Optimization of rice straw hydrolysis to convert ligno-cellulose to simple sugars

Abstract

Development and use of renewable energy is a key option towards poverty alleviation and to mitigate global warming. Annual rice straw yield of 2.7 million metric tons in Sri Lanka is the most abundant lingo-cellulosic crop residue for bioconversion into ethanol. This research focused on the hydrolysis phase in ethanol production to optimize the rice straw digestion in the saccharification process. In this study, four anaerobic leaching columns in which the base filled with gravel and sand were used. Except for the control, the other three contained an additional soil layer of 60 mm that was obtained beneath a pile of decomposing rice straw. Then, 500g of rice straw was added to each column. In one of the soil added columns, elute was re-circulated. Rice straw to water ratio was maintained at 1:20 by adding water and straw. Samples were taken at daily basis, except for non-recycled one with analysis of initial and final day for pH, conductivity, TDS, salinity, brix, TS, VS, VSS, TSS, VSS, and BOD. The non-recycled, soil added column gave increased values of conductivity, TDS, salinity, TSS, VSS, and well-grown white mycelia on straw. Highest values of measured parameters were reported in re-circulated column. The increment of cumulative conductivity, TDS, salinity, VS, TSS, and VSS, of non-recycled, soil added column was significant rather than the control. In applying Michaelis Menten kinetics, Vmax and Km for the column without soil are 0.33 and 26.7, with soil were 0.42 and 18.37 and with re-circulation were 7.22 and 0.15, respectively. The latter with higher rate of conversion with less inhibition, proved better affinity between substrate and enzyme. The elusions of TS for without and with soil were 58.0g and 71.5g. These converted to 40.1g and 50.8g of simple sugars and they were 8.0% and 10.1% of total straw weight, expressing in cellulose mass, varied between 22.9%-26.7% and 29%-33.8% of simple sugars. Thus, deduced energy contents are 1.03-1.40 MJ/kg and 1.30-1.77 MJ/kg. Hence hydrolysis of rice straw with soil, has a potential to recover 175 million liters of ethanol.