Chemosphere 53 (2003) 773–778 www.elsevier.com/locate/chemosphere Influence of eutrophication on the coagulation efficiency in reservoir water Wen Po Cheng a,*, Fung-Hwa Chi b a Department of Safety, Health and Environmental Engineering, National Lien-Ho Institute of Technology, Miaoli 36012, Taiwan, ROC b Department of Environmental Engineering, Kun Shan University of Technology, Tainan 710, Taiwan, ROC Received 3 May 2002; received in revised form 25 April 2003; accepted 30 April 2003 Abstract Water from the three reservoirs, Min-ter, Li-yu-ten and Yun-ho-shen, was examined for concentration of chloro- phyll a, ultraviolet absorption (UV254), fluorescence intensity (FI), concentration of dissolved organic carbon (DOC), and fractionation of dissolved molecules by molecular weight. The water samples were collected over the change from spring to summer (May to July but before the typhoon season) when the water temperature and extent of eutrophi- cation increase. Analytical results indicate that the concentration of DOC is proportional to the concentration of chlorophyll a, but not to the values of UV254 and FI. Therefore, eutrophication, extraneous contaminants of small molecules, and the extracellular products of algae cause an increase in DOC, but a decrease in the proportion of large organic molecules such as of humic substances. The fraction of DOC with a molecular weight of less than 5000 Da increases with the concentration of chlorophyll a. All these data suggest that changes in the quality of water after eutrophication make the treatment of drinking water more difficult. The method of enhanced coagulation was recently developed for removing DOC. However, the results of this paper demonstrate that the efficiency of DOC removal falls as the degree of eutrophication increases. When the percentage of DOC with small molecules excreted by algae in- creased by 1%, the efficiency of DOC removal decreased by approximately 1%, implying that enhanced coagulation are not able to remove the DOC excreted by the algae during eutrophication, and resulting an increased concentration of trihalomethanes formation in water disinfections process. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Molecular weight fractionation; Eutrophication; Aluminum sulfate; Coagulation; THM 1. Introduction rate of dissolved organic carbon (DOC) accumulation exceeds that of its consumption by microorganisms, Reservoirs are the main sources of drinking water in such that the composition of DOC is changed. Restated, Taiwan. These sources can become contaminated by the ratio of large molecules to small molecules declined agricultural activity, domestic wastewater discharge, and as eutrophication continued, causing problems in the industrial effluent, which promote eutrophication, and treatment of drinking water (Owen et al., 1995; Volk cause the blooming of algae in the reservoirs. Also, the et al., 2000). The large molecules such as humic acid could be easily removed by the method of coagulation (Amy, 1987; Amy et al., 1992), but the extraneous or- ganics and the contaminant of small molecules resulting * Corresponding author. Tel.: +886-3-7332543x26; fax: +886- from the propagation of algae are difficult to remove 3-7333187. from raw water by the methods of traditional drinking E-mail address: [email protected] (W.P. Cheng). water treatment. 0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00510-1 774 W.P. Cheng, F.-H. Chi / Chemosphere 53 (2003) 773–778 Several methods have been recently developed to (Dohrmann phoenic 8000) was used to analyze the non- treat serious contamination problems with drinking purgeable DOC in raw water to determine the concen- water, and especially to increase the removal efficiency tration of DOC, which was filtered and adjusted to pH 7 of DOC. Among these methods, the enhanced coagu- in advance. lation differs from the traditional coagulation that fo- cuses on decreasing turbidity. Enhanced coagulation 2.1.2. Fluorescence analysis overuses coagulant to increase DOC removal to improve The amount of humic acid in water sample was the quality of drink water, and thus prevent the for- determined by measuring the fluorescence intensity mation of trihalomethanes (THM), without degrading (FI) using a fluorescence spectrophotometer (Hitachi, the quality of the water (Krasner and Amy, 1995; F-2000) at an excitation wavelength of 315 nm and an Chowdhury et al., 1997; Edwards, 1997; White et al., emission wavelength of 417 nm (Mopper and Schultz, 1997). Notably, Volk et al. (2000) indicated that the 1993; Hautala et al., 2000; Cheng, 2002). The FI was enhanced coagulation is useful only for removing large calibrated using 0.3 mg/l quinine sulfate. The FI is af- and hydrophobic organic molecules, but not for small or fected by solution pH, especially in the low pH range. hydrophilic molecules. Accordingly, following eutroph- However, the standard method of NIEA W940.50T ication, the ratio of large organic compounds to smaller (Standard method of the Environmental Protection compounds decreases, and then the efficiency of removal Agency of the Republic of China) states that filtering of organic matter decreases. and adjusting solution to pH 7 before measurement can Biological and chemical indicators of water quality minimize the error. are used to compare the degree of eutrophication of the three reservoirs Min-ter, Li-hu-ten and Yun-ho-shen to 2.1.3. Fractionation of molecular weight elucidate the relationship between eutrophication and The fractionation of molecular weight was conducted enhanced coagulation. The total concentrations and cha- by ultrafiltration using equipment purchased from racteristics of organic matter were analyzed by DOC, Millipore Company with a membrane with a molecu- ultraviolet absorption (UV254), fluorescence and the lar weight cutoff of 5000 Da. The sample was filtered fractionation of molecular weight. The formation po- through a 0.45 lm membrane and then recycled by tential of trihalomethanes (THMFP) of water following pumping into the ultrafiltration system, until the resid- coagulation was examined using chlorination (Koch ual volume of the sample was 1/3 of the original volume. et al., 1991). Experiments realized the following goals of Then, both filtrate and retentate were analyzed it DOC this research. (1) The effect of eutrophication on the concentration and the recovery percentage was calcu- concentration of organic matter was understood by lated. If the recovery ratio was between 85% and 115%, measuring the concentration of chlorophyll a. (2) The the data were considered effective. fractionation of organic matter in eutrophication. (3) The removal efficiency of organic matter by enhanced 2.1.4. Trihalomethanes analysis coagulation was compared to the degree of eutrophica- The method used to measure the concentration of tion (the concentration of chlorophyll a) to elucidate THM is NIEA W785. 50B (Standard method of the the relationship between enhanced coagulation and eu- Environmental Protection Agency of the Republic of trophication. (4) The effect of different coagulant dos- China), which is originally used to measure the con- ages on the THMFP and the fractionation of organic centration of volatile organic compounds in drinking matter in water after coagulation were clarified. Overall, water by purge and trap/GC/MS. The stock solution was this study aims to gain further insight into how eu- prepared by diluting 0.1 ml of standard 200 mg/l (Su- trophication affects the removal efficiency of enhanced pelco) with methanol 100 ml to a concentration of 200 coagulation. lg/l. The calibration curve was obtained by subse- quently diluting the standard solutions with Milli-Q water to various concentrations. The correlation coeffi- 2. Material and methods cient of calibration curve was higher than 0.995 and the accuracy of laboratory checking sample was con- 2.1. Analysis of water quality trol between 80% and 120%. Gas chromatography/MS (Agilent Model 6890/Agilent 5973 mass selective detec- 2.1.1. SUVA254 tor) with Purge & Trap (Tekmar Dohramann-3100) Edzwald and Van Benschoten (1990) demonstrated were used to measure THM. Extra pure nitrogen gas that the specific ultraviolet absorbance (SUVA) relate was introduced as the purge gas at a flow rate of 40 ml/ closely to the amount of removable dissolved aromatic min and under a pressure of 138 kPa. The purge time organic matter (e.g., humic acid) in raw water. SUVA254 was 11 min and the dry purge time was 4 min. The is the UV absorbance at 254 nm per mass of carbon trapped THM in Tenax adsorbent was desorbed at a (DOC in mg/l). The total organic carbon analyzer temperature of 108 °C for 4 min, into a narrowbore W.P. Cheng, F.-H. Chi / Chemosphere 53 (2003) 773–778 775 capillary column for the measurement of concentration of halomethane (CHCl3, CHBrCl2, CHBr2Cl, CHBr3). The flow rate of the carrier gas (extra pure helium) was 2 ml/min. The temperature was held at 40 °C for 5 min, and then increased by 5 °C/min to 200 °C at which it was held for 5 min. The scanning range of mass spectrometry was 35–260 amu at EI 70 ev; the interface temperature was 280 °C. The scanning time will not be less than 0.7 s. 2.1.5. Measurement of THM formation potential THMFP analyses were conducted according to USEPA method 510.1. The USEPA THMFP procedure estimates the maximum formation of THMs in a water sample. The water sampled from each reservoir was stored in three 40 ml bottles. Sodium hypochlorite sol- ution (9–12 mg/l as Cl2) was injected into water sample, and then the samples were put in an incubator for seven days THM formation period at 25 °C. After which time, Hach DR-2000 spectrophotometer was used to analyze Fig. 1. Relationship between DOC and chlorophyll a concen- the residual chlorine. The samples between 3 and 5 mg/l tration in the raw water from the three reservoirs, at various of residual chlorine were chosen for the analysis of times.