DON'T WASTE THE HEAT - USING ADVANCED DIGESTION (TPAD) WITH HEAT RECOVERY TO PRODUCE CLASS A BIOSOLIDS AT THE NINE SPRINGS WWTP

INTRODUCTION Construction is nearing completion in Madison, Wisconsin for improvements that will convert conventional digestion facilities to sequential batch Temperature Phased Anaerobic Digestion (TPAD). The upgraded facilities are the culmination of facilities planning initiated in 1998 by the Madison Metropolitan Sewerage District for its 50-mgd Nine Springs Wastewater Treatment Plant (NSWWTP). At that time, digester loadings exceeded design capacity and digester foaming was a recurrent problem during wintertime operation. In addressing the capacity and foaming problems being experienced, the District concluded that it would be prudent to design the digester improvements to enable production of Class A biosolids, significantly diversifying their beneficial reuse program. Based on bench-scale studies conducted at the Nine Springs laboratory by Reusser et al. (2002), the District concluded that TPAD would be the most suitable process alternative for future digester operation. Subsequent lab-scale studies at Iowa State University, sponsored by Black & Veatch, confirmed the stability and performance of a sequential-batch TPAD system (Santha et al.; 2003). Operation in the sequential-batch mode (withdrawing a portion of the digester contents, refilling the digester, and holding the contents for up to 24 hours without further feeding at temperatures of approximately 131°F) appeared to be a workable solution for meeting Class A standards at the NSWWTP.This paper will discuss the design challenges associated with converting this large-scale digestion facility to comply with the requirements for Class A pathogen reduction, and the lessons learned in completing the design. The paper should be of interest to wastewater utilities that are contemplating upgrading their digestion facilities to produce Class A biosolids.The conversion of the existing mesophilic digestion process to a TPAD system consisted of digesters at thermophilic temperatures (˜131°F) in the first stage, followed by a second stage of digestion at mesophilic temperature (95°F). The challenging design aspects related to converting the existing facilities to a TPAD design focused primarily on process heating and heat recovery, digester feeding, and digester mixing.