ANAEROBIC CO-DIGESTION OF THERMO-CHEMICALLY PRE-TREATED MUNICIPAL SOLID WASTE

Abstract

Municipal solid waste (MSW) is chiefly composed of biodegradable waste (50%), like food waste (FW) and yard waste, construction waste (29%), glass and metal waste (5%), cloth waste (7%), plastic and paper waste (9%). However, as sustainably recovering energy from MSW is the need of the hour, the need of treating the organic fraction of municipal solid waste (OFMSW), which is mainly the biodegradable waste of the MSW is also one of the priorities in developing countries. One of the most feasible technologies for bioenergy recovery from OFMSW is by anaerobic digestion (AD), which involves digesting the organic waste in the absence of oxygen to produce energy-rich biogas and nutrient-rich digestate. However, due to the complex and heterogeneous composition and structure of OFMSW, the AD is difficult. For efficient AD of organic substrate, there are some pre-requisites of the parameters involved: the pH required for methanogenesis is between 6.5 to 7.5, the C/N ratio must be between 20 to 30 and there should be no volatile fatty acid (VFA) accumulation. Digesting OFMSW alone does not fulfil to the basic process parameters requirements, due to high carbohydrate content, low alkalinity, high C/N ratio and lack of micro and macro nutrients. Therefore, co-digesting it with a co-substrate sewage sludge (SS) was suitable as it would provide a balance in low C/N ratio, low ammonia content, high alkalinity and abundant macro and micro nutrients. Thus, in the first part of the study the substrate mix of OFMSW and SS was optimised in terms of mixing ratio and total solid (TS)%. It was observed that at varying mixing ratio of OFMSW:SS (50:50, 70:30, 80:20 and 90:10), the maximum biogas yield of 167 mL/gVS was obtained at 80:20 and the biogas production was also higher by 45, 22, and 30% as compared to 50:50, 70:30 and 90:10 mixing ratios, respectively. Similarly, the biogas yield at a TS of 9% was 56, 14 and 69% higher than 6, 12 and 15% TS, respectively. Thus, for anaerobic co-digestion (AcoD) of OFMSW+SS, mixing ratio of OFMSW:SS of 80:20 ratio and TS 10% was optimum. The substrate as optimised in the previous study was then pretreated in thermal and thermal alkali modes in order to optimise the reaction time and alkali dosage, respectively. In the first stage the substrate was subjected to varying temperature (100, 125, 150, 175 and 200℃) and reaction time (30, 60, 90 and 120 min) (to optimise the reaction time). Based on the solubilisation of organics (chemical oxygen demand (COD), proteins and carbohydrates) and recalcitrant formation, the reaction time of 30 min was considered as optimum. The COD and protein solubilisation increased as the temperature increased from 100℃ upto 200℃, and stabilised when the reaction time was longer than 30 min. However, the carbohydrate solubilisation increased as the temperature increased from 100℃ to 150℃, and remained constant or increased slightly from 150℃ to 175℃. However, it decreased further with an increase in temperature up to 200℃ due to the generation of furans by the dehydration of hexose and pentose, and the Maillard reaction between soluble carbohydrates and proteins. High temperature thermal pretreatment also leads to the formation of recalcitrant compounds, 5- Hydroxy Methyl Furfurals (5-HMF) and furfurals due to the Maillard reaction occurs between soluble proteins and carbohydrates, which is intensified with increasing the temperature. No HMF and furfural formation was occurred below 150℃. Their production started from 150℃ onwards, and increased with increasing the pretreatment temperature and time. Thus, the reaction time of 30 min for thermal pretreatment was considered optimum for further experiments. Batch experiments of the substrate at varying temperature (100 to 200℃) at 30 min reaction time, yielded highest biogas yield (474 mL/gVS) at 125℃, which kept reducing at 150, 175 and 200℃ (442, 365 and 117 mL/gVS, respectively) due to formation of 5-HMF and furfurals. In the next stage the substrate was subjected to varying temperature (100, 125, 150, 175 and 200℃) and alkali dosage (1, 3, 5 and 7 g/L-NaOH) (to optimise the alkali dosage). A minor improvement in organics solubilisation was noticed after increasing the NaOH dosage above 3g/L as higher NaOH concentration leads to the formation of protection film on the cell surface, restricting organic degradation and due to Maillard reaction, which produces dark brown coloured melanoidins, posing difficulties in biodegradability and solubilisation of organics. Thus, 3g/L NaOH was observed to be the optimum dosage for thermal-chemical pretreatment before AD. Batch experiments of the substrate at varying temperature (100 to 200℃) at 30 min’s reaction time and 3g/L NaOH dosage, yielded highest biogas yield (618 mL/gVS) at 125℃, which kept reducing at 150, 175 and 200℃ each with 3g/L NaOH (574, 531 and 310 mL/gVS, respectively) due to formation of 5-HMF and furfurals.