Research PaperObtaining green energy from dry-thermophilic anaerobic co-digestion of municipal solid waste and biodiesel waste
Introduction
High energy demand and waste production have been brought to light as a result of increasing public health concerns and environmental awareness. As a result, biodiesel waste and municipal solid waste have become an ecological problems. The average solid waste generation rate in 23 developing countries is 0.77 kg person−1 d−1 (Troschinetz & Mihelcic, 2009) and this is increasing. Anaerobic digestion (AD) represents an opportunity to decrease environmental pollution whilst at the same time providing biogas and material that can be used as organic fertiliser or carrier material for biofertilisers (Demirel et al., 2009, Feng et al., 2017, Owamah and Izinyon, 2015, Scarlat et al., 2015, Vlyssides et al., 2012, Wang et al., 2012, Wang et al., 2015). Using AD, waste is converted into methane (CH4) and carbon dioxide (CO2). AD includes four steps and is performed by a microbial consortia comprising both Eubacteria (hydrolytic-acidogenic bacteria (HAB)) and acetogenic bacteria (acetogens) and Archaea (H2-utilising methanogens (HUM) and acetate-utilising methanogens (AUM)). Studies on the AD of municipal solid waste (MSW) have shown that the C/N ratio of this waste presents average values of 10:1, below the optimum for anaerobic digestion (25:1) (Fiore et al., 2016, Forster-Carneiro et al., 2008), while methane production (MP) is reduced due to the washing of microorganisms and not to overloading (Zahedi et al., 2013a, Zahedi et al., 2013b). Therefore, an increase in the loading rate of the AD process via the addition of readily biodegradable organic substances such as glycerol, a major by-product of biodiesel production, could be an ideal strategy (Fountoulakis and Manios, 2009, Fountoulakis et al., 2010, Zahedi et al., 2017b). Biodiesel manufacturing worldwide has gained importance for several reasons: (i) The unavailability of fossil fuels due to demographics and political instability; (ii) Modern methods of biodiesel production and new catalyst formulations producing higher biodiesel yields; (iii) the breeding and cultivation of new varieties of oil crops with higher yields of lipid; (iv) an increase in the cultivation of inedible oilseed plants on waste land; (v) new engine designs that can utilise biodiesel and its admixtures as fuel; and (vi) stringent regulations for reducing global greenhouse gas emissions (Avhad and Marchetti, 2015, Dwivedi et al., 2011, Khuntia et al., 2017). Every 100 kg biodiesel yields approximately 10 kg glycerol or biodiesel waste (BW) as a waste stream (Razaviarani, Buchanan, Malik, & Katalambula, 2013), which makes its disposal a fundamental environmental concern. Recent studies have also shown the effectiveness of single-phase dry-thermophilic AD of real municipal solid wastes (MSW) in methane production (MP) (Zahedi, Sales, Romero, & Solera, 2013a), and that biodiesel wastes (BW) supplementation (0.5% v/v) improves the methane production steps in thermophilic-dry dark fermentation of MSW in batch mode (Zahedi et al., 2017b). However, no previous studies have been published on the effects of adding BW to the single-phase dry-thermophilic AD of MSW in a semi-continuous feeding regime on MP or on the effect on the different microbial groups involved in the digestion process. The present study was accordingly carried out, the two objectives of the study being: (1) to establish the optimal conditions (organic loading rate (OLR) or hydraulic retention time (HRT)) to maximise CH4 production; and (2) to investigate the population dynamics in dry-thermophilic anaerobic co-digestion of MSW and BW (0.5% v/v).
The study was carried out in a laboratory-scale reactor under high OLRs ranging from 10 to 34 g [VS] l−1 d−1 or low HRTs ranging from 10 to 3 d. The structure and dynamic of the anaerobic consortia that developed during the different operating periods (OLRs or HRTs) were analysed by fluorescent in situ hybridisation (FISH), employing different oligonucleotide probes. Moreover, the effect of the variations of the operational parameters (OLR or HRT) on pH, alkalinity, organic matter consumed, biogas production, microbiological population dynamics and microbial activity were considered to establish the optimal conditions for the single-phase dry-thermophilic AD of MSW and BW under low HRT. These aspects are important, as short pre-treatment times are preferable for practical applications, because they reduce the volume of the required pre-treatment tank (Zahedi, Icaran, Yuan, & Pijuan, 2017a). Also, it is worth noting that the AD of MSW and BW allows a reduction in greenhouse gas emissions to the atmosphere and has the economic advantage of sharing equipment and costs to treat two different wastes in the same digester simultaneously.
Section snippets
Experimental equipment and operating conditions
Single-phase dry-thermophilic AD producing CH4 in a continuously stirred tank reactor (CSTR) at the laboratory scale was employed in thus study. A 5 l reactor without biomass recycling was used for this purpose, the HRT and the Solid Retention Time (SRT) being equal. Thermophilic conditions (55 °C) were maintained by circulating water through the jacket from a thermostatic water bath. A PRECISTERM 6000142/6000389 (SELECTA S.A.) bath was used, with a maximum capacity of 7 l water. The reactor
Results and discussion
In this section, the effect of varying the operational parameters (OLRs or HRTs) on pH, Alkalinity, SCOD, VFA, VS, organic matter consumed, biogas production and population dynamics are reported.
Conclusions
Anaerobic co-digestion of MSW with brief supplementation of readily biodegradable BW (0.5% v/v) to enhance biomethane production at low HRT is feasible, since the bioenergy of MSW could enhance by more than 50%. Dry-thermophilic anaerobic co-digestion of MSW and BW could operate without jeopardizing the safety of the process between at HRTs of between 10 and 4.4 d. At HRTs of between 10 and 4.4 d, VS removal rates ranged between 50 and 72% and average stable values of Eubacteria (93–95%) and
Acknowledgements
This work was supported by the Spanish Ministry of Economy and Competitiveness, specifically via project CTM 2012-35654, financed by the European Regional Development Fund (ERDF), entitled “Valorización integral de residuos sólidos urbanos y subproductos de la producción de biocombustibles”.The authors wish to express their gratitude to Abengoa Bioenergía, S.A. and U.T.E. Las Calandrias (Jerez de la Frontera) for providing the BW and the MSW used in this study. Zahedi also wishes to thank her
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