News | October 22, 1999

Enviromega Selected to Assess New Anaerobic Digester Mixing Technology

Contact: Mr. Hugh Monteith

A primary anaerobic sludge digestion tank at a Western New York publicly owned treatment works (POTW) was selected as a demonstrate site for the vortex ring mixing technology. The potential benefits of the vortex ring mixer are better suspension of heavy settleable particles and light floating materials in the actively mixed zone of the digester, at substantially reduced energy requirements, relative to standard digester mixing equipment. Reduction of active mixing zone in a digester due to heavy solids and a floating scum layer can result in a poorly digested sludge, unstable digester operation, and reduced production of biogas for energy recovery.

The consortium of Canadian and American firms marketing the technology retained Enviromega Inc. to assess the mixing efficiency of the digestion tank, both before and after the installation of the vortex ring mixer. The evaluation is performed using non-intrusive hydraulic tracer procedures, which are based on sound chemical engineering principles.

The objective of the study was to characterize the mixing efficiency of the North Tonawanda digester, identifying any deviations from ideal mixing conditions if they existed. An initial test serves to establish a baseline condition for evaluating any change in the hydraulic mixing regime within the digester following retrofitting with the vortex ring mixing system. A second follow-up tracer test will demonstrate whether the vortex ring mixer has improved the digester mixing efficiency

A pulse injection of lithium chloride tracer was injected into a primary anaerobic digester at the plant site on April, 1999. Sampling of the digester effluent continued through June.

From the tracer washout curve, the actively mixed volume of the digester was calculated to be 80%, with lower and upper confidence limits of 75% and 86%, respectively. The calculated mean average retention time of the digester, based on the total concentration database and sludge pumping records, was 19 days. The overall mass recovery of the injected tracer was 81%.

The digester is currently being retrofitted with the new mixing technology. The confirmatory test to assess potential improvement in mixing performance will take place within 6 months to a year following installation of the vortex ring mixer.