Influence of Eutrophication on the Coagulation Efficiency in Reservoir

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 chlorophyll 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 eutrophication 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 increased 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 organics 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.

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 solution (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. THM. and the composition of dissolved organic matter change 2.2. Coagulation experiment greatly. As shown in Fig. 1, the changes of DOC concentration were observed to be proportional to the Water collected from the entrance points of the water changes of chlorophyll a in three reservoirs. The amount treatment plants of Min-ter, Li-yu-ten and Yun-ho-shen of organic matter significantly increased as the algae reservoirs was used in the experiment of coagulation. began to propagate. Fig. 2(a) and (b) show the values of The coagulation experiments, using Al2(SO4)3 14H2O UV254/DOC (SUVA254) and FI/DOC decreased as the as coagulant, were performed on the raw water (1 l) concentration of DOC increased. Generally, the benzene collected at different times from each reservoir. The ring and the double bonds of C@O, C@N and N@O can dosage of the coagulant was increased by 10 mg/l in each absorb UV at 254 nm, and the conjugated double bonds experiment. The maximum dosage was determined in the benzene compounds of organics can emit fluo- under the condition that the solution pH following co- rescence after excited by UV (Edzwald and Van Ben- agulation will not be less than 6.3. The jar test procedure schoten, 1990; Hautala et al., 2000). The humic acids (Black et al., 1957) involved rapid mixing at 120 rpm for contain many aromatic compounds. Both the UV254 and 1 min, followed by slow mixing at 30 rpm for 30 min. FI methods are highly sensitive in qualitatively and After settling for 30 min, the supernatant was collected quantitatively measuring the humic acids. Therefore, the to measure of pH and turbidity. The filtrates were tested results that SUVA254 and FI/DOC decreased as the for dissolved residual organic matter and characterized concentration of DOC increased demonstrate that eu- according to the fractions of organic carbon, THMFP, trophication could thus be concluded to affect the frac- UV254, FI, and DOC. tionation of organic matter. In addition, the changes in the DOC composition observed from the fractionation of molecular weight also indicate the extent of eutroph- 3. Results and discussion ication. Theoretically, the portion of small molecules increases with the extent of eutrophication, because the 3.1. The effect of eutrophication on the quality of raw contaminant molecules from surrounding environment water of the reservoirs and the organics excreted from algae are small. As shown in Fig. 3, the percentage of the The entry of contaminants from surrounding en- DOC with molecular weight less than 5000 Da was vironment can change the composition of DOC in proportional to the concentration of total DOC in the reservoirs. The nitrogen and phosphate in those con- raw water. More importantly, the increased amount of taminants can promote eutrophication, resulting in the organic matter cannot be removed easily by traditional blossoming of algae. In that case, the degradation of coagulation. Most researchers agree that traditional organic matter is slower than its formation by the me- coagulation can remove only large molecules, such as tabolism of biomass. Accordingly, the concentration those with a molecular weight greater than 5000 Da 776 W.P. Cheng, F.-H. Chi / Chemosphere 53 (2003) 773–778

3.2. Eﬀect of eutrophication on coagulation

To investigate the effect of eutrophication and coagulant dosage on the removal of DOC in drinking water, two coagulant dosages were used in this study without pH control. In general, the turbidity in the three reservoir water were removed efficiently at a coagulant dosage of 20 mg/l, but the removal of DOC under the same dosage were not so effectively as shown in Fig. 4. In another case, when a coagulant dosage of 120 mg/l was used in coagulation, the DOC removal efficiencies of the three reservoirs increased. It is found that a higher extent of eutrophication usually accompanied with a higher concentration of DOC and the fraction of small molecules increases in proportion to the extent of eutrophication, and the small molecules are unlikely to react with the coagulant (Amy et al., 1992; Huang and Shiu, 1996). Therefore, even when high dosage of coagulant was used, the efficiency of DOC removal decreased dramatically as the concentration of DOC in raw water increased (Fig. 4). The preliminary result shown in Fig. 5(a) further demonstrates that the removal efficiency of DOC decreased as the percentage of DOC with molecular weight less than 5000 Da increased (suggesting of eutrophication). The slope of regression also indicates while the percentage of DOC with mo- Fig. 2. (a) Relationship between DOC concentration and lecular weight less than 5000 Da increased by 1%, the UV254/DOC in the raw water from the three reservoirs, at removal efficiency by enhanced coagulation decreased various times. (b) Relationship between the DOC concentration by approximately 1%. Thus, the organic matter released and FI/DOC in the raw water from the three reservoirs, at various times. from eutrophication could not be removed by enhanced coagulation. The selectivity of coagulation in removing organics results in a higher removal efficiency for large molecules than for small molecules (Amy et al., 1992). In Fig. 5(b), the percentage of the DOC with molecular

Fig. 3. Relationship between the percentage of DOC with molecular weight less than 5000 Da to the total DOC concentration in the raw water of the three reservoirs, at various times. Fig. 4. Removal eﬃciencies of DOC by coagulation for the (Amy et al., 1992). However, the small organics may be water from the three reservoirs, at two diﬀerent dosages of removed by activated carbon. coagulant. W.P. Cheng, F.-H. Chi / Chemosphere 53 (2003) 773–778 777

Fig. 6. Comparison of THMFP and DOC residuals under diﬀerent coagulant dosages in the water from the three reservoirs.

the THMFP removal efficiency were higher than those of DOC, which were only 8.6–10.1% and 32.2–35.4% for traditional coagulation and enhanced coagulation, re- spectively. These observations imply that coagulation process are effective mainly in removing large and hydrophobic molecules, such as humic acids in drinking water, and benzene rings or aromatic hydrocarbons Fig. 5. (a) Removal efficiency of DOC with molecular weight less than 5000 Da by enhanced coagulation versus the DOC in large organic matter, which are the precursors of concentration of the raw water. (b) Percentage of DOC with THM (Boyce and Hornig, 1983; Christman et al., 1983; molecular weight less than 5000 Da before and after coagula- Fleischacker and Randtke, 1983; Larson and Weber, tion. (The solid circle represents a coagulant dosage of 20 mg/l, 1994). Usually the THMFP per unit DOC in humic and the open circle represents a coagulant dosage of 120 mg/l). acids exceeds that in non-humic acids (Huang and Yeh, 1997; Graham et al., 1998). Therefore, as the fraction of small molecules increased after coagulation, a higher weight less than 5000 Da of the raw water (represented removal efficiency for THMFP than for DOC was by RI) was plotted against that of the coagulated water observed. Nevertheless THMFP increased with DOC (represented by RC). For either the traditional coagu- concentration. The DOC excreted after eutrophication lation with a coagulant dosage 20 mg/l or the enhanced from the propagation of algae was probably not easily coagulation with a coagulant dosage 120 mg/l, all the removed by traditional coagulation or filtration, so the points are located above the dash line (RC:RI ¼ 1:1 amount of disinfectant or chlorine used was then in- showing that the ratio does not change after coagula- creased. Therefore, it resulted a higher concentration of tion, in other words, the organic matters were removed THM, and worse the quality of drinking water. with a same percentage in both fractions). This result shows that RC exceeded RI, suggesting that more large molecules were removed by coagulation than small ones. It also demonstrates the enhanced coagulation (solid 4. Conclusions circles) leaving the RC:RI ratio far from the 1:1 linear relationship than traditional coagulation, that is, the The concentration of small organic molecules ex- enhanced coagulation could remove higher fraction of creted by the metabolism of algae, measured by DOC, is large molecules than does the traditional coagulation, proportional to the concentration of chlorophyll a in the but its ability to remove the small organic matters is water. This metabolism indirectly decreased the frac- limited (Volk et al., 2000). tion of large organic molecules such as humic acids Comparing the THMFP of the raw water with dif- and the values of UV254/DOC and FI/DOC in the raw ferent coagulant dosages in the three reservoirs (Fig. 6), water. It was also observed that the small organic the removal efficiencies increased from 31–48%, by tra- molecules produced from the metabolism of algae could ditional coagulation at dosage of 20 mg/l, to 74–85% by not be easily removed by coagulation. The efficiency of enhanced coagulation at a dosage of 120 mg/l. However, DOC removal of raw water was inversely related to 778 W.P. Cheng, F.-H. Chi / Chemosphere 53 (2003) 773–778 the concentration of DOC in the test of enhanced co- Christman, R.F., Norwood, D.L., Millington, D.S., Johnson, agulation. The experimental results show that the J.D., 1983. Identity and yields of major halogenated removal efficiency of organic matter by enhanced coag- products of aquatic fulvic acid chlorination. Environ. Sci. ulation decreased by 1% as the percentage of DOC with Technol. 17 (10), 625–628. a molecular weight of less than 5000 Da increased by Edwards, M., 1997. Predicting DOC removal during enhanced coagulation. J. Am. Water Works Assoc. 89 (5), 78–89. 1%. This suggests that the organic matter excreted from Edzwald, J.K., Van Benschoten, J.E., 1990. Aluminum coag- algae could not be removed by enhanced coagulation, ulation of natural organic matter. In: Proc. 4th Interna- and the quality of water after eutrophication could lead tional Guthenburg Symposium on Chemical Treatment, to difficulty in controlling the formation of disinfection Madrid, Spain. by-products, such as THM and carcinogenic matter. Fleischacker, S.J., Randtke, S.J., 1983. Formation and control of organic chlorine in public water supplies. J. Am. 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