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A waste autoclave is a form of solid waste treatment that uses 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 steam pressure in the vessel is maintained up to 6 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.
Some autoclaves, also referred to as waste 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 associated with operating pressure vessels. However, steam that is not 97% saturated may not contain sufficient energy to kill the spores that may be on the surface of the items it comes in contact with.
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 bulk 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 (rottable), cellulose and lignin elements of the waste stream. 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.
Some systems claim to 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 system claims to be able to dry the cellulose during processing using heat, and another 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, 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 typically completed within 90 minutes. In a typical batch-type configuration, 2 10-ton units operating side by side would treat 440 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 heavy weight of the vessel may cause equipment failures.
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.
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 230,000 metric tons (253,531.6 U.S. tons) of municipal solid waste every year. . Babcock is operating under a license from EstechUSA LLC, who operated a commercial twin vessel plant in Wales beginning in 2004. Pictures from that operation, showing a pair of characteristic green and silver autoclaves, have been used by many startups as advertising literature.
Estech began its autoclave business in 1998, and built its first full scale demonstration (single vessel system) in Sheffield UK. This is identified as the Model 1 system, and is now on display at Estech's headquarters in Ohio. A full size twin vessel facility, Model 2, was built in 2000 and operated in Bridgend, Wales. Recognizing the existing deficiencies in autoclave systems, Estech reviewed the world-wide state of the art of autoclave and other emerging waste technologies, assembled an engineering group with a broad background in technology development (from NASA, DoD, etc., and privately funded a multi-million investment in improvements to produce its vessel-based Model 3 system. Others, including the UK, have determined that autoclave technology is the best solution for municipal waste management. The Model 3 system is built around its a high-throughput autoclave/dryer (FiberCycle), which has been realized and demonstrated in several prototype vessels. PowerCycle produces steam and commercial electricity from the dried/cleansed biomass. WaterCycle captures, cleans, and reuses water. Post-vessel separations cull out ferrous, non-ferrous, and plastic goods for commercial sail. The system is being commercialized globally. Much of combined equipment is available off-the-shelf, but the assembly has proprietary and specialized technological improvements. Estech is now finishing the construction of a proprietary Model-3 unit for a specialty municipal waste stream in the EU.
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 (but for a period of shut-down for inspection by the Health and Safety Executive following an explosion in January 2011, see below) and processes mixed household waste. The twin autoclaves are commercial scale, processing 100,000 metric tons (110,231.1 U.S. tons) per year 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 metric tons (264,554.7 U.S. tons) per year, processing residual waste as a substitute for landfill. The company also has two other sites in the UK: in Cardiff, Wales and in Essex near London. The Sterecycle process is protected by a series of patents although it may be noted that autoclave patents are notoriously difficult to reinforce as a consequence of "prior-art".
In July 2011 the plant was visited by His Royal Highness The Duke of Kent. An additional third autoclave was installed in October 2011 to increase capacity. In December 2011, completed a funding round raising £5.1m in order to expand its waste processing capacity at the Rotherham plant.
WastAway of McMinnville, Tennessee, U.S.A., has operated a small autoclave facility since 2003. 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 first WastAway commercial plant processed all the MSW of Warren County, Tennessee from 2003 through 2008. 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 (136 metric ton) per day facility in Aruba which began operations in July, 2009. 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 claims that it 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. It is claimed that fluff from these two facilities will be used to fuel a new power plant being constructed on St. Croix.
- Fibrecycle Process Environment Agency Waste Technology Data Centre Evaluation of Estech Fibrecycle Process
- "Sterecycle - Assignee". Retrieved 20 January 2012.
- "Third autoclave fabricated for Sterecycle plant". Let's Recycle. Retrieved 20 January 2012.
- "WastAway: Our Process". Retrieved 22 January 2012.
- "WastAway: Projects". Retrieved 22 January 2012.