The rainy winter months in Forest City, North Carolina, posed a major challenge for the city’s wastewater treatment plant. In addition to treating wastewater, the city also recycled and stored biosolids — a byproduct of the wastewater treatment process.
After treating the biosolids, the city worked with a local contractor to haul the material to neighboring farms where it was spread on pastures and used as fertilizer at no cost to the farmers. However, during the rainy season the material could not be shipped away and the city was forced to store the biosolids for weeks on end.
The city had limited space to hold the material for extended periods of time. To make matters worse, at times the fecal coliform counts found in the biosolids continued to increase the longer the material was stored, exceeding the limits set by the U.S. Environmental Protection Agency for Class B biosolids.
Faced with limited storage space and a biosolids product that might not meet Class B certification, the city needed an alternative method to produce and store its biosolids.
In May 1997, Forest City turned to USFilter to help eliminate its biosolids storage problem. USFilter supplied the city with its Dragon Dryer™ Indirect Rotating Chamber (IRC) — an automated heat drying system that produces Class A biosolids. The Dragon Dryer, engineered by USFilter’s Davis Products of Thomasville, Georgia, safely dries biosolids and decreases the volume they occupy by 75 percent or more.
In the past decade, heat drying has become widely used because it minimizes the sheer volume of biosolids, eliminates harmful bacteria, and produces a higher quality product.
Previously, Forest City strove to produce Class B biosolids, which have less stringent standards for treatment and contain various amounts of pathogens. The pathogens found in Class B material are generally reduced and diluted once the material is spread and exposed to natural sunlight, wind and soil microbes. For the city to achieve Class A biosolids certification, it would have to incorporate treatment that included either heating or composting to remove bacteria from the biosolids to legislative limits before the material left the facility.
Since the Dragon Dryer operates at temperatures at or near 212 degrees Fahrenheit, the high heat kills pathogens found in the biosolids during the drying process. Consequently, the Dragon Dryer produces only Class A biosolids, which meet and exceed the EPA’s standards for pathogen free material and vector attraction.
The single-pass dryer produces biosolids in granule sizes perfect for use as a fertilizer and capable of being stored in the facility’s 6,000 cubic foot storage silo. Having found such a comparable solution to its biosolids problems, the Forest City wastewater treatment plant was well on its way to becoming a more efficient facility.
Prior to installation of the dryer, the city was storing its biosolids in roll-off containers all over its property. Today, the city can safely store 30 days worth of processed biosolids in one silo for up to four weeks without removal. The Dragon Dryer has the capacity to dry up to 50 wet tons of biosolids per 24 hours.
With the Dragon Dryer, the facility now is producing Class A biosolids, enabling the city to distribute the material for use as a fertilizer for agriculture and residential lawns or gardens. The system was designed so farmers can drive to the plant, place their truck under the silo, and load the dried biosolids for a $20 fee. Prior to the installation of the system, the plant had to hire contractors to deliver the wet biosolids during the fair weather days of winter, which were few and far between.
The stand-alone Dragon Dryer system comes ready to use. Once installed, wet biosolids are conveyed to the dryer and enter a rotating dehydration tube. The tube is heated on the outside by three natural-gas-fired burners — never putting the material in direct contact with the heat source. The counter-current rotation of the internal auger versus the external tube creates extra agitation of the material and ultimately more efficient heat drying.
While the biosolids pass through the drying stage, temperatures reach 212 degrees Fahrenheit for a retention time of approximately 2-3 hours. These temperatures quickly kill pathogens in the biosolids. The retention time exceeds the requirement for vector attraction reduction. Finally, the dried biosolids emerge in the silo in the shape of 1 to 4 mm pellets.
The system is environmentally safe, isolating the drying chamber from combustion gases to minimize air emissions, while clean-burning, high-efficiency gas burners meet all federal and state air-quality regulations.