Sodium hydroxide solution pretreatment effect on anaerobic digestion process under mesophilic conditions

Biogas, latent energy inside organic components, is a by-product of anaerobic digestion process. Biogas validity depend on methane composition value. Some organic compounds are not easily accessible to active microorganisms in anaerobic digestion process due to their nature and structure. These compounds are made available to microorganisms by pretreatment and methane composition is increased. In this study, alkaline pretreatment of solid organic wastes by sodium hydroxide solution was studied through the anaerobic digestion laboratory apparatus. A mixed feed of organic solid waste, previous stage digested matter and bovine excrement were considered. Three tests were designed. First test, feed was without pretreatment. Solid organic wastes of the second and third tests were immersed in 0.5 M and 0.1 M sodium hydroxide solution for 24 hours in the laboratory, respectively. The set up was subjected to -0.75 barg vacuum and mesophilic temperature conditions. The mechanical stirrer in digester homogenized substrate medium. The digestion period lasted 40 days. The percentage composition of dry matter, organic matter, humidity, ash, carbon and nitrogen of the feed and digested were measured. Produced biogas analyzed by gas chromatography. According to the results, produced biogas and methane volume respectively at 3rd test compare to 1st test increased 15.52% and 21.31% and 3rd test compare to 2nd test 19.52% and 2.3 times. Vice versa, produced biogas and methane volume respectively at 2nd test compare to 1st test decreased 3.32% and 63.7%. Sodium hydroxide solution destroys hard tissue of cellulose lignin and hydrolyzes organic polymers. However, in high concentrations acetogenic and methanogenic microorganisms are inhibited by produced volatile fatty acids and saponification.