concentration and fractionation of NOM, where the structural features of the various MW fractions were studied. In the current research, more extensive studies of not only the chemical characteristics, but also the disinfection behaviour, of the MW fractions were conducted. Separation of the sample was conducted on a large diameter silica-based HPSEC column, with fraction collection based on semi-resolved peaks of the HPSEC chromatogram. Nine MW fractions were collected by this method.After concentration and dialysis to remove the buffer salts in the HPSEC mobile phase, each fraction was re-analysed by analytical HPSEC-UV254[subscript] and showed a single Gaussian shaped peak, indicating discrete MW fractions had successfully been collected. Analysis of the collected MW fractions indicated that 57 % of the organic carbon was in Fractions 3 and 4, with 41 % in Fractions 5-9, leaving only 2 % in Fractions 1 (highest MW) and 2. For each of the nine MW fractions, chorine demand and 7 day trihalomethane formation potential (THMFP) were measured on dilute solutions of the same DOC concentration, and solid state 13[superscript]C NMR spectra were recorded on some of the solid isolates obtained after Iyophilisation of the separate or combined dialysis retentates. The larger MW Fractions 3 and 4 were found to contain a greater proportion of aromatic and carbonyl carbon, and the lower MW Fractions 5 and 6 and Fractions 7-9 contained greater proportions of aliphatic and O-aliphatic carbon, by this technique. Chlorine demand experiments on each individual fraction with a normalised DOC concentration indicated that the largest MW fraction (Fraction 1) had the lowest chlorine demand. It was concluded that material in this fraction may be associated with inorganic colloids and unavailable for reaction with chlorine. Fraction 3 had the highest chlorine demand, just over two times more than the next highest chlorine demand (Fraction 4) and approximately three times the chlorine demand of Fraction 2. The organic material in Fraction 2 was postulated to contain a mixture of the reactive material present in Fraction 3 and the colloidal associated material present in Fraction 1.NMR analysis indicated that the difference between Fraction 3 and Fraction 4 was a reduction in reactive aromatic carbon and hence the lower chlorine demand in the latter fraction. Fractions 5-8 had similar chlorine demands, lower than Fraction 4, while Fraction 9 had a very low chlorine demand similar to that of Fraction 1. For Fractions 5-9, the lower aromatic carbon content most likely resulted in the lower chlorine demand. The 7 day THMFP experiments showed some clear trends, with Fraction 1 and Fraction 2 producing the least amounts of THMs but having the greatest incorporation of bromine. Fractions 3 and 4 produced the greatest concentration of THMs with the lowest bromine incorporation, perhaps as they contained fast reacting THM precursors and the higher chlorine concentrations resulted in greater amounts of chlorinated THMs. Fraction 5 and Fraction 6 produced similar levels of THMs over 7 days to Fractions 7-9 (approximately 75% of the amount formed by Fractions 3 and 4), however, Fractions 7-9 formed these THMs more quickly than Fractions 5 and 6, with slightly greater amounts of bromine incorporation. It was thought that the increased speed of formation was due to the smaller MW of these fractions and a simpler reaction pathway from starting material to formation of THMs, as well as some structural differences. This research marks the first report of significantly resolved MW fractions being isolated and their behaviour in the presence of a disinfectant being determined. While the high MW fractions had the greatest chlorine demands and THMFPs, these fractions are also the easiest to remove during coagulation water treatment processes, as shown in Chapter 3. The lowest MW material formed significant amounts of THMs, and also formed THMs more quickly than other MW fractions.This has important implications from a water treatment perspective, as the lowest MW material is also the most difficult to remove during conventional treatment processes. Solid samples of NOM were isolated from water samples taken from four points at the Wanneroo GWTP using ultrafiltration and subsequent Iyophilisation of the retained fractions, as described in Chapter 6. The sampling points were following aeration (Raw), following treatment by MIEX®, following treatment by MIEX®-C and following treatment by EC. Elemental analysis, FTIR spectroscopy, solid state 13[superscript]C NMR spectroscopy and HPSEC-UV254[subscript]-0CD analysis were used to compare the four isolates. Treatment with MIEX®-C was found to remove the greatest amount of NOM. Additionally, treatment with MIEX®-C was able to remove the largest MW range of NOM, with the remaining material being depleted in aromatic species and having a greater proportion of aliphatic and O-aliphatic carbon. EC treatment completely removed the NOM components above 5000 Da, but NOM below this was not well removed. NOM remaining after the EC train had a lower aromatic content and more aliphatic oxygenated organic matter than the RW. The remaining organic matter after MIEX® treatment contained less aromatic material compared to the RW, but had a greater aromatic content than either of the EC or MIEX®-C samples. HPSEC was a significant analytical technique used throughout this research. Initial optimisation of an HPSEC method was an important development which allowed improved resolution of various MW fractions. The application of this technique and comparison of three detection systems for the study of DOC removal showed, for the first time, the performance of MIEX® treatment at a full scale groundwater treatment facility.The use of various HPSEC detection systems allowed significant characterisation of the MW fractions, more information than had previously been gathered from such a sample set. This work demonstrated the need for OCD when applying HPSEC to the study of NOM. As such, a system was constructed that built on previously developed systems, with the use of a small detector cell enabling detection limits capable of measuring even the most dilute natural and treated water samples. To study the individual MW fractions in detail, preparative HPSEC was applied and, for the first time, the disinfection behaviour of various MW fractions was examined. Interestingly, the lowest MW fractions, acknowledged to be the most recalcitrant to conventional water treatment processes, produced significant quantities of THMs. Also the formation kinetics of THMs from the low MW fractions indicated that THMs were formed as quickly as, or perhaps even at faster rates than from the larger MW fractions. Finally, structural characterisation of NOM at four stages of the Wanneroo GWTP indicated MIEX®-C treatment was superior to EC, of significant interest for the water industry

Effect of IX dosing on polypropylene and PVDF membrane fouling control

The performance of ion exchange (IX) resin for organics removal from wastewater was assessed using advanced characterisation techniques for varying doses of IX. Organic characterisation using liquid chromatography with a photodiode array (PDA) and fluorescence spectroscopy (Method A), and UV254, organic carbon and organic nitrogen detectors (Method B), was undertaken on wastewater before and after magnetic IX treatment. Results showed partial removal of the biopolymer fraction at high IX doses. With increasing concentration of IX, evidence for nitrogen-containing compounds such as proteins and amino acids disappeared from the LC-OND chromatogram, complementary to the fluorescence response. A greater fluorescence response of tryptophan-like proteins (278nm/343nm) for low IX concentrations was consistent with aggregation of tryptophan-like compounds into larger aggregates, either by self-aggregation or with polysaccharides. Recycling of IX resin through multiple adsorption steps without regeneration maintained the high level of humics removal but there was no continued removal of biopolymer. Subsequent membrane filtration of the IX treated waters resulted in complex fouling trends. Filtration tests with either polypropylene (PP) or polyvinylidene fluoride (PVDF) membranes showed higher rates of initial fouling following treatment with high IX doses (10mL/L) compared to filtration of untreated water, while treatment with lower IX doses resulted in decreased fouling rates relative to the untreated water. However, at longer filtration times the rate of fouling of IX treated waters was lower than untreated water and the relative fouling rates corresponded to the amount of biopolymer material in the feed. It was proposed that the mode of fouling changed from pore constriction during the initial filtration period to filter cake build up at longer filtration times. The organic composition strongly influenced the rate of fouling during the initial filtration period due to competitive adsorption processes, while at longer filtration times the rate of fouling appeared to depend upon the amount of biopolymer material in the feed water

A new organic carbon detector for size exclusion chromatography.

A novel organic carbon detector (OCD) for size exclusion chromatography (SEC), and its application to the characterisation of aquatic natural organic matter (NOM) in natural and treated potable water samples, is described. The instrument uses a conventional UV-persulfate oxidation technique to convert organic carbon to CO2. The novelty of the technique is detection of the evolved CO2 using a sensitive Fourier transform infrared (FTIR) spectroscopy 'lightpipe' detector originally designed for detection of analytes after gas chromatographic separation. With the exception of the lightpipe, the OCD system was constructed using simple, inexpensive, readily available components. The system was designed to minimise deadvolume, allowing for use of smaller sample sizes and smaller columns, substantially shortening analysis time, while maintaining chromatographic integrity through the OCD system. Downscaling resulted in some loss of separation but it was shown that this was caused by thelower separation efficiency of the smaller capacity column, rather than from sample dispersion within the OCD system

Understanding drivers and barriers : the key to water use behaviour change

In southwest Victoria, like many other regions in Australia, drought, climate change and population growth have exposed gaps in water supply. To develop effective demand management strategies for rural and regional areas, this paper investigates the drivers and barriers to water saving in southwest Victoria. Although the majority of people felt water saving was important, the drivers for water saving differed between different groups. Residential users were saving water for altruistic reasons, while for farmers the drivers were farm viability and productivity. Although the barriers differed between property types, common barriers included lack of understanding of the impact their water use has on supplies, lack of knowledge, the pricing system and distrust of the water authority. The findings provide information for effective demand management strategies for the region.<br /

Plastic tastes in drinking water: factors affecting the chemistry of bromophenol formation

A “plastic” or “chemical” taint, has recently emerged as a problem in drinking water in Perth, Western Australia. The taste occurs intermittently in zones receiving blends of treated groundwaters from several sources, generally only in boiled water. The compound primarily responsible is 2,6-dibromophenol (taste threshold concentration 0.5 ng/L). It was established that the relative ratios of phenol, bromide and chlorine, and pH are important determinants in whether the taste would or would not form and that the primary sources of phenol are plastic appliances, especially kettles and refrigerators (Heitz et al., 2001). However, bromophenol formation varied widely between waters from different sources, even though reaction conditions were ostensibly identical, and it was concluded that another, as yet unknown, factor must influence the reaction rate. This could account for observations that plastic taste only occurred in some groundwaters, but not in others. In the present study the effects of organic and inorganic nitrogen-containing compounds on phenol bromination rates were examined, with the view that this might give some insights into the nature of the unknown factor discussed above. These compounds slowed the rate of bromophenol formation, and results suggested that disinfection using chloramine, rather than chlorine, could prevent plastic taste problems.</jats:p

Understanding the behaviour of molecular weight fractions of natural organic matter to improve water treatment processes

Water utilities have experienced increasing pressure to minimise the formation of disinfection by-products (DBPs), as reflected in the increasingly stringent regulations and guidelines for the concentrations of DBPs in drinking water. Understanding the disinfection characteristics and molecular weight (MW) distribution of natural organic matter (NOM) will assist in the optimisation of drinking water treatment processes to minimise the formation of DBPs. This study investigated the disinfection behaviour of MW fractions of NOM isolated from a Western Australian source water. The NOM was fractionated and separated using preparative size exclusion chromatography (SEC) and the fractions were chlorinated in the presence of bromide ion. The larger MW fractions of NOM were found to produce the highest concentrations of DBPs (trihalomethanes, haloacetic acids, haloacetonitriles, haloketones, and haloaldehydes), with the low MW fractions still producing significant amounts of these DBPs. The results also showed a trend of an increasing proportion of brominated DBPs with decreasing MW and aromatic character. Considering that the smaller MW fractions of NOM produce significant amounts of DBPs, with a higher relative contribution from brominated DBPs, water treatment processes need to be optimised for either bromide removal or the removal of aliphatic, small MW fractions of NOM, in order to meet DBP guidelines and regulations.</jats:p

Size Exclusion Chromatography to Characterize DOC Removal in Drinking Water Treatment.

A full-scale (110 ML/d) potable water treatment plant (WTP) based on the MIEX process, an innovative new process based on a strong base anion-exchange resin with magnetic properties, has been operating in Perth Western Australia since 2001. This plant has been configured so that a combined MlEX-coagulation (MIEX-C) process can be operated in parallel with a conventional enhanced coagulation (EC) process, allowing comparison of the performance of the two processes. Here, we report the use of size exclusion chromatography (SEC) to compare the removal of different apparent molecular weight (AMW) fractions of DOC by the two processes. Water was sampled from five key locations within the WTP, and SEC was carried out using three different on-line detector systems, DOC-specific detection, UV absorbance detection at lambda= 254 nm, and fluorescence detection (lambda(ex) = 282 nm; lambda(em) = 353 nm). This approach provided information on the chemical nature of the DOC comprising the various AMW fractions. The study showed that the MIEX-C process outperformed the EC process with greater removal of DOC in each of the eight separate AMW fractions identified. While EC preferentially removed the fractions of highest AMW, and those exhibiting the greatest aromatic (humic) character, MIEX-C removed DOC across all AMW fractions and did not appear to discriminate as strongly on the basis of differences in aromatic character or AMW. The results demonstrate the benefits of combining these complementary treatment processes. The study also demonstrates the utility of SEC coupled with multiple detection systems in determining the characteristics of various AMW components of DOC