How would you like to have all the money you needed and never have to work?
An equally intriguing question is: How would you like to have digesters that destroyed all the volatile solids and pathogens you wanted them to without costly additional tankage or renovation?
There are several technologies being offered that claim to improve anaerobic digestion by destroying the cell walls in waste activated sludge prior to digestion. One paper presented at the New York Water Environment Association Annual Conference described two technologies. The paper was titled; "Ultrasonic and Pulsed Power Technologies for Improving Anaerobic Digestion" by Al Saikkonen, PE, CDM and Kelly Saikkonen, CDM.
The benefits claimed by manufacturers include:
Increased gas production which can then be used to offset heating and energy costs,
• Increased volatile solids destruction which results in less dry tons of biosolids needing transport, disposal or recycling,
• Reduced filamentous bulking and foaming
• Improved dewatering
• Pathogen reduction
There are various types of technologies used to destroy cell walls (cell lysis) including but not limited to: Ultrasonic cavitation, electric pulse, chemical conditioning and mechanical sheer forces.
The Saikken paper examined ultrasound and electric pulse technologies. The full scale electric pulse technology demonstration showed a 37% increase in gas production and a 27% decrease in weight of dewatered biosolids.
In the electric pulse technology, pulses of high voltage electricity bombard waste as it flows between two electrodes. Two plants in Germany showed a 30% increase in gas production. However, a demonstration of the same technology in the US showed only minimal increase in gas production. The reasons the author's gave were a high ratio of primary to WAS in the US plant and insufficient ultrasound energy input for a large volume of solids.
Another paper reported the results of a research project by Christopher Muller; Brown and Caldwell, and John Novac; Virginia Tech Dept. of Civil and Environmental Engineering, which was partially funded by the Mid Atlantic Biosolids Association. This research collected data on performance of ultrasonic and mechanical shear technologies. The data showed that results were extremely variable for both technologies. Muller and Novac conducted an in depth investigation into floc structure, protein-sugar ratios, the impact of iron and aluminum on biodegradation and came to the following conclusion.
“The selection and application of any disintegration technology is going to be sludge specific. Digested sludges rich in iron and poor in aluminum are best suited for mechanical disintegration, in which it is thought that particle size is reduced and surface area is increased allowing for additional chemical iron reduction, organics release and areas for enzymatic attack and microbial adhesion. Unlike mechanical shear, ultrasonic disintegration is suited for sludge poor or devoid of trivalent metal. The recoagulation of released proteins by aluminum hydroxides and the potential for attenuation of ultrasonic irradiation by ferrous sulfides in the floc matrix results in a reduction in cavitation efficiency and thus disintegration. Thus, when selecting a wet sludge disintegration technology to improve digester performance some knowledge of floc composition and structure will help identify which facilities are best suited for the technology under consideration. ”
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