Taking Waste Out of WAS: Sludge Pretreatment for Beneficial Uses
- Matthew Van Horne, Mark Bottin, Ya-Chi Tsao, and James Grandstaff - Hazen and Sawyer
The waste activated sludge (WAS) from biological treatment processes at wastewater treatment plants (WWTPs) has typically been regarded as a waste product and can require substantial processing to efficiently dispose of this material. With the emerging view of WWTPs as resource recovery facilities and a continuing desire to optimize their operations, alternative uses for WAS are generating more interest. Pretreatment of WAS can be implemented at wastewater treatment plants to achieve a variety of goals. Typically these goals are to increase volatile solids destruction, increase digester gas production or to generate a supplemental carbon source to fuel the denitrification process.
This paper will review two pilot testing case studies of a pulsed electric field pretreatment technology at two treatment facilities, each with a different goal. The first case study is with the Philadelphia Water Department at the Southwest Wastewater Treatment Plant. This is a 194 mgd facility with an interest to generate more digester gas that would be beneficially used to offset operational costs at their sludge drying facility as well as a reduction in sludge to be subsequently dewatered, dried and receive final disposal. Two digesters of the twelve existing digesters are being isolated to receive a blend of primary sludge and pre-treated WAS to allow the unit gas generation resulting from the pretreatment to be compared to the gas generation under the baseline conditions. Ultimately, an economic analysis will be performed to determine if the pretreatment technology generates sufficient benefits to justify the costs of its installation and operation.
The second case study is at the Henrico County (Virginia) Water Reclamation Facility. Strict effluent discharge requirements for this facility related to Chesapeake Bay restoration efforts require denitrification. For the specific characteristics of the wastewater at this facility, this process requires a supplemental carbon source to drive the denitrification reaction. Current operations utilize a glycerin product for this application but there is a potential to use the same WAS pretreatment technology on thickened and un-digested WAS to release bioavailable carbon from the WAS and introduce it to the anoxic zones of the biological treatment process to replace this significant chemical addition and potentially result in a cost savings. This potential benefit has to be weighed against the potential negative impacts. This application has the potential for significant process impacts if not implemented correctly due to the return of additional solids to the biological treatment process and the possibility to increase the ammonia loading to the treatment facility. The pilot testing monitoring plan includes an assessment of these specific considerations to ensure that there are no adverse impacts on the final effluent from the facility.
These two pilot testing studies represent significant potential for wastewater treatment plants to optimize their operations and reduce overall costs by maximizing their use of materials generated through the treatment process. The available pilot testing results will be presented and will be used to demonstrate if the technical and economic conditions should be considered for other projects.
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