Assimilable Organic Carbon Levels Research Articles

Effect of residual chlorine on the interaction between bacterial growth and assimilable organic carbon and biodegradable organic carbon in reclaimed water

Reclaimed water can significantly reduce household water consumption. However, microbial growth introduces several problems to reclaimed water, including health concerns, aesthetic deterioration and biofouling. Biological stability refers to the potential of organic matter or nutrients in water to support heterotrophic bacterial growth (HPC). Assimilable organic carbon (AOC) and biodegradable organic carbon (BDOC) are often used as indicators to evaluate the biological stability of water. This study investigated the effect of residual chlorine on the interaction between bacterial regrowth and AOC or BDOC and the variation of AOC after chlorine disinfection in reclaimed water. The results show that the HPC level is insensitive to AOC concentration when residual chlorine is >0.5 mg/L; however, the effects are more pronounced when residual chlorine is <0.5 mg/L. A residual chlorine concentration of >0.5 mg/L therefore maintains biological stability in reclaimed water. When residual chlorine was <0.5 mg/L, HPC levels were found to be limited when AOC was <128 μg/L or BDOC was <0.25 mg/L; and when residual chlorine was >0.5 mg/L, HPC levels were found to be limited when AOC was <796 μg/L or BDOC was <0.85 mg/L in reclaimed water. After chlorine disinfection, AOC contents initially increase and then decrease gradually, reaching minimum levels around day 20, and then increase in both greywater reclaimed water and mixed wastewater reclaimed water. Maintaining the chlorine demand and controlling the AOC level is critical for producing an effect on microbial regrowth and stabilising reclaimed water. The results of this study are conducive to the popularisation of reclaimed water use and also provide reference for reuse standards of reclaimed water.

Occurrence of Legionella spp. in Water-Main Biofilms from Two Drinking Water Distribution Systems.

The maintenance of a chlorine or chloramine residual to suppress waterborne pathogens in drinking water distribution systems is common practice in the United States but less common in Europe. In this study, we investigated the occurrence of Bacteria and Legionella spp. in water-main biofilms and tap water from a chloraminated distribution system in the United States and a system in Norway with no residual using real-time quantitative polymerase chain reaction (qPCR). Despite generally higher temperatures and assimilable organic carbon levels in the chloraminated system, total Bacteria and Legionella spp. were significantly lower in water-main biofilms and tap water of that system ( p < 0.05). Legionella spp. were not detected in the biofilms of the chloraminated system (0 of 35 samples) but were frequently detected in biofilms from the no-residual system (10 of 23 samples; maximum concentration = 7.8 × 104 gene copies cm-2). This investigation suggests water-main biofilms may serve as a source of Legionella for tap water and premise plumbing systems, and residual chloramine may aid in reducing their abundance.

Assimilable organic carbon (AOC) determination using GFP-tagged Pseudomonas fluorescens P-17 in water by flow cytometry.

One of the newly developed methods for Assimilable organic carbon (AOC) determination is leveraged on the cell enumeration by flow cytometry (FC) which could provide a rapid and automated solution for AOC measurement. However, cell samples staining with fluorescence dye is indispensable to reduce background and machine noise. This step would bring additional cost and time consuming for this method. In this study, a green fluorescence protein (GFP) tagged strain derived of AOC testing strain Pseudomonas fluorescens P-17 (GFP-P17) was generated using Tn5 transposon mutagenesis. Continuous culture of this mutant GFP-P17 showed stable expression of eGFP signal detected by flow cytometry without staining step. In addition, this GFP-P17 strain displayed faster growth rate and had a wider range of carbon substrate utilization patterns as compared with P17 wild-type. With this strain, the capability of a new FC method with no dye staining was explored in standard acetate solution, which suggests linear correlation of counts with acetate carbon concentration. Furthermore, this FC method with GFP-P17 strain is applicable in monitoring GAC/BAC efficiency and condition as similar trends of AOC level in water treatment process were measured by both FC method and conventional spread plating count method. Therefore, this fast and easily applicable GFP-P17 based FC method could serve as a tool for routine microbiological drinking water monitoring.

Effect of disinfectant residual on the interaction between bacterial growth and assimilable organic carbon in a drinking water distribution system

Public health is threatened by deteriorated water quality due to bacterial regrowth and uncontrolled growth-related problems in drinking water distribution systems (DWDSs). To investigate the scope of this problem, a two-year field study was conducted in south China. The amount of assimilable organic carbon (AOC), total cell concentrations (TCC), and intact cell concentrations (ICC) of water samples were determined by flow cytometry. The results indicated that ICC was significantly correlated to AOC concentration when the chlorine concentration was less than 0.15 mg/L, and ICC was lower at chlorine concentrations greater than 0.15 mg/L, suggesting that free chlorine level had effect on AOC and ICC. To further analyze the effect of disinfectant on AOC and bacterial growth, we designed an orthogonal experiment with different dosages of two commonly used disinfectants, chlorine and chloramine. The results demonstrated that high concentrations of free chlorine (＞0.15 mg/L) and chloramine (＞0.4 mg/L) were associated with relatively low proportions of intact cells and cultivable bacteria. Compared with chlorine, chloramine tended to cause lower AOC level and intact cells, likely because the chlorinated disinfection byproducts (DBPs) were more easily absorbed by bacteria than the chloraminated DBPs. Based on the statistical analysis of 240 water samples, ICC was limited when AOC concentration was less than 135 μg/L, while temperature and the number of small-size particles showed positive effects on ICC (P＜0.05). We conclude that the use of chloramine and controlling particle numbers should be suitable strategies to limit bacterial regrowth.

Assessment of the microbial growth potential of slow sand filtrate with the biomass production potential test in comparison with the assimilable organic carbon method

Slow sand filtration is the final treatment step at four surface-water supplies in the Netherlands. The microbial growth potential (MGP) of the finished water was measured with the assimilable organic carbon (AOC) method using pure cultures and the biomass production potential (BPP) test. In the BPP test, water samples were incubated untreated at 25 °C and the active-biomass concentration was measured by adenosine tri-phosphate (ATP) analysis. Addition of a river-water inoculum improved the test performance and characteristic growth and maintenance profiles of the water were obtained. The maximum ATP concentration attained within seven days and the cumulative biomass production after 14 days of incubation (BPC14, d ng ATP L−1) showed highly significant and strong linear relationships with the AOC in the slow sand filtrates. The lowest AOC and BPC14 levels were observed in the supplies applying dune filtration without ozonation in post treatment, with AOC/TOC = 1.7 ± 0.3 μg acetate-C equivalents mg−1 C and BPC14/TOC = 16.3 ± 2.2 d ng ATP mg−1 C, corresponding with 1.2 ± 0.19 ng ATP mg−1 C. These characteristics may represent the lowest specific MGP of natural organic matter achievable by biofiltration at temperatures ≤20 °C. The AOC and BPC14 concentrations in the slow sand filtrate of the supply treating lake water by ozonation with granular-activated-carbon filtration and slow sand filtration as post treatment increased with decreasing temperature. The BPP test revealed that this slow sand filtrate sampled at 2 °C contained growth-promoting compounds that were not detected with the AOC test. These observations demonstrate the utility of the BPP test for assessing the MGP of drinking water and show the performance limits of biofiltration for MGP reduction.

Development of an ATP luminescence-based method for assimilable organic carbon determination in reclaimed water

Assimilable organic carbon (AOC) is an important indicator of the biological stability of reclaimed water. In this study, a new rapid and more stable method for AOC measurement in reclaimed water was proposed. Indigenous microbial culture from secondary effluent was used as the inoculum, and bacterial growth was determined by the quantity of adenosine triphosphate (ATP) in the form of luminescence instead of plate count. ATP luminescence had a high correlation with biogrowth both in pure acetate solutions and reclaimed waters. ATP luminescence analysis could be determined in 5 min. Three days of 10000 cells/mL inoculum incubated at 25 °C were enough for the bacteria to reach the stationary phase. The good correlations between ATP luminescence and the added acetate-C concentration illustrated the applicability of monitoring AOC level by luminescence method. And in reclaimed water samples, indigenous microbial culture produces the highest AOC results compared with the pure strains. This indicated that the integrity of indigenous microbial culture ensured the full utilization of matrix carbons, which demonstrated the advantage of indigenous microbial culture compared with the selected pure bacteria in the traditional AOC test. The average ATP content per cell of 3.95 × 10−10 nmol/cell was derived, and this value was stable in both the acetate solutions and reclaimed waters. Furthermore, the average yield coefficient of 1.5 × 105 RLU/μg acetate-C (4.1 × 10−3 nmol ATP/μg acetate-C) was obtained from different indigenous cultures. Additionally, the indigenous microbial cultures from different secondary effluents would produce the similar AOC results for the same water sample, indicating the consistency of this assay. The ATP luminescence-AOC assay provides a faster, more stable and accurate approach for monitoring the biological stability of reclaimed waters.

Application of flow cytometry to monitor assimilable organic carbon (AOC) and microbial community changes in water

Flow cytometry is an efficient monitoring tool for rapid cell counting, and can be applied to research on water quality and treatment. In this study, a method that employs flow cytometry and a natural microbial inoculum to determine assimilable organic carbon (AOC) was adapted for use with challenging surface waters that have a high organic and particle content, and subsequently applied in a long term river water study. AOC method optimization showed that river water bacteria could pass through a 0.2μm membrane filter, and therefore membrane filtration combined with heat treatment was required for sample sterilization. Preparation of the natural river inoculum with an acceptable yield value could only be achieved when grown using the natural water source, since growth was limited on different types of inorganic minimal media and in natural spring water. The resulting flow cytometry AOC method was reliable and reproducible, and results were comparable to the standard plate count AOC method. Size exclusion chromatography showed that both high and low molecular weight organic matter fractions were utilized by the natural AOC inoculum. Flow cytometry was used to measure both AOC levels and total cell counts in a long term study to monitor the water quality of a river which was used as a drinking water source. The method could distinguish between high nucleic acid (HNA) and low nucleic acid (LNA) groups of bacteria, and HNA bacteria were found to respond faster than LNA bacteria to seasonal changes in nutrients and water temperature.

PEX and PP Water Pipes: Assimilable Carbon, Chemicals, and Odors

Eleven brands of plastic drinking water pipe were evaluated for assimilable organic carbon (AOC) release at 23°C for 28 days: polyvinyl chloride, high‐density polyethylene, polypropylene (PP), and cross‐linked polyethylene (PEX) pipes. Three of eight PEX pipe brands exceeded a 100 µg/L AOC microbial regrowth threshold for the first exposure period, and no brands exceeded this value on day 28. No AOC increase was found for PP or PEX‐a1 pipes; the remaining pipe brands contributed marginal AOC levels. Pipe water quality impacts were more fully evaluated for two PEX‐b brands and one PP brand. PEX pipes caused greater odor than the PP pipe and released more organic carbon as well as volatile and semi‐volatile organic compounds. Water quality impacts were less after 30 days. Regulated and unregulated contaminants were found in three PEX plumbing systems. Drinking water odors were attributed to toluene, ethyl‐tert‐butyl ether, and unidentified contaminants.

Effect of different molecular weight organic components on the increase of microbial growth potential of secondary effluent by ozonation

Ozonation has been widely applied in advanced wastewater treatment. In this study, the effect of ozonation on assimilable organic carbon (AOC) levels in secondary effluents was investigated, and AOC variation of different molecular weight (MW) organic components was analyzed. Although the removal efficiencies were 47%–76% and 94%–100% for UV254 and color at ozone dosage of 10mg/L, dissolved organic carbon (DOC) in secondary effluents was hardly removed by ozonation. The AOC levels increased by 70%–780% at an ozone dosage range of 1–10mg/L. AOC increased significantly in the instantaneous ozone demand phase, and the increase in AOC was correlated to the decrease in UV254 during ozonation. The results of MW distribution showed that, ozonation led to the transformation of larger molecules into smaller ones, but the increase in low MW (<1kDa) fraction did not contribute much to AOC production. The change of high MW (>100kDa and 10–100kDa) fractions itself during ozonation was the main reason for the increase of AOC levels. Furthermore, the oxidation of organic matters with high MWs (>100kDa and 10–100kDa) resulted in more AOC production than those with low MWs (1–10kDa and <1kDa). The results indicated that removing large molecules in secondary effluents could limit the increase of AOC during ozonation.

Increase of microbial growth potential in municipal secondary effluent by coagulation

Microbial growth is a big issue of concern in the use of reclaimed water. In this study, the variation of microbial growth potentials of municipal secondary effluents after coagulation was evaluated by measuring assimilable organic carbon (AOC). Surprisingly, the AOC levels increased significantly (55–667%) after coagulation with poly-aluminum dosages of 60mgL−1 for the samples investigated in this research. By ultrafiltration membrane fractionation, the microbial growth potentials of the fractions with different molecular weight (MW) were measured. The results revealed that the maximum cell densities of microbial growth in secondary effluents were lower than those in their fractions with MW<10kDa. Meanwhile, the organic component with MW>10kDa in biological treated effluents was proved to have an inhibitory effect on microbial growth. Therefore, the removal of those high MW organic matters was the main reason for the increase of microbial growth potential in secondary effluents during coagulation. Furthermore, polysaccharides and/or proteins in secondary effluents were easily removed by coagulation and were thought to be the possible key organic substances affecting the microbial growth potential during coagulation. It is suggested that post treatments would be needed after coagulation to maintain the biological stability of reclaimed water.

Biostability in distribution systems in one city in southern China: Characteristics, modeling and control strategy

This study investigated the bacterial regrowth in drinking water distribution systems receiving finished water from an advanced drinking water treatment plant in one city in southern China. Thirteen nodes in two water supply zones with different aged pipelines were selected to monitor water temperature, dissolved oxygen (DO), chloramine residual, assimilable organic carbon (AOC), and heterotrophic plate counts (HPC). Regression and principal component analyses indicated that HPC had a strong correlation with chloramine residual. Based on Chick-Watson's Law and the Monod equation, biostability curves under different conditions were developed to achieve the goal of HPC ⩽ 100 CFU/mL. The biostability curves could interpret the scenario under various AOC concentrations and predict the required chloramine residual concentration under the condition of high AOC level. The simulation was also carried out to predict the scenario with a stricter HPC goal (⩽ 50 CFU/mL) and determine the required chloramine residual. The biological regrowth control strategy was assessed using biostability curve analysis. The results indicated that maintaining high chloramine residual concentration was the most practical way to achieve the goal of HPC ⩽ 100 CFU/mL. Biostability curves could be a very useful tool for biostability control in distribution systems. This work could provide some new insights towards biostability control in real distribution systems.

Chlorine requirement for biologically stable drinking water after nanofiltration

There is considerable interest in minimizing the chlorine residual in Japan's tap water because of increasing consumer complaints about the chlorinous odor of drinking water. However, minimization of the chlorine residual requires stricter control of biodegradable organics in finished water to ensure biological stability during water distribution. In this context, we investigated the improvement of biological stability of drinking water by nanofiltration (NF). The removal of assimilable organic carbon (AOC) was 52% on average, but showed large seasonal variation. It was found to be difficult to maintain the acceptable AOC level for biologically stable water by NF. In addition, significant bacterial regrowth in NF permeates was still observed without chlorination, although 52% AOC was removed. Then, the chlorine concentration required to prevent bacterial regrowth during distribution of nanofiltrated water was determined using continuous-flow reactor systems. The heterotrophic plate counts (HPC) densities in biofilm were monitored at different chlorine conditions. The results indicated that the NF process could significantly reduce the chlorine requirement of the finished water. The relationship between free chlorine residual and HPC in biofilm formed from 1 μgC AOC showed that a trace level of chlorine residual, &amp;lt;0.1 mg/L, could inactivate heterotrophic bacteria and prevent biofilm formation.

Biofilm formation and permeate quality improvement in Gravity Driven Membrane ultrafiltration

The effects of biofilm development on ultrafiltration membranes with regard to permeate stability and permeation rates were investigated using Gravity Driven Membrane (GDM) filtration. The first part of the study aimed at evaluating the influence of the biofilm on permeate flux quality and quantity with regard to Assimilable Organic Carbon (AOC) degradation. In addition, two types of biological pre-treatments were evaluated: slow sand filtration and packed bed bio-reactor, compared to a control (no treatment). Biofilm formation helped to decrease the AOC content of permeate water, compared to the influent. Both pre-treatments additionally reduced the AOC level in the permeate and thus increased its biological stability, however none of the systems were able to guarantee microbiologically stable water. Removal of AOC before the GDM filtration reduced the biofilm growth potential, which in turn influenced its physical structure and enhanced the permeation rates. Influence of inorganic particle removal by pre-sedimentation and its effect on biofilm structure were also studied. Pre-sedimentation of particle populations selected fine and homogeneous particle fractions, which led to the formation of a homogeneous biofilm structure characterised by an increased hydraulic resistance. This was clearly visible between horizontally and vertically installed membranes where the latter ones had a significantly reduced flux despite lower deposited particle mass. The presence of larger, heterogeneous particle fractions counterbalanced the negative effects of the fine particles, which overall resulted in enhanced permeation rates.

Removal of DOM and AOC in a full-scale advanced water treatment plant: effects of operational periods of BAC filters

Water utilities in Japan want to reduce residual chlorine levels so as to reduce disinfection byproducts and the chlorine smell, but bacterial regrowth is a concern. In advanced water treatment plants using ozonation and biological activated carbon (BAC), BAC plays the most important role in reducing assimilable organic carbon (AOC). Therefore, we analyzed the removal of dissolved organic matter (DOM) and AOC by full-scale BAC filters with different operational periods. Seasonal variation in AOC in raw water was wide, with a maximum of 306 μg acetate-C/L in November, and ozonation increased AOC. Nonetheless, BAC filters that had operated for 6 years produced water with lower AOC than ones with shorter operational periods, although they released a small amount of aromatic DOM. A diagram was constructed to derive the required levels of residual chlorine for no bacterial regrowth at various AOC levels of BAC effluents, which were linked with the operational periods of BAC filters and AOC in the ozonation effluents. Although BAC filters operated for longer than 100 weeks can effectively reduce AOC, lowering AOC levels in the ozonation effluent was indispensable for preventing bacterial regrowth in the water supply network at lower levels of residual chlorine than at present.

A rapid technique for assessing assimilable organic carbon of UV/H2O2-treated water

A rapid technique was adopted and customized to assess the assimilable organic carbon (AOC) content of water for UV/H2O2 treatment applications. Indigenous consortium was used as inoculum and cells grown were counted using flow cytometry. Residual H2O2 after UV/H2O2 treatment was found to inhibit microorganisms’ regrowth, leading to incorrect AOC measurement. Therefore, Catalase immobilized on a polymeric support was used to eliminate the residual H2O2 prior to the AOC bioassay without affecting the water quality and the AOC. The choice of microbial consortium was found to influence the result of the AOC bioassay, suggesting that indigenous inoculum should be used for each individual sample set. Three days of incubation at 30°C were enough for the microorganisms to reach their stationary phase. Addition of minerals was found to be necessary, affecting the outcome of the AOC test. The method presented in this article will be valuable for monitoring AOC levels at different stages of water treatment train, especially when UV/H2O2 is used as one of treatment steps.

A rapid bioluminescence-based test of assimilable organic carbon for seawater

The accumulation of biological materials and bacteria on water purification membranes, termed biofouling, is associated with decreased membrane performance and increased cost of operation. One strategy to minimize biofouling is pretreatment of the influent water. In this regard, tools and indicators that can assess the influent water are required, enabling an optimum selection of pretreatment methods. One parameter directly linked to biofouling potential is the concentration of assimilable organic carbon (AOC) in the feed-water. High AOC levels are associated with increased growth potential of the microbial fouling community. This work focused on the development of a new method for rapid and accurate quantification of AOC concentration in seawater. The method is based on the quantification of the bioluminescence response of the marine bacterium Vibrio fischeri MJ-1. Compared to previous methods, this new V. fischeri method was rapid (within 1h), sensitive (detection limit=0.1μg-C glucose equivalents/L) and highly suitable for seawater samples. V. fischeri method was evaluated using real seawater samples. The results showed positive reproductive AOC values. The new V. fischeri AOC method developed has a highly promising potential to be practically adopted as a rapid indicator of AOC concentration and hence biofouling potential of influent marine water.

Nontuberculous Mycobacteria, Fungi, and Opportunistic Pathogens in Unchlorinated Drinking Water in the Netherlands

The multiplication of opportunistic pathogens in drinking water supplies might pose a threat to public health. In this study, distributed unchlorinated drinking water from eight treatment plants in the Netherlands was sampled and analyzed for fungi, nontuberculous mycobacteria (NTM), and several opportunistic pathogens by using selective quantitative PCR methods. Fungi and NTM were detected in all drinking water samples, whereas Legionella pneumophila, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Aspergillus fumigatus were sporadically observed. Mycobacterium avium complex and Acanthamoeba spp. were not detected. Season had no influence on the occurrence of these organisms, except for NTM and S. maltophilia, which were present in higher numbers in the summer. Opportunistic pathogens were more often observed in premise plumbing water samples than in samples from the distribution system. The lowest number of these organisms was observed in the finished water at the plant. Thus, fungi, NTM, and some of the studied opportunistic pathogens can multiply in the distribution and premise plumbing systems. Assimilable organic carbon (AOC) and/or total organic carbon (TOC) had no clear effects on fungal and NTM numbers or on P. aeruginosa- and S. maltophilia-positive samples. However, L. pneumophila was detected more often in water with AOC concentrations above 10 μg C liter(-1) than in water with AOC levels below 5 μg C liter(-1). Finally, samples that contained L. pneumophila, P. aeruginosa, or S. maltophilia were more frequently positive for a second opportunistic pathogen, which shows that certain drinking water types and/or sampling locations promote the growth of multiple opportunistic pathogens.

Impact of AOC and chlorine residual on regrowth of microbes in a model distribution system receiving UV-treated potable water

The microbial ecology of distribution systems is complex and is influenced by a number of factors. This study assessed whether or not during drinking water treatments it is important to add a chemical disinfectant after ultraviolet (UV) disinfection in order to control the regrowth of microbes in distribution systems. Results showed that low UV doses did not impact assimilable organic carbon (AOC) significantly. No definite relationship between UV dose and steady-state biofilm heterotrophic (HPC) bacteria was evident. AOC and chlorine residual are two important factors affecting biofilm and suspended HPC growth in distribution systems. When the AOC concentration was 0, 5 and 10 μg AC/L (μg acetate-C/L), a chlorine residual was not necessary to keep the fluid HPC under 500 CFU/mL. When the AOC concentration reached between 100 and 200 μg AC/L, the regrowth of microbes in the model distribution system necessitated a chlorine residual of over 0.5 mg/L. At lower AOC levels (0, 10–20 μg AC/L), the AOC appeared to have a strong effect on controlling biofilm formation and suspended HPC concentration. In contrast, at higher AOC levels (50, 100 and 200 μg AC/L), a chlorine residual was indispensable when controlling the regrowth of HPC in the distribution systems. AOC and the required chlorine residual in these systems were interrelated.

Assimilable Organic Carbon (AOC) in Soil Water Extracts Using Vibrio harveyi BB721 and Its Implication for Microbial Biomass

Assimilable organic carbon (AOC) is commonly used to measure the growth potential of microorganisms in water, but has not yet been investigated for measuring microbial growth potential in soils. In this study, a simple, rapid, and non-growth based assay to determine AOC in soil was developed using a naturally occurring luminous strain Vibrio harveyi BB721 to determine the fraction of low molecular weight organic carbon in soil water extract. Calibration of the assay was achieved by measuring the luminescence intensity of starved V. harveyi BB721 cells in the late exponential phase with a concentration range from 0 to 800 µg l−1 glucose (equivalent to 0–16.0 mg glucose C kg−1 soil) with the detection limit of 10 µg l−1 equivalent to 0.20 mg glucose C kg−1 soil. Results showed that bioluminescence was proportional to the concentration of glucose added to soil. The luminescence intensity of the cells was highly pH dependent and the optimal pH was about 7.0. The average AOC concentration in 32 soils tested was 2.9±2.2 mg glucose C kg−1. Our data showed that AOC levels in soil water extracts were significantly correlated (P<0.05) with microbial biomass determined as microbial biomass carbon, indicating that the AOC concentrations determined by the method developed might be a good indicator of soil microbial biomass. Our findings provide a new approach that may be used to determine AOC in environmental samples using a non-growth bioluminescence based assay. Understanding the levels of AOC in soil water extract provides new insights into our ability to estimate the most available carbon pool to bacteria in soil that may be easily assimilated into cells for many metabolic processes and suggest possible the links between AOC, microbial regrowth potential, and microbial biomass in soils.

Molecular Weight Distribution of Organics in Raw and Treated Water

Levels of organic matters in raw water from Huangpu River and treated water from different processes have been investigated. Dissolved organic carbon (DOC) and assimilable organic carbon (AOC) have been determined on ultrafiltrate (UF) (MW cut-off of 1K and 10K Daltons) samples. The organics in raw water are majority of lower MW (&lt;1K Daltons) fractions. The conventional treatment processes are effective in removing higher MW (&gt;10K Daltons) organics, while granular activated carbon (GAC) adsorption can reduce organics with lower MW significantly. The lower MW fractions correspond to 60%~70% of the AOC in raw and treated waters, and much higher than other MW fractions. This indicates that the AOC is mainly related to organics with lower MW. In order to decrease AOC level to achieve biostability in drinking water, the processes which can reduce lower MW organics effectively must be employed.