Improvement of anaerobic digester performance by wastewater recirculation through an aerated membrane.

Authors: Loughrin, John; Lovanh, Nanh; Cook, Kimberly - Kim

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2013
Publication Date: 11/20/2013
Citation: Loughrin, J.H., Lovanh, N.C., Cook, K.L. 2013. Improvement of anaerobic digester performance by wastewater recirculation through an aerated membrane. Transactions of the ASABE. 56(5):1675-1681.

Interpretive Summary: Wastewater from animal rearing operations is malodorous and produces greenhouse gases that contribute to global warming. However, the gases (biogas) from animal waste represent a valuable source of energy if economical means can be found to improve its quality. Swine wastewater from an anaerobic digester was recirculated through a silicone hose located in an external aeration chamber to determine its effect on wastewater odors and biogas composition. The silicone hose acted as a membrane for the passage of small molecules. In the first experiment, wastewater was continuously recirculated through the silicone membrane. Improvements in wastewater malodorants were modest. For instance, concentrations of cresol, a typical malodorant of animal wastes, were approximately one-third their concentration in treated digesters than they were in control digesters. However, improvements in biogas quality occurred. Recirculation of the wastewater lowered carbon dioxide concentrations in the biogas, which helped to improve the heating value of the biogas. Biogas volume was decreased by the treatment, however, with control digesters producing 490 liters of biogas and 346 liters of methane as compared to treated digesters, which produced 313 liters of biogas and 261 liters of methane. In the second experiment, wastewater was recirculated through the silicone hose for only the first twenty-three days of the experiment to limit loss of methane through the silicone membrane. No improvements in wastewater malodorants were achieved but biogas production was enhanced. Control digesters produced 803 liters of biogas and 483 liters of methane while treated digesters produced 884 liters of biogas and 539 liters of methane.

Technical Abstract: Swine wastewater from an anaerobic digester was recirculated through a silicone hose located in an external aeration chamber to determine its effect on wastewater malodorants and biogas composition. The silicone hose acted as a semipermeable membrane for the passage of small molecules. In the first experiment, wastewater was continuously recirculated through the silicone membrane with a hydraulic retention time of 2.6 days. Improvements in wastewater malodorants (simple aromatics and volatile fatty acids) were modest. For instance, peak cresol concentrations in treated digester wastewater were approximately 67,000 ng L-1 whereas in control digesters, wastewater cresol concentrations were over 150,000 ng L-1. However, improvements in biogas quality occurred. In control digesters, CO2 concentrations were 224,000 ppm while in treated digesters CO2 concentrations were 143,000 ppm. Control digesters produced 490 L of biogas and 346 L of CH4 while treated digesters produced 313 L of biogas and 261 L of CH4. In the second experiment, wastewater was recirculated through the silicone hose for only the first twenty-three d of the experiment to limit loss of CH4 through the silicone membrane. No improvements in wastewater malodorants were achieved but biogas production was enhanced. Control digesters produced 803 L of biogas and 483 L of CH4 while treated digesters produced 884 L of biogas and 539 L of CH4.