Stephenson R, Hoy P and Price K
Paradigm Environmental Technologies


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 key to exploiting the maximum economic, energy, and environmental value of WAS is to efficiently convert it to biogas in an anaerobic digester. In contrast with the on-going expense of hauling and land applying or land filling or incinerating residual solids, digester gas can be converted to electricity and heat on site to displace otherwise purchased energy. This provides direct environmental, energy, and economic benefits to wastewater treatment plants (WWTPs).

With conventional mesophilic anaerobic digestion (CMAD), anaerobic digesters deliver only a small fraction of the potential volatile solids reduction (VSr) during practical residence times. Under-performing digesters produce relatively small amounts of methane and large quantities of sludge that require trucking and disposal. Only partially digested sludge generates the odours that are the lightning rod for public opposition to sludge management. Poor digester performance results in high capital and operating costs and contributes to the public’s growing concerns regarding negative environmental impacts and risk to public health posed by sludge residuals.

If anaerobic digestion of WAS could be made to be effective, three main benefits could be realized: (1) Solids destruction would be increased, lowering the quantity of residuals for disposal. (2) Biogas production would be increased for conversion to electricity and/or heat. (3) Greenhouse gas (GHG) emissions would be reduced due to fewer residuals that would degrade and emit global warming gases. However, the microbial cells of WAS have tremendously resilient cell membranes that protect the cells against external forces. Consequently, a pre-treatment process that disrupts these membranes to lyse the cells is needed to enable anaerobic digesters to efficiently convert WAS microbes into biogas.

By first efficiently extracting the maximum biogas potential, cell lysis would enable anaerobic digesters to extract the highest value from WAS. Correspondingly, the quantity of residuals would be minimized, and therefore the costs for drying, land application, land filling or incineration would be the lowest possible.

Various technologies are available to pre-treat sludge prior anaerobic digestion. The net economic benefit of each cell lysis technology depends on the following factors: (1) the volatile solids reduction in the digester, (2) the cost of residuals disposal, (3) the polymer requirements and dewaterability of digested residuals (4) the efficiency of the biogas-to-energy recovery system, (5) the purchase price of electricity, (6) the value of the electrical and heat energy recovered, and (7) the cost to build and operate anaerobic digesters, dewatering equipment, and equipment to convert biogas to energy.

KEYWORDS Anaerobic digestion, biogas energy, biosolids, cell disruption, cell lysis, greenhouse gas mitigation, SAS, volatile solids reduction, WAS, waste activated sludge

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