DESIGN FOR CONVERSION OF DIGESTERS AT MADISON (WI) MSD TO ENABLE SEQUENTIAL BATCH TPAD

In 1998, the Madison Metropolitan Sewerage District (MMSD) initiated facility planning for the 10th addition to the 50 MGD Nine Springs Wastewater Treatment Plant (NSWWTP). The modifications comprised of several upgrades including a new headworks facility, modifications to the existing digestion process, and construction of new biosolids processing facilities, revisions to energy utilization and energy recovery facilities and other miscellaneous improvements. The anaerobic digestion facilities at the plant were included in the evaluation because the digester loadings were exceeding design values, resulting in a nominal 14-day solids retention time. The digester capacity related issues were being further accentuated by recurrent foaming problems in the gas-mixed digesters.Though not mandated, it was thought prudent to ensure Class A compliance with the capacityrelated digester improvements as increased public scrutiny and growing resistance to land application of Class B biosolids in many parts of the country have raised questions about the continued reliance on this practice. Temperature-Phased Anaerobic Digestion (TPAD) and Acid/Methane phased digestion emerged as the leading candidates, as research studies have indicated the potential of these processes to reduce pathogens. Laboratory studies conducted at the Nine Springs laboratory by Reusser et al. (2001) found that TPAD was capable of higher volatile solids (VS) destruction with total detention times of 10 and 12.5 days than any of the other systems at 15 to 20 days. But MMSD recognized that it would need to operate the thermophilic stage of TPAD in a batch, sequential batch, or similar mode to ensure that “every particler” has been exposed to conditions stipulated under CFR Part 503 Regulations for Class A operational compliance. Operation in a sequential-batch mode appeared to be a workable solution for meeting the time-temperature requirements for Class A pathogen reduction at NSWWTP. Lab-scale studies at Iowa State University sponsored by MMSD's design consultant, Black & Veatch, confirmed the stability and performance of the sequencing-batch TPAD scheme.With the critical process questions answered, the next task was working out the design details and completing plans and specifications for bidding the 10th Addition project.