A thermal screw steriliser treats bagged clinical waste by shredding it, then heating it indirectly on oil-heated screws so that the waste’s own moisture turns to steam and sterilises it. Unlike a steam autoclave, it dries the waste as it goes, so the treated material weighs less and costs less to transport and dispose of. A 750 kg/h design for a UK clinical waste facility used spare steam from an existing pyrolysis plant to provide the heat.
The challenge: sterilising “orange bag” waste without the weight
Orange-bag clinical waste is the portion of healthcare waste that does not have to be incinerated by law, but still must be made safe before disposal. A conventional autoclave sterilises it with injected steam. The waste leaves wet and heavy, and transport and disposal are charged by weight.
The facility in question already ran a pyrolysis plant and had a source of heat. The question was how to use that heat to sterilise and dry low-grade clinical waste in one step.
The approach: shred, heat on screws, filter the exhaust
Step by step
- Feed: a belt conveyor lifts bagged waste to a single-shaft rotary shredder. Shredding opens the bags, increases the surface area for heat transfer and destroys the waste’s structure so that it cannot be reused. The shredder is held under negative pressure so dust and pathogens do not escape.
- Sterilising chamber: shredded waste drops into a stainless steel chamber with two hollow thermal screws that move it along. The screws are heated by thermal oil. The chamber is insulated and discharges at low level. The unit was a reconditioned thermal screw conveyor.
- Heat source: thermal oil is heated primarily by steam from the existing pyrolysis plant. A second heater fired on light diesel provides back-up when steam is reduced or unavailable. A bypass lets operators choose the primary source and balance the system.
- Steam and exhaust: heat drives water out of the waste. The steam sterilises it, and a blower draws the vapour through the system, keeping the inlet under negative pressure. The vapour passes a coarse particulate filter, a HEPA filter and a stainless steel condenser.
The numbers behind it
| Item | Value |
|---|---|
| Capacity | 750 kg/h per line |
| Chamber temperature | 130°C |
| Pathogen destruction | 99.99% |
| Coarse filter | F8 classification |
| HEPA filter | H13, 99.997% at 0.3 micron |
| Safety standard | ATEX and the industry DSEAR code for waste management |
The design was a two-line system. One thermal oil heater could supply two sets of screws, so a second line could be added later at reduced cost.
Why it works
Drying is the point
A sterilised, dried waste is lighter than a sterilised, wet one. The weight saving is the main commercial argument for a thermal screw over an autoclave, and it relies on a free or cheap heat source. Where steam is already available, as it was here, the economics improve.
Containment
The negative-pressure design is the key safety feature. From shredder to blower, any leak draws air in and does not push contaminated air out. The HEPA filter is the last line of defence before release.
Fuel flexibility
A dual-heat design avoids shutting the steriliser down when the pyrolysis plant is off. It does mean carrying a diesel heater that is used only as back-up.
What the supplier did not include
As is normal for equipment supply, the proposal left a number of items to the client:
- condensate management after the condenser;
- a compactor, baler or other storage for the treated waste;
- site preparation, foundations, earthing and lightning protection;
- steam and diesel connections, including any diesel storage;
- the electrical supply to the control panel;
- analysis of treated waste, third-party approvals, and any safety integrity or DSEAR studies.
What to consider for your own project
- Look at your heat first. Spare steam, thermal oil or waste heat makes a thermal screw much more attractive.
- Plan the treated waste route. You still need a licensed outlet and a way to compact or bale the output.
- Prove the treatment. Validation testing of treated waste was outside the supplier’s scope and must be planned separately.
- Design for two lines. Planning for expansion early costs little.
- Check regulations. In England, the Environment Agency regulates waste treatment permits. DSEAR duties are explained by the Health and Safety Executive.
FAQ
How is a thermal screw steriliser different from an autoclave?
It heats waste indirectly on screws and uses the waste’s own water as steam, so it also dries the waste.
What happens to the steam?
It is drawn through particulate and HEPA filters and condensed. The condensate must be handled separately.
Why shred the waste first?
Shredding opens the bags, improves heat transfer and renders the waste unusable.
What if there is no steam available?
A diesel-fired thermal fluid heater can supply all the heat, or top it up.
Treating clinical waste on site?
Talk to our team about your waste volumes and available heat.