Predicting the effects of integrating mineral wastes in anaerobic digestion of OFMSW using first-order and Gompertz models from biomethane potential assays
Introduction
Worldwide annual production of municipal solid wastes (MSW) was 2.01 billion tonnes in 2016 [1], and growing economic activity and urbanisation is expected to increase MSW generation to 3.4 billion tonnes in 2050 [1]. The organic fraction of municipal solid waste (OFMSW) represents a significant part of MSW. In developing countries OFMSW is predominantly derived (∼70–80%) from food waste [2].
OFMSW comprises both dry and wet components. The dry fraction is environmentally ‘managed’ via incineration converting it to energy as an effective alternative to landfill disposal. The wet or putrefied fraction of OFMSW can be used for the production of renewable energy via AD [3]. Generally, solid residues from municipal solid waste incineration (MSWI) plants represent at least one fifteenth of the volume of the MSW [4]. The residues from an MSWI plant can be subdivided into incineration bottom ash (IBA) and air pollution control (APC) solid residues. The latter comprise fly ash (FA) and boiler ash (BA). Similarly, the construction and demolition industry also generate a substantial solid waste stream, mainly composed of hydrated cement waste containing a high level of calcium and other minerals. Cement-based waste (CBW), which accounts for about 35.5–67% of construction demolition waste (CDW) also has a notional CaO content of about 10–20% [5] and contains calcium silicate and hydroxide minerals.
The residues from MSWI plants and CDW are often recycled to be used for the production of aggregates for construction, or they are used as a daily cover layer for landfills [6]. A major concern for these wastes is the leaching of heavy and alkali metals after disposal. A study by Hjelmar [7] investigated different management scenarios prior to disposal of mineral wastes (MW) into landfills, which included enhancing leaching rates prior to disposal to reduce the duration of the active care period.
MW are enriched with minerals and heavy metals [[8], [9], [10], [11], [12]] and as such, have a reasonable acid neutralising capacity. This property makes MW promising for applications related to AD processes, and potentially, MW could be amended directly to anaerobic digesters to stabilise pH [10,13].
If adopted, such use of MW for anaerobic co-digestion of OFMSW would generate an economic value to MSWI and CDW wastes, and decrease the amount of MW need to be disposed in landfills. Additionally, anaerobic co-digestion of MSWI and CBW wastes with OFMSW would bring about a controlled release (stripping) of metal contaminants from those wastes prior their ultimate disposal.
The aim of the current study was to evaluate the stability and productivity of co-digesting organic and mineral wastes in laboratory-scale experiments in order to predict the outcome of this application in full-scale digesters. Experimental methane production data obtained from BMP experiments (BMPs co-digested mineral and organic wastes) were modelled with first-order and Gompertz models to investigate the effects of integrating MW with OFMSW. This include the kinetics of methane production and microbial growth rates of OFMSW at 37 °C, and the maximum methane potential values obtained from BMP assays. These were compared with 1) the theoretical maximum values obtained from compositional analysis of the organic waste, 2) the published values in the literature.
Section snippets
Feedstock and seed inoculum
In order to minimise the effect of heterogeneity in the feeding substrate composition [14], the substrate used for BMP assays was a synthetic organic waste (SOW). It was composed of 79% cooked food leftovers (such as rice 13.6%, meat 1.5%, beans 5.6% and fat 1.4%), 20% uncooked fruit and vegetable wastes (such as apple 1.3%, orange 1.7%, banana 2%, lemon 1.2% and pomegranate 1.4% and herbs ∼ 6%), and 1% packaging cardboard, and simulated a typical OFMSW going to landfill [13]. The substrate
Substrate and inoculum analyses
The macro-parameters including total and volatile solids (TS and VS) showed the TS of the substrate (SOW) was 18.6% (W/W), and the VS accounted for 92% of TS (Table 2), which is similar to the typical food waste composition of OFMSW (18%–23% (W/W) and VS accounted for 91–95% of TS, [30]). The C:N ratio of the SOW (Table 2) was 22.4:1, and therefore slightly below the optimal C:N ratio of 25–30 suggested for mesophilic AD at 37 °C [31]. The molecular formula of the substrate (SOW) was estimated
Conclusions
The first- order and Gompertz fitting models predicted biomethane potential and maximum methane production rates for the AD of OFMSW with and without co-digestion with MW. Results showed that MW originating from MSWI and CDW could be integrated into the digestion of OFMSW to improve microbial growth and activity to promote biogas production and improve methane yield in continuously operated digesters. MW can release soluble micronutrients (trace elements) essential for the growth and activity
Author contributions section
Burhan Shamurad: carried out the experiment, data analysis, and wrote the manuscript. Paul Sallis: supervised the study and reviewed the manuscript. Neil Gray: supervised the study and reviewed the manuscript. Evangelos Petropoulos, Shamas Tabraiz and Edward Membere: they helped with data analysis.
Declaration of competing interest
We have no conflicts of interest to disclose.
Acknowledgement
We would like to acknowledge the Waste-to-Energy (Teesside, UK) and Springwell Quarry (Gateshead, UK) companies for providing the mineral wastes.
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