Abstract
Background:The increased energy consumption from fossil fuels with its attendant gas emissions and environmental problems has provided the impetus to exploit new energy source that are renewable and environmentally-friendly.Objective:This work focused on the investigation and evaluation of the single or individual effects of feed-inoculum ratio, temperature, and agitation speed (i.e.operating variables) on biomethanization of the mixture of cattle manure, pig manure and poultry manure (mixed animal wastes) co-digested with pineapple fruit waste and content of chicken-gizzard (inoculum) as well as to model the kinetics of biomethanization at these different operating variables and to determine the thermodynamic properties of the biomethanization process.Method:The biomethanization experiments were carried out in anaerobic biodigesters at operating variables of feed/inoculums ratio that ranged from 1:1 to 3:1, temperature from 25 to 60°C, and agitation speed from 30 - 70 rpm using one factor at a time (OFAT) method. The biodigesters were incubated for 70 days retention time.Result:The feed/inoculum ratio, temperature and agitation speed had positive impact on cumulative biogas yield, biomethane content and start-up time of biomethanization. The cumulative biogas yield and biomethane content achieved with agitation speed of 30 to 70 rpm was respectively higher than the biogas yield and biomethane content attained without agitation. Minimum cumulative biogas yield and biomethane content was respectively obtained with feed/inoculum ratio of 1:1, temperature of 25°C and agitation speed of 70 rpm; while maximum cumulative biogas yield with its biomethane content was attained with feed/inoculum ratios of 1:3 and 3:1, temperature of 60°C and agitation speed of 30 rpm, respectively. Modified Gompertz and Exponential Rise to Maximum kinetic models fitted very well to the data and thus showed better correlation of cumulative biogas production. The thermodynamic parameters of Gibbs free energy, enthalpy, entropy change and activation energy of biomethanization were estimated and evaluated, and was found that the biomethanization process was thermodynamically feasible, spontaneous and endothermic in nature suggesting hydrogenotrophic methanogenesis pathway. The activation energy of the biomethanization process was found to be 3.324 kJ/ mol. The specific heat capacity at constant volume and constant pressure, specific internal energy and specific enthalpy of the biogas and biomethane content increased with increase in temperature.Conclusion:Biogas/biomethane production from the biomethanization of mixed animal wastes co-digested with fruit waste and inoculum is a feasible, viable and sustainable renewable energy option that can be simulated by kinetic models and influenced by operating variables.
Highlights
Biomass and their wastes, depending on their characteristics, can be converted into energy and/or fuel by combustion, gasification, co-firing with other fuels and by anaerobic digestion [1]
Fig. (3) shows the effect of operating variables on the start-up time of the biomethanization process of cattle manure, pig manure and chicken manure mixture co-digested with pineapple fruit waste and content of chicken-gizzard
The chromatographic analysis of biogas obtained from the biomethanization of cattle manure, pig manure and chicken manure mixture co-digested with pineapple fruit waste and content of chicken-gizzard at different temperature of 25, 30, 40, 50 and 60 oC indicated that the gas constituents are CH4, CO2, H2S, CO and NH3
Summary
Biomass and their wastes, depending on their characteristics, can be converted into energy and/or fuel by combustion, gasification, co-firing with other fuels and by anaerobic digestion [1]. The process and mechanism of anaerobic digestion (biomethanization) is carried out by a consortium of micro-organisms and the activity of biomethanization process and biogas yield depends on various factors like temperature, pH, concentration of substrate/nutrients, agitation, pretreatment of feedstock, hydraulic retention time, total solid content, organic loading rate and the carbon: nitrogen (C/N) ratio [5, 6]. All these factors need proper monitoring and control to achieve maximum yield of biogas [7]. The increased energy consumption from fossil fuels with its attendant gas emissions and environmental problems has provided the impetus to exploit new energy source that are renewable and environmentally-friendly
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