Methane production test of the anaerobic sludge from rice parboiling industries with the addition of biodiesel glycerol from rice bran oil in Brazil

Experimental research and energy plans show that the residues of rice parboiling industries in Brazil have the energy potential to promote the sector's self-sufficiency. The studies investigate the production of biogas from the anaerobic digestion of wastewater and its anaerobic sludge. There are no reports of the investigation to optimize such production by adding glycerol. This addition can increase methane production; however, high proportions of crude glycerol can cause inhibition of microbial activity. Considering that anaerobic sludge acts as inoculum in the digestion of effluent in treatment plants, the main goal of this study was to test the feasibility of adding crude glycerol from the production of rice bran biodiesel in anaerobic digestion with sludge. The reactors operated with different proportions of crude glycerol (0%, 1%, 2%, and 3%). The highest methane production occurred in the system with 1% glycerol, 945.23 ± 5.34 mL, in 168 h – a production 539.45% higher when compared to the system without glycerol. Higher doses of glycerol resulted in fast interruption of methanogenic activity. The study determined that the addition of 1% glycerol was able to prolong the duration of the stationary phase of methanogenic microorganisms. Thus, the high energy potential of the sector can be significantly increased with the addition of the biodiesel by-product in anaerobic digestion. Such a possibility could guarantee greater security, energy autonomy, the possibility of generating revenue from the sale of surplus methane or electricity, and an environmentally proper destination for crude glycerol.