Enhancement of biogas production from rice straw through AcoD with sewage sludge and chemical and enzyme pre-treatment.
Creators: name: Youyong, Wichuta
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400513
subject: Water resources engineering
classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)
classificationCode: 401199
subject: Environmental engineering not elsewhere classified

description: Rice straw is the third largest agricultural residue in the world behind wheat and corn. The world annual rice straw production in 2014 was about 741 million tons, of which approximately 50% is used as animal feed. Currently, the common practice is to dispose of rice straw through field burning, but this has adverse effects on the air quality and consequently people’s health. Rice straw has potential for bioenergy generation, especially providing good opportunities for using residues and an improved environment due to their abundance. Rice straw is composed of a heterogeneous complex of carbohydrate polymers. Therefore, a pre-treatment process is essential to break the lignin seal to make cellulose and hemicellulose available for anaerobic microorganisms. The use of chemical and enzyme pre-treatments has been investigated and showed potential to enhance bioenergy production. However, most research has focused on specific agricultural residues such as silage and wheat straw and little has been done on rice straw. Therefore, the aim of this research is to investigate the potential of enhancing methane production from rice straw through pre-treatment and anaerobic co-digestion (AcoD) with sewage sludge. Several pre-treatment methods, including chemical using NaOH, enzyme and combined chemical and enzyme were investigated in this study.<br><br>Wastewater treatment plants (WWTPs) use anaerobic digestion (AD) to stabilise sludge and generate energy that can be used onsite. However, the operation of AD is costly and usually does not provide enough energy to offset the cost of operation and disposal of digestate generated during the AD process. Many WWTPs implemented the strategy of AcoD involving sewage sludge with other organic substrates available in the vicinity of the treatment plant. Therefore, this project aims to assess the potential of using rice straw as a co-substrate in the feedstock to AD at WWTPs. It also assesses the effect of pre-treatment on biogas yield from these anaerobic digesters.<br><br>To achieve the aims of this project the potential of rice straw as a feedstock for biogas production was investigated using biochemical methane potential (BMP) tests. To enhance biogas yield from rice straw, the effect of chemical and enzymes pre-treatment on biogas yield was also assessed. Chemical pre-treatment was carried out using NaOH of 1%-2%, whereas enzyme pre-treatment was assessed using 15 FPU/g dried substrate. The effectiveness of pre-treatment was evaluated in terms of compositional changes such as: (i) the composition of rice straw in terms of water soluble, lignin, cellulose and hemicellulose all measured according to the NREL method; (ii) the crystallinity determined using Transform Infrared Spectrophotometer (FTIR) and Nuclear Magnetic Resonance Spectroscopy (NMR); (iii) the structure based on scanning electron microscopy (SEM) for before and after pre-treatment; and (iv) effect on biogas yield. The BMP tests showed rice straw pre-treated using combined 2% NaOH with Viscamyl had the highest biogas production of 394 mL/gVS (74% CH4), whereas the untreated rice straw produced 223 mL/gVS (66% CH4).<br><br>The potential of enhancing methane production from rice straw, through AcoD with sewage sludge, was investigated both for untreated and pre-treated rice straw. BMP tests were carried out to determine the biogas yield for rice straw and sewage sludge as single substrates and mixed at different rations of sewage sludge: rice straw (SS:RS) of 40:60, 30:70, 20:80, 10:90. The tests were designed using response surface methodology (RSM) for different solids loadings (between 16% and 40% (W/W)), C/N ratios (between 15 and 42) and different concentrations of NaOH pre-treatment. The results showed that AcoD of rice straw with sewage sludge had a synergistic effect on methane yield, where the AcoD of SS:RS at 20:80 was 164% and 130% higher compared to the single substrates rice straw and sewage sludge, respectively. This enhanced yield is most likely achieved by improving the feedstock’s C/N content and nutrients’ availability in the digester. The highest biogas yield of 264 mL/gVS was achieved for C/N of 26. The RSM analysis showed that the optimum pre-treatment conditions of rice straw is using 3.6% NaOH at 121ᵒC for 30 minutes; under these conditions the maximum biogas yield of 305 mL/gVS can be achieved.<br><br>The second phase of this study evaluated the effect of AcoD of sewage sludge with rice straw (untreated and pre-treated) under semi-continuous conditions. These tests investigated the effect of organic loading rates (OLRs) on biogas yield using different types of inoculum, i.e. inoculum collected from a mesophilic anaerobic digester at a WWTP and inoculums acclimated to mixed feedstock at three different rations of SS:RS 100:0, 40:60 and 20:80. The results showed the reactors that started-up using inoculum acclimated to a feedstock of sewage sludge: rice straw at 20:80 elicited the highest biogas production throughout this phase of the experiments.<br><br>Also, it was observed that all semi-continuously fed anaerobic digestion reactors that received pre-treated rice straw had higher biogas yield than those receiving untreated rice straw. Combined pre-treatment (2% NaOH and Viscamyl) showed the highest biogas yield of 418 mL/gVS (73% CH4), compared with the yield from the semi-continuous reactors receiving untreated rice straw (250 mL/gVS, 66% CH4). The results indicated the optimum OLR for semi-continuous digestion at 1.26 kgVS/m3.d. The performance of the AcoD reactors were assessed in terms of how much was removed (53% of TS, 55% of VS, and 60% of tCOD).<br><br>In addition, to assess the effect of pre-treatment on the composition of the digestate, the digestates collected daily from the semi-continous AcoD rectors were characterised in terms of lignocelulosic main fractions (cellulose, hemicellulose, lignin and water soluble). It was observed that cellulose was converted into hemicelluloses and hot water soluble. The results provided more hot water soluble to the digestate. The relationship between biogas production and the amount of hot water soluble was one where increasing the amount of these solubles led to enhanced biogas production. The proportion of cellulose was degraded better than lignin, hence a higher proportion of lignin presented in the digestate. The percentage of lignin in the feedstock (18-23%) and the digestate (22-24%) cannot be degraded.