عنوان مقاله [English]
Anaerobic Digestion (AD) is an alternative process for the treatment of organic waste that enables the energy production in biogas. In addition, the total volume of waste transported to landfills decreases, thus increasing the lifespan of landfills. Considering the different sources of organic waste, food waste (FW) is of great importance because large quantities are produced and buried. For this purpose, co-digestion of FW and green waste (GW) with five mixing ratios of raw materials was performed to evaluate biogas production. Increasing the percentage of GW in the substrate led to increased methane yield. The highest biogas production was determined in the ratio of FW and GW 50:50. About 90% of methane yield was obtained after 21 days of digestion with a total yield of 425 ml/g VS. Based on the preferred ratio, the effect of total solids (TS) on co-digestion of FW and GW ranged from 5 to 25% TS. The results showed that the methane yield from digestion of high-solids AD (HS-AD) (15 to 20% TS) was higher than that of liquid AD (L-AD) (5-10% TS), while further increasing the TS content to 25%, disrupted the function of methanogenesis. This inhibition may be due to overloading of the organic charge and the formation of excess ammonia.
The use of organic biomass to obtain energy helps to meet two major challenges: rising energy consumption and environmental pollution from these wastes, which has become a major problem worldwide (Dong et al., 2020; Richards et al., 2012). Food waste and green waste are the booklet of the largest municipal solid waste streams in metropolitan areas that require efficient and environmentally friendly management (Vickers, 2017). In the current situation, anaerobic digestion is one of the alternatives that is used in addition to the treatment of organic waste and the renewable energy production in the form of biogas.
Some organic wastes have a high potential for biogas production and increase the performance of the AD process (Brown & Li, 2013). Thus, FW has the potential for high organic content in its composition, also high water content (more than 80%) and is easily degradable (Jiang et al., 2012). Because these important properties have made FW a valuable substrate for energy recovery (Woon & Lo, 2016). However, FW is not suitable for the single AD process due to some of its characteristics such as low pH values, high nitrogen content and low C/N ratio (Ahmadi-Pirlou et al., 2017; ZHANG & SU, 2014). From this point of view, anaerobic co-digestion of FW with GW residues leads to increased biogas performance due to the possibility of improving the C/N ratio equilibrium and creating favorable conditions for anaerobic microorganisms (Oleszek et al., 2014). Energy recovery from lignocellulosic materials may be difficult due to its structural properties. However, its application in the anaerobic co-digestion is constantly growing (Zou et al., 2018). Thus, only a few studies have examined the effect of adding lignocellulosic substrates, such as FW with garden waste (Brown & Li, 2013). Some previous studies, including (Ahmadi-Pirlou et al., 2017; Borowski & Weatherley, 2013; Fitamo et al., 2016; Serrano et al., 2017), Have suggested that co-digestion provides efficient AD process and higher biogas production due to nutrient balance. Therefore, it can be used to help achieve higher digestive function.
Considering this background, the aim of this study was to investigate the mixed digestion ratios of FW and GW and then to investigate the effects of different percentages of TS on biogas and methane production in batch reactors with mesophilic temperature conditions. Therefore, first, the effect of different mixing ratios on the co-digestion performance of FW and GW was evaluated. Then, compare high-solids and liquid AD FW and GW were examined based on the preferred ratio of the first series of experiments. Experiments were performed with an experimental approach on a pilot scale with a focus on quantitative and qualitative analysis of biogas and methane.
In this study, the raw materials used were the organic part of FW and GW. GW used was prepared by the predecessor of Imam Ali (AS) Military University. GW was collected from the campus of Imam Ali (AS) Military University and mainly included cuts of green space on the campus and fallen leaves. After preparation, the raw materials were stored in plastic at 4 °C. To increase the microbial population as inoculum, cattle manure was prepared from one of the industrial farms in Samian village of Ardabil province and when loading 20% of the digestion workload, it was loaded from inoculum and the rest from substrate (Lee et al., 2019).
Each batch AD system consisted of a 1000 ml glass bottle with a working volume of 650 ml, a 2 liter gas collection plastic bottle and a 1000 ml graduated liquid collection cylinder. Digestive bottles were loaded with raw materials and inoculated.
Two sets of experiments were performed in a batch AD system with a reaction time of 30 days. The first set of experiments studied the effect of mixing ratios of FW and GW on biogas production through anaerobic co-digestion with five mixing ratios of FW and GW. In this series of experiments, based on the initial TS contents of FW, GW and inoculum were added to each of the water digestions so that the TS content of the mixture inside the digestive system reached 10%. After the first set of experiments, the preferred mixing ratio of FW and GW was determined to be optimal for optimal biogas production: 50:50 (50% of FW and 50% of GW based on VS). In the second set of experiments, based on the preferential ratio of the first series of experiments, the effect of TS content on the co-digestion of FW and GW was investigated. FW and GW were digested at five TS levels (5, 10, 15, 20 and 25% TS). After preparation of each digester, nitrogen gas was used for 1 minute in each digester to establish anaerobic conditions (Fang et al., 2014) and digestion was then incubated at 37 °C.
Optimal yield for co-digestion of FW and GW was obtained at different mixing ratios of 50:50. Based on this preferential ratio, the effects of TS content (5 to 25% TS) on anaerobes co-digestion in batch systems were investigated. The results showed that methane yield was higher than HS-AD (15 and 20% TS) or comparable to L-AD output (5 and 10% TS), while the lowest methane production was observed at 25% TS. Considering the volumetric production of methane, HS-AD systems (15 to 25% TS) showed an increase of 36.7, 27.3 and 37.9% compared to L-AD in 5% TS.
Keywords: "Anaerobic Digestion", "Total Solids", "Lignocellulosic Wastes", "Methane", "Mixing Ratio"