DOM chemodiversity pierced performance of each tandem unit along a full-scale “MBR+NF” process for mature landfill leachate treatment

Highlight

• Contribution and function of each unit were recognized along the “MBR+NF” process.

• The “MBR+NF” process removed 83.2%-92.2% of DOM molecules in terms of richness.

• Preferential removal of lignin/CRAM-likes with relatively high saturation.

• Lignin/CRAM-likes, lipids and aliphatic/proteins played key roles in sCOD and DOC.

• Bio-derived aliphatic/proteins were resistant and positively correlated with DON.

Abstract

Mature landfill leachate contains a substantial fraction of recalcitrant dissolved organic matters (DOM) that is a challenging for conventional wastewater treatment that is typically focused on the removal of biodegradable organic matter. “Biological treatment + membrane treatment” has been widely employed to treat complex leachate. However, the performance of each unit based on both conventional bulk indicators and molecular information has not been well understood. Therefore, the fate of DOM chemodiversity along the full-scale treatment process across ten sampling points over three different seasons were analyzed to determine the efficiency of every unit process with the assistance of ultra-performance liquid chromatography coupled with hybrid quadrupole Orbitrap mass spectrometry. Results showed that the process performance, visualized through the molecular signals, were relatively stable in the temporal dimension. The process removed 83.2%-92.2% of DOM molecules in terms of richness, where lignin/carboxyl-rich alicyclic compounds (CRAM)-likes with relatively high saturation was preferentially removed, while newly generated bio-derived N-containing compounds (N/C_wa 0.15-0.17) became resistant. The relationship between conventional bulk physicochemical indicators and molecular indexes suggested that soluble chemical oxygen demand (sCOD) and dissolved organic carbon (DOC) were contributed by the refractory DOM with high weighted average double bond equivalents (DBE_wa), which was distributed in the region of O/C 0.2-0.5 and H/C 1.2-1.8. This refractory DOM required ultrafiltration and nanofiltration for removal. DOM molecules were positively correlated with five-day biochemical oxygen demand (BOD₅) and revealed that approximately 96.9%-98.4% of the DOM could be removed or transformed in the primary anoxic zone. In addition, the bio-derived aliphatic/proteins, lipids and lignin/CRAM-likes (O/C > 0.2) with condensed aromatization were the sources of dissolved organic nitrogen (DON) and still remained in the final effluent. The present study suggests that the design and operation of the combination process with biological and membrane treatment could be specifically optimized based on the DOM molecular characteristics of the wastewater.

Introduction

Landfilling is a predominate method for the management of municipal solid waste. One of the primary issues for landfill operation is the generation of highly concentrated and complex leachate (Qiu et al. 2020b, Zhan et al. 2017). The properties of landfill leachate changes over time, usually indicated by a gradual decrease in biodegradability and increase in ammonia-nitrogen concentration as leachate transitions from young to mature (Kurniawan and Lo 2009, Renou et al. 2008). The characteristics of mature leachate (i.e., low biodegradability and high concentration of ammonia-nitrogen) adds to the difficulty in treating this leachate. Dissolved organic matter (DOM) is the dominant contributor to total organic carbon of landfill leachate (Liu et al. 2015, Lü et al. 2009). In addition, the composition and characteristics of DOM change when undergoing diverse treatment factors, affecting effluent quality and subsequently the selection of advanced treatment technologies (Komatsu et al. 2020, Tang et al. 2020, Wagner et al. 2015). Meanwhile, more and more sophisticated combinations of different treatment technologies can be applied, raising the question of whether or not such combinations are redundant and too complicated and if the process can be simplified to some extent and still maintain the proper effluent quality. Hence, meeting a more stringent wastewater discharge requirements at a more cost-effective level is needed and it is important to trace the behavior of DOM in mature leachate during various treatment processes.

In order to uncover the characteristics of leachate DOM, significant efforts have focused on exploring analytical techniques to characterize these complex compounds (Lü et al. 2009, Qiu et al. 2020b, Yuan et al. 2017). To date, high-resolution mass spectrometry (HRMS) is recognized as the most promising analytical method to decipher the complex structure of DOM particularly at the molecular level, which has been universally employed in the environmental domain (Rathgeb et al. 2017, Wagner et al. 2015, Zhang et al. 2019). The combination of ultra-performance liquid chromatography with mass spectrometry ultrahigh-resolution (UPLC-HRMS) can provide additional information necessary to dissect DOM by accurate separation time and precise m/z (Lu et al. 2018, Petras et al. 2017). This technique has successfully confirmed the fingerprint characteristics of leachate DOM collected from different landfill sites (Qiu et al. 2020b) and the transformation of fresh leachate DOM during simultaneous denitrification and methanogenesis (Qiu et al. 2020a), therefore it is a proven powerful tool for tracing the transformation of mature leachate DOM during treatment processes in the field.

Recently, the endeavor of using molecular tools to uncover the characteristics of wastewater DOM and to improve the DOM removal efficiency gradually shifted from lab-scale or pilot-scale to field-scale processes. Biological treatment combined with membrane treatment is primary methods utilized for leachate treatment (Fudala-Ksiazek et al. 2018, Yang et al. 2019). For example, an anoxic-oxic membrane bioreactor with nanofiltration process for fresh leachate was examined by Wang et al. (2020), and membrane bioreactor with reverse osmosis (MBR-RO) process for mature leachate by Chen et al. (2020). However, these previous studies only evaluated the influent and effluent samples of the biological step, and the influent and effluent samples of the membrane step. Therefore, this methodology of sampling only the influent and effluent neglects the individual contribution of each unit in the complex biological steps (e.g. anaerobic unit, anoxic unit, oxic unit, MBR unit, sludge recycling unit) and in the complex membrane step (e.g. NF, the tandem of NF and RO). Typically, only direct data mining strategies have been applied, which only illustrate the molecular profile of either the influent or effluent DOM (e.g. the relative richness and abundance of DOM compounds). This limited information only provides the quantitative contribution of DOM removal via each unit. Furthermore, the molecular changes in DOM at the microscopic scale is rarely connected with the macroscopic performance indicated by traditionally bulk indicators like COD, which impairs the initial motivation of optimizing process performance through molecular tools.

Therefore, by using UPLC-HRMS to investigate the transformation of DOM on molecular level along a complex field-scale leachate treatment process, this study has the following goals: 1) to provide a comprehensive report on the fate of DOM at the molecular level in each treatment unit, 2) to quantify the contribution of each treatment unit in treating DOM, by applying our previously proposed mathematical method based on the absence/presence of DOM compounds (Qiu et al. 2020a), 3) to monitor the temporal changes of DOM in each treatment unit, and 4) to determine the performance efficiency of each unit by correlating the bulk indicators with the molecular DOM characteristics.