Assessment of ozonation as a pretreatment to increase the methane production potential of dairy manure wastewater

Abstract

Brazil has one of the largest cattle herds in the world. Consequently, cattle manure wastewater production is also substantial in the country. Due to the presence of hardly-biodegradable organic molecules such as lignin and cellulose, hydrolysis is the rate-limiting step of the anaerobic digestion of cattle manure wastewaters. Within this context, ozonation has been widely studied as an alternative to promote the solubilization of refractory organic compounds and improve their biodegradability. Given these facts, the major objectives of this work were to assess the effects of ozonation as a pretreatment for increase methane production in anaerobic digestion of dairy manure wastewater (DMW) and determine optimal conditions for a treatment system constituted of pre-ozonation followed by anaerobic digestion, based on the energy balance of the system. For this, a systematic literature review was carried out to assess the effect of ozone pretreatment on methane production potential of wastes/wastewaters and on process energy balance. Subsequently, a bench-scale study was performed to assess the effect of different ozone doses (20 mg O3. g-1 VS, 40 mg O3. g-1 VS, 100 mg O3. g-1 VS e 180 mg O3.g-1 VS) on the methane production potential of a DMW at natural pH and on the energy balance of the proposed system. The literature review evidenced that ozonation tends to have a positive effect on the methane production potential of wastes and wastewaters. In general, substrates with a very low degradability or experiments under conditions unfavorable to anaerobic digestion (e.g., acidic pH or low temperatures) had positive energy balances when low ozone doses were applied. High ozone mass transfer efficiencies were observed for all ozone doses applied in the experimental stage. The dose of 100 mg O3. g-1 VS significantly increased the methane production potential of DMW. However, the energy balance was negative in all studied scenarios. Even so, ozone pretreatment resulted in potential benefits such the reduction in anaerobic digestion lag phase time, which indicates an acceleration in hydrolysis, and a high stabilization of the final effluent.