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The rate-limiting step for anaerobic digestion of waste activated sludge is the destruction of the cell membrane of each microbe. Anaerobic digestion of WAS is both slow and incomplete because the individual cell membranes are not significantly degraded in conventional mesophilic (35 to 37°C) anaerobic digesters that rely on enzymes to promote cell lysis. Consequently, anaerobic digesters deliver only a fraction of the potential cell destruction during practical residence times. This leads to high capital and operating costs, and contributes to the public's growing concern regarding odours, negative environmental impacts, and public health of the undigested residuals.

MICROSLUDGE™ is a chemical and pressure pretreatment process that significantly changes both the rate and the extent that waste activated sludge (WAS) is degraded in an anaerobic digester. MICROSLUDGE uses alkaline pretreatment to weaken cell membranes and an industrial scale homogenizer to provide an enormous and sudden pressure change to burst the microbial cells. The resulting liquefied WAS is readily converted to biogas in a conventional mesophilic anaerobic digester.

The heart of the process is an industrial scale homogenizer that provides a large and abrupt pressure drop. At 12,000 psi (82,700 kPa), WAS in the cell disruption homogenizing valve is accelerated up to 1,000 feet per second (305 meters per second, nominally the speed of sound) in about 2 microseconds. This high velocity flow causes enormous fluid shear and cavitation, and then impinges on an impact ring, disrupting the cell membranes and producing a liquefied WAS homogenate.

The first full-scale prototype MICROSLUDGE plant was commissioned in January 2004 in Chilliwack (near Vancouver), British Columbia, Canada. The Chilliwack WWTP serves a population of approximately 70,000, and a single 4,000 litre per hour homogenizer operated approximately 12 hours per day to process all of the thickened WAS (TWAS) generated at this facility.

The full-scale demonstration clearly demonstrated that MICROSLUDGE pretreatment of waste secondary sludge significantly increases both the rate and the extent that thickened WAS is degraded in a conventional mesophilic anaerobic digester. In a 65:35 primary: secondary sludge mix at an HRT of 13 days, volatile solids reductions of up to 90% were achieved, averaging 78% volatile solids reduction. This implies a volatile solids reduction of greater than 90% for secondary sludge alone. When conventionally treating the same mix of primary sludge and untreated waste activated sludge under similar operating conditions, the anaerobic digesters at the Chilliwack WWTP achieved an average volatile solids reduction of 60%.

MICROSLUDGE requires considerable energy to liquefy the WAS as a result of the extremely high pressure (82,700 kPa, or 12,000 psi) in the homogenizer. Assuming a unit power cost of US0.05/kWh, the power cost of homogenization is in the order of US38 per dry tonne of TWAS processed.

The energy input with MICROSLUDGE results in significantly greater solids conversion to biogas than in a conventional anaerobic digester. The total energy output is the sum of the electrical energy and heat energy generated from biogas conversion, and the heat energy due to high-pressure homogenization. Overall, approximately net 915 kWh/dry tonne WAS of electricity and 2,075 kWh/dry tonne WAS of heat can be recovered by anaerobic digestion with MICROSLUDGE pretreatment. The total value of the net energy from MICROSLUDGE and anaerobic digestion is in the order of US128 per dry tonne of WAS processed.
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Document Type: Research Article

Publication date: 2005-01-01

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