Alkaline pretreatment of food waste with grass clippings for the enhancement of methane production in anaerobic digestion processes

The proper management of solid urban waste is essential to avoid the contamination of natural resources and greenhouse gas emissions. In Brazil, approximately 50% of municipal solid waste is made up of food and garden waste. Anaerobic co-digestion can convert this waste into energy and biofertilizer, but hydrolysis of the lignocellulosic components is a barrier and pre-treatment is necessary. This study sought to identify the biochemical methane potential of the co-digestion of food waste with garden waste (grass clippings) subjected to alkaline hydrolysis. 3% sodium hydroxide (NaOH) (wet state) was added to the grass clippings for 12, 24 and 48 hours. The biochemical methane potential tests followed the recommendations of the German standard VDI 4630 (2006) for 31 days at a mesophilic temperature (37 ºC ± 1). Total and volatile solids, chemical oxygen demand, total Kjeldahl nitrogen, total organic carbon, carbon/nitrogen ratio, carbohydrates, cellulose, hemicellulose, and lignin were analyzed. The samples followed a ratio of ≤ 0.5 with the inoculum and 1:1 for substrate and cosubstrate (on VS basis). The results showed that the co-digestion of food waste with grass clippings was advantageous due to the balance of nutrients, raising the carbon/nitrogen ratio 21 to 25. The greatest generation of biogas and methane was seen in the co-digestion 12h sample, which showed the greatest reduction in solids. Alkaline hydrolysis was efficient, reducing the lignin content by 40.95% and increasing the substrate conversion and maximum production rates by an average of 1.25 and 1.05 times. The improvement in the digestibility of food waste co-digestion samples with NaOH pre-treated grass clippings was evident in all configurations. It was possible to conclude that grass clippings subjected to pre-treatment with 3% of NaOH for 12 hours are recommended as a co-substrate with food waste, due to the improvement observed in methane generation.