General waste

Find out how general waste is processed in West Sussex.

Overview

The county’s general waste is collected and transported to our Mechanical Biological Treatment (MBT) facility in Brookhurst Wood, north of Horsham. This includes:

  • 'black bag' household waste
  • some commercial waste
  • mixed household waste from all 11 recycling centres in West Sussex

Here it is sorted and separated using a variety of machines and over 120 conveyor belts, known as mechanical pre-treatment.

The purpose of the MBT is not to sort and separate recyclable materials. It sorts and treats any leftover waste that cannot be moved up the waste hierarchy through prevention, reuse or recycling. 

The MBT ensures the amount of waste sent to landfill in West Sussex is minimised. This protects the environment and reduces the amount of greenhouse gases emitted from landfill sites.

Sorting

Mechanical pre-treatment

When waste arrives at the MBT facility, it is emptied into one of two 10-metre deep pits. These pits can hold over two thousand tonnes of waste. Using a crane grab, it is then dropped into a shredder and moved on conveyor belts to start the first stage of processing. This processing is known as mechanical pre-treatment.

This process separates the waste into four types:

  1. Biodegradable waste (such as food waste)
  2. Refuse derived fuel (RDF) (such as paper, plastic, cardboard and textiles disposed of in the general waste bin, that could have been recycled)
  3. Metals 
  4. Inert materials (such as bricks, glass and rubble)

Each waste type goes through a separate treatment route. This enables each part to be processed in an eco-friendly way and to recover its value.

The RDF is mainly comprised of a mixture of paper, plastics and cardboard. When combusted it gives off energy and can be used as a fuel in energy recovery facilities to produce electricity.

Any metal that is extracted at this stage is recycled.

Any other materials from the MBT process are sent for landfill disposal at the Redhill landfill site. These include:

  • inert materials such as bricks and rubble
  • rejects, items not fit for treatment, such as mattresses
  • residues, material left over once the treatment has been completed

Due to this efficient process only 5% of everything that gets collected for processing in West Sussex goes to landfill. There's always more that can be done to reduce waste that's collected in the first place. To find out more, visit our waste prevention page for hints and tips.

Biodegradable waste processing

After the mechanical pre-treatment has separated the waste into one of the four groups, the biodegradable waste undergoes further treatment.

Wet pre-treatment

Organic (biodegradable) waste is first refined in the wet pre-treatment area. Here the waste is mixed with water to form a slurry, and impurities that have found their way through the mechanical sorting are removed. These are things like small pieces of plastic, sand, grit, stones and glass shards.

Once extracted, the small plastic pieces can be sent back to mechanical pre-treatment to join other products that form the refuse derived fuel output. 

Anaerobic digestion

The remaining slurry is then pumped into our hydrolysis holding tank to start the biological breakdown process. After hydrolysis the slurry is pasteurised for one hour above 70⁰C to kill any pathogenic bacteria, such as e. coli or salmonella.

The slurry is then pumped to the anaerobic (absence of air) digester tanks, where it remains for around 20 days. During this time micro-organisms breakdown the remaining organic material. As they do they produce biogas, a methane-rich gas similar to cooking gas used in homes.

By-products of biodegradable waste

Compost-like output

The anaerobic digestion process works in a similar way to composting, just without oxygen. After the micro-organisms have had their fill in the anaerobic digester tanks, a compost-like material remains. This is dewatered and a drier reduces its volume further by removing more moisture. The water is put back into the system to be used again. 

The end product of this procedure is dry, with a high nutrient content. This material is used as a replacement for soil in land or landfill restoration. It could also potentially be dried and used with refuse derived fuel as a fossil fuel alternative.

Biogas

The biogas produced from the anaerobic digestion is collected, and combusted on-site to produce heat and electricity.

The heat is used to dry and pasteurise the compost-like output to reduce its volume and make it more marketable. 

Some of the electricity is used to power the facility and the rest is exported to the national grid to be used in homes.

At full capacity the biogas will produce around 4.5MW of energy - the equivalent power use of 10,000 homes.

Changes to the MBT

Recent changes under section 45A of the Environment Act 2021 and the Government’s Simpler Recycling policy mandate the separate collection of food waste.

To find out more about how the MBT will be changing in order to process this food waste visit our about the MBT page.

See also