Replacement of the Multiple Hearth Furnaces by the Fluid Bed Furnaces - The R.L. Sutton WRF Experience

The Cobb County Water System Authority operates a wastewater treatment facility at the R.L. Sutton Water Reclamation Facility (WRF) in Smyrna, Georgia and has been using Multiple Hearth thermal oxidation as their disposal option for the last twenty six years. The plant serves Smyrna, Marietta and the surrounding areas. It has a design capacity of 60 MGD (40 MGD actually permitted) on an annual average. To accommodate expansion, the plant has experienced many improvements throughout the years, starting with aerobic digestion and drying beds (1971), anaerobic digestion and Multiple Hearth Furnaces (MHF, 1982) and today with primary and secondary treatments and Fluid Bed Furnaces (FBF). Today, it incorporates advanced activated sludge treatment with ammonium nitrogen removal (less than .5 mg/liter) and UV for disinfection. Sludge dewatering is via centrifuge decanter to about 28% dry solids and the incinerator combusts a 60/40 mix of primary and activated sludge. Sludge is primarily municipal with less than 5% from industrial sources.Thermal oxidation has been used as the sludge disposal method since 1982 when the first MHF was built to dispose of digested sludge. In 1991, digestion was eliminated, and a second MHF was installed to accommodate the increase in sludge loading. In 2003, the Authority conducted a Facilities Planning Study examining future disposal options. This study showed thermal oxidation to continue to be the recommended option, based on economics and other factors. A modern thermal oxidation system using two FBFs, 53 dry tons per day, each was recommended and became operational in March 2008.Both FBF units are of type hot-windbox with refractory arch air distributor and refractory-lined wind-box. External shell and tube heat exchangers are used for combustion/fluidizing air preheat to minimize auxiliary fuel use and venturi scrubbers/tray towers as air pollution control devices (APCD).The R.L. Sutton sludge facility was designed with provision of importing sludge from surrounding waste water plants. In waiting for the importing sludge, R.L. Sutton is actually running one FBF at a time. Normal operation of the system is 16 hours per day, 7 days per week. After the daily shutdown, it took 20 minutes every day to restart on sludge. The new units have shown flexibility of operation and perfectly suitable to intermittent operation. The plant is using olivine sand and has not experienced any noticeable sand loss since startup. The plant is running at capacity, with minimal auxiliary fuel during normal operating condition.Both units satisfied their emission requirements by a wide margin upon initial performance testing.A detailed comparison is provided of operational experience with the differences between the new FBF systems and the old MHF systems. The text provides the performance test results of both the new FBF and the old MHF units.The successful operation at the R.L. Sutton WRF has shown the modern fluid bed thermal oxidizer system to be an economical, environmentally acceptable sludge disposal method satisfactory to the Authority, the permitting agencies and the general public.