WPCA-SOLIDS DRYING PROJECT In 2006, The Stamford Water Pollution Control Authority completed an upgrade and expansion of the wastewater treatment plant to a 24 million gallon per day (MGD) average daily flow (30-MGD peak daily flow) biological nitrogen removal facility. That project was completed on time and on budget and cost $105 million. The plant is performing very well and is the largest seller of nitrogen credits in Connecticut. All wastewater treatment plants produce residuals as part of the process. The cost of disposing of these residuals is expensive and carries a high environmental risk if not done correctly. Until recently, the WPCA hauled the residuals long distances to an out of state landfill or to an incinerator. In 2005, the WPCA received approval to construct a solids drying facility at the WPCA site.
Thermal drying of wastewater solids has been used in the United States since the 1930s, when the first heat dryer was installed in Milwaukee, marketed under the name of Milorganite R and sold throughout the United States as a soil conditioner.
Thermal drying is a process that evaporates water from solids. The technology was used originally in the materials and chemical industries and then adapted and perfected for wastewater solids. The key process components of the direct thermal drying system include a heat supply (gas burner), rotary dryer, recycle bin, air/solids separator, screen, and RTO (regenerative thermal oxidizer odor control system).
The thermal drum drying process (Figure 1) is based on the evaporation of moisture through direct contact of material with a stream of hot air/exhaust. The hot air required for the heat drying process is produced in a furnace, which is typically gas-fired. The furnace produces a stream of hot air at an approximate temperature of 800 degrees F. Dewatered sludge is mixed with recycled dry product to create approximately 60% dry solids. Solids and hot air pass through an air/solids separator, where solid particles fall out and separate from the air stream in a two-step separation process.
Dry pellets are separated into oversize, product, and fine size fractions on a vibrating screen. Product size pellets (typically 1 to 4 mm diameter) (Figure 2) are cooled in a water-based plate cooler and then pneumatically conveyed to storage silos. Large size pellets are crushed, combined with fine size pellets and passed through the recycle bin into the mixer for blending with dewatered cake. Most of the exhaust air is recycled to the drying drum, while approximately 10% of the flow is treated in the air emissions control system and discharged to the atmosphere. A regenerative thermal oxidizer is used for odor control.
Figure 1 Solids Drying Process Schematic
Thermal drying delivers a product with significant volume and mass reduction of the sludge, pathogen destruction, and vector attraction reduction. Since Stamford is a commercial city rather than industrial, the metal concentrations in the sludge are low and meet the limits for Exceptional Quality Biosolids in 40 CFR Part 503. Additionally, the thermal drying process meets the time and temperature requirements stipulated by Part 503, which means the dried sludge would be classified as Class A biosolids with respect to pathogen reduction requirements. Furthermore, by producing a dried sludge with over 90 percent solids content, the vector attraction requirements would be met.
Figure 2 Pellets Produced in Drying System
The dried pellets are used as a fertilizer supplement or soil conditioner in products that are distributed and marketed for horticultural purposes. The value of such products is usually based upon the nitrogen content of the product. The expected nitrogen content of a pellet produced from Stamford’s sludge is 4 to 5 percent. The pellets typically are land applied for agricultural purposes, used on golf courses and athletic fields, or on citrus fruit groves. The Stamford pellets will initially go to the fertilizer market but by the end of 2009 will be used to generate electricity. Currently the WPCA is conducting a Department of Energy research project leading to the use of this material as a renewable energy source.
The project is housed in the abandoned 1958 Incinerator, which was renovated as part of this project. Figure 3 shows the outside of the renovated building. The structures include storage silos, truck loading system and RTO odor control.
The inside of the building contains a control room and all processing equipment including the dryer (Figure 4) and the wet cake bin (Figure 5). The project was competitively procured as a design/build/operate delivery method and has been completed from design to operation in less than two years. The project is funded by WPCA Revenue Bonds and with a $500,000 grant from US-EPA. The process is on line in the testing stage and is producing dried pellets on a daily basis. This is the first dryer to be installed in the State of Connecticut and is considered State of the Art technology.
Figure 3 Solids Drying Facility-Northwest Elevation
Figure 4 Solids Drying Unit
