FREE NITROUS ACID (FNA) PRETREATMENT TO ENHANCE BIOMETHANATION OF AGRO-WASTE

Abstract

Agro-residues having lignocellulosic biomass are considered most effective (heating value 16 MJ/kg) for energy production through anaerobic digestion (AD), i.e., bio-methanation (CH4). Anaerobic digestion is a promising approach to concurrently fulfill a percentage of the community’s energy requirements, curb greenhouse gas (GHG) emissions, and cope with agricultural residues. Anaerobic co-digestion (ACoD) of Agro-waste (wheat straw-WS) with food waste (FW), and cattle manure (CM) could enhance the process potential to achieve higher methane yield while offering combined treatment of three problematic wastes. However, complex organic materials (lignocellulosic fraction) in Agro-waste obstruct their biotransformation and are a process's rate-limiting step. Before AD, a pretreatment step could enhance the process efficiency through a higher hydrolysis rate, allowing a higher organic loading rate and enhanced methane generation. This research investigates the effect of free nitrous acid pretreatment on enhancement in substrate solubilization and methane yield through AD of FNA pretreated WS and ACoD of WS with FW-CM mixed under mesophilic operation. Recently, a novel pretreatment approach using Free Nitrous Acid (FNA) has been tested on substrates such as algal biomass, waste-activated sludges, etc. It has been found to improve and enhance biomethanation. Thus, to study the effect of FNA pretreatment on Agro-waste, wheat straw samples (particle size 2 mm) were pretreated for 12, 24, 36, and 48 hours at 1.77, 3.54, and 5.31 mg-HNO2/L concentrations. Under two phases, BMP tests were performed to evaluate the biogas yield from these FNA pretreated wheat samples. Phase I comprised of monodigestion of FNA pretreated wheat samples. Phase II comprised of co-digestion of FNA pretreated wheat samples with food waste and cattle manure. Model-based evaluations (First Order Model, Modified Gompartz Model, Transference Model, Monod Type and Dual Pool First Order Model) were carried out to understand the biogas yield and hydrolysis kinetics. BMP and model-based evaluation concluded that the highest biogas yield of 571.84 ml/ gVS was obtained for mono-digestion for wheat samples pretreated at 1.77 FNA for 12 hours. The percent increment in biogas yield was 13.43 %. Similarly, from co-digestion highest biogas yield of 517.37 ml/g VS added was obtained at 1.77 FNA for 24 hours. The percent increment of biogas yield was 8.76 %. Dual Pool First Order model resulted in the best fit for all the conditions and was from the model-based analysis, it was observed that for Phase I, peak biogas yields of 576.16, 563.65 and 565.44 ml/g VS added were observed at 1.77, 3.54 and 5.31 FNA concentrations, and the percentage increment in biogas yields were 10.87 %, 8.846 % and 8.80 % at 1.77, 3.54 and 5.31 FNA concentrations respectively.