Abstract
Enhancement of methane production by co-digestion of Napier grass and Napier silage with food waste was investigated in batch and repeated batch modes. First, the ratios of Napier grass to food waste and Napier silage to food waste were varied at different g-volatile solids (VS) to g-VS at an initial substrate concentration of 5 g-VS/L. The optimum ratios of Napier grass to food waste and Napier silage to food waste were 1:4 and 3:2 (g-VS/g-VS), respectively. This gave maximum methane yields (MY) of 411 and 362 mL-CH4/g-VSadded, respectively. Subsequently, the suitable ratios were used to produce methane at various substrate concentrations. A maximal MY of 403 and 353 mL CH4/g-VS were attained when concentrations of Napier grass co-digested with food waste and Napier silage co-digested with food waste were 15 g-VS/L and 20 g-VS/L, respectively. Under the optimum substrate concentration, the maximum MY from co-digestion of Napier grass with food waste was 1.14 times higher than that of Napier silage with food waste. Thus, co-digestion of Napier grass with food waste was further investigated at various organic loading rates (OLRs) in a 10.25 L horizontal reactor with a working volume of 5 L at an optimal ratio of 1:4 (g-VS/g-VS) and substrate concentration of 15 g VS/L. An OLR of 1.5 g-VS/L∙d gave a maximum methane production rate and MY of 0.5 L CH4/L∙d and 0.33 L-CH4/g-VSadded, respectively. Under the optimum OLR, the predominant methane producers were Methanoregula sp., Methanotorris sp., Methanobacterium sp., Methanogenium sp. and Methanosarcina sp. An energy production of 11.9 kJ/g-VSadded was attained.
Highlights
Methane is an alternative energy resource that has received considerable attention due to its high heating value of 55.5 MJ/kg [1], which is equivalent to 15.4 kWh of electricity and 1.2 times higher than an equal mass of liquefied petroleum gas (LPG) [2]
The variation of mixing ratios of Napier grass to food waste and Napier silage to food waste resulted in a variation of methane production potential (MP), methane production rate (MPR), and methane yield (MY) (Table 1)
A slight increase in the pH at the end of the fermentation could have been caused by the degradation of protein in food waste and subsequent release of ammonia into the fermentation broth
Summary
Methane is an alternative energy resource that has received considerable attention due to its high heating value of 55.5 MJ/kg [1], which is equivalent to 15.4 kWh of electricity and 1.2 times higher than an equal mass of liquefied petroleum gas (LPG) [2]. In Thailand, various kinds of agricultural feedstock have been used to produce methane, e.g., biomass residues from palm oil milling [3,4], cassava pulp [5], Napier grass and Napier silage [6], and rice straw [7]. Napier grass (Pennisetum purpureum) contains high content of cellulose, hemicellulose [6,8] and other nutrients which made it suitable to use as the substrate for energy and animal feed. Napier grass is rapidly regenerated after cutting with a high yield per unit area, and fast growth. The utilization of Napier grass to energy does not compete with the utilization as animal feed. In Thailand, Napier grass is promoted as Energies 2018, 11, 3200; doi:10.3390/en11113200 www.mdpi.com/journal/energies
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