Rev. Feb 1999
New methods for the disposal of medical waste are evolving, and Vincent is in the thick of it.
Historically infectious biological waste has been disposed of by incineration. It is expensive to build an incinerator as well as to operate one. Another factor that mitigates against incineration is the amount of attention received from environmental regulators.
Although the waste has some BTU value, fuel is required to maintain the high temperatures needed to assure complete incineration. There are many materials in the waste that contain chlorine, everything from antiseptics to plastic scissors and syringes. When these burn hydrochloric acid is generated and must be removed in a scrubber. Up to 500 hp worth of fans are needed to overcome the pressure drop through the scrubber. The acid is corrosive to all of the equipment in the system, and a considerable amount of caustic is used to neutralize it.
Using gas or oil, the cost of incineration is 14 to 22 cents per pound. An alternative, burning with plasma, is even more expensive.
Sterile Technologies, an Indiana firm, has developed an alternative to incineration. Their first installation was sold to Bridgeview, Inc., a processor of medical waste with multiple treatment facilities. It is handling 1,500 pounds per hour of medical waste.
The system starts with a ram to force boxes of "red bag" waste through two stages of low speed shredders, the second of which has 3/4" teeth. Before and during the shredding process, the waste is doused with a solution of water and Sodium Hypochlorite (NaOCl).
Next the shredded and chemically treated waste falls into the hopper of a Vincent CP-10 press.
The effluent that squeezes out through the press screen is filtered, recharged with enough NaOCl to maintain a minimum 1000 ppm concentration, and used again.
The dewatered waste drops from the press into a Steam Clave (continuous autoclave) manufactured by Vincent. The Steam Clave is a 24' long, totally enclosed and insulated, elevating screw conveyor. There are steam injecting nozzles all along the conveyor and thermocouple ports at the beginning and end. Enough steam is introduced to maintain a temperature between 200 and 210º F. The screw turns slow enough that the resident time in the conveyor is about 30 minutes.
By the time the waste exits the Steam Clave into the back of a truck, it has been rendered inert and ready to be taken to a landfill.
The cost to process infectious waste using this system is expected to be less than 10 cents per pound.
The two main problems we encountered were with the press.
First, the medical waste dewatered so easily and quickly that it bound inside the press. This was countered by using a specially designed screw to "open up" the press. This design configuration was named the "Sterile Screw". It has become our standard in other applications that involve pressing "high freeness" materials.
The other problem was abrasion. The glass containers and hypodermic needles wore down the screw and screens at a surprising rate. This was controlled by using harder, more abrasion resistant materials and by opening up the screw-to- screen clearance. (Ceramic coatings failed in short order due to the flexing of the screw flights.)
Sterile Technologies planed to market the system nationwide, but they met with little success. A Vincent press was to be tested by another firm in Virginia that has a smaller scale batch system for use on-site at the hospital. It also failed to proceed, due to a lack of funding.
Overall we have been disappointed by the medical waste market. It appears that excess disposal capacity was put in place in anticipation of environmental regulations.