Enhancement of biogas production from sunnhemp using alkaline pretreatment

This study investigates enhancing the biogas production of sunnhemp by pretreatment, before the anaerobic digestion and co-digestion processes, to address the complex and recalcitrant structure of the plant. Fresh sunnhemp harvested at a cutting interval of 50 days is used in the study. Five systems (each with a 5 litre useable volume) are operated semi-continuously with five different ratios of the feedstock by feeding separate feedstocks every five days with a hydraulic retention time (HRT) of 40 days. The system operates at room temperature (30 °C). The study uses sunnhemp as 20% of the feedstock and also considers sunnhemp mixed with cow manure at different ratios, with the weighed sunnhemp being pretreated with dilute sodium hydroxide. Pretreatment of sunnhemp before digestion produces a methane (CH₄) yield 89% greater than that of the untreated sunnhemp. It requires 3.597 kg of dry sunnhemp to produce 1 m³ of CH₄ and the annual CH₄ yield per hectare is 19,015 m³. In the pretreatment of sunnhemp before co-digestion, the increased CH₄ yield depends on the amount of pretreated sunnhemp in the feedstocks. However, the %CH₄, the CH₄ production level and the system stability depend on the optimal ratio of the sunnhemp to cow manure. The initially prepared sunnhemp to cow manure ratio is recommended at 10 g:10 g in 80 mL of water. At this ratio, the %CH₄ and the CH₄ yield are 53.84% and 313 kg chemical oxygen demand (COD) removed, respectively, and the COD removal efficiency is 56.4%. Sunnhemp has high potential and it is worth pretreating before producing biogas. Using sunnhemp to produce biogas is recommended to decrease greenhouse gas emissions and mitigate global warming.