Effect of dissolved oxygen and agitation rate on microaerated hydrolysis of silage corn straw

Abstract: Corn straw belongs to lignocellulose biomass, and its cell walls are interwoven by cellulose, hemicellulose and lignin. Lignin can't be degraded under anaerobic conditions, which hinders the contact of extracellular enzymes with cellulose and hemicellulose. The degradation of lignin is an aerobic reaction. Microporous aerator is the main equipment for microbial micro-aeration hydrolysis process. In this process, the air supplied by air compressor is sent to the micro-porous aerator distributed at the bottom of the aeration tank through the pipeline system at the bottom of the aeration tank. The compressed air is blocked by micro-holes on the diaphragm of the micro-porous aerator to form micro-bubbles with a diameter of less than 3 mm. The micro-bubbles are distributed and diffused into the water of the micro-aeration hydrolysis pretreatment tank to fully contact with the hydrolysate so as to effectively dissolve oxygen. To meet the needs of microbial metabolism and biochemical degradation in the hydrolysate of water supply, the purpose of pretreatment of corn straw is achieved. In the presence of molecular oxygen, lignin can be degraded by aerobic hydrolysis microorganisms. Therefore, the aerobic hydrolysis of straw in the anaerobic fermentation of straw is conducive to improve the biodegradability of straw. In this study, a wet micro-aerated hydrolysis pretreatment process with continuous feed-in and feed-out was used to treat corn straw. By controlling the water content of the aerobic hydrolysis fermentation system, the fermentation reactants can be fluidized by the pretreatment with micro-aeration hydrolysis. Stirring can promote heat and mass transfer, the extracellular hydrolase produced by microorganisms can be fully contacted with corn straw, and the occurrence of local over-degradation can be effectively prevented. In this process, by controlling temperature, microbial metabolism in micro-aeration hydrolysis, the pretreatment can be ensured to have high activity, so that the hydrolysis pretreatment reaction can run steadily. In a reactor with effective volume of 30 L, silage corn straw, hydrolysate and tap water with length of 2-3 cm and shredded and kneaded silk were mixed. The hydrolysate was added according to the volume ratio of 10%. The optimum design was carried out with the dissolved oxygen content(1, 2, 3, 4, 5, 6 mg/L) of the hydrolysate and the agitation rate (50, 100 r/min) as the changing factors, and the temperature was controlled at 35-38 ℃ and total solid (TS) was 5%. The test was carried out according to 30% of the daily feed and output, and the hydraulic residence time of the material was 3.3 days. It was found that the pH value and oxidation-reduction potential (ORP) values tended to be stable after 8 days operation. Under the conditions of agitation rate of 50 r/min, dissolved oxygen of 1-4 mg/L and agitation rate of 100 r/min, dissolved oxygen of 1-2 mg/L, the accumulation of fatty acids was significant with the increase of aeration rate. Stirring and aeration can promote the accumulation of acetic acid, propionic acid and n-butyric acid. When the stirring rate was 100 r/min, the concentration of soluble chemical oxygen demand (sCOD) was higher than that when the agitation rate was 50 r/min. Taking cellulose degradation as an example, when the agitation rate was 100 r/min and dissolved oxygen was 2 mg/L, lignocellulose had a high degradation rate of 48%. Continue to increase dissolved oxygen to 6 mg/L, although the degradation rate of lignocellulose still slightly increased, but the increase was small.