Origins and implications of drinking water odours in lakes and reservoirs of British Columbia, Canada

Production of enzymes for the removal of odorous substances in plant biomass

Residual plant biomass collected from agricultural, technical or biopharmaceutical processes contains odorous substances. The latter are often unacceptable for customers if the biomass is used in sustainable products such as building materials, paints, glues or flame-resistant foils. The objective of this study was to identify enzymes that can prevent the formation or facilitate the degradation of odorous substances such as butanol, eugenol or ethyl acetate and their derivatives in residual biomass. We used plant cell packs (PCPs) as a small-scale screening platform to assess the expression of enzymes that break down odorous substances in tobacco biomass. First, we compiled a list of volatile compounds in residual plant biomass that may give rise to undesirable odors, refining the list to 10 diverse compounds representing a range of odors. We then selected five monomeric enzymes (a eugenol oxidase, laccase, oxidase, alkane mono-oxidase and ethyl acetate hydrolase) with the potential to degrade these substances. We transiently expressed the proteins in PCPs, targeting different subcellular compartments to identify optimal production conditions. The maximum yield we achieved was ∼20 mg kg-1 for Trametes hirsute laccase targeted to the chloroplast. Our results confirm that enzymes for the removal of odorous substances can be produced in plant systems, facilitating the upcycling of residual biomass as an ingredient for sustainable products.

Management of biogenic taste and odour: From source water, through treatment processes and distribution systems, to consumers

Biogenic taste and odour (T&O) have become a global concern for water utilities, due to the increasing frequency of algal blooms and other microbial events arising from the combined effects of climate change and eutrophication. Microbially-produced T&O compounds impact source waters, drinking water treatment plants, and drinking water distribution systems. It is important to manage across the entire biogenic T&O pathway to identify key risk factors and devise strategies that will safeguard the quality of drinking water in a changing world, since the presence of T&O impacts consumer confidence in drinking water safety. This study provides a critical review of current knowledge on T&O-causing microbes and compounds for proactive management, including the identification of abiotic risk factors in source waters, a discussion on the effectiveness of existing T&O barriers in drinking water treatment plants, an analysis of risk factors for biofilm growth in water distribution systems, and an assessment of the impacts of T&O on consumers. The fate of biogenic T&O in drinking water systems is tracked from microbial production pathways, through the release of intracellular T&O by cell lysis, to the treatment of microbial cells and dissolved T&O. Based on current knowledge, five impactful research and management directions across the T&O pathway are recommended.

Off-Flavors in Aquacultured Fish: Origins and Implications for Consumers

Off-flavors in fish and water are considered a worldwide problem. Several factors, such as the presence of phosphorus, micronutrients, and organic matter, contribute to phytoplankton proliferation and the production of off-flavors. Geosmin and 2-methylisoborneol are the most common off-flavors that confer the smell of earth or mold to water and fish. These metabolites are not considered toxic, but they can be easily transferred from water to living organisms and accumulate in the biota, up the trophic levels and to consumers, including fish species. Numerous processes have been studied to eliminate or reduce the presence of off-flavors in recirculating aquaculture systems. Managing off-flavors must be eco-friendly and consumer-friendly. Strategies against off-flavors must be efficient and low-cost. However, these solutions may be different for each fish production system. We review herein the main compounds produced by cyanobacteria that can accumulate in fish used in aquaculture that can affect the quality of food, as well as production costs and consumer preference.

Relationship between algae diversity and water quality- a case study: Chah Niemeh reservoir Southeast of Iran

Algae are known with many detrimental impacts on drinking water quality. Discharge of municipal and agricultural wastewater into the receiving water resources make desirable conditions for algae growth and consequently cause eutrophication phenomena. Water samples were withdrawn monthly from 5 stations in Zabol City within spring and summer seasons. To identify algae species, micronutrients, and physical parameters such as temperature, depth of Secchi disk (SD) and pH on their growth were evaluated. The average phosphate in spring and summer were observed to be 0.034 and 0.028 mg/L, respectively. The results obtained from the present study indicated that the volume and depth of the water reservoirs were less critical on total phosphorus compared with the concentration of algal cells and total nitrogen. The mean pH for water samples taken from Chah Niemeh (CN) in spring and summer were observed to be 8.4 which is suitable for algae growth. Furthermore, the mean temperature (>20 °C) in both seasons were found to be desirable for the growth of algae, especially cyanobacteria in the CN. Moreover, the mean SD in spring and summer samples was 96.16 m and 119.83 m, respectively. As a result, the reservoir had low transparency in terms of algal growth. Totally, most of the identified algae were green algae (50%), algal flagella (19%), cyanobacteria (15.4%) and diatoms (15%). Therefore, cyanobacteria are most possible responsible for the taste and odor in the CN water reservoir. Future efforts should be directed toward preventive measurements for protecting water reservoirs from municipal and agricultural wastewaters and algae control.

Effective removal of odor substances using intimately coupled photocatalysis and biodegradation system prepared with the silane coupling agent (SCA)-enhanced TiO2 coating method

Intimately coupled photocatalysis and biodegradation (ICPB) combining photocatalysis with microbial degradation is an attractive wastewater treatment technology. However, when prepared in conventional ways, the supported-photocatalysts aggregate frequently, detach easily from carriers, and prohibit the colonization of microorganisms inside the carriers. To overcome these challenges, silane coupling agent (SCA)-enhanced TiO₂ coating method is developed in this study. The coupling agent γ-glycidoxypropyltrimethoxysilane (KH560) greatly enhanced the adhesion between photocatalysts and the carrier through ether and Ti-O-Si linkages. The dense TiO₂ layer was firmly adhered to the carrier outer surface, and the loading amount reached 351.8±8.2 mg/g, over ten times higher than using the powder sintering method (31.5±2.4 mg/g). In the ICPB system constructed with the KH560-enhanced TiO₂-supported polyurethane sponge (KH560-TiO₂-PU) carriers, removal efficiencies of two model odor substances, 2-methylisoborneol (2-MIB) and geosmin (GSM), reached 88.9±0.3% and 85.0±1.0% in 12 h at an initial concentration of 500 ng/L respectively, which were 17.7±0.6% and 19.4±0.4% greater than those of the ICPB system prepared with the powder sintering method. After 5 operating cycles, the novel ICPB system remained stable with high 2-MIB and GSM removal efficiencies, reaching 89.9±0.8% and 86.1±0.2% respectively after 12h, while TiO₂ peeling ratio was as low as 5.0±2.8%. Biofilms attached onto the carrier inner surface were resilient over the operating cycles with the increase of both richness and diversity of microbial communities. Analysis of biofilm microbial community and pollutant degradation pathways revealed the enhanced removal of 2-MIB and GSM in the novel ICPB system might be attributed to multiple factors. First, the alleviated aggregation and increased adhesion of photocatalysts onto carriers improved the overall photocatalysis efficiency. Second, biofilm inside of the carrier was protected and the microbial activity was well remained. Third, photocatalytic intermediate products were efficiently biodegraded by the enriched functional microbial populations, such as Thauera and Flavobacterium, with little concern of excessive oxidation. Collectively, this research provides a new technological solution that synergizes photocatalysis and biodegradation for effective removal of odorous substances in polluted natural water.

Trapping and preconcentration of volatile organic sulfur compounds in water samples by portable and battery-powered trapping device prior to gas chromatography-sulfur chemiluminescence determination

A simple, portable and battery-powered trapping device (iTrap) consisting of a purification tube, a trapping unit and a miniature air pump was developed for the pre-concentration of volatile organic sulfur compounds (VOSCs). The tested VOSCs, including methanthiol (MT), ethanethiol (ET), dimethyl sulfide (DMS), diethyl sulfide (DES) and dimethyl disulfide (DMDS), were firstly purged from water samples and then in situ pre-concentrated with the iTrap prior to their analysis by thermal desorption gas chromatography coupling with a sulfur chemiluminescence detector (TD-GC-SCD). Twenty-six adsorbents were studied to find the most suitable adsorbent for the efficient pre-concentration of VOSCs. Under optimal conditions, limits of detection of 6, 8, 6, 2 and 3 ng L^-1 were obtained for MT, ET, DMS, DES and DMDS, respectively. The precisions were better than 5.3% (relative standard deviations, RSDs). The iTrap was successfully applied for the analysis of VOSCs in Certified Reference Materials, several surface water, underground water and wastewater samples collected from Pengzhou city, Sichuan, China. Moreover, the VOSCs trapped in the iTrap were much more stable than those directly stored in water samples and the recoveries for all samples could be maintained at acceptable levels (>73%), even their preservation time as long as 8 h.

Causality analysis and prediction of 2-methylisoborneol production in a reservoir using empirical dynamic modeling

2-Methylisobornel (MIB) is one of the most widespread and problematic biogenic compounds causing taste-and-odor problems in freshwater. To investigate the causes of MIB production and develop models to predict the MIB concentration, we have applied empirical dynamic modeling (EDM), a nonlinear approach based on Chaos theory, to the long-term water quality dataset of Kamafusa Reservoir in Japan. The study revealed the dynamic nature of MIB production in the reservoir, and determined causal variables for MIB production, including water temperature, pH, transparency, light intensity, and Green Phormidium. Moreover, EDM established that the system is three-dimensional, and the approach found elevated nonlinearity (from 1.5 to 3) across the whole study period (1996-2015). By taking only one or two candidate predictors with varying time lags, multivariate models for predicting MIB production (best model: r = 0.83, p < 0.001, root mean squared error = 3.1 ng/L) were successfully established. The modeling approach used in this study is a powerful tool for causality identification and odor prediction, thus making important contributions to reservoir management.

The relationships between odors and environmental factors at bloom and non-bloom area in Lake Taihu, China

Lake Taihu has been experiencing taste and odor (T&O) events recently. And for the purpose of seeking the environmental factors having great influences on T&O compounds and supplying theory information for preventing the occurrence of T&O problems, Redundance analysis (RDA) was conducted for the dissolved and particle-bound forms of T&O compounds. And the whole lake was divided into the blooming and non-blooming areas. Results indicated that environmental factors, including biotic and abiotic factors, made great contributions to the variation of T&O compounds in Lake Taihu. The key biotic factors included Microcystis, Oscillatoria and chlorophyll-a. Microcystis made great contribution of these compounds in the blooming area and had close relationship with those particulate forms of odorants in Taihu. Oscillatoria made great and absolute contribution to odorants in the non-blooming area. Chlorophyll-a influenced greatly the odorants in the blooming area and had significant relationship with the particle-bound fractions in the whole lake. Dissolved oxygen and water temperature were the dominant abiotic factors with large contributions.

Factors affecting the water odor caused by chloramines during drinking water disinfection

Chloramine disinfection is one of the most common disinfection methods in drinking water treatment. In this study, the temporal variability of water odors during monochloramine auto-decomposition was investigated to elucidate the characteristics of odor problems caused by adopting chloramine disinfection in tap water. Odor intensities and dominant odorant contributions were determined using the flavor profile analysis (FPA) and odor active value (OAV), respectively. During auto-decomposition of monochloramine, Cl₂/N molar ratio, pH, temperature, and the presence of NOM all affected odor intensity and odor temporal variation in drinking water. In general, decreasing pH from 8.5 to 6.0 led to increasing perceived odor intensity due to the formation of dichloramine. The major odorants responsible for chlorinous odor under acidic and non-acidic conditions were dichloramine and monochloramine, respectively. Chloraminated water with a Cl₂/N molar ratio of 0.6 or NOM concentration <2 mg-C L^-1 inhibited odor intensity. Furthermore, the influence of rechlorination on chlorinous odor intensity for chloraminated water should not be neglected. The results of this study will be beneficial for the control of chlorinous odors caused by chloramine disinfection in drinking water.

Biochemistry and genetics of taste- and odor-producing cyanobacteria

Cyanobacteria are one of the principal sources of volatile organic compounds (VOCs) which cause offensive taste and odor (T&O) in drinking and recreational water, fish, shellfish and other seafood. Although non-toxic to humans, these T&O compounds severely undermine public trust in these commodities, resulting in substantial costs in treatment, and lost revenue to drinking water, aquaculture, food and beverage and tourist/hospitality industries. Mitigation and control have been hindered by the complexity of the communities and processes which produce and modify T&O events, making it difficult to source-track the major producer(s) and the factors governing VOC production and fate. Over the past decade, however, advances in bioinformatics, enzymology, and applied detection technologies have greatly enhanced our understanding of the pathways, the enzymes and the genetic coding for some of the most problematic VOCs produced by cyanobacteria. This has led to the development of tools for rapid and sensitive detection and monitoring for the VOC production at source, and provided the basis for further diagnostics of endogenous and exogenous controls. This review provides an overview of current knowledge of the major cyanobacterial VOCs, the producers, the biochemistry and the genetics and highlight the current applications and further research needs in this area.

Eutrophication mediates a common off-flavor compound, 2-methylisoborneol, in a drinking water reservoir

Off-flavors, such as 2-methylisoborneol (MIB) and geosmin, cause drinking water to have earthy or musty tastes and odors. Humans can detect such compounds at minute concentrations (10 and 30 ng/L for MIB and geosmin, respectively), and, although not a health risk, off-flavors can promote consumer distrust. Removal of these compounds is costly and often unreliable or only suitable under certain conditions. Minimizing off-flavor production at the watershed-scale may be more cost-effective in addition to improving ecosystem health and aesthetics. Cyanobacteria are considered to be the primary drivers of off-flavors in freshwater systems. Due to their ability to produce toxins, cyanobacteria have been under particular scrutiny, and environmental factors promoting cyanobacterial blooms are relatively well-studied. Using this body of literature, we conducted a seven-week, limnocorral experiment where we manipulated nitrogen and nitrogen-to-phosphorus concentrations to influence phytoplankton community structure and off-flavor production. The addition of a single nutrient across broad ranges (nitrogen or phosphorus) had no effect on MIB. However, the addition of both nitrogen and phosphorus promoted high concentrations of MIB relative to treatments that received no nutrients (448% increase) or only nitrogen or phosphorus (722% increase). Interestingly, cyanobacteria waned during the experiment and were replaced by diatoms, which were the dominant taxa by the end of the experiment. Our findings clearly show that eutrophication affects MIB production, but mechanisms leading to the production of this compound may differ from what has been previously predicted.

Geographical and seasonal patterns of geosmin and 2-methylisoborneol in environmental water in jiangsu province of china

This study was conducted to obtain the basic data of two common odorants-geosmin and 2-methylisoborneol (GSM and 2-MIB)-in environmental water. More specifically, the headspace solid-phase microextraction coupled to gas chromatography mass spectrometry (HS-SPME/GC-MS) was applied to determine the levels of GSM and 2-MIB in water samples, and the samples were collected depending on water sources, conventional treatment processes, and seasons. The significant difference was shown for the 2-MIB levels of source water (P < 0.05), the concentrations of GSM and 2-MIB decreased significantly as treatment process of tap water moved forward (P < 0.0001), and the significant differences for the levels of GSM and 2-MIB were observed among three sampling periods (P < 0.01). The levels of GSM and 2-MIB in all water samples were lower than 10 ng L(-1), the odor threshold concentration (OTC), and the conventional treatment process plays a significant role in removing odorants in tap water.

Mechanism of Fishy Odor Compounds Metabolized by Algae

Fishy odor, part of the taste and odor existing in the drinking water, often cause people to reflect poor water quality, and even lead to social panic. The typical fishy compounds are primarily amines and polyunsaturated aldehydes (PUAs), which are widely found in surface water, particularly during the algae blooms. The properties of the small molecular weight, low odor threshold and difficult removing, have received a great concern in the field of environment protection. Because of their environmental hazards, the source, physical-chemical properties, genotoxicity and generation mechanism of the fishy compounds were introduced in this paper in order to remove the compounds by controlling the precursors and generation pathway. The results show that amines are produced by the nitrogenous compounds in catalysis of microorganisms and enzymes, while PUAs are produced by polyunsaturated fatty acids (PUFAs). The algal cells cleavage is a key factor of producing fishy odor.

Dissolved organic carbon and trihalomethane precursor removal at a UK upland water treatment works

The removal of dissolved organic carbon (DOC) during potable water treatment is important for maintaining aesthetic water quality standards, minimising concentrations of micro-pollutants, controlling bacterial regrowth within distribution systems and, crucially, because it contains a sub-component that can act as trihalomethane (THM) precursors. In this study, the concentration and characteristics of raw water DOC and THM formation potential (THMFP) entering an upland potable water treatment works were analysed over twelve months. Correlations between raw water DOC characteristics, standardised THMFP (STHMFP) and % DOC removal were also investigated. DOC and THM precursor removal during a series of treatment stages was examined over this period, as well as potential selectivity in the removal of DOC fractions, to assess the importance of different treatment stages for DOC removal and THM amelioration. Though THMFP removal remained high and fairly stable throughout the study period (83-89%), the data suggest that this was mostly the result of high DOC removal rates rather than the selective removal of THM precursors. Whilst this chemical agnosticism makes DOC removal more robust, it may make the overall process more vulnerable to exceeding permissible THM concentrations under changing climatic conditions. The kinetics of the reaction between DOC and chlorine appeared to vary seasonally, indicating temporal changes in the proportions of fast- and slow-reacting precursors with implications for THM concentrations at the point of delivery to the consumer. The initial treatment stages, comprising coagulation-flocculation and dissolved air floatation (DAF) were by far the most important in terms of bulk DOC removal and the preferential removal of THM precursors, though, surprisingly, DOC quality was also modified following chlorination and secondary rapid gravity filtration (RGF). Though net THM concentration decreased following initial treatment stages, a doubling in the proportion of brominated THMs (BrTHMs), which are reported to be more carcinogenic, was also observed.

Analysis of Five Earthy-Musty Odorants in Environmental Water by HS-SPME/GC-MS

The pressing issue of earthy and musty odor compounds in natural waters, which can affect the organoleptic properties of drinking water, makes it a public health concern. A simple and sensitive method for simultaneous analysis of five odorants in environmental water was developed by headspace solid-phase microextraction (HS-SPME) coupled to chromatography-mass spectrometry (GC-MS), including geosmin (GSM) and 2-methylisoborneol (2-MIB), as well as dimethyl trisulfide (DMTS), β -cyclocitral, and β -ionone. Based on the simple modification of original magnetic stirrer purchased from CORNING (USA), the five target compounds can be separated within 23 min, and the calibration curves show good linearity with a correlation coefficient above 0.999 (levels = 5). The limits of detection (LOD) are all below 1.3 ng L(-1), and the relative standard deviation (%RSD) is between 4.4% and 9.9% (n = 7) and recoveries of the analytes from water samples are between 86.2% and 112.3%. In addition, the storage time experiment indicated that the concentrations did not change significantly for GSM and 2-MIB if they were stored in canonical environment. In conclusion, the method in this study could be applied for monitoring these five odorants in natural waters.

Orthogonal Design Study on Factors Affecting the Determination of Common Odors in Water Samples by Headspace Solid-Phase Microextraction Coupled to GC/MS

Geosmin and 2-MIB are responsible for the majority of earthy and musty events related to the drinking water. These two odorants have extremely low odor threshold concentrations at ng L(-1) level in the water, so a simple and sensitive method for the analysis of such trace levels was developed by headspace solid-phase microextraction coupled to gas chromatography/mass spectrometry. In this study, the orthogonal experiment design L32 (4(9)) was applied to arrange and optimize experimental conditions. The optimum was the following: temperatures of extraction and desorption, 65°C and 260°C, respectively; times of extraction and desorption, 30 min and 5 min, respectively; ionic strength, 25% (w/v); rotate-speed, 600 rpm; solution pH, 5.0. Under the optimized conditions, limits of detection (S/N = 3) were 0.04 and 0.13 ng L(-1) for geosmin and 2-MIB, respectively. Calculated calibration curves gave high levels of linearity with a correlation coefficient value of 0.9999 for them. Finally, the proposed method was applied to water samples, which were previously analyzed and confirmed to be free of target analytes. Besides, the proposal method was applied to test environmental water samples. The RSDs were 2.75%~3.80% and 4.35%~7.6% for geosmin and 2-MIB, respectively, and the recoveries were 91%~107% and 91%~104% for geosmin and 2-MIB, respectively.

Spatio-temporal variability of tastes and odors of drinking water within a distribution system

The threshold of human perception in the detection of tastes and odors (T&O) relating to compounds in drinking water is variable. For example, chlorine can be detected at the ppm level and geosmin can be perceived at the ppt level. In this paper, sensory tests (using a human panel), physicochemical analyses (total and free residual chlorine, temperature, metals, geosmin, and 2-methylisoborneol (2MIB)) and microbiological analyses (algae, Actinomycetes and heterotrophic plate count) were performed for water samples collected during a seventeen-month period at ten different locations of a municipal distribution network of Quebec City (Canada). The results showed that different flavors(1) assessed by a panel and aggregated into global flavor intensity (GFI) vary considerably spatially and seasonally. Multiple regression analysis showed that the factors best explaining the variability of GFI are (in order) the season, the location, the concentration of total residual chlorine and the presence of cyanobacteria. Results also demonstrate that chlorine has a masking effect on other T&O.

Simultaneous determination of eight common odors in natural water body using automatic purge and trap coupled to gas chromatography with mass spectrometry

Production and fate of taste and odor (T&O) compounds in natural waters are a pressing environmental issue. Simultaneous determination of these complex compounds (covering a wide range of boiling points) has been difficult. A simple and sensitive method for the determination of eight malodors products of cyanobacterial blooms was developed using automatic purge and trap (P&T) coupled with gas chromatography-mass spectrometry (GC-MS). This extraction and concentration technique is solvent-free. Dimethylsulfide (DMS), dimethyltrisulfide (DMTS), 2-isopropyl-3-methoxypyrazine (IPMP), 2-isobutyl-3-methoxypyrazine (IBMP), 2-methylisoborneol (MIB), β-cyclocitral, geosmin (GSM) and β-ionone were separated within 15.3 min. P&T uses trap #07 and high-purity nitrogen purge gas. The calibration curves of the eight odors show good linearity in the range of 1-500 ng/L with a correlation coefficient above 0.999 (levels=8) and with residuals ranging from approximately 83% to 124%. The limits of detection (LOD) (S/N=3) are all below 1.5 ng/L that of GSM is even lower at 0.08 ng/L. The relative standard deviations (RSD) are between 3.38% and 8.59% (n=5) and recoveries of the analytes from water samples of a eutrophic lake are between 80.54% and 114.91%. This method could be widely employed for monitoring these eight odors in natural waters.

Development of predictive models for geosmin-related taste and odor in Kansas, USA, drinking water reservoirs

The presence of taste and odor compounds can greatly reduce the quality of drinking water supplies. Because the monetary costs associated with the removal of these compounds can be high, it is impractical for most facilities to continuously treat their raw water. Instead, new tools are needed to help predict when taste and odor events may be most likely to occur. Water quality data were collected between June and October in 2006-2007 from five Kansas (USA) reservoirs in order to develop predictive models for geosmin, a major taste and odor compound; two of these reservoirs were also sampled during specific taste and odor events in December 2006 and January 2007. Lake trophic state alone was not a good predictor of geosmin concentrations as the highest average geosmin concentration was observed in the reservoir with the lowest nutrient and chlorophyll a concentrations. In addition, taste and odor events were not confined to summer months; elevated geosmin concentrations were observed in several reservoirs during the winter. Growth limitation by inorganic phosphorus appeared to be the primary determinant of geosmin production by algal cells in these reservoirs.

Developing hexanal as an odor reference standard for sensory analysis of drinking water

There are many analytical and sensory methods to analyze drinking water for flavor and off-flavors before it reaches consumers. Flavor profile analysis (FPA) is one of the most comprehensive methods. A well-trained panel is essential for FPA and although taste standards are well established, FPA training lacks an odor reference standard. In search of an odor reference standard, four different panel groups were trained and tested for n-hexanal at various concentrations (1-1000 microg/L) over 14 months. The Weber-Fechner plots for n-hexanal showed a linear and overlapping relationship for all panels. Analytical measurements demonstrated that the headspace concentration of n-hexanal was constant after 5 sniffs at 45 degrees C and it remained constant during FPA sessions for up to 4 h. The panelists liked the grassy odor of n-hexanal, which did not result in fatigue, and testing demonstrated that approximately 95% of the population can detect n-hexanal's odor. n-Hexanal is proposed as an odor reference standard for FPA training to define odor intensities because it is chemically stable, follows Weber-Fechner law, mimics grassy odors found in drinking water, and was acceptable to the human panelists.

Investigation of assimilable organic carbon (AOC) in flemish drinking water

The aim of the study was to investigate the drinking water supplied to majority of residents of Flanders in Belgium. Over 500 water samples were collected from different locations, after particular and complete treatment procedure to evaluate the efficiency of each treatment step in production of biologically stable drinking water. In this study assimilable organic carbon (AOC) was of our interest and was assumed as a parameter responsible for water biostability. The influence of seasons and temperature changes on AOC content was also taken into account. The AOC in most of the non-chlorinated product water of the studied treatment plants could not meet the biostability criteria of 10 mug/l, resulting in the mean AOC concentration of 50 microg/l. However, majority of the examined chlorinated water samples were consistent with proposed criteria of 50--100 microg/l for systems maintaining disinfectant residual. Here, mean AOC concentration of 72 microg/l was obtained. Granular activated carbon filtration was helpful in diminishing AOC content of drinking water; however, the nutrient removal was enhanced by biological process incorporated into water treatment (biological activated carbon filtration). Disinfection by means of chlorination and ozonation increased the water AOC concentration while the ultraviolet irradiation showed no impact on the AOC content. Examination of seasonal AOC variations showed similar fluctuations in six units with the highest values in summer and lowest in winter.