Cheng-Ching Lake water treatment plant (CCLWTP), the largest water treatment plant in southern Taiwan serving the Kaoping region, uses the Kaoping River water as the source water. The plant has encountered both technical and managerial challenges to implement advanced water treatment system since 2004 in order to provide high quality drinking water to the residents living in the Kaoping metropolitan area and to meet future stringent drinking water standards.
Granular activated carbon (GAC), derived from wood, bituminous coal, lignite, or other carbon-containing materials, and is the most widely utilized adsorbent for treating water and wastewater. It is usually used after the sand filtration process in water or wastewater treatment plant; the exhausted GAC is re-activated by a combustion process. Moreover, biological activated carbon (BAC) filtration (biofiltration) has become one of the advanced treatment techniques applied in the water treatment plant. In general, BAC offers a large internal surface area for the adsorption of taste, odor, and color compounds, excess chlorine, toxic and mutagenic substances (e.g., bromide, chlorinated organic compounds, including trihalomethanes), trihalomethane precursors, pesticides, phenolic compounds, dyes, toxic metals, and substances that cause biological after growth. After the biofiltration process, a final disinfection is necessary to ensure the microbial quality of the treated water. Because biofiltration is usually not capable of removing biorefractory substances, pre-oxidation with ozone is usually applied for oxidizing the most biorefractory organic matters and also improving their biodegradability before the water is treated in the BAC process. Hence, using ozone pre-oxidation will greatly enhances the effectiveness of the subsequent BAC process.
The CCLWTP effluent meets the current drinking water quality established by Taiwan Environmental Protection Administration (TEPA). However, the microbial regrowth due to the residual minute quantity of organic carbons causes pipe corrosion, and the formation of disinfectant by-products (DBPs) in the distribution system leading to potential contaminations of the clean water after it enters into the distribution system. Thus, monitoring the water quality in water distribution systems necessity to develop appropriate strategies for managing both the treatment plant and following distribution systems.
Chlorine is often used in municipal water treatment plant for disinfecting drinking water; it can react with naturally occurring organic matter to form trihalomethanes (THMs), e.g. chloroform, bromodichloromethane, chlorodibromomethane and bromoform that causes long-term health hazards to consumers through oral ingestion, dermal absorption and inhalation. The lifetime cancer risk and the hazard index of THMs through oral ingestion, dermal absorption, and inhalation exposure from tap water in 9 districts in Kaohsiung City are estimated.
In the first part of this study, water samples were periodically collected from each treatment process of Cheng-Ching Lake Water Treatment Plant (CCLWTP) to assess the AOC (assimilable organic carbon) removal. In the second part of this study, the role of BAC filtration used in advanced water treatment plant and its capability to remove pollutants (AOC, bromide, bromate, and iron) were evaluated. Additionally, the efficiency of biofiltration process using GAC and anthracite as the fillers was also assessed with a bench-scale GAC adsorption column. In third part of the study, the distribution system of CCLWTP was selected for conducting the case study for understanding the fate and transport of water quality indicators in the distribution system. The last part of the study concentrated on undertaking multipathway exposure assessment based on the concentrations of various THMs found in the water samples collected at various locations of Kaohsiung City water supply system.
The AOC removal efficiency of the advanced water treatment processes of the CCL was assessed using data collected in the field during the first phase of this study. However, the effect of two different filling materials on the efficiency of biofiltration process was evaluated using a laboratory bench-scale column study. Results of both laboratory study and field investigation show that a significant AOC removal efficiency was achieved by the BAC system implemented in CCLWTP. Conclusions of this study are summarized as follows:
1. Significant AOC removal efficiency was achieved in CCLWTP and the AOC concentrations in the effluent could meet the current established standards.
2. The increased AOC concentrations after the treatment of preozonation and chlorination may be caused by the oxidation of organic matters to more biodegradable and assimilable products.
3. The removal of AOC is correlated with the decrease in concentrations of other drinking water indicators, e.g., coliform, TPC, TDS, and particle counts).
4. The addition of sodium thiosulfate in water samples could enhance the performance of the AOC analysis (the accuracy and reliability).
5. The BAC filtration has been demonstrated to play an important role in the removal of the trace AOC. Thus, the application of BAC for AOC removal is feasible and should be included as a required treatment unit in the advanced WTP.
The field study completed in the second part assessed the removal efficiencies of AOC and other water quality indicators in CCLWTP, while the effects of using two different filling materials on the efficiency of biofiltration process and microorganisms growing were evaluated using a laboratory bench-scale column study. Conclusions of this study include the following:
1. The BAC filtration system is capable of removing trace pollutants including organics and metals.
2. Significant overall treatment efficiency can be achieved in the CCLWTP, and concentrations of the water quality indicators in the effluent will meet the drinking water standards established by TEPA.
3. The increased AOC concentrations after ozonation and chlorination processes may be caused by the oxidation of organic matters into more biodegradable and assimilable organic products.
4. GAC is a more appropriate filling material than anthracite in the biofiltration system for the removal of AOC.
5. More microorganisms were observed in GAC column than in BAF column. This may be due to the effect that GAC has more specific surface area than anthracite. Additionally, more microbial growth was observed at depth of 5 cm than 0 and 40 cm in both columns indicating that 5 cm below the column surface is rich in both dissolved oxygen and biodegradable that causes higher microbial populations.
6. The BAC filtration plays an important role in the removal of the trace AOC; it should be included as a required treatment unit in future advanced WTP. Additionally, the BAF filtration column filled with anthracite is not as effective as the GAC-filled column in removing AOC. Thus, GAC should be used for the proposed BAF filtration unit.
7. The oxidation process using ozone will increase the amount of carbonyl group organics in the oxidized water leading to poor biological stability. Therefore, the oxidation should be combined with a subsequent GAC or biological process to minimize the AOC formation potential.
The third study, Using the oxidation/reduction potential (ORP) along with other water quality parameters to indicate the water quality in the CCLWTP distribution systems was assessed and focused. Behavior of water quality parameters by monitor and investigate was made a replacement of corrosive pipe line. The results reveal that the treated water leaving CCLWTP (clear water) meets the drinking water standards in Taiwan. However, the water is re-contaminated by a number of factors including the corrosion of old pipes while it is flowing in the distribution system. Major conclusions of this study are summarized in the following sections:
1. The free residual chlorine concentration in CCLWTP distribution system is adequate to meet the drinking water standards established by TEPA.
2. The residual AOC concentration is well correlated with the TOC concentration in the samples collected at various sites in different administrative areas.
3. Ratios of AOC/TOC in six administrative areas were higher than 9%, indicating that the biofilms were fall and increased organic matter of tap water distribution systems.
4. The average AOC concentrations were increased with followed variations of UV-254 value.
5. A number of factors (AOC, pH, redox potential, TOC, UV-254, and chlorine residual) control the growth of microorganisms on pipe surfaces.
6. DO have a negative relationship between THMs and HAAs concentrations. Because that oxygen have higher electronegative than chlorine and bromine, and apt utilization of organic carbon.
7. Results were shown of pH, DO and ORP had a positive relationship (Need to be more specific about the correlationship.
8. Major chemical reactions in the distribution system involve both electrons and protons transfers; they are pH- and Eh (ORP)-dependent. Therefore, chemical reactions in pipe net often can be characterized by pH and Eh together with the activity of dissolved chemical species.
9. The results reveal that the non-scaling water in LSI of distribution systems of CCL close to saturation (LSI = 0) (Cannot be understood). As show the other results, located K and M1 areas in LSI¡@were -0.002 and -0.012, respectively. The appearance of the pipe in K and M1 areas were corrosion and undersaturated with CaCO3 (needs to be re-written).
10. The RSI value was between 7.0 and 7.5 showing potential corrosion and prioritizes replacement of the pipe.
11. The DO value has a correlation with the reverse in Fe and Fe3+ concentrations.
12. High oxidation conditions and elevated Fe3+ concentrations of exist inside the corrosion scales of the corroded water distribution pipes.
13. The Fe concentrations in the samples collected in various administrative areas exceed the TEPA drinking water standards.
14. The appearance of CCL distribution system of shows severe corrosive and oxidized conditions.
The last part of this study concentrated on evaluating the association between trihalomethanes (THMs) exposure through three different pathways and long-term health risks. The results show that the consumer has a higher risk of cancer through Inhalation route. This is different from the results reported by other research. Because most residents living in Taiwan are accustomed to drinking boiled water, the lifetime cancer risks through oral ingestion of water-borne CHBrCl2, and CHBr2Cl in tap water in all 9 districts were higher than 10-6. By oral ingestion the lifetime cancer risk for total THMs was highest in the 7th district, while the lowest lifetime cancer risk for total THMs was in the 4th district.
Chloroform poses a higher cancer risk to Kaohsiung City residents through dermal exposure than the other three THMs. This study showed that residents in 7th district had the highest cancer risk through inhalation of chloroform among the 9 districts, and the residents in 6th district had the least cancer risk. Residents in 7th district has the highest risk of cancer due to exposure of THMs during showering and bathing as compared with residents in 4th district Males have a higher cancer risk than females through dermal absorption when exposed to THMs.
The results of noncarcinogenic risk assessment for THMs indicate that if the main pathways are through oral ingestion and dermal absorption, 7th district has the highest hazard index of the four chemicals, while 4th district has the lowest hazard index.
According to the above results, the quality of drinking water in Kaohsiung City is in general in accordance with the guidelines for drinking water quality as recommended by the World Health Organization. A better drinking water quality can be achieved by reducing the quantity of disinfection by-products (DBPs) through the removal of DBP precursors using modified treatment practices. Coagulation, granular activated carbon, membranes and ozone-biofiltration can all remove natural organic matter. Additionally, source water protection and control are effective non-treatment alternatives to control water-borne precursors. Optimized applications of disinfectants as primary and secondary disinfectants can further be implemented to control DBPs.
Although research efforts continue to develop new treatment methods that will reduce the levels of DBPs during disinfection, it is generally accepted that risks to health caused by water-borne DBPs in drinking water are relative small in comparison with risks associated with water-borne diseases due to inadequate disinfection. Thus, it is important that the disinfection process should not be compromised in attempting to control water borne DBPs.
The predominant DBPs group has been shown to be THMs, with chloroform and BDCM as the most dominant THMs. Although THMs are only one subgroup of the many DBPs formed during chlorination, they are useful as indicators of the overall DBP formation. It is concluded that, given the current state of knowledge, a risk assessment based on THMs would provide the greatest level of confidence regarding the ability of a drinking water guideline to protect against risks of cancer and other long-term health hazards.
In conclusion, in order to reduce the cancer risk and hazard as indexed by THM concentrations in the drinking water, some methods could be used including controlling to reduce THMs precursors and microbial contaminants in raw water, and aged pipeline, optimizing all treatment processes to ensure that concentrations of disinfectant are adequate, using alternative disinfectants and reducing water age in distribution system. The potential human risks associated with drinking water disinfection are largely unknown, even though some information is available from toxicological and epidemiological studies. More research is needed to determine the risks associated with DBPs. The next progress will facilitate a more realistic assessment of risk due to drinking water contaminants without increasing the levels of uncertainty in risk estimates.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0213107-142258 |
| Date | 13 February 2007 |
| Creators | Chien, Chuan-Chi |
| Contributors | Wen-Liang Lai, Chih-Ming Kao, Cheng-Di Dong, Min-Chao Wang, Jy-Gau Sah |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0213107-142258 |
| Rights | withheld, Copyright information available at source archive |
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