Optimization of biogas yield through co-digestion of sugarcane vinasse and pre-treated maize stalks

Abstract

Sugarcane vinasse contains toxins which pose serious challenges to the environment if not disposed properly. Previous studies have shown that anaerobic digestion (AD) of vinasse is a viable option for producing biogas and organic load reduction, however, biogas yield from anaerobic digestion (AD) is as low as 16%. The main objective for this study was to co-digest vinasse and pre-treated maize stalks (maize residues are abundant in most farms as a result of it being the major crop grown by most farmers) in order to find the optimum biogas yield. Specifically, the study aimed at characterizing the substrates; optimize operating conditions for biogas production and to characterize the gas and the resultant sludge (digestate). The vinasse was collected from Muhoroni Sugar Company and the maize stalks from maize farms in Uasin-Gishu county-Kenya. To characterize the substrates, the pH, moisture content, chemical oxygen demand (COD), total solids (TS), total suspended solids (TSS), total dissolved solids (TDS), total organic carbon (TOC) and nitrogen content were determined based on standard methods. The experiment design was done based on response surface methodology specifically Box-Benhken experiment design was used to optimize the biogas production by determining the experimental design matrix. The pH was varied from 6.5 to 7.5, temperature from 350C to 400C and ratio of substrates (maize stalks ratio to vinasse) from 25% to 75%.The biogas was analyzed using gas chromatography on flame ionization detector (FID) and the digestate potential hydrogen (pH), TS, TSS, TDS and COD were determined based on standard methods. The study established that the pH, moisture content, COD,TS, TSS, TDS, TOC and nitrogen content for vinasse were 4.34, 93.91%, 71.28gO/l, 7.05%, 6.04%, 1.01%, 2.23g/l and 2.7g/l respectively. The pH, moisture content, TS, TSS, TDS, TOC and nitrogen content for maize stalks were 7.52, 9.52%, 91.50%, 90.12%, 7.38%,

49.51g/kg and 1.28g/kg respectively. Consequently, the optimum yield was 3.99ml/ml of substrate at pH, temperature and substrate ratio conditions of 7.0, 37.5oC and 50% respectively. The study further found that biogas yield from co-digestion was 3.99ml/vol. of substrate while digesting the substrates (Vinasse and maize stalks) independently yielded 1.32ml/ml of substrate and 1.54ml/ml of substrate respectively. Co–digestion bioreactors operating at 40oC yielded shorter start-up times of 4 days while the ones operating at 37.5oC and 35oC were 7 and 10 days respectively. Analysis of the co-digestion product indicated that the average methane yield was 61.91%. In conclusion, the study established that the characteristics of the substrates and the optimum conditions made favorable conditions for multiplication of bacteria; in addition it also inferred that co-digestion enhances efficiency of the digester and subsequently more biogas yield is attained. The study recommends pH adjustment during different stages of methanogenesis for higher biogas yield and also it puts forward a room for co-digestion of vinasse with more other substrates.