Improvement of Anaerobic Digester Performance by Wastewater Recirculation through an Aerated Membrane

Abstract

<abstract><title><italic>Abstract.</italic></title> Animal wastes are a source of nuisance odors and greenhouse gases. Swine wastewater from an anaerobic digester was recirculated through a permeable silicone hose located in an external aeration tank to determine if this affected wastewater composition, malodorants, or greenhouse gases. Treated wastewater had a higher pH than did control wastewater, but this could not be explained by differences in either ammonium or volatile fatty acid concentrations between control and treated digesters. The higher pH of treated wastewater resulted in higher-quality biogas, with carbon dioxide averaging 143,000 ppm in treated biogas and 224,000 ppm in control biogas. Methane averaged 677,000 ppm in control biogas and 697,000 ppm in treated biogas. The improvement in biogas quality was offset by a loss of biogas production: control digesters produced 490 L of biogas, and treated digesters produced 313 L of biogas. Improvements in wastewater malodorants were modest. For instance, peak cresol concentrations were approximately 67,000 ng L^-1 in treated wastewater and 150,000 ng L^-1 in control wastewater. In a second experiment, wastewater was recirculated through the silicone hose for only the first 23 d of the experiment to limit loss of CH₄ through the silicone membrane. No improvements in aromatic malodorants were achieved, but biogas production was enhanced. Control digesters produced 803 L of biogas, while treated digesters produced 884 L of biogas. After wastewater recirculation was turned off, the biogas produced by treated digesters was not of higher quality than that produced by control digesters. Results show that recirculation of wastewater through a permeable membrane can be used to manipulate wastewater composition and the amount and composition of greenhouse gases.