Characteristics of DOM in 14 AAO processes of municipal wastewater treatment plants

Highlights

• DOM characteristics and removal in AAO wastewater treatment system were investigated.

• The specific fluorescence intensity may be used to monitor of DOM's molecular weight.

• The regional fluorescence proportion may predict hydrophobic/hydrophilic contents.

• Regional distribution of DOM was related to economy, population and water quality.

Abstract

The characteristics of dissolved organic matter (DOM) such as chemical composition, molecular weight (MW) distribution and hydrophobic/hydrophilic distribution can affect wastewater treatment efficiency, effluent quality and ecological risk. Fluorescence spectroscopy could provide a quick estimate of DOM characteristics during the monitoring of wastewater treatment plants (WWTPs). In this study, the characteristic and quantitative correlation of DOM from 14 anaerobic-anoxic-oxic (AAO) processes of WWTPs located in different provinces (municipalities) of China were investigated. The results showed that DOM of MW <1 kDa was the largest group of DOM in influent and secondary effluent, and DOM removal increased as the MW increased. Hydrophilic (HPI) fraction and hydrophobic acid (HPO-A) comprised the major portion of DOM in influent and secondary effluent and exhibited the lowest rate of removal. In addition, DOM concentrations in the northern provinces were higher than in the southern provinces, which were related to the water quality, economy and population. There were positive correlations between specific fluorescence intensity (SFI) and the MW <1 kDa, 1–5 kDa and <10 kDa fractions. The smaller the molecular weight, the better the correlation. Strong positive correlations between regional fluorescence proportion (f_i) and HPI were found. SFI and f_i may be explored as potential indicators of the MW fractions and the hydrophobic/hydrophilic distribution of DOM in AAO processes WWTPs.

Introduction

Dissolved organic matter (DOM) is a highly heterogeneous mixture that can profoundly impact wastewater treatment systems (Cao et al., 2019; Cohen et al., 2014; Wang and Chen, 2018). DOM is the main substance removed during treatment and a limiting factor for wastewater reclamation and reuse applications (Phungsai et al., 2016; Qi et al., 2018; Wang and Chen, 2018) in areas where China experiences a shortage of water resources (Qu et al., 2019). Various biological treatment techniques to remove DOM in wastewater have been investigated (Liang et al., 2008; Kang et al., 2014).

DOM is inextricably related to various processes during wastewater treatment. DOM substances may impact coagulation efficiency (Rosario-Ortiz et al., 2007), disinfection by-product formation (Ike et al., 2020; Kimura and Ortega-Hernandez, 2019), membrane fouling (Jacquin et al., 2018; Ly and Hur, 2018) and microbial activity (Ishii and Boyer, 2012; Kwon et al., 2005) during treatment. The DOM's molecular weight distribution and hydrophobic/hydrophilic distribution reflect the source of the DOM, the transformation of the DOM (Mangal et al., 2020), and the biodegradability and biotoxicity of the DOM (Podgorski et al., 2018; Zhang et al., 2010) during wastewater treatment processes. For example, hydrophobic acids and hydrophobic neutral fractions of the secondary effluent were the key fractions related to anti-estrogenic activity that can enhance the potential ecological risk after the anaerobic-anoxic-oxic (AAO) process (Tang et al., 2013). A sub-fraction of hydrophobic acid (HOA) with MW less than 1 kDa can result in an increase in genotoxicity (Wu et al., 2010). The DOM's molecular weight distribution is an important index of wastewater treatability and can be used to evaluate the suitability of chemical, physical and biological processes to remove DOM in wastewater (Tran et al., 2015). The DOM's hydrophobic/hydrophilic distribution can affect the separation process such as interfacial adsorption and phase partitioning, and interaction between the DOM and surfaces/particles/colloids/solutes in wastewater (Tran et al., 2015; Xiao et al., 2016). Therefore, rapid monitoring of the MW distribution and hydrophobic/hydrophilic distribution of DOM would provide a comprehensive understanding of the state of wastewater treatment and will help to optimize the process operation to effectively remove DOM from wastewater. However, a comprehensive source of information is not available regarding DOM characteristics in typical municipal wastewater treatment plants (WWTPs).

It was inconvenient to measure the MW distribution and hydrophobic/hydrophilic distribution of DOM. Conventional measurement methods for MW distribution (such as adsorption column chromatographic fractionation) and hydrophobic/hydrophilic distribution (such as sequential ultrafiltration) are generally complicated and time-consuming (Leenheer and Croué, 2003; Xu and Guo, 2017; Xue et al., 2009). In comparison, excitation-emission matrix (EEM) fluorescence spectroscopy is a rapid and sensitive tool that can provide abundant fingerprinting information for DOM properties (Saadi et al., 2006; Cuss et al., 2016; Lee et al., 2019; Li et al., 2020b). The EEM technique has been applied as a monitoring tool in wastewater quality parameters such as biochemical oxygen demand and total organic carbon (TOC) (John Bridgeman et al., 2013, Carstea et al., 2016) and the removal of emerging contaminants (Sgroi et al., 2017). EEM fluorescence spectroscopy has even been applied to reveal the relationship between genotoxicity change and contaminant removal (Chai et al., 2018). It is worth further exploring if fluorescence spectroscopy could be used to indicate MW distribution and hydrophobic/hydrophilic distribution of DOM over a range of AAO processes that occur in WWTPs.

The AAO process constitutes the main municipal wastewater treatment technology in China (Zhang et al., 2016). This study investigated the characteristics of DOM in 14 municipal WWTPs using AAO processes located in different Chinese provinces (municipalities). Chemical composition, MW distribution and hydrophobic/hydrophilic distribution of DOM were analyzed. Multiple factor analysis (MFA) was conducted to reveal the effect of factors in the distribution of DOM in WWTP influent. Statistical analyses were employed to further explore if there are any statistical relationships between fluorescence spectral properties and MW distribution or hydrophobic/hydrophilic distribution of DOM in the AAO processes of WWTPs. The results may provide new clues for monitoring MW distribution or hydrophobic/hydrophilic distribution of DOM in wastewater treatment systems using fluorescence spectral properties.