عنوان مقاله [English]
A large number of biogas plants fed by municipal solid waste (MSW) are installed around the world. However, extensive research is being conducted with the aim of maximizing the performance of the anaerobic digestion process, in order to further compare it with fossil fuels. One of the most important problems of this process and the main obstacle to this transformation is the complex structure of lignocelluloses and the difficulty of its biological transformation. For this purpose, the effect of different concentrations of nanoparticles (NPs) of zero valent iron (ZVI) (50, 70, 90 and 110 ppm ZVI NPs) on maximizing biogas and methane production was evaluated. In this study, the ratio of co-digestion mixing MSW and sewage sludge (SS) was determined from our previous studies as the optimal ratio (MSW: SS: 60:40). The results showed that the highest methane yield was obtained by adding 90 ppm ZVI NPs based on statistical analysis (at least a significant difference using Duncan test) compared to the control treatment (p<0.05). Iron nanoparticles (Fe NPs) also led to an 85% increase in cellulose, a 64% decrease in lignin and a 33% decrease in hemicellulose, indicating an increase in biodegradability due to NPs. The highest methane production was obtained at 90 ppm ZVI NPs concentration which increased by 45% compared to the control treatment and the highest reduction of TS and VS in this digester was 31 and 17% compared to the control digester, respectively.
Anaerobic Digestion (AD), as one of the most important methods used to convert organic waste into renewable energy in the form of methane by-products (as a form of fuel), may reduce the cost of treatmen (Holm-Nielsen et al., 2009; Wang et al., 2012). Also, anaerobic digestion is a desirable technology with high biological evolution to fight organic waste and to be used for electricity generation, residential heating and cooking, etc. to save energy and produce renewable energy (Yin et al., 2014). Hence, an increasingly popular method has been used for landfilling organic waste such as municipal solid waste (Krishna, 2013), food waste (Sreekrishnan et al., 2004) and industrial wastewater (Qiang et al., 2012), all of which have promising results. The overall result of anaerobic digestion is the conversion of biodegradable organic matter to methane, carbon dioxide, hydrogen sulfide, ammonia and new bacterial biomass (Hendroko et al., 2014). Different biological and chemical additives are subject to different operating conditions. The use of additives in anaerobic digestion significantly improves its performance. The suitability of an additive strongly depends on the type of substrate (Sreekrishnan et al., 2004).
Iron nanoparticles are due to their super paramagnetic properties, high inductance, non-toxicity and biocompatibility and have been used in anaerobic digestion to increase methane production and substrate degradation (Demirel & Scherer, 2011; Gustavsson et al., 2013). The addition of Fe has been reported to stimulate and stabilize anaerobic digestion and thus improve biogas production performance (Hanay et al., 2009). Iron nanoparticles are unstable and can slowly dissolve and supply iron ions. In fact, iron ions are the main constituents of cofactors (biochemicals) and enzymes, and their addition to anaerobic digestion increases the activity of Archaea microorganisms (the most important metanogenic microorganisms) (Hanay et al., 2009; Hao et al., 2017). Although the addition of iron nanoparticles increases biogas production in the first 24 to 48 hours, its high concentration has a toxic effect on bacteria, which further leads to a decrease in biogas production (Yanfeng et al., 2008). Accordingly, iron nanoparticles are a good candidate for maintaining the optimal concentration of iron during anaerobic digestion due to their unsaturation and bioavailability.
The aim of this study was to anaerobic co-digestion of municipal solid waste organic wastewater and sewage sludge and to investigate the effects of different concentrations of zero valent iron nanoparticles on methanogenic activity and thus increase biogas and methane production in batch reactors with mesophilic temperature conditions.
The raw materials for testing the organic Fraction of municipal solid waste (OFMSW) and sewage sludge. To increase the microbial population, cow manure was used as an inoculum. Finally, after preparing the raw materials, they were stored at 4 °C until used.
Batch laboratory digestion consisted of glass bottles (1000 mL) of which 667 mL were sealed with substrate and 1/3 empty. Nitrogen gas was used to anaerobic the conditions inside the digestors at the beginning of the experiments (for 1 minute) (Angelidaki et al., 2009). The substrate contained 15% organic solid. Digesters were loaded in a hot water bath equipped with a digital temperature control system under mesophilic conditions (37 °C). Each day, before determining the volume of gas produced and sampling, the digestions were stirred for approximately 30 seconds to improve mixing (Angelidaki et al., 2009).
The organic loading rate at the beginning of the process was 16.4 gVS/L. Anaerobic digestion tests were performed under mesophilic conditions with a hydraulic retention time of 30 days. Experiments including: (1) control treatment was selected based on our previous studies which was determined as the optimal mixing ratio of municipal solid waste and sewage sludge  (MSW without nanoparticles) and (2) municipal waste with different concentrations of nanoparticles ZVI (50, 70, 90, and 110 ppm). Concentrations of nanoparticles were added at the beginning of the anaerobic digestion process.
The aim of this study was to evaluate the effects of zero-valent iron nanoparticles on biogas and methane production from the co-digestion of municipal solid waste and sewage sludge in batch digesters with mesophilic temperature conditions. The highest production of biogas and methane in the digester was observed by adding nanoparticles with a concentration of 90 ppm ZVI, which was 46% and 262% higher than the control sample, respectively, and also the highest biodegradability (reduction of TS and VS) was obtained at this concentration. The results showed that zero-valent iron nanoparticles can effectively increase the yield of biogas and methane and be a new approach to optimize biogas production from municipal waste. Further research is needed on the anaerobic digestion process, including the mechanism for recycling residual iron from digestion.