Comparison of formation of disinfection by-products by chlorination and ozonation of wastewater effluents and their toxicity to Daphnia magna

Highlights

• Comparison on disinfection by-products by chlorine and ozone is made.

• Less HAAs than THMs and less chloroform than other THMs in chlorinated effluents.

• Formaldehyde was produced more than acetaldehyde during ozonation of the effluents.

• The greatest concentration of formaldehyde in this study shows some ecotoxicity.

Abstract

This study compared the two most frequently used disinfectants (i.e., chlorine and ozone) to understand their efficiency in wastewater effluents and the ecotoxicity of disinfection by-products created during chlorination and ozonation. Four trihalomethanes (THMs) and nine haloacetic acids (HAAs) were measured from a chlorine-disinfected sample and two aldehydes (i.e., formaldehydes and acetaldehydes) were analyzed after ozonation. Chlorination was effective for total coliform removal with Ct value in the range of 30–60 mg-min/L. Over 1.6 mg/L of ozone dose and 0.5 min of the contact time presented sufficient disinfection efficiency. The concentration of THMs increased with longer contact time (24 h), but that of HAAs showed little change with contact time. The measured concentration of formaldehyde at the ozone dose of 1.6 mg/L and the contact time of 9 min showed the greatest value in this study, approximately 330 μg L⁻¹, from which the corresponding ecotoxicity was determined using an indicator species, Daphnia magna. The ecotoxicity results were consistent with the toxicological features judged by occurrence, genotoxicity, and carcinogenicity. Both the disinfection efficiency as well as the DBP formation potential should therefore be considered to avoid harmful impacts on aquatic environments when a disinfection method is used for wastewater effluents.

Introduction

Emerging contaminants such as endocrine disrupting chemicals (EDCs), non-steroidal anti-inflammatory drugs (NSAIDs), and pharmaceutical and personal care products (PPCPs) have gained a significant amount of attention in recent years. These contaminants are widely used in daily life and their existence in trace concentrations has been in aquatic environments. Their effects on public health and the environmental are unknown, but it is supposed that they are harmful (Oneby et al., 2010, Noutsopoulos et al., 2015 In addition, un-metabolized antibiotics, which are found in medicine taken by humans and animals and excreted after digestion, have also become of great concern because they reach wastewater treatment plants (WWTPs) through sewage and are suspected to develop antibiotic resistance among bacteria in the biological process in the WWTPs. Fiorentino et al. (2015) mentioned that the release of antibiotic-resistant bacteria (ARB) in the receiving water from wastewater effluents occurs when either a disinfection process is not applied or less effective treatment processes, such as conventional disinfection, are used.

Conventional WWTPs consist of primary settling, biological processing using activated sludge, secondary settling, and disinfection. The conventional processes have been constructed to mainly remove organic pollutants from domestic sewage. Most of the emerging pollutants, however, are only marginally removed through biological wastewater treatment (Samaras et al., 2013, Stasinakis et al., 2013). In addition, increases in the demand for indirect potable reuse due to the shortage of fresh water resources in an arid area require high-quality effluent from WWTPs (Oneby et al., 2010). Novel technologies to meet stringent water quality standards include ozone disinfection, advanced oxidation processes with hydrogen peroxide, ozone, and sunlight, and UV light and ionizing radiations (Oneby et al., 2010, Fiorentino et al., 2015, Lee et al., 2015).

Chlorination is still the most widely applied disinfection method to inactivate pathogenic microorganisms in wastewater. Chlorination also can achieve effective removal of EDCs and NSAIDs including nonylphenol, bisphenol A, and ibuprofen when the ambient solution is maintained at a proper pH range (Noutsopoulos et al., 2015). The authors, however, mentioned that much research is still needed to understand removal mechanisms of the chemicals under natural conditions due to variances in their properties; for example, Xu et al. (2002) noted that chlorination was ineffective in removing some epidemic microorganisms such as Cryptosporidium and Giardia at low doses. Furthermore, undesirable by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) are produced during chlorination. Disinfection by-products (DBPs) are formed when disinfectants, such as chlorine, react in general with natural organic matter. The THMs and HAAs are known to be carcinogenic to human beings. Richardson et al. (2007) reviewed the toxicity of 85 disinfection by-products emphasizing occurrence, genotoxicity, and carcinogenicity. The authors categorized the DBPs into three categories: some or all of the toxicological characteristics of human carcinogens, DBPs that occur at moderate concentrations and are genotoxic, and DBPs that occur at moderate concentrations for which little or no toxicology data are available. Among the DBPs from chlorination, the bromodichloromethane (BDCM), dichloroacetic acid (DCCA), and dibromoacetic acid (DBAA) were categorized as having some or all of the toxicological features of human carcinogens.

Ozone has been recognized, especially in Europe, as one of the most effective disinfectants for drinking water, as an alternative to chlorination (Xu et al., 2002, Facile et al., 2000). Oneby et al. (2010) noted the renewed interest in the use of ozone in wastewater treatment due to more stringent regulations, more frequent water shortages, and public's perception of it providing higher quality water. Xu et al. (2002) showed that the total ozone demand of between 2 and 15 mg/L was sufficient to meet the World Health Organization standard for irrigation, i.e., 1000 FC/100 mL and to provide total removal of enteroviruses in the secondary effluent with 71 mg/L of chemical oxygen demand and 18 mg/L of suspended solid concentrations. One of bacterial aerobic spores, Bacillus subtilis, was shown only 0.0001 of surviving fraction with 1.16 mg/L of initial ozone residual (Facile et al., 2000). Sigmon et al. (2015) investigated inactivation efficiencies of ozone on four pathogenic viruses and found that the viruses are easily inactivated at the Ct (concentration × time) values lower than 0.1 mg-min/L. In addition, a typical ozone dose for disinfection removes numerous EDCs and PPCPs below detection limits (Snyder et al., 2007). However, ozonation creates several disinfection by-products because ozone is a highly active oxidant and greatly reacts with organic matter in the water. The typical DBPs from ozonation are formaldehyde, acetaldehyde, glyoxal, methylglyoxal, and trichloroacetaldehyde (i.e. chloral hydrate). The study by Richardson et al. (2007) found that these aldehydes were detected at concentrations up to 30.6 μg/L during ozone disinfection. The formaldehyde and acetaldehyde were categorized as the DBPs having some or all of the toxicological features of human carcinogens although they are not legally regulated in most countries. The authors argued the importance of examination of DBPs formed by alternative disinfection such as chloramine and ozone due to the complexity of reactions with wastewater content.

This study, therefore, investigated efficiencies of two disinfection methods (i.e. either chlorination or ozonation) and the extent of disinfection by-products formation using the water collected from an operating WWTP. The four THMs and nine HAAs were measured from chlorine disinfection and two aldehydes, i.e. formaldehyde and acetaldehyde were analyzed after ozonation. An evaluation of toxicity of the DBPs to the aquatic environment was conducted using the indicator species, Daphnia magna, which is commonly used for testing ecotoxicity in Korea and worldwide.