Autoclaving, also known as Mechanical Heat Treatment
Autoclaving is a pretreatment technology which, when normally offered, comprises the first stage of an MBT system. Autoclaving technology is one of the Waste Technologies which also falls under the category of MHT or Mechanical Heat Treatment.
Autoclave technology is a well established technology in the small scale which has been used to sterilise certain hospital type wastes (clinical waste) for many years and is essentially a steam treatment process. Some processes include mechanical action during the treatment to break open bags and break down the organic portion of the waste into into a fibrous material.
When this process is used to pretreat Municipal Solid waste the Waste (MSW) may also be shredded first. The MSW is processed in a pressurised sealed drum under the action of steam. After around an hour of processing the waste is reduced to a 'flock' like material, with metals and glass partially cleaned for extraction as recyclables
The process deforms plastics making them either more or less difficult to recycle, depending on the process and the polymer type. The remaining material may be sorted and the highly calorific fraction thermally treated as a type of Refuse Derived Fuel (RDF). Remaining material has also been proposed by some suppliers for use in a fibrous inert board for building use, although as is so often the case, such uses are impeded by the category of the material remaining as "waste" under the Waste Regulations.
There will typically be a residue for disposal from mixed MSW processing after Autoclaving, and as for other processes this will be sent to landfill.
MHT is often offered in systems where it is followed by anaerobic digestion (AD) or hydrolysis of the organic fraction. The autoclaving process removes a high proportion of the organic content, labels and wrapping of tins and plastic do tend also tend to be removed and enter the organic fraction.
A disadvantage is the heat and other energy used in pressurisation. Hence, the incorporation of AD into the MBT system with power generation is often proposed to ensure that the necessary heat (often CHP heat) and power for pressurisation is readily available on site.
An advantage of Autoclaving lies in hygiene. The system was originally devised for clinical waste, and largely sterilises the waste.
The fact that the low risk of injury from the hand sorting stage seen in most MBRs and MBT Plants, where materials are picked from a slowly moving conveyor, is largely avoided, and is replaced by mechanical equipment processing, after autoclaving. Thus the Health and Safety issues regarding hand picking from a moving conveyor in the presence of the waste materials are avoided.
An example of Autoclaving Technology in the ESTECH Plant which is still in negotiation (Autumn 2006) for Hereford's waste.