EFFECTS OF TOTAL SOLIDS CONCENTRATIONS OF POULTRY, CATTLE, AND PIGGERYWASTE SLURRIES ON BIOGAS YIELD

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The effects of total solids concentrations of poultry, cattle and piggery waste slurries on biogas yield wasinvestigated. Twelve laboratory-size anaerobic batch digesters with 25 L volume were constructed and used for theexperiments. Three replicates of 5%, 10%, 15%, and 20% TS concentrations of poultry, cattle, and piggery waste slurrieswere anaerobically digested for a 30-day detention period and gas yield was measured by the method of waterdisplacement. Temperature variation within the digesters was measured with a maximum and minimum thermometer.Anaerobic digestion of the slurries was undertaken in the mesophilic temperature range (20-40C). The carbon: nitrogenratio of each of the slurries digested was determined. The carbon content was determined using the wackley-Blackmethod, and nitrogen content was determined by the regular kjeldhal method. The pH was measured weekly during theperiod of digestion from a digital pH meter. Gas quality (% methane fraction) was also measured weekly from ananalyser. Coefficient of variation was computed to ascertain the status of the digestion process. Analysis of variance wasused to determine the significant difference in gas yield at p < 0.05. Duncans New Multiple Range Test at p < 0.05 wasused to analyze the difference in gas yield among the various TS concentrations of the slurries investigated. The resultsindicate that biogas yield is of the order: 5% TS > 10% TS > 15% TS > 20% TS. This result shows that gas yieldincreases with decreasing TS concentration of the slurries. The ANOVA showed that the gas yield from the various TS %was significantly different (p < 0.05). DNMRT showed that there was significant difference in gas yield from the slurriesand wastetypes investigated. Poultry waste slurries had the greatest gas yield (L CH4/kg TS) as the gas yield from thewaste types was of the order: Poultry > Piggery > Cattle. The pH of the slurries was of the range 5.5 to 6.8 (weaklyacidic). The C:N of the slurries varied between 6:1 and 9:1. The Coefficient of Variation (CV) for 10 consecutive days ofdigestion was less than 10% indicating a steady state in all the digesters.