VERTAD™, AUTO-THERMOPHILIC AEROBIC DIGESTION: PILOT TEST RESULTS

VERTAD™ is an auto-thermophilic aerobic digestion (ATAD) process being evaluated by the Technology Assessment Program of the King County Wastewater Treatment Division, Seattle, WA. 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 225–900 dry kg solids per day (5000 population equivalent). Compressed air is introduced into the digester at depths of 46 m and 92 m to provide oxygen for biological degradation of organics and digester mixing via airlift circulation The relatively high pressure achieved in the oxidation zone (10 atm) facilitates high oxygen transfer efficiency (>50%). Digested solids pass through an unaerated plug-flow zone at the bottom of the reactor (confirmed by tracer studies) to insure compliance with Class A time and temperature requirements.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 were met at a 4 day detention time. The system readily achieved above 40% volatile solids destruction; organic nitrogen destruction was 45–50%; fats, oil and grease (FOG) destruction was over 80%. Pathogen destruction was excellent with fecal coliform and salmonella generally below detection limits. In addition, the VERTAD product was easily float thickened with dissolved CO2 to 8–12% solids. The thickened product was estimated to be able to achieve over 30% solids with relatively low polymer demand.Bench-scale anaerobic digestion of the VERTAD product was tested to evaluate the “dual-digestion” approach using VERTAD as the preliminary aerobic step. Total volatile destruction efficiency exceeded 63% with a 4-day SRT in VERTAD and an 11-day mesophilic anaerobic SRT. Dewaterability of the final product was estimated to achieve 24% cake solids with relatively low polymer demand.The results of this demonstration project provided the basis for development of full-scale design parameters and cost estimate. 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 potential significant savings in dewatering and haul cost of King County biosolids.