EFFECT OF GROUND INSULATION AND FEED STOCK ON PERFORMANCE OF FIXED DOME BIOGAS DIGESTER
Abstract
The aim of the study is to determine the effect of ground insulation and feedstock on performance of fixed dome biogas digester. To achieve this, six identical fixed dome biogas digesters with volume of 32 liters were designed, constructed and used to carryout anaerobic digestion of three feedstocks namely cow abdominal waste, poultry droppings and 1:1 mixture of cow abdominal waste and poultry droppings. Three digesters were buried underground for insulation while the remaining three kept on the surface uninsulated. The ANOVA result showed that there were significant differences in the slurry temperature, volume of biogas produced and slurry pH respectively at 5% significant levels. The range of pH, Volume of biogas and slurry temperature (ST) in all the digesters with the three feedstocks were 6.987-7.045, 2.342-3.460 liters and 27.975-28.717oC, respectively. The range of values for measured physicochemical parameters {(Ground insulated digester): (Uninsulated digesters)} were: TS{(4.55-5.56: (3.23-4.14)%}, VS{(3.79-4.87): (2.51-3.38)%}, BOD5 {(42.7-60.3): (36.8-54.4)mg/l}, TVC{(6.78E+05-1.09E+06): (5.44E+05-9.82E+05)cfu/ml}, Protein {(1.92-2.26): (1.64-1.86)%}, Ash{( 0.1-0.4): ( 0.1-0.3)%}, Moisture {(93.8-94.62): (95.7-95.91)%}, Fibre {(0.2-0.6): (0.2-0.5)%}, Fat {(0.45-0.8): (0.35-0.65)%}, Carbon {(3.07-4.79): (2.85-4.20)%}, COD {(160 -234.4): (149.6 -213.6)mg/l}, P{(0.22-0.28): (0.18-0.25)%} , K{(0.26-0.32): (0.24-0.29)%}, and Calorific Value {(13860-18552): (12004-17120)kJ/kg}. The total solid, volatile solid, protein contents, crude fat, BOD5, Carbon Content, COD, Phosphorus, Potassium, Total Viable Count and Calorific Value of ground insulated digester were higher than that of uninsulated surface digesters for the three different feedstocks at the end of the 30 days digestion. GC analysis of biogas showed that the percentage compositions of methane produced were 68.39%, 64.33%, 66.41%, 61.79%, 57.74%, and 59.24% for underground with cow abdominal waste, underground with poultry droppings, underground with1:1 mixture of cow abdominal waste and poultry droppings, uninsulated with cow abdominal waste, uninsulated with poultry droppings, and uninsulated with1:1 mixture of cow abdominal waste and poultry droppings, respectively. The underground insulated biogas digester produced more methane than their counterpart uninsulated digesters containing the same feedstock signifying the importance of temperature regulation through insulation. In terms of the feedstock, the cow abdominal waste performed better than the poultry droppings in both the underground and uninsulated digesters. They mixture of 1:1 Cow abdominal waste to poultry dropping also showed better performance in methane production than the single anaerobic digestion of poultry dropping alone indicating efficiency and importance of co-digestion of feedstocks.
