Seasonal variations in natural organic matter and its impact on coagulation in water treatment

In the past decade, a number of UK and US water utilities have been experiencing operational difficulties connected with the increased dissolved organic carbon (DOC) levels during the autumn and winter periods. This has been observed as an increase in the production of disinfection-by-products (DBP), and a greater coagulant demand. Resin adsorption techniques were used to fractionate raw water and investigate the variation in surface charge and coagulant-humic interactions over a 36-month period. A change in the natural organic matter (NOM) composition throughout the year was observed, with the fulvic acid fraction (FAF) increasing from 36% in September to 61% in November. However, a reduction in treatment performance is not simply due to an increase in DOC concentrations (from 4.3 to 14.5 mg L-1), but also a change in the charge density of the NOM. It was found that hydrophilic NOM fractions possess negligible charge density (<0.06 meq g-1DOC), and it is the hydrophobic NOM fractions, FAF in particular, that exert the greater dominance on coagulation control. The hydrophilic NOM fractions are less amenable to removal through conventional coagulation with metal salts, and are therefore likely to indicate the DOC residual remaining after treatment. Understanding the seasonal changes in NOM composition and character and their reactivity with coagulants should lead to a better optimisation of the coagulation process and a more consistent water quality.     

Impact of recycling filter backwash water on organic removal in coagulation-sedimentation processes

The overall purpose of this research was to examine the impacts of filter backwash water (FBWW) and membrane backwash water (MBWW) recycles on water quality in coagulation-sedimentation processes. Specifically, the impact of recycling 5 or 10% by volume of FBWW and MBWW with surface water on the removal of natural organic matter (NOM) was evaluated at bench-scale using a standard jar-test apparatus and measurement of specific water quality parameters including total organic carbon (TOC), dissolved organic carbon (DOC), UV254, turbidity, total aluminum and zeta potential. The results of jar test conducted on a source water with a specific UV absorbance (SUVA) value within the range of 2-4 mg/L m showed a significantly higher removal of DOC from the raw water that was blended with 5 and 10% by volume of FBWW as compared to control trials where backwash water was not added. Increasing rates of MBWW that did not contain destabilized hydroxide precipitates did not significantly change DOC concentrations in the settled water samples as compared to the control trials. For source waters that are characterized as having low turbidity with medium SUVA values, these results could hold particular significance for plants that have reached treatment ceilings in terms of dissolved NOM removal using conventional coagulation designs.     

Comparison of NOM character in selected Australian and Norwegian drinking waters

Observations from many countries around the world during the past 10-20 years indicate increasing natural organic matter (NOM) concentration levels in water sources, due to issues such as global warming, changes in soil acidification, increased drought severity and more intensive rain events. In addition to the trend towards increasing NOM concentration, the character of NOM can vary with source and time (season). The great seasonal variability and the trend towards elevated NOM concentration levels impose challenges to the water industry and the water treatment facilities in terms of operational optimisation and proper process control. The aim of this investigation was to compare selected raw and conventionally treated drinking water sources from different hemispheres with regard to NOM character which may lead to better understanding of the impact of source water on water treatment. Results from the analyses of selected Norwegian and Australian water samples showed that Norwegian NOM exhibited greater humic nature, indicating a stronger bias of allochthonous versus autochthonous organic origin. Similarly, Norwegian source waters had higher average molecular weights than Australian waters. Following coagulation treatment, the organic character of the recalcitrant NOM in both countries was similar. Differences in organic character of these source waters after treatment were found to be related to treatment practice rather than origin of the source water. The characterisation techniques employed also enabled identification of the coagulation processes which were not necessarily optimised for dissolved organic carbon (DOC) removal. The reactivity with chlorine as well as trihalomethane formation potential (THMFP) of the treated waters showed differences in behaviour between Norwegian and Australian sources that appeared to be related to residual higher molecular weight organic material. By evaluation of changes in specific molecular weight regions and disinfection parameters before and after treatment, correlations were found that relate treatment strategy to chlorine demand and DBP formation.     

The effect of ozonation on natural organic matter removal by alum coagulation

Natural organic matter (NOM) was extracted from a moderately colored, eutrophic surface water source (Forge Pond, Granby, MA), and fractionated into quasi-homogeneous fractions. Fulvic acid (FA) and hydrophilic neutrals (HN) were the two most abundant NOM fractions that were isolated. Adsorption affinity of the isolated NOM fractions on preformed aluminum hydroxide flocs increased with increase in specific organic charge of the fractions, except for the two most highly charged fractions, FA and hydrophilic acids (HAA), which showed less adsorption affinity than expected based on their specific organic charge. Prior ozonation of FA and HN fractions resulted in a decline and an increase, respectively, in their adsorption affinity on aluminum hydroxide surface. Prior ozonation of Forge Pond raw water resulted in a progressive decline in dissolved organic carbon (DOC) removal by alum coagulation with increase in ozone dose. It appeared that ozone applied to raw water reacted preferentially with the humic fraction of NOM, resulting in the detrimental effects of ozonation on subsequent NOM removal by alum coagulation being magnified. Forge Pond raw water was pre-coagulated to remove humic substances. Ozonation of the pre-coagulated water demonstrated the beneficial effects of ozonation on the removal of non-humic NOM through alum coagulation. A strategy for staged coagulation with intermediate ozonation was proposed for waters containing both humic and non-humic NOM for maximum DOC and specific UV absorbance at 254nm (SUVA) removal.     

Comparison of polyaluminum silicate chloride and electrocoagulation process,in natural organic matter removal from surface water in Ghochan,Iran

Removal of natural organic matter (NOM) is one of the most important objectives of water treatment plants but reducing these pollutants either present in water as dissolved or suspended form is not as efficient as is required in conventional treatment plants. The purpose of this study was comparison performance of composite polyaluminum silicate chloride (PASiC) and electrocoagulation (EC) process by aluminum electrodes in NOM removal from raw surface water. In this study, the effects of turbidity, total organic compounds (carbons) (TOC), adsorption at a wavelength of 254 nm (UV254), chemical oxygen demand (COD), alkalinity, residual aluminum in finished water by application of EC process and PASiC were investigated. The results demonstrate that PASiC coagulant at optimum concentration of 1–5 ml/L was capable of removing TOC, COD, U.V., and turbidity from raw water by 93.77, 93.5, 63 and 95 %, respectively. In contrast, EC process, removed TOC, COD, UV and turbidity from raw water by 89, 99.75, 37 and 50%, respectively. The pilot-scale results demonstrated the significant advantage of PASiC compared to EC process in removal of NOM and turbidity form raw water. Residual aluminum in finished water was below the recommended WHO guidelines 0.2 mg/L for both processes. Finally it can be concluded that PASiC and EC process are reliable, efficient and cost-effective methods for removal of NOM from surface water.     

Enhanced coagulation for high alkalinity and micro-polluted water: the third way through coagulant optimization

Conventional coagulation is not an effective treatment option to remove natural organic matter (NOM) in water with high alkalinity/pH. For this type of water, enhanced coagulation is currently proposed as one of the available treatment options and is implemented by acidifying the raw water and applying increased doses of hydrolyzing coagulants. Both of these methods have some disadvantages such as increasing the corrosive tendency of water and increasing cost of treatment. In this paper, an improved version of enhanced coagulation through coagulant optimization to treat this kind of water is demonstrated. A novel coagulant, a composite polyaluminum chloride (HPAC), was developed with both the advantages of polyaluminum chloride (PACl) and the additive coagulant aids: PACl contains significant amounts of highly charged and stable polynuclear aluminum hydrolysis products, which is less affected by the pH of the raw water than traditional coagulants (alum and ferric salts); the additives can enhance both the charge neutralization and bridging abilities of PACl. HPAC exhibited 30% more efficiency than alum and ferric salts in dissolved organic carbon (DOC) removal and was very effective in turbidity removal. This result was confirmed by pilot-scale testing, where particles and organic matter were removed synergistically with HPAC as coagulant by sequential water treatment steps including pre-ozonation, coagulation, flotation and sand filtration.     

Long term case study of MIEX pre-treatment in drinking water; understanding NOM removal

Removal of natural organic matter (NOM) is a key requirement to improve drinking water quality. This study compared the removal of NOM with, and without, the patented magnetic ion exchange process for removal of dissolved organic carbon (MIEX DOC) as a pre-treatment to microfiltration or conventional coagulation treatment over a 2 year period. A range of techniques were used to characterise the NOM of the raw and treated waters. MIEX pre-treatment produced water with lower concentration of dissolved organic carbon (DOC) and lower specific UV absorbance (SUVA). The processes incorporating MIEX also produced more consistent water quality and were less affected by changes in the concentration and character of the raw water DOC. The very hydrophobic acid fraction (VHA) was the dominant NOM component in the raw water and was best removed by MIEX pre-treatment, regardless of the raw water VHA concentration. MIEX pre-treatment also produced water with lower weight average apparent molecular weight (AMW) and with the greatest reduction in complexity and range of NOM. A strong correlation was found between the VHA content and weight average AMW confirming that the VHA fraction was a major component of the NOM for both the raw water and treated waters.     

Bench-scale testing of a magnetic ion exchange resin for removal of disinfection by-product precursors

The objective of this research was to compare enhanced coagulation with anion exchange for removal of disinfection by-product (DBP) precursors (i.e. natural organic matter (NOM) and bromide). Treatment with a magnetic ion exchange resin (MIEX((R))) was the primary focus of this study. Raw waters from four utilities in California were evaluated. The waters had low turbidity, low to moderate organic carbon concentrations, a wide range of alkalinities, and moderate to high bromide ion concentrations. The treated waters were compared based on removal of ultraviolet (UV) absorbance, dissolved organic carbon (DOC), trihalomethane formation potential (THMFP), and haloacetic acid formation potential (HAAFP). The results indicated that treatment with MIEX is more effective than coagulation at removing UV-absorbing substances and DOC. Treatment with MIEX and treatment with MIEX followed by coagulation yielded similar results, suggesting that coagulation of MIEX-treated water does not provide additional removal of organic carbon. MIEX treatment reduced the THMFP and HAAFP in all waters, and did so to a greater extent than coagulation. Treatment with MIEX was most effective in raw waters having a high specific UV absorbance and a low anionic strength. Following MIEX treatment, subsequent chlorination resulted in a shift to the more brominated THM and HAA species as compared to chlorination of the raw water. MIEX also removed bromide to varying degrees, depending on the raw water alkalinity and initial bromide ion concentration.     

In-line coagulation with low-pressure membrane filtration

The objectives were to investigate the effects of in-line coagulation on removal of natural organic matter (NOM) and turbidity and on fouling of membranes during ultrafiltration (UF). Coagulants were added prior to UF, without intermediate use of conventional processes for removal of particles. Synthetic and natural raw waters were examined. Charge neutralization, sweep floc, and under-dose conditions (with respect to conventional treatment) were all effective for removal of NOM and turbidity by UF. Most coagulation conditions resulted in decreased resistance to filtration and improved hydraulic removal of filter cake, including conditions that would be ineffective prior to conventional settling and rapid-filtration. Flocs that were produced under charge neutralization conditions and acidic under-dose conditions produced lower hydraulic resistance and were less compressible than for sweep floc conditions.     

A pilot plant evaluation of the Magnetic Ion EXchange® process for the removal of dissolved organic carbon at Draycote water treatment works

Natural organic matter (NOM) in water sources is undesirable for customers because it reacts with chlorine to form trihalomethanes (THMs), which can be harmful to health, and other organochlorine compounds that can cause taste complaints. A new Magnetic Ion EXchange process (MIEX^® DOC) has been reported as being successful in removing NOM from raw water. The aim of this research was (a) to evaluate the performance of the MIEX^® process in removing NOM from hard, lowland water on a pilot plant scale and (b) to compare the performance of the MIEX^® process with low pH coagulation DAF, in particular, with respect to reducing THM levels. A 45 and 40% reduction in the amount of NOM and in the levels of THMs, respectively, in chlorine‐dosed water was achieved when the MIEX^® process was used with dissolved air flotation (DAF).     

Combination of ferric and MIEX for the treatment of a humic rich water

Seasonal periods of high rainfall have been shown to cause elevated natural organic matter (NOM) loadings at treatment works. These high levels lead to difficulties in removing sufficient NOM to meet trihalomethane (THM) standards, and hence better alternative treatments are required. Here the removal of NOM was investigated by a new ion exchange process (MIEX) using both bulk and fractionated NOM. Initial results showed that in excess of 80% of the raw water dissolved organic carbon (DOC) and greater than 85% of the UV absorbance from the bulk raw water could be removed by the use of MIEX alone. It was also seen that the removal of the more recalcitrant isolated fractions was increased. When MIEX was combined with a significantly reduced dose of coagulant a slight improvement on the overall DOC and UV removals was observed, however a significant decrease in the amount of THM formation potential (THMFP) in the final water was seen. This combined with the reduction in coagulant would imply a more efficient process during the times when the water becomes increasingly difficult to treat.     

混凝-超滤工艺处理滦河水的中试研究

采用混凝—超滤工艺进行了处理滦河水的中试研究,考察了混凝剂投量和混凝反应时间对膜出水水质及跨膜压差的影响。结果表明,在三氯化铁投量为6 mg/L、混凝反应时间为7.5min时,系统对污染物的去除效果较好,对CODMn的去除率为48.7%,膜出水的CODMn〈2.0 mg/L,浊度〈0.1 NTU;此时的跨膜压差相对较小且随运行时间增长缓慢。在高温、高藻期,预氯化有助于提高系统对有机物的去除率并可减缓膜污染;EFM清洗方式可使膜系统长时间在较低的跨膜压差下运行,是延缓膜污染的有效手段。     

常规工艺对浊度的去除效率及浊度预警水平

分析了原水泵站及水厂各工艺单元出水一整年的浊度数据，发现1月-4月的原水浊度较低，如果碱铝投加量低于一定限值（如8．88mg／L）则易造成出水浊度偏高，需要控制混凝剂投加量以保证混凝效果。清水池内浊度容易升高，这可能与滤池初滤水的水质较差有关。根据各工艺单元对浊度的一般去除效率和出厂水浊度标准，提出了原水和工艺单元出水的浊度预警水平，这对于水厂常规工艺的运行具有指导性意义。     

不同条件下低温低浊水混凝试验研究

为了保证饮用水水质并实现节能降耗,宁夏宁东水厂采用调节原水pH值,投加高锰酸钾的方式来强化常规处理工艺。结果表明,调节pH值对于处理偏碱性的低温低浊水具有良好的净化效果。当原水pH值调节为中性时,混凝效果达到最优;适当使用聚丙烯酰胺可以优化混凝效果,但由于聚丙烯酰胺具有毒性,在实际运行中要慎重使用;高锰酸钾预氧化可有效提高低温低浊水的处理效果,对于提高冬季低温低浊水处理效果具有重要影响;通过滤池反冲洗水回流的方法．可以提高原水中颗粒物的数量．有助于提高水处理过程中颗粒碰撞效率,改善混凝效果。     

Performance of selected anion exchange resins for the treatment of a high DOC content surface water

The objective of this study was first to compare the performance of four strong anion exchange resins (AERs) (MIEX from Orica Pty Ltd, DOWEX-11 and DOWEX-MSA from DOW chemical and IRA-938 from Rohm and Haas) for their application in drinking water treatment (natural organic matter (NOM), mineral anions (nitrate, sulfate and bromide) and pesticide removal) using bench-scale experimental procedures on a high DOC content surface water. The efficiency of MIEX for NOM and mineral anions removal was furthermore evaluated using bench-scale dose-response experiments on raw, clarified and post-ozonated waters. NOM removal was assessed using the measurement of dissolved organic carbon (DOC), UV absorbance at 254 nm (UV254) and the use of high-performance size exclusion chromatography with UV (HPSEC/UV) and fluorescence detection (HPSEC/FLUO). The MIEX and IRA938 anionic resins exhibit a faster removal of NOM and mineral anions compared to the DOWEX11 and MSA AERs. All the resins were found to be very effective with similar performances after 30 to 45 min of contact time. As expected, only limited sorption of atrazine and isoproturon (C0=1 microg/L) occurred with MIEX, DOWEX11 and MSA AERs. MIEX resin proved to be very efficient in eliminating NOM of high-molecular weight but also a large part of the smallest UV absorbing organic compounds which were refractory to coagulation/flocculation treatment. Remaining DOC levels after 30 min of contact with MIEX were found similar in raw water, clarified water and even post-ozonated water implying no DOC benefit can be gained by employing conventional treatment prior to MIEX treatment. Removal of bromide (initial concentration 110 microg/L) was also observed and ranged from 30% to 65% for resin dose increasing from 2 to 8 mL/L. T     

Magnetic ion-exchange resin treatment: impact of water type and resin use

Three raw waters of fundamentally different natural organic matter (NOM) character were treated by magnetic resin using a bench-scale method designed to mimic how the resin is used in continuous operation. Increasing water hydrophobicity resulted in reduced dissolved organic carbon (DOC) removal with removal of 56%, 33% and 25% for waters containing 21%, 50% and 75% hydrophobic NOM, respectively. Study of consecutive resin uses showed that the NOM in the hydrophobic water had high affinity for the resin shown by DOC removal of 65% after the first use of the resin. This dropped to 25% DOC removal after 15 consecutive resin uses. For the more hydrophilic waters, NOM removal remained consistent after each resin use. The hydrophobic sample contained higher MW NOM that was capable of blocking resin sites that prevented continual adsorption of organics on to the resin. The hydrophilic NOM containing a large proportion of hydrophilic acids was consistently removed to around 60%. The water containing algogenic-derived NOM was poorly removed by magnetic resin. Subsequent coagulation showed higher removal with increasing hydrophobicity.     

Seasonal variations of disinfection by-product precursors profile and their removal through surface water treatment plants

A sampling program has been undertaken to investigate the variations of disinfection by-products (DBPs) formation and nature and fate of natural organic matter (NOM) through water treatment plants in Istanbul. Specific focus has been given to the effect seasonal changes on the formation of DBPs and organic precursors levels. Water samples were collected from the three reservoirs inlet and within three major water treatment plants of Istanbul, Turkey. Changes in the dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV(254)), specific ultraviolet absorbance (SUVA), trihalomethane formation potential (THMFP), and haloacetic acids formation potential (HAAFP) were measured for both the treated and raw water samples. The variations of THM and HAA concentrations within treatment processes were monitored and also successfully assessed. The reactivity of the organic matter changed throughout the year with the lowest reactivity (THMFP and HAAFP) in winter, increasing in spring and reaching a maximum in fall season. This corresponded to the water being easier to treat in fall and an increase in the proportion of hydrophobic content. Understanding the seasonal changes in organic matter character and their reactivity with treatment chemicals should lead to a better optimization of the treatment processes and a more consistent water quality.     

基于分形理论的混凝控制研究

依据分形理论,针对混凝过程中原水水质、水量的变化,考察了絮凝体分形维数与混凝剂投量和沉后水浊度间的变化关系。结果证实,分形维数的变化能够很好地反映絮凝程度及混凝处理效果,从而为建立以分形维数作为主控特征参数的混凝投药系统提供了依据,并通过大量的基础试验为分形维数模型控制提供了参考。     

预氧化联合污泥回流处理低浊微污染水试验研究

研究了采用高锰酸钾预氧化与污泥回流联用工艺对低浊微污染原水的处理效果。结果表明,采用污泥回流可有效改善沉后水浊度,最佳回流比为60%,沉后水浊度从无回流时的1.91NTU下降到1.51NTU;粉末活性炭与污泥一同回流能改善滤后水有机物的去除效果。投加高锰酸钾进一步提高了处理效果,污泥回流则强化了高锰酸钾的助凝除浊效果,使有机物去除作用更显著。     

Degradation of DBPs' precursors in river water before and after slow sand filtration by photo-Fenton process at pH 5 in a solar CPC reactor

The generation of disinfection by-products during water treatment can be controlled by reducing the levels of precursor species prior to the chlorination step. The Natural Organic Matter (NOM) is the principal organic precursor and conventional removal of pollutants such as coagulation, flocculation and filtration do not guarantee the total NOM removal. In this study the degradation of NOM model compounds (dihydroxy-benzene) as well as the removal of NOM from river water via photo-Fenton process in a CPC solar photo-reactor is presented. The effect of solar activated photo-Fenton reagent at pH 5.0 before and after a slow sand filtration (SSF) in waters containing natural iron species is investigated and the details reported. The results showed that the total transformation of dihydroxy-benzene compounds along a mineralization higher than 80% was obtained. The mineralization of the organic compounds dissolved in natural water was higher than in Milli-Q water, suggesting that the aqueous organic and inorganic components (metals, humic acids and photoactive species) positively affect the photocatalytic process. When 1.0 mg/L of Fe³⁺ is added to the system, the photo-Fenton degradation was improved. Therefore the photo-Fenton reagent could be an interesting complement to other processes for NOM removal. Comparing the response of two rivers as media for the organic compounds degradation it was observed that the NOM photo-degradation rate depends of the water composition.