Enhancing biogas production from lawn grass by optimizing selected factors involved in anaerobic digestion

M.Tech. (Chemical Engineering)Abstract: Biogas technology via anaerobic digestion (AD) is considered as an alternative option to reduce the emission of greenhouse gases, improve energy efficiency and address increasing dependency on imported fossil fuel. Although biogas can be produced easily, numerous researches have shown that the efficiency of methane production, especially from grass, is not optimal due to some factors affecting the AD rocess directly and indirectly. Therefore, this master’s dissertation focuses on evaluating the techniques that can be used to enhance the biodegradability of lawn grass, and subsequently improves biogas and methane production under anaerobic condition. The effects of operational parameters such as temperature, pH, loading rate, and retention time were evaluated. Techniques including recirculation of digestate, alkaline (NaOH) pre-treatment and co-digestion were also investigated. The characteristics of the substrate were determined in order to verify the suitability of the substrate for AD process. Biogas production from lawn grass was studied in the batch laboratory scale digester (1000 mL) with the working volume of 800 mL. Automatic Methane Potential Test System (AMPTS II) was used to measure bio-methane potential and methane content was measured using gas chromatography (GC, claurus 580). Although all the techniques have proven to enhance biogas and methane production from lawn grass, co-digestion has been considered as the most preferable method for enhanced biogas and methane production. The highest biogas yield was obtained when grass was co-digested with cow dung, while the maximum methane was observed when pig manure was a co-substrate with grass at the ratio of 1:1 (25:25 g). The suitable environmental condition for AD on this study was found to be at 45 ºC and pH of 6.5, while the optimal retention time was found to be 8 days, regardless of the effect of temperature and pH. An increase of biogas and methane production by 10% was observed when the digester loading rate was increased from 20 g/L to 60 g/L, whereas higher methane content was obtained in a digester with 30 % recycled solid digestate. It was also found that pre-treating lawn grass with 0.093 M NaOH at 40 and 55 ºC prior the AD process resulted in a significant increase in bio-methane potential from 813 to 1204 NmL and methane content from 40 to 52 %