| Poultry manure from a commercial farm (caged layers) was fed to three 5-liter, intermittently-mixed, mesophilic anaerobic digesters on a daily basis. Large batches of manure were frozen, then thawed and diluted as needed to give a consistent composition through each phase of the study. Volatile solids (VS) destruction, methane yield (Yma), alkalinity, and other parameters were evaluated at fifteen operating configurations, with feed concentrations of 4.0, 5.5, and 7.0% VS and hydraulic retention times (HRTs) ranging from 10 to 50 days. Effluent was partially decanted in two configurations to increase the solids retention time over the HRT by about 50%, and one configuration was a replicate to estimate experimental error.;Digestion was successful at loading rates as high as 5.6 g VS per liter-day, although acclimation times as long as 5-7 HRTs were needed at start-up and between step-changes. The main problem at high loading rates was foam formation, which was controlled by chemical addition and increased mixing frequency. Grit and feathers were not serious problems at bench scale.;VS destruction averaged 51.4% and was largely unaffected by HRT or feed concentration. Yma averaged 241 mls methane/g VS added, with low values in the 4.0% VS, 10-20 day HRT range. Yma also dropped 6% as the feed rose from 5.5% to 7.0%. Batch (200 ml) bioassays indicated that the acetoclastic methanogens were not inhibited by volatile acids up to 5 g/l as HAc. Decant operation improved VS destruction slightly but had little effect on Yma. Comparison with data of other researchers showed that VS destruction and Yma varied more with feed composition than with the operating variables.;A double-endpoint alkalimetric technique was developed to rapidly and easily monitor digester performance. Titration to pH 5.75 ("PA") gave a rough estimate of bicarbonate concentration, and titration from pH 5.75 to pH 4.30 ("IA") gave a semi-quantitative estimate of total volatile acids. The dimensionless IA/PA ratio was a sensitive process indicator, rising rapidly at the onset of digester stress and falling below 0.3 during stable operation. |
