Four Different Pathways to Advanced Biosolids Stabilization Assessing Drivers and Results To-Date

Abstract

With increased public scrutiny and growing local regulatory resistance to land application of Class B biosolids, many utilities are raising questions about the continued reliance on this practice. Consequently, they have started exploring options for enhancing the performance of existing solids treatment facilities to achieve the highest level of stabilization, Class A. With composting losing much of the luster it held in the 1980s, much of the focus for achieving Class A stabilization in the U.S. has been on advanced digestion, thermal drying or chemical stabilization. Treatment facilities, that have traditionally used mesophilic anaerobic digestion for solids stabilization, have shown greater interest in achieving Class A pathogen reduction within the digestion process by incorporating high temperature schemes and through creative operational control of the thermophilic stage of digestion. Facilities seeking a more attractive product for distribution and marketing prefer further post-digestion processing such as heat drying to meet the Class A standards. In contrast to the U.S. practice, European counterparts have embraced more innovative technologies, including pre-pasteurization and thermal hydrolysis of digester feed to meet enhanced treatment standards. This poster will focus on the drivers, the key design elements and the results to-date from four projects in the U.S. and Europe, operating different processes aimed at meeting Class A performance standards. TEMPERATURE-PHASED ANAEROBIC DIGESTION (TPAD)Madison Metropolitan Sewerage District (MMSD) completed improvements to its digester complex for operation in the sequential-batch temperature-phased mode. The conventional mesophilic digesters at the Nine Springs Wastewater Treatment Plant (WWTP) WI were capacity limited and were operating at a nominal 14-day solids retention time (SRT). The capacity issues were further accentuated by recurrent foaming episodes in the gas-mixed digesters during the winter months. There was no immediate need for the Madison to upgrade its biosolids to Class A standards, but the uncertainty shrouding the practice of land application in many parts of the country made it prudent to ensure Class A compliance with the capacityrelated digester improvements.In developing an advanced digestion scheme for Madison, the focus was on achieving timetemperature conditions specified in Part 503 and, thereby, satisfying the requirements for Class A pathogen reduction by "definition" under Alternative 1. This called for isolating small batches of feed to ensure sufficient retention time for satisfying Part 503 operational requirements. THERMAL DRYINGNot all utilities are convinced that merely enhancing pathogen reduction will be sufficient to ensure a robust biosolids management program. They believe that the treatment method must 2090 WEFTEC®.06