In June 2010, West Sussex County Council and Biffa signed a 25-year waste contract. This partnership was designed to help treat the county’s ‘Black Bag’ waste, and ultimately reduce the volume sent to landfill. Mechanical Biological treatment is one of the many ways we can treat waste, and in West Sussex this is carried out at the MBT facility in Brookhurst, Horsham.
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Transport
The facility here collects the county’s household general waste, as well as commercial waste and the mixed waste containers from the 11 recycling centres in West Sussex. It is then taken to one of many transfer stations, positioned strategically around the county. The waste is bulked up into larger vehicles, before transportation to the MBT in Brookhurst. This process is to reduce the otherwise vast Carbon Dioxide emissions from Transport.
Inside the MBT
Upon arrival at the MBT, the waste is emptied into one of two 10-metre deep pits, which have a capacity of over 2,000 tonnes of waste. Via a crane grab, it is dropped into a shredder and sent along conveyor belts for the first stage of processing: sorting.
Types of waste
During this process, waste is separated into 4 groups: Biodegradable waste, Refuse Derived Fuel, metals, and inert materials. From here on, each type of waste is sent through a different treatment route, with this efficient system resulting in only 5% of general waste in landfill. This small proportion is made up of residue from treatment, inert materials such as bricks and glass, and rejected items not fit for treatment, such as mattresses.
Refuse Derived Fuel
The Refuse Derived Fuel (RDF), is comprised mainly of a mixture of paper, card, plastics and textiles. When combusted, it releases energy and it can also be used to provide fuel for an energy recovery facility. However, many of these materials in the RDF are recyclable, which is a much better place for your paper and card.
Metals
The metals separated out are then sent back to Ford MRF for recycling, which you can read more about here.
Organic Waste
The organic (biodegradable) waste is first refined in the wet pre-treatment, where the waste is mixed with water to form a slurry. Impurities such as glass shards, sand, stones, and grit are removed. After extraction, the plastic particles are sent back to the mechanical pre-treatment and then on to form refuse-derived fuel.
The slurry is pumped into the hydrolysis tank to start the biological breakdown process. After hydrolysis, it is pasteurised at 70’C for an hour to kill any pathogenic bacteria, such as E. Coli and Salmonella.
The pasteurised slurry is then sent to the anaerobic digester tanks, where it is held for around 20 days. Over this period, micro-organisms break down the organic materials. In doing this they produce biogas, a methane-rich gas similar to cooking gas in homes.
By-Products of digestion
The anaerobic digestion works in a similar way to composting, except as the name suggests the process is completely oxygen-free. After the digestion has taken place, a compost-like material is left behind. This is dewatered and dried, allowing the water to be used again.
This in turn creates a dry, nutrient-rich product, which can be used as a replacement for soil in landfill restoration. It could be used with refuse-derived fuel as an alternative to fossil fuels.
The biogas released from digestion is combusted to provide heat and electricity. The heat is used to dry and pasteurise the compost-like material to reduce its volume, and to make it more marketable.
Some of the electricity produced is used to power the MBT, and the rest is sent to the national grid to be used in homes. When at full capacity, the biogas produces 4.5MW of energy: enough to power 10,000 homes!