Find out how general waste is processed in West Sussex.
The county’s 'black bag' household waste, commercial waste and mixed waste from all 11 recycling centres in West Sussex is collected and transported to our mechanical biological treatment (MBT) facility in Brookhurst Wood, north of Horsham.
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, but rather to sort and treat any leftover waste that cannot be moved up the waste hierarchy through prevention, reuse or recycling.
The aim of the MBT is to ensure the amount of waste sent to landfill in West Sussex is minimised, protecting the environment and reducing the amount of greenhouse gases emitted from landfill sites.
When it arrives at the MBT facility, waste 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, known as mechanical pre-treatment.
This process separates the waste into four types:
- Biodegradable waste (such as food waste)
- Refuse derived fuel (RDF) (such as paper, plastic, cardboard and textiles)
- Inert materials (such as bricks, glass and rubble)
Each waste type goes through a separate treatment route that enables each part to be processed in an environmentally friendly way and to recover its economic 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 inert materials, rejects and residues from the MBT process are sent for landfill disposal at the Redhill landfill site.
Biodegradable waste processing
After the mechanical pre-treatment has separated the waste into one of the four groups, the biodegradable waste undergoes further 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 (such as small pieces of plastic, sand, grit, stones and glass shards) that have found their way through the mechanical sorting are removed.
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.
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
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.
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.