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Commercial Scale Results of the Microsludge Process to Liquefy Waste Activated Sludge: Waste to Energy Economics

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Abstract:

The key to exploiting the economic value of waste activated sludge is to efficiently convert it to biogas in an anaerobic digester. In contrast with the on-going expense of hauling and land application or land filling, or incineration of biosolids, digester gas can be converted to electricity and heat on site to displace otherwise purchased energy, providing a direct environmental, energy, and economic benefit to wastewater treatment plants.

Waste activated sludge (WAS) is comprised of billions upon billions of individual microbial cells that are inherently formed as a by-product of secondary wastewater treatment. The cell membranes of the microbial cells of WAS protect the cells and are tremendously resilient against external forces, including those exerted by the anaerobic bacteria that are resident in anaerobic digesters.

MicroSludge™ is a chemical and pressure pre-treatment process that profoundly changes both the rate and the extent that WAS is degraded in an anaerobic digester. MicroSludge uses alkaline pre-treatment to weaken cell membranes and 12,000 psi pressure to liquefy the cells. This enables an anaerobic digester to metabolize biosolids into biogas from which to generate electrical power and/or produce heat.

Each tonne of volatile solids destroyed in an anaerobic digester produces nominally 936 m3 of biogas. Biogas averages 64% methane and methane has an energy value of 37,300 kJ/m3. For typical WAS with a volatile solids content of 80%, the energy content of WAS is therefore potentially 4,965 kWh/dry tonne. Because MicroSludge enables an anaerobic digester to achieve 95% volatile solids destruction of WAS, 95% of the energy value of WAS or 4,720 kWh/dry tonne WAS is recovered.

The total recoverable net energy output is the sum of the electrical and heat energy from biogas conversion and the heat energy generated by high-pressure homogenization, minus the electrical energy for MicroSludge processing. At 35% electrical conversion efficiency, the electricity produced is 1,652 kWh/dry tonne of WAS minus 536 kWh/dry tonne of WAS for MicroSludge processing, or net 1,116 kWh/dry tonne of WAS. At 40% recovery of waste heat from biogas conversion to electricity, 1,890 kWh/dry tonne WAS is gained. Also, MicroSludge increases the temperature of WAS by 22 to 25 C°, resulting in an additional heat gain of 425 kWh/dry tonne of WAS.

The first commercial prototype MicroSludge plant was commissioned in January 2004 in Chilliwack (near Vancouver), British Columbia, Canada. The Chilliwack wastewater treatment plant (WWTP) serves a population of 70,000. During the January to November 2004 demonstration program, a single 4,000 litre per hour homogenizer operated 8 to 10 hours per day to process all of the thickened WAS (TWAS at 4 to 5% total solids) generated at this facility. Treated thickened WAS was then anaerobically digested at 37°C with primary solids at an HRT of 13 days. Volatile solids, volatile fatty acids (VFAs), alkalinity, BOD, COD, gas composition, electrical consumption, chemical consumption, pathogens, ammonia, TKN, and the dewaterability of digested solids were monitored throughout.

This commercial scale demonstration verified that MicroSludge greatly increases both the rate and the extent that WAS is degraded in a conventional mesophilic anaerobic digester (CMAD). An average volatile solids reduction (VSr) of 78% was achieved in a mesophilic anaerobic digester operating at an HRT of 13 days when treating a blended sludge (65% primary solids and 35% MicroSludge treated WAS, measured as dry solids). Assuming an average VSr of 65% in the primary sludge alone, the average VSr in the WAS alone was estimated to be greater than 90%. At 13 days HRT without MicroSludge, the total VSr averaged 60%, implying a 36% VSr of WAS alone. By way of comparison, the Water Environment Manual of Practice No. 8 estimates that VSr in a conventional mesophilic anaerobic digester (CMAD) at 15 days HRT is approximately 55% for primary solids only and 15% for WAS only.

Document Type: Research Article

DOI: https://doi.org/10.2175/193864705783977538

Publication date: 2005-01-01

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