Process engineering business Enpure is responsible for installing mechanical biological treatment plants at two of Greater Manchester’s sites, reports Bernadette Redfern.
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Back in 2005 process engineering contractor Enpure began talking to waste consortium Viridor Laing about the potential of mechanical biological treatment (MBT) plants in Manchester. These conversations led to visits by the clients to see aspects of the process in operation in Europe and eventually led to Enpure being asked to construct two MBT facilities under a £57M contract.
“We originally talked to Viridor Laing about one project but they then decided to put two projects with us,” explains Enpure business development director Peter Harvey.
“Costain was then appointed as EPC contractor for the overall project, so we became part of the team working with Costain. This was an advantage to us because we have worked with Costain a lot in the water sector, so they knew the measure of our capabilities and we respected the measure of theirs,” he says.
“We have worked with Costain a lot in the water sector, so they knew the measure of our capabilities and we respected the measure of theirs.”
Peter Harvey, Enpure
The two MBT schemes Enpure is responsible for are at Reliance Street in central Manchester and Bredbury Parkway in Stockport. Reliance Street is due to complete in February 2011 and Bredbury in November 2011.
As the name suggests the first step of the MBT process is mechanical treatment. This preparation of waste gives rise to materials that can be recycled and also produces a fraction with a high degree of organics.
“There are a number of ways that you can biologically treat that [smaller fraction], composting for example or anaerobic digestion, which can then create methane for electricity production. That particular approach is what we have on our two MBT projects,” says Peter Harvey. The larger of the two schemes is Bredbury with a throughput of 110,000tpa.
The process begins at the materials recovery facility whereby black bag waste goes through a very coarse shredder that reduces waste to 250mm to 300mm pieces. At that point the waste is screened and material of less than 80mm the organic fraction that goes off for anaerobic digestion. But first it needs further separation.
“If we didn’t clean it up then grit, pieces of plastic and bits of rubble could cause blockages so we have a very sophisticated wet separation process called the hydro-pulper,” says Enpure bid manager Phil Harvey.
“Between these two facilities there is enough to power 10,000 homes from the biogas alone.”
Peter Harvey, Enpure
It is this pulper which has not been used before in the UK. It comes from German firm BTA International. “It is a vessel with a high speed impellor in the centre that will transfer any organic material, paper, cardboard, food and organic matter into a sludge, which we can then pass through a screen and into the digestors,” says Phil Harvey.
Just under 5MW of electricity will be generated by the Reliance Street and Bredbury facilities to fuel the MBT process and feed back into the grid. “Between these two facilities there is enough to power 10,000 homes from the biogas alone,” says Peter Harvey.
As for the larger components, these are sorted for recycling in the case of metals and plastics, or for sending on as Refuse Derived Fuel (RDF) to Runcorn.
“The fraction above 80mm is then put through a density separation process so that we can eliminate any hard core, rugged material from that size fraction. Then there is metals removal. After this it is shredded to 30mm ready for RDF production. We also have an air knife which blows off plastics into the RDF as well,” explains Phil Harvey.
At Reliance Street the treatment process will handle 100,000t of waste per annum, but only 63,000t of this will go through the MBT plant. The rest, larger particles over 45mm, is sorted out and sent off for recycling. “The only real difference between the two sites is the degree of sorting that happens before the wet preparation stage,” says Peter Harvey.
Because Reliance Street is right in the city it uses a sophisticated odour control process called regenerative thermal oxidation. The odour is collected via ducts and blown into the treatment system which uses heat in the presence of a catalyst to thermally oxidise the odorous compounds in the air. The heat from the treated air is captured using silicon tiles. “Bredbury has a more conventional odour system, we operate the buildings under negative pressure so that if there are any leaks they go into the building rather than out and all that air is treated with biofilters,” says Peter Harvey.