Waste autoclave

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A waste autoclave is a form of solid waste treatment that utilises heat, steam and pressure of an industrial autoclave in the processing of waste. Waste autoclaves process waste in batches. Saturated steam is pumped into the autoclave at temperatures around 160°C ^[1]. The pressure in the vessel is maintained at 5 bar gauge for a period of up to 45 minutes to allow the process to fully 'cook' the waste. The autoclave process gives a very high pathogen and virus kill.

[edit] Process results

The 'cooking' process causes plastics to soften and flatten, paper and other fibrous material to disintegrate into a fibrous mass, bottles and metal objects to be cleaned, and labels etc. to be removed. The process reduces the volume of the waste by ~60%. After 'cooking', the steam flow is stopped and the pressure vented via a condenser. When depressurised, the autoclave door is opened, and by rotating the drum the 'cooked' material can be discharged and separated by a series of screens and recovery systems.

In early systems, the primary product was cellulose fibres. This comprises the putrescible, cellulose and lignin elements of the waste stream. The biodegradability of the waste has not been affected by the autoclave and so must undergo further treatment to reduce its reactivity prior to landfilling. The fibres can be fed into anaerobic digesters to reduce the biodegradability of the waste and to produce biogas. Alternatively the fibre could be used as biofuel.

Newer technology systems wash out hydrolysed hemicellulose sugars and most of the protein as water-solubles. The remaining materials, after simple physical separation (trommel screen) has several valuable uses. One newer system is able to dry the cellulose during processing using heat, and another newer system is able to dry the cellulose (much more economically) using pressure and steam kinetics.

After fibre separation, the secondary streams comprise of mixed plastics, which have normally been softened and deformed which eases separation, a glass and aggregate stream, which can be exceptionally clean of both plastic and paper, and separate ferrous and non ferrous metals. The heat and steam and rotating action of the autoclave vessel strip off labels and glues from food cans leaving a very high quality ferrous/non-ferrous stream for recycling.

With the removal of water, fibre, metals, and much of the plastics, the residual waste stream for disposal may be less than 10% by weight of the original stream, and is essentially devoid of materials that decompose to produce methane. Systems in Europe meet and exceed all of the European waste treatment and recycling requirements.

The full process of loading, treatment and sorting is normally completed within 90 minutes in earlier models, and with the advent of newer technology, the cycle time has been decreased to one hour. In a typical "new" configuration, 2 10-ton units operating side by side would treat over 400 tons per day with time for preventative maintenance.

The size of the vessel varies between vendors. Experience shows that "small" vessels are not productive enough; while if the vessel is too large, the pressures in the vessel and the heavy weight of the vessel can cause equipment failures.