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Abstract
Fixed-dome biodigesters are the most widely deployed small-scale anaerobic digestion technology in the developing world, yet their performance in unheated tropical installations is governed almost entirely by slurry temperature. This study presents the engineering design and construction specification for a 10 cubic metre fixed-dome biodigester fed on cattle dung, and quantifies the effect of temperature on its expected performance under the ambient regime of southeastern Nigeria. A design-and-analysis approach was adopted, in which the digester was sized from substrate loading and hydraulic retention time, its geometry derived from spherical and cylindrical volume relations, and its temperature-dependent output modelled using a modified Arrhenius formulation with a temperature coefficient of 1.10. The resulting plant comprises a 2.60 metre diameter cylindrical chamber of 1.22 metre wall height capped by a hemispherical dome of 1.30 metre radius, giving a digester volume of 9.6 cubic metres and a gas storage volume of 1.5 cubic metres at a maximum gauge pressure of 9.81 kilopascals. Design gas output is 3.60 cubic metres per day at a reference slurry temperature of 27 °C. Modelling across the expected buried-digester slurry range of 24 to 30 °C predicts output between 2.70 and 4.79 cubic metres per day, a seasonal variation of 1.77 fold about the design figure. Raising the slurry to 35 °C would more than double output to 7.72 cubic metres per day; an energy balance indicates that approximately 66 per cent of the additional energy would be retained after heating losses. The study concludes that passive thermal measures offer the better return in this climate and specifies a verification protocol.