Document Type

Dissertation - Open Access

Award Date

2023

Degree Name

Doctor of Philosophy (PhD)

Department / School

Civil and Environmental Engineering

First Advisor

Guanghui Hua

Keywords

Biofiltration, Dihaloacetonitriles, Disinfection Byproducts, Drinking Water, Pre-chlorination, Unknown DBPs

Abstract

Drinking water disinfection is essential to protecting public health from waterborne diseases. However, the reaction of disinfectants with natural organic matter (NOM) can form carcinogenic disinfection byproducts (DBPs). Drinking water treatment facilities employ precursor control and alternative disinfectants to minimize regulated DBPs formation to comply with Environmental Protection Agency (EPA) guidelines. Despite their efficacy, some limitations have been observed during the application, such as increased cost of enhanced coagulation, high demand for sludge disposal, inconsistency in precursor removal, and formation of more toxic unregulated and unknown DBPs. Therefore, it is important to develop more efficient and safe strategies for DBPs control. This study proposed a new strategy “pre-chlorine/biofiltration/post-chlorine treatment” to control DBPs formation in drinking water. The main objective of this study is to systematically analyze this strategy on overall DBPs control. The following tasks were performed to achieve the project objective: 1) investigating the potential of biofiltration technology for different groups of DBPs control and evaluating the impact of disinfect switch from chlorine to chloramine on biofilters performance on different DBPs removal; 2) studying the key factors that impact the performance of the biofilter on total organic halogen (TOX), unknown DBPs (UTOX), haloacetic acids (HAAs), dihaloacetonitriles (DHANs) removal, developing kinetic models and temperature activity coefficients to predict different DBPs species removal rates under various conditions; and 3) evaluating the pre-chlorination - DBP biofiltration - post-treatment water treatment strategy in DBPs removal and overall DBPs formation potential control, and investigating the impact of backwash on biofilters performance on DBP removal. The results of long-term biofiltration experiments using the City of Brookings drinking water indicate that biofiltration is an effective technology for DBPs control. Biofilters can consistently remove approximately 52% of the total organic halogens (TOX), 97% of haloacetic acids (HAAs), 14% of trihalomethanes (THMs), and 63% of unknown DBPs (UTOX) in chlorinated drinking water. Biofilters also effectively remove 46% of TOX, 14% of THMs, 96 % of HAAs, and 48% of UTOX from chloraminated drinking water. The two activated carbon biofilters (GAC 300 and 200) exhibited better DBP biofiltration efficiencies than sand and anthracite biofilters. The switch from chlorine to chloramine had little impact on HAAs biodegradation in GAC biofilters. However, it decreased UTOX biodegradation by 15%, indicating that the UTOX formed in the chloraminated water is less biodegradable than those formed in chlorinated water. Moreover, TOX spikes was observed in GAC biofilter effluents after switching to chloraminated water, which was mainly attributed to the leaching of THMs. Therefore, attention should be paid to potential DBPs leaching from GAC biofilters when switching disinfectant from chlorine to chloramine. The DBP biofiltration kinetic study results indicate that a first-order model can adequately describe the degradation of different groups of DBP in GAC biofilters, including HAAs, unknown DBPs, and Dihaloacetonitriles (DHANs), across varied empty bed contact times (EBCTs) (R2 > 0.95). The biodegradation rate constants (k values) for these DBPs increased with increasing temperatures. Extending the EBCT is an effective method to improve DBPs removal, particularly at lower temperatures. In addition, the biodegradation rate constants for the studied DBPs at 5 - 20 °C were 0.106 - 0.273 min-1 for DCAA, 0.081 - 0.235 min-1 for TCAA, 0.020 - 0.046 min-1 for chlorinated UTOX and 0.018 - 0.040 min-1 for chloraminated UTOX. DHANs, including dichloroacetonitrile (DCAN), bromochloroacetonitrile (BCAN), and dibromoacetonitrile (DBAN), show effective biodegradability with k values between 0.098 and 0.427 min-1. In conclusion, the observed biodegradability order of the studied DBPs is: DCAN > BCAN > DBAN > DCAA > TCAA > Cl2-UTOX > NH2Cl-UTOX > Chloroform. The developed first-order model and temperature activity coefficients offer a tool for predicting DBPs biodegradation rates under varying temperatures and EBCTs. Furthermore, the results indicate that the DBP-preformation - biofiltration - post-treatment strategy effectively controls the formation potential of various DBP groups. Biofiltration could remove up to 57% of preformed TOX, mainly attributed to the degradation of unknown DBPs and HAAs. Although post-treatment with chlorine and chloramine would lead to some DBP re-formation, this strategy was able to reduce the overall formation potentials of different groups of DBPs. Compared to pre-chlorine and post-chlorine treatment, the pre-chlorine/biofiltration/post-chlorine treatment reduced 71% of HAAs, 37% of DHANs, 44% of UTOX and 17% of THMs. When post-chloramine was used, the corresponding formation potential reductions were 90% of HAAs, 83% of DHANs, 43% of UTOX and 10% of THMs at an EBCT of 15 min. Therefore, the DBP-preformation - biofiltration - post-treatment strategy can significantly decrease the overall exposure of DBPs within the distribution system.

Publisher

South Dakota State University

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Rights Statement

In Copyright