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Mechanical Biological Treatment

Residual waste is taken to one of our five Mechanical Biological Treatment facilities (MBT), four with anaerobic digestion (AD).

These are located at:

Bredbury Parkway, Stockport

Cobden Street, Salford

Longley Lane, Sharston

Reliance Street, Manchester

Arkwright Street, Oldham (Mechanical process only)

 

Waste delivered to the facilities is separated and treated to generate electricity. The Refuse Derived Fuel (RDF) produced from this process is sent by rail to Runcorn Recovery facility.

MBT process

Reception/pre-treatment

Oversize materials are separated sent to the bulky crusher and dispatched to landfill or energy recovery. The remaining material goes through to a shredder which breaks open the refuse bags.

Mechanical separation

A drum screen separates the waste into 3 streams: under 120mm, 120mm – 300mm and over  300mm.  The over 300mm fraction is either sent back to be re-shredded or to the bulky crusher and sent for energy recovery.

Metal removal

The two smaller fractions are then passed under a magnetic separator to remove ferrous (iron –based) metals. They then go over an eddy current separator to remove non-ferrous metals. These metals are sent for recycling.

Reciprocating screen star sieve

Any materials < 40mm fall through the bottom and are passed under a further overband magnet to remove any remaining ferrous materials. They are then passed to the mixers to be prepared for the anaerobic digestion stage (AD). Before the organic fraction goes to the mixers it passes over a vibrating screen to aid the removal of plastic and fabrics. All the larger materials go to the air classifiers.

Air classifiers

The remaining material consists of two fractions: 40mm – 150mm and 150mm – 300mm.  Air classifiers then separate them into heavy and light fractions by blowing air over a curved belt; the lights are blown over the belt and the heavies drop before the belt.

Heavies

Both heavy fractions are passed by a near infra-red (NIR). This is programmed to select certain plastics, card and Tetra Pak which are sent to the secondary shredder and then compacted as high calorie RDF.

Lights

All the light material goes to a secondary shredder and is then compacted as high calorie RDF.  The 40 – 150mm fraction goes straight to compaction and the 150 – 300mm fraction goes through the secondary shredder.

Pre-digestion wet treatment

All of the under 40mm material is sent to three mixers where the organic material is mixed with water to create a slurry like material, this is to aid the suspension of the organic material within the mixture.

Sandscreen filters

An Archimedes screw pulls the material through the sand filters in which the lightest particles float and are sent as low calorie RDF. The heaviest particles sink to the bottom; this is mainly glass, stones and rubble and is sent for recycling. The remaining material is then sent to the Rotacut pumps.

Rotacut pumps (macerators)

These shred the remaining material into very fine pieces to form a slurry. This slurry then goes to the hydrolysis tank. Rotacuts are positioned before the pumps; this is designed to protect the pump against failure.

Hydrolysis tank

This is a buffer or holding tank which feeds the digesters at regular intervals. Chemicals are added here to control hydrogen sulphide.

AD process

Digester tanks

The slurry is heated to around 37°C  as it goes into the digester tanks. Waste heat from the gas engines is used to heat the slurry. If for any reason the engines are not running and there is no waste heat available then heat is supplied from a heating station. In the digestion tanks, anaerobic bacteria break down the slurry in the absence of oxygen. The bacteria produce a biogas which has a methane content of 60%-65%. This gas is sent to the gas storage bubble.

Gas

The gas stored in the bubble is purified and compressed before being sent to the two combined heat and power (CHP) engines.

Engines

Each CHP engine produces approximately 1MW of electricity which is sent to the national grid (2 MW in total) the waste heat from the engines is used for heating the slurry in the digesters and also for drying the digestate.

Aeration/strip tank

The material left over from the digestion process is sent to the aeration/strip tank where it is cooled and oxygen is added to destroy the bacteria.

Drying

The digestate is then returned to the building and into the decanters (which work in a similar way to a centrifuge). The decanters remove some of the moisture and reduce the digestate to about 35% dry solids (65% moisture). This then goes to the dryers which work like a tumble dryer to reduce it to 70% dry solids (30% moisture). The digestate can then be used as a low calorific value RDF in a combined heat and power plant (CHP). The dust from the dryers is also collected and sent as low calorific value RDF.

Environmental Protection

All the MBT’s have been designed to mitigate environmental impacts. These include:

Odours

The building is under negative pressure which means that fans (air curtains) operate when the doors are opened. The air is sucked into the building by the fans and the air curtains prevent the immediate escape of the air. This keeps the odours inside. The air is then filtered and piped outside into the acid scrubbers where diluted sulphuric acid flows on the air to remove odours. The air is then pumped into the VocsiBoxes® which are a type of clay aggregate which is heated to 900°C which works like a catalytic converter to flamelessly oxidise virtually all the volatile organic compounds which may cause bad odours. The cleaned air leaves by a stack which is monitored by the site and Environment Agency.

Water use

The process water is held in a storage tank and is continually recycled. However there are some parts of the process which require clean water. The process water is cleaned by reverse osmosis. Any concentrate remaining after this process is tankered off to the sewerage works.

Spillage

Each of the digester tanks holds 4 million litres of fluid. A concrete wall has been constructed to act as a bund and contain any spillage to avoid an environmental accident. The bund is capable of holding 110% of any one of the tanks.

 

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