What is Autoclaving?

The EU Landfill Directive demands that before waste can be sent to landfill it must be pre-treated. Firstly, this is to reduce the environmental impact of the waste that does get sent to landfill, and secondly, to further increase the amounts of materials recycled.

Autoclaving is a waste treatment process that uses a combination of heat, steam and pressure, along with the mechanical action of rotation to treat residual waste (waste that has not been collected for recycling or composting and has been placed in wheelie bins/black bags). Ultimately, the aim of waste autoclave process is to reduce the impacts residual waste has on the environment when sent to landfill. This article will take a closer look at what the waste autoclave process involves, the outputs of the process, as well as taking into consideration the process’ potential advantages and disadvantages.

The Autoclave Process

The waste autoclave process begins with the preparation of residual waste. Such preparation involves the removal of any large, bulky items of waste in order to make it more suitable to be placed in an autoclave. In some cases it is also possible that residual waste undergoes further mechanical waste preparation techniques such as shredding in order to facilitate the autoclave process. The main purpose of shredding waste as a preparation technique for autoclaving is to split open refuse bags in order to release the waste within them.

Following waste preparation, the remaining unsorted residual waste is then placed and sealed in an autoclave, a large, enclosed vessel similar to that of a fuel tanker that uses mechanical rotation to agitate and mix the waste (FOE). Pressured steam is then pumped into the autoclave vessel which ‘cooks’ the waste at temperatures of around 160° for approximately 45 minutes. The process results in the biodegradable/organic fraction of residual waste (green waste, food waste, paper/card waste) being broken down into a sanitised recyclable fibre known as ‘floc’, and the inorganic fraction being steam cleaned to produce sanitised recyclates (glass, plastics, metals) and residual waste for final disposal.

After the autoclave process, the fully sanitised waste is released from the autoclave vessel and further processed by mechanical separation techniques. This separation batches together the different material types (metals, plastics) ready to be sent for recycling. There are a number of different techniques that can be used to separate the sanitised waste, all of which can be used in combination to achieve specific end use requirements for the different materials. Trommel screens, magnetic separation, eddy current separation, and air classification are all types of different waste separation techniques used. Such techniques utilize the different properties of the waste materials, properties such as shape, magnetism, electric conductivity and weight, in order to separate them.

Outputs of Autoclaving

The main output of the waste autoclave process is ‘floc’. ‘Floc’ is a fibre like material created from the breakdown of the biodegradable fraction of residual waste during the autoclave process. This is the main output of the waste autoclave process as biodegradable waste makes up the largest proportion of residual waste (FOE). If however floc was to be sent to landfill for disposal, it would still biodegrade (breakdown and release the harmful greenhouse gas methane into the atmosphere). This means that further treatment of floc is necessary before it can be sent to landfill.

Floc can be biologically processed using either aerobic composting or anaerobic digestion. Floc treated with anaerobic digestion will produce biogas which can either be used as a natural gas substitute, or more commonly, to fuel generators in combined heat and power (CHP) applications to generate electricity, as well as heat (DEFRA). Floc treated with aerobic composting will produce a compost-like output (CLO), a residue similar to normal compost that can be used to spread on previously developed land, or as landfill cover. This compost-like output is not suitable for using as compost on agricultural land or for horticulture as it is not compliant with the UK specification for compost, BSi PAS 100. This is because the compost-like output is not as clean as normal compost as it may have, for example, high concentrations of some heavy metals due to metal materials being present in the autoclave process (FOE).

Potential Advantages

• Autoclaving reduces the volume of residual waste by ~60%, reducing the amount of waste requiring final treatment and disposal. This reduction in the volume of waste results in lower waste disposal costs for local authorities.
• Poor levels of waste management, particularly recycling, can see recyclable materials become part of the composition of residual waste. Such recyclable materials within residual waste can be captured by the waste autoclave process and sent for reprocessing.
• Autoclaving enhances the quality of recyclable materials by fully sanitising them, in turn increasing their market value.
• Autoclave plants can be built in large numbers on a small scale, reducing the need to bring waste in from wide areas. This supports the proximity principle which suggest waste should be disposed of as near as possible to its place of generation.

Potential Disadvantages

• The market for the main output of the autoclave process, ‘floc’, either as refused derived fuel or a secondary material is underdeveloped in the UK.
• The compost-like output (CLO) produced from the aerobic composting/digestion of ‘floc’ is not compliant with BSi PAS 100.
• Autoclaving is energy intensive which questions its environmental and economic feasibility.
• Autoclaving does not reduce the biodegradable content of residual waste; instead, the biodegradable content is broken down into a biodegradable fibre like material (floc) that requires additional processing or disposal in landfill.