The city of Baltimore contracted with Wheelabrator Water Technologies Inc. in 1995 to design, construct and operate a biosolids heat drying facility at the 70 mgd Patapsco Wastewater Treatment Plant. The drying process produces a product which is beneficially used as fertilizer.
Biosolids entering the Patapsco Pelletizer Facility are dewatered by use of centrifuges and conveyed to a mixer where previously dried recycled pellets are coated with dewatered cake material and conveyed to a rotary drum dryer.
In the dryer, air, which is heated in the heat exchanger, dries the wet pellets and evaporates water. After the air-solids separation, the drying air is divided into two gas streams. One stream is scrubbed and recycled back to the air heater?s combustion burner before exiting the stack. The other is recycled back to the dryer after being reheated in the heat exchanger.
Air vented from process equipment (conveyors, hoppers, screens, etc.) is cleaned in a separate fabric filter and also ducted to the air preheater?s combustion zone for thermal treatment. The pellets leave the system and pass through a vibratory screen which separates the pellets by size.
The system is designed to provide:
- Energy efficiency
- Separation of combustion products from biosolids and drying air
- Reduction of oxygen content which reduces fire potential
- Air emission and odor control
- Minimal environmental impact.
The resultant pelletized fertilizer product is clean, easy to handle and store, and can be sold as a fertilizer, soil conditioner or alternate fuel product.
As part of the contract, Bio Gro receives two kinds of fees ? a base tipping fee and a service tip fee, according to Karl von Lindenberg, Plant Manager of the Patapsco and Back River Plants.
The base tipping fee, also called a monthly capital recovery fee, covers the capital cost of the project amortized over the term of the service agreement, which is 20 years. The service tip covers operation and maintenance costs, and profit.
The service fee is based on the dry tons that the pelletizing facility produces. According to the fee structure, the more thoroughly the biosolids are dewatered, the more the facility benefits.
The pellets are sold primarily to brokers and bulk blenders of commercial fertilizers for sale to farmers as a source of slow-release organic nitrogen. The nutrient content is 5 percent nitrogen, 3 percent phosphorus, 0 percent potash. Trace nutrients also are present.
In addition to meeting EPA?s Class A criteria under Part 503 Regulations, the Baltimore pellets meet quality standards which allow their sale without restriction for use on all crops in most states.
About the odor control system:
Patapsco?s pelletizing process is carried out in an enclosed building to control noise and odor. The air quality and odor control process is a high-efficiency dust removal system comprised of a mechanical cyclone, fabric filter, and wet condenser. No drying air is released to the atmosphere without passing through high-temperature zones, resulting in thermal destruction of odorous compounds. Full recirculation and reheating of drying air results in high thermal efficiency and reduction in emissions. A dedicated wet scrubber controls emissions from the factory building.
Patapsco facility at a glance:
Bio Gro Division owns and operates the facility under the terms of the 20-year contract. It has been in commercial operation since July 1997. It operates 24 hours per day and seven days per week, processing up to 110 dry tons per day. The system can receive and process liquid and semi-liquid forms of biosolids. The process is called Swiss-Combi/Direct Drying with Full Air Recirculation, organized into two independent process trains.
