The Landfill Directive requires biodegradable waste to be diverted from landfill to help protect the environment and reduce the potential for greenhouse gases to be emitted from landfill sites. Mechanical Biological Treatment (MBT) is one way of doing this.
The West Sussex facility has had to pass multiple tests before commencing waste treatment; all tests were completed and passed in summer 2015. Our facility takes the county’s “black bag” household, commercial waste and the mixed waste containers from all 11 household waste recycling sites in West Sussex. The waste is taken to transfer stations strategically placed around the county to be bulked up into larger vehicles before transportation to the MBT facility, this is to reduce the amount of vehicles we have on the road; thus reducing our carbon footprint.
The kerb-side waste collected is emptied from the vehicles into one of two 10-metre deep pits in the reception hall of the MBT facility. These can hold over two thousand tonnes of waste. A crane grab can pick up five tonnes of waste at a time, dropping it into a shredder to break it down. It then drops onto a moving conveyor to start the first stage of processing which is known as Mechanical Pre-Treatment. This splits the waste into 4 parts:
- Biodegradable waste (e.g. food waste)
- Refuse derived fuel (e.g. paper, plastic, cardboard, textiles)
- Metals (e.g. magnetic – mostly steel, and non-magnetic – mostly aluminium)
- Inert materials (e.g. bricks, glass, 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 economic value from each part.
Organic (biodegradable) waste is refined in our wet pre-treatment area. Here the waste is mixed with water to form a slurry. Impurities that have found their way through the mechanical sorting are taken out; items such as small plastic pieces, 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.
The slurry is then pumped into our hydrolysis holding tank to start the biological breakdown process. After hydrolysis the slurry is pasteurised for 1 hour above 70⁰C to kill any pathogenic bacteria such as E-Coli or Salmonella. The slurry is then pumped to the anaerobic digester tanks, and stays in there for around 20 days.
Inside the digester tanks bacteria eat the food particles. As the organic particles are digested the bacteria produce biogas; a methane rich gas similar to cooking gas used in homes. The biogas is combusted on-site to produce heat and electricity. The heat is used in the pasteurisation process and to dry the compost like output that could be used as a replacement for soil in land or landfill restoration. Some of the electricity is used to power the facility and some is exported to the national grid to be used in homes.
At full capacity the biogas will produce around 4.5MW of energy; which is the equivalent power use of 10,000 homes!