Psychrophilic anaerobic treatment of dairy wastewater in attached-film packed-bed reactors

Retention of microorganisms in anaerobic digesters by providing an attachment medium potentially can increase their productivity at lower operating temperatures. The objective of this work was to investigate the effectiveness of attached-film bioreactors for psychrophilic anaerobic digestion of dairy manure. Eight digesters were maintained in an environmental chamber and studied at temperatures of 37 and {dollar}rm 10spcirc C.{dollar} Two digesters were packed with limestone gravel, two with pieces cut from non-woven polyester matting, two with a combination of limestone gravel and polyester pieces, and two had no packing. Biogas methane content averaged 66% at {dollar}rm 37spcirc C{dollar} and 74% at {dollar}rm 10spcirc C{dollar} and was generally consistent across all reactors. Because of this all measures of reactor performance showed similar trends for a given reactor type. The polyester medium with its high porosity and surface to volume ratio had the best overall performance at both 37 and {dollar}rm 10spcirc C.{dollar}; The performance of attached-film reactors under psychrophilic conditions with reduced organic loading rates (OLRs) and decreasing hydraulic retention times (HRTs) was evaluated. The reactors packed with mixed media (limestone and polyester) were used. Although the methane yields seem to increase when the OLRs were reduced for lower temperatures, much of this increase may have been due to previously accumulated solids. In general it was seen that a reactor subject to decreasing OLRs performs poorly in terms of waste treatment and energy production. The influence of bioaugmentation on the performance of attached-film reactors, under psychrophilic conditions {dollar}rm (10spcirc C){dollar} was also studied. Bioaugmentation improved biogas and methane production at {dollar}rm 10spcirc C{dollar} when a very low OLR of 0.01 kg VS/{dollar}rm m3{lcub}cdot{rcub}d{dollar} was used.; Recovery of digester performance at mesophilic {dollar}rm (37spcirc C){dollar} conditions after temperature cycling to {dollar}rm 10spcirc C{dollar} was evaluated using two digesters packed with limestone gravel, two with pieces cut from non-woven polyester matting, and two with no packing. Two control reactors were maintained continuously at {dollar}rm 37spcirc C.{dollar} None of the reactors regained their original performance in terms of biogas and methane production characteristics or pollution abatement after the temperature was cycled to {dollar}rm 10spcirc C{dollar} then back to {dollar}rm 37spcirc C{dollar}; however, the control reactors also showed a decline in performance during the period of study. It was concluded that the lack of complete recovery of reactor performance could not be attributed to the temperature cycle.