Treatment of unsorted ‘black bag’ waste using autoclaves will begin in August when Sterecycle’s 100,000-tonnes per year plant in Rotherham starts operating.
The plant is expected to treat unsorted commercial waste at first, while the output will be used largely for energy recovery in either anaerobic digestion or biomass combined heat and power plants, according to Duncan Grierson, the company’s chief executive.
Two other autoclave plants are due to be operational by 2010, according to an ENDS survey (see table). Meanwhile in June, the VT Group submitted a planning application for a 150,000-tonnes per year capacity facility in Wakefield.
Autoclaving has been used to treat clinical waste since the 1990s and has been touted as a way to treat unsorted municipal and commercial waste for several years (ENDS Report 356, p 17 ). Four years ago Estech announced proposals to build three plants in Herefordshire and Worcestershire, but none of them has emerged. The company was bought by the VT Group earlier this year.
The process involves treating unsorted waste with steam at 160°C, and elevated pressure, in rotating vessels. This sterilises and softens recyclable materials, while breaking down the cellulose in all organic matter to produce a "mush". This does not lower its biodegradable content. After the material is taken from the vessels, recyclables are removed using magnets and screens and the organic matter is also separated.
The outputs are about 64% organic fibre, 17.5% recyclables, 2.5% aggregate and 16% other materials suitable for landfill.
The steam for the process can be generated either using gas-fired boilers - an approach to be used at Graphite Resources’ Gateshead plant - or it can be produced using the moisture in the waste itself. In the latter approach, oil in the vessels’ jackets is heated, which in turn heats the waste to produce steam. Such technology, which will be used by Sterecycle’s facility and is part of Yorwaste’s plans for a plant in Bradford, are more efficient and do not require boiler quality water.
The principal benefit of autoclaving is the variety of uses for the organic fibre. It can be put through anaerobic digestion or biomass plants to generate electricity and heat, used as a soil improver, or the long-fibres - some 60% of the total - can be separated and used to make paper pulp.
But one of the enduring questions around autoclaving has been the life-cycle greenhouse gas emissions of the process given its high energy use.
According to Yorwaste, its autoclave could require 285 kilowatt hours of gas and 40kWh of electricity to treat each tonne of waste, although it expects to reduce these figures. This compares with 70kWh of electricity for mechanical-biological treatment plants.
ENDS approached all autoclave companies to ask for figures on their greenhouse gas balance but only Sterecycle obliged. The others mostly said they would supply a greenhouse gas balance once their process had been optimised or they had undertaken an independent life-cycle analysis. VT Group could not provide a spokesperson.
If all recyclables are recovered and a biomass CHP plant is used to process the organic fibre, Sterecycle says some 220 kilograms of carbon dioxide equivalent would be saved per tonne of waste. However, the company would not say what this is compared with - presumably it is landfill.
According to Environment Department (DEFRA) figures in last year’s waste strategy, MBT processes that produce a refuse-derived fuel can save up to 570kg CO2e per tonne of input.
A recent report by Eunomia on greenhouse gas balances of waste technologies rated autoclaving highly. But it only looked at autoclaving when it was followed by gasification of the organic fraction (ENDS Report 397, p 19 ).
The news of Sterecycle’s plant coming on-stream came soon after it was revealed Veolia had dropped plans to build a 160,000-tonne per year capacity plant in Rainham to process waste from Tower Hamlets. The decision was made last year, but only came to light in June when Tower Hamlets said it was drawing up a new waste strategy.
Veolia says that it dropped the plant, initially put forward by Cleanaway (ENDS Report 375, pp 20-21 ), because its planning consent required its energy needs to be met from on-site renewables, such as biomass, and the cost of this was "prohibitive".
Richard Mair of Graphite Resources denied this problem would necessarily apply to other autoclave firms. "We have considered using the organic output on-site to generate our energy needs, but we felt we’d be perceived as backdoor incineration if we did that. It is something we’d look to do in the future."