VERTAD™ – AUTO-THERMOPHILIC AEROBIC DIGESTION: DEMONSTRATION-SCALE TEST RESULTS

VERTAD™ is an auto-thermophilic aerobic digestion process, employing a subsurface vertical reactor to aerobically digest mixed primary and secondary wastewater treatment solids. High metabolic activity results in heat generation, which enables the production of Class A Biosolids at short solids retention times (SRT). Currently the Technology Assessment Program of the King County Wastewater Treatment Division, Seattle, WA, is evaluating the VERTAD™ process. A demonstration facility was constructed in 1998 and operated through 1999 at the County's South Treatment Plant in Renton, WA. The process consists of a deep vertical reactor (107 m deep, 50 cm diameter) with a capacity of 225 to 900 dry kg solids per day (5000 population equivalent). Successful results in demonstration-scale tests have prompted the County to consider VERTAD™ as a retrofit for existing facilities, and for future projects.The process was tested at detentions times from 2 to 6 days and temperatures of 56 to 67°C. The requirements for Class A Biosolids (40 CFR 503.32 Alternative 1) were met at an average detention time of 4 days and 60°C. With a 4 day solids detention time, the VERTAD™ reactor achieved greater than 40% volatile solids destruction; 45-50% organic nitrogen destruction; and fats, oil and grease (FOG) destruction greater than 80%. Pathogen destruction was excellent, with fecal coliform and salmonella generally below detection limits. An oxygen transfer efficiency (OTE) of 50% was attained easily in the system. In addition, the VERTAD™ product was simply float thickened with dissolved CO2 to 8-12% solids. The thickened product dewatered to over 30% solids with a relatively low polymer demand. Bench-scale anaerobic digestion of the VERTAD™ product resulted in 67% total volatile destruction with a 4 day SRT in VERTAD™ and an 11 day mesophilic anaerobic SRT.The results of this demonstration project provided the basis for development of full-scale design parameters and cost estimates. The present worth of capital and operating costs for a system using VERTAD™ as a pretreatment to anaerobic digestion was similar to that of traditional mesophilic digestion, primarily due to the significant operating cost savings with low energy and polymer requirements. Improved dewaterability lowered the associated haul and disposal costs of King County biosolids. The minimal footprint requirement for VERTAD™, and production of Class A Biosolids, make the technology an ideal retrofit for current Class B Biosolids facilities.