Sanitary landfilling is the most widely applied solid waste management method (Kaza et al., 2018). In landfills, waste, liquid present in the waste and percolated rain water interact and result in leachate. The leachate has high chemical oxygen demand (COD) content, high ammonium nitrogen content and lasting toxicological characteristics (Alslaibi et al., 2011; Li et al., 2014; Regadio et al., 2012; Yang et al., 2013). When the liners of landfill failed, the leakage of leachate often leads to the contamination of groundwater and soil. Landfills are considered to be important sources of groundwater contamination. The major groundwater contaminants from landfill include ammonia, organic matter, such as COD, heavy metals and so on (Alslaibi et al., 2011; Milosevic et al., 2012; Regadio et al., 2012; Smahi et al., 2013; Tian et al., 2005). These contaminants pose great threat to human health.
Over the last few decades, a variety of different physical, chemical and biological technologies have already been used to remove or degrade contaminants from groundwater(Grover et al., 2011; Zhang and Zhou, 2008). In situ bioremediation is one of the most widely used groundwater remediation techniques (Azubuike et al., 2016). it is an eco-friendly and low cost approach. Specifically, in situ bioremediation is achieved by providing substrates to stimulate the growth of native microbial species at first, and then the microorganisms transform contaminants into less harmful daughter products (Albers et al., 2015). Aeration is the most common method to stimulate microbial growth. However, traditional aeration consumes more than 50% of the total energy requirement in wastewater treatment plants (Belloir et al., 2015). Sander et al.(2017), Xiao and Xu (2020) found that fine bubbles aeration could save approximately 20% operating cost than conventional aeration. As a kind of fine bubbles, micro-nano-bubbles (MNB) has sm all diameter (with a diameter of 200 nm-50 µm), large specific surface area and low rising velocity in liquid, and it persists for long periods and significantly improves gas solubility (Feng et al., 2020; Hu and Xia, 2018). With the superior mass transfer characteristics, MNB has received continuous attention in the fields of aeration(Ye et al., 2019; Zhang and Guiraud, 2017).
Some studies about contaminant removal efficiency under MNB aeration have shown satisfactory performance. As a pretreatment technique, MNB has been shown to be highly beneficial for downsizing the water/wastewater treatment plants and improving the quality of product water (Kazuyuki et al., 2010; Kazuyuki et al., 2009). In aerobic biofilm system, air nanobubble aeration accelerated the growth of the biofilm and achieved better removal efficiency of COD and ammonia nitrogen. The dehydrogenase activity was as maximum as six times higher than that of traditional aeration (Xiao and Xu, 2020). In membrane bioreactor, oxygen MNB markedly enhanced the removal efficiency of contaminants compared with conventional air aeration, and the pure oxygen provided a high dissolved oxygen condition which would influence the biomass activity by affecting the enzymatic activities (Zhuang et al., 2016). Due to the characteristic of high mass transfer efficiency (Bai et al., 2021; Xiao and Xu, 2020), the strong of bubble migration (Kristen et al., 1993; Li et al., 2014) and high pollutant degradation efficiency, MNB aeration has become a promising measure for wastewater remediation.
As mentioned above, groundwater contamination resulting from the landfill leachate is widespread. However, little investigation has been done about treatment of landfill leachate contaminated groundwater by MNB aeration. Different from industrial pollution sites, there are compound contaminants such as organic matter, inorganic salt and heavy metal in solid waste landfill site. For landfill leachate contaminated groundwater, the main contaminants, such as COD, ammonia nitrogen and heavy metal, are degradable and transformable, and their concentrations fluctuate greatly in time and space. Therefore, it is very important to study the contaminant removal efficiency on landfill leachate contaminated groundwater under MNB aeration and its mechanism.
The objective of the present work was to evaluate the effect of oxygen MNB aeration on the organic contaminant removal and composition variation of landfill leachate contaminated groundwater. Furthermore, the difference between oxygen MNB and common bubble (CB) aeration was investigated. Specifically, the groundwater samples were treated by oxygen MNB or CB aeration at first, respectively. The mass transfer efficiency of MNB and CB aeration were estimated by the dissolved oxygen (DO) value variations during aeration. Then, according to the concentration variation of COD, 5-day biochemical oxygen demand (BOD5), ammonia nitrogen and dehydrogenase in groundwater samples before and after MNB or CB aeration, the contaminant removal and microbial activity enhancement efficiency of MNB aeration on landfill leachate contaminated groundwater were evaluated. After that, the composition variation of dissolved organic matter (DOM) in groundwater before and after MNB or CB aeration were characterized by ultraviolet-visible (UV-VIS) absorption spectrum and fluorescence excitation-emission matrix (EEM). At last, the energy consumption of MNB and CB aeration were estimated.