Humic Acid and Trihalomethane Breakdown with Potential By-Product Formations for Atmospheric Air Plasma Water treatment

Abstract

Over the past century chlorine has been widely used as an oxidant in water and wastewater treatment. Chlorine’s efficacy is demonstrated for microbial inactivation of a wide variety of pathogens along with oxidation of various chemical contaminants. However, the potential formation of disinfection by-products (DBPs), such as trihalomethane, is a concern [1]. DBPs, including trihalomethanes, can be formed as a consequence of the reaction of chlorine with natural organic matter (NOM) present in both surface and ground water [2]. NOM comprises of two fractions; humic substances (HS), which are composed of humic acids, fulvic acids, and non-humic substances (non-HS), which include carbohydrates, lipids, and amino acids [3]. Furthermore, humic acids constitute a major fraction of NOM, of which the soluble portion (aromatic compounds) of humic substances may react with chlorine to form trihalomethanes [4]. It has been reported that these THMs are carcinogens [5]. Epidemiologic studies in humans suggest a weak association with bladder, rectal and colon cancer [6,7] along with reproductive and developmental effects [8,9]. Due to these health concerns, many jurisdictions specify maximum allowable concentrations. The United States Environ- mental Protection Agency [10] specifies maximum levels for trihalomethanes (THMs) and haloacetic acids (HAAs) of 80 and 60 mg/L, respectively [11]. European Union regulations limit THMs to 100 mg/L. However, many national reports find frequent occurrence of THM exceedance [12]. It is important to limit THMs and THMs causing substances (humic acids) in water. Several approaches such as nanofiltration, ultrafiltration, reverse osmosis coagulation [13], activated carbon adsorption [14], Fenton treatment [15], nano-TiO2 photocatalysis [16], membrane filtration [17], biological treatment [18], and ozonation [19] have been employed to remove humic substances and THM’s. Biological processes, including the use of bio-filters may lead to the accumulation of suspended solids and release of bacteria [20]. Separation technologies such as biofilms and membrane filtration may be limited due to fouling [21]. Ozone’s efficacy for humic substance breakdown in water has been demonstrated, however the process may result in the formation of brominated by-products [22]. In addition, the use of such advanced oxidation processes (AOP) which are capable of oxidizing some of the NOM present in raw water sources require an additional step for mineralization increasing the operational cost. Therefore, there is a need to develop novel approaches to remove humic substances and THMs in energy efficient manner