###### Abstract  
Primary coffee processing generates important by-products--the pulp, husk and mucilage--while producing the green coffee beans. These by-products represent a large quantity of biomass and might create an adverse impact on environment if they are left to uncontrolled natural decay. In this study, the bio-methane formation potential of coffee husk, pulp and mucilage was examined in batch assays performed at 21 \({}^{\circ}\)C, 30 \({}^{\circ}\)C and 37 \({}^{\circ}\)C. The mean specific methane yield (SMY) from husk, pulp, and mucilage were 159.4, 244.7 and 294.5 L kg\({}^{-1}\) volatile solids(VS), respectively, for a fermentation temperature of 37 \({}^{\circ}\)C; 156.8, 234.8 and 287.1 L kg\({}^{-1}\) VS, respectively, for 30 \({}^{\circ}\)C; and 139.9, 196.2 and 255.9 L kg\({}^{-1}\) VS, respectively, for 21\({}^{\circ}\)C. Two kinetic models, namely, the modified Logistic model (LOG) and the modified Gompertz model (GOM), were applied to fit experimental data and the respective kinetic constants were generated. Both models exhibited a very good fit to the measured data points (R\({}^{2}\) > 0.987). The relationship of kinetic constants of substrates with fermentation temperatures was established and inserted into the LOG and GOM models; thus, generalized LOG and GOM models were obtained to predict SMY of the substrates at any temperature between 21 \({}^{\circ}\)C and 37 \({}^{\circ}\)C.  
c 2019 9 412 2019 2019  
exposing neighboring populations to offensive odor, health risks, and the proliferation of insects [1; 5; 6]. The by-products, however, could be utilized employing proven biomass conversion technologies to produce renewable energy carriers, like biogas, ethanol, and biodiesel [7; 8]. Biogas technology plays a vital role in sustainable environmental management and the proper handling of agro-industrial by-products [9]. The biogas yield from coffee processing by-products has been investigated by many researchers, but the bio-methane yield potential reported varies significantly [10; 11; 12; 13]. Mixing different parts of the coffee by-products as a feedstock to a digester improve the bio-methane yield by buffering acid accumulation and supplementing of trace elements [14]. The rate and extent of bio-degradation are crucial in the anaerobic fermentation of agricultural residues, which in turn depends on the content and properties of lignocellulosic and other factors (pH, temperature, moisture etc.) [15]. However, in a first approximation, lignin content has been proven to adequately predict the methane potential from agro-industrial wastes [16]. The modified Gompertz and logistic equations are the most widely used kinetic models in anaerobic fermentation to demonstrate bio-methane production. Several researchers applied the modified Gompertz model to fit experimental datasets from vinasse [17], cattle manure [18], municipal solid waste [19], water hyacinth [20] and poultry manure [21]. Likewise, the modified Logistic models have also been applied for different substrates [17; 22; 23]. The cumulated biogas yield from both models exhibits a sigmoidal shape. The S-shaped curve produced by the Gompertz model is asymmetrical while the Logistic model is symmetrical about their points of inflection [24; 25]. Temperature is one of the most influential parameters in anaerobic digestion. It affects the growth and metabolic activities of microorganisms [26]. Anaerobic microorganisms function under three temperature ranges: psychrophilic (< 20 \({}^{\circ}\)C), mesophilic (20-45 \({}^{\circ}\)C) and thermophilic (45-60 \({}^{\circ}\)C). Rapid temperature change (\(\pm\)2 \({}^{\circ}\)C h\({}^{-1}\)) is harmful to methanogens, especially for the thermophile consortia [27]. Several models are applied to describe the effect of temperature on the rate of anaerobic fermentation. The Arrhenius model, for instance, was applied to estimate bacterial growth and product formation rate. However, the limitation of the model was the indefinite increase of the growth rate with increasing temperatures.