Sewage sludge as a potential resource integrated in landfill biocovers: an experimental study

Sewage sludge (SS) management, comprising its treatment and final use, recovery, or disposal, represents a major issue for wastewater treatment facilities. SS management can account for a relevant or predominant portion of the total costs sustained for the entire wastewater treatment process. Recent findings have indicated that integration of SS in landfill cover soils, consequently identifiable as biocovers, may improve the abatement of diffuse CH4 fluxes uncollected by existing landfill gas (LFG) extraction systems, thus contributing to reduce the emission of one of the major greenhouse gases into the atmosphere. In fact, SS integration in cover soils can enhance the activity of several microbial species, including methanotrophic bacteria, by providing many organic and inorganic micro- and macro-nutrients. In a perspective of circularity, the integration of SS as a secondary resource in landfill cover soils can save an equivalent amount of vegetal soil (primary resource), leading also to a consequent economic advantage for multi-utility companies generally operating both wastewater and waste (including landfill) facilities. Moreover, total operational costs in landfill management are significantly impacted by the treatment and disposal of the generated landfill leachate (LL) that, alternatively, could be recirculated within the landfill body to speed up deposited waste degradation and enhance production rates of LFG. Despite the mentioned benefits potentially expected from the integration of SS in landfill biocovers (eventually sprayed with LL), a standardised protocol for the configuration and operation of these cover systems is still lacking and, particularly, their effective contribution in degrading a wide spectrum of potentially harmful compounds, that are still contained in LFG (in addition to CH4 and CO2), is poorly known so far. In the overall framework of sustainability and circularity of sludge management and landfill operation, this study investigated the potential of SS integration as a material resource in biocovers (eventually in combination with LL spraying) to degrade CH4 and nonmethane volatile organic compounds (VOCs) expected in LFG. Specifically, two laboratory experiments were conducted based on a developed prototype simulating a miniaturised landfill, which consisted of (i) a maceration chamber (20 L) representing the waste body under predominant anaerobic degradation conditions, connected to (ii) a soil column (105 cm high polypropylene cylinder) representing the cover soil’s vertical profile. The first, preliminary experiment, which lasted 27 days, implemented two prototypes: (1) a blank, with the column filled with vegetal soil; and (2) a biocover, with the column filled with a mixture of vegetal soil and anaerobically stabilised and dewatered SS (SSans-d) in a mass ratio of 10:1, periodically sprayed with LL. The subsequent experiment, which lasted 40 days, implemented four prototypes: (1) a blank with vegetal soil; two biocovers with a mixture of vegetal soil and SSans-d in the mass ratio of (2) 10:1 and (3) 20:1, respectively; and (4) a final biocover (vegetal soil to SSansd mass ratio of 20:1) being further periodically sprayed with LL. In both experiments, the organic substrate in maceration chambers consisted of degradable food waste. Gas samples were periodically collected from the maceration chambers and multiple depths of the soil columns, and analysed in terms of main components (CH4, CO2, H2, N2, Ar, and O2), VOCs, and carbon isotope signatures (13C/12C) in CO2 and CH4. The preliminary experiment showed that simulated biocover with SSans-d integration and LL spraying had greater efficacy in degrading CH4 and selected VOCs compared to vegetal soil only. The subsequent experiment showed that the efficacy in abatement of LFG components by simulated biocovers was influenced to a larger extent by SSans-d integration as compared with LL spraying.