The indirect drying method uses a heating medium (in this case, thermal fluid) to heat a metal surface to moderate temperatures. The biosolids are transported over the metal surface, and moisture is evaporated. Bio Gro uses a vertical multitray dryer capable of simultaneously drying and pelletizing the biosolids to avoid the need for additional mechanical pelletizing.
The heat drying process is simple: The biosolids first are dewatered in centri-fuges, then moved to a mixer that blends the material with previously dried pellets. The dried, recycled pellets are coated with dewatered cake material and conveyed to the indirect dryer units. The multiple hollow trays inside the vertical dryer are filled with MultiTherm IG-2 heat transfer oil from MultiTherm Corp., Colwyn, PA. Heated by a natural gas burner, the thermal fluid is circulated to the trays at 500°F (288°C), which then heat the biosolids to 220°F (104°C). An overhead-mounted rotating arm mixes and rakes the biosolids, further drying them and transferring them to lower trays until they reach the bottom of the dryer. A vibrating screen classifier separates the sizes of the pellets. Air and odor is controlled though a series of scrubbers. A completely closed-loop system prevents contamination of the fluid.
As a first-time user of thermal fluids, Bio Gro has not had a problem with MultiTherm IG-2. Walter Simmons, Bio Gro's operations manager, noted, "It's a completely closed-loop system. Nothing can get in to contaminate the fluid. Also, because the pump almost never shuts off and the fluid is always in motion, we don't have to worry about the problems typically related to multiple system startups and stops."
Thanks to the indirect drying system, the city of Baltimore has a reliable method of recycling its biosolids, Bio Grow produces a beneficial, profitable fertilizer, and area farmers and growers benefit from the product's organic and nutrient content.