Waste autoclave

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 either in batches or in continuous-flow processes. In batch processes, saturated steam is pumped into the autoclave at temperatures around 160°C. 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 rate although the fibrous products which come from the process are susceptible to bacteria and fungus as they are high in starch, cellulose and amino acids.

Modern autoclaves, also referred to as converters, can operate in the atmospheric pressure range to achieve full sterilization of pathogenic waste. Super heating conditions and steam generation are achieved by variable pressure control, which cycles between ambient and negative pressure within the sterilization vessel. The advantage of this new approach is the elimination of complexities and dangers associated with operating pressure vessels.

Process results

In 'batch system" autoclave processes, 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 continuous flow autoclave processes, glass, stones, and metals are removed from the waste stream before the "cooking" process begins, thus saving considerable energy, labor and equipment costs. With this process, waste enters and the product leaves the autoclave without the loss of temperature or pressure in the vessel. The material moves continuously through the process via computer-controlled process conveyors. After the waste is loaded onto the initial conveyor, the entire process is automated and it does not require human intervention to clean the inside of the vessel.

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 consist 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, 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. Continuous flow systems complete the process in approximately twenty-five minutes. In a typical "new" batch-type configuration, 2 10-ton units operating side by side would treat over 400 tons per day with time for preventative maintenance. Continuous flow systems are modular and are specifically designed to match the necessary capacity.

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.

Commercial application

Sterecycle claims to be the first company in the world to build and operate a full scale autoclave plant for household waste. In late 2005 the company started construction of the first vessel for their industrial scale autoclave plant to process household waste. Under contract from 3 local authorities, the Sterecycle plant has been in full commercial operation in Yorkshire, England since June 2008 and processes mixed household waste 24/7. The twin autoclaves are commercial scale, processing 100,000 tonnes per annum of waste from Barnsley, Doncaster and Rotherham councils under a long term contract for up to 10 years. The plant is currently being expanded to 240,000 tonnes per annum, processing residual waste as a substitute for landfill. The company also has 2 other sites in the UK, in Cardiff Wales and in Essex near London. The Sterecycle process is claimed to be protected by a series of patents. Sterecycle suffered a setback in January 2011 when one of their autoclaves in Rotherham suffered a rapid depressurisation, resulting in the death of one worker and injuring another. The plant restarted operations in March 2011. His Royal Highness the Duke of York visited the site in July 2011. An additional 3rd autoclave was installed in October 2011 to increase capacity.

Ambiensys has built a plant based on an autoclave for the treatment of municipal waste using steam, the city of Barbera del Valles near Barcelona (Spain). System designed by Rafael Ribas and Oscar Ribas, processes household waste without prior selection. The equipment has a capacity of 25,000 tons and is the first in a series that will work in parallel. The plant also has a section for selecting materials resulting from the process, especially biomass, obtained very clean and fit for further use.The plant was installed in an old textile factory and it could use existing facilities to produce the necessary steam.

WastAway of McMinnville, Tennessee, U.S.A., has operated an autoclave facility on a commercial scale since 2003. WastAway's process is a continuous-flow process. (Smaller scale operations began before 1997.) The first WastAway commercial plant processed all the MSW of Warren County, Tennessee from 2003 through 2008 and the facility continues to process on a commercial basis in Morrison, Tennessee. Product from this facility has been used predominantly as a soil amendment in the commercial horticulture industry. The company constructed a 150 U.S. ton per day facility in Aruba which began operations in July, 2009. The WastAway System processes waste on a continuous-flow basis with a complete processing time, from unseparated waste to final product, of approximately 25 minutes. The System separates metals and inert materials from the waste stream and produces an environmentally-friendly, non-malodorous, bio-stabilized material that the company has trademarked, "Fluff". WastAway will soon begin manufacture of twin 200 ton per day facilities for St. Thomas, U.S. Virgin Islands and St. Croix, U.S. Virgin Islands. Fluff from these two facilities will be used to fuel a new power plant being constructed on St. Croix.

Babcock In 2010 Babcock built a single autoclave as a proof of concept for the company's waste processing solution, the autoclave is capable of processing around eleven tonnes of unsorted waste per hour. This autoclave will be one of three autoclaves to go into a new facility at South Kirkby part of a PFI Project for the City of Wakefield Metropolitan District Council which will ensure that the Council achieves its strategic waste targets for diversion and recycling. The construction and commissioning of the new primary facilities at South Kirkby is scheduled to be completed in late 2012, the facilities at Denby Dale Road will be completed earlier during the same year. Together these facilities will treat c230,000 tonnes of municipal solid waste every year.

There are a number of companies seeking to enter the autoclaving arena, however there are significant barriers to entry including significant build costs, need for waste streams and intellectual property/patent issues.

External links

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• Babcock Waste Processing Autoclave System