Monday, December 28, 2009

Mechanical Biological Treatment Process with AD Explained

The New Civil Engineer magazine has published (19 November edition 2009) a refreshingly down to earth description of the process which will soon start to mechanically and biologically treat a part of Manchester’s residual (black bag) Municipal Solid Waste (MSW). The following was stated by Peter Harvey in his role as the Business Director of Enpure (Process Engineering). Enpure is the encumbent EPC Contractor, in the Greater Manchester Waste Disposal Authority’s huge city-wide PFI waste management contract.

This multi-billion contract was finally let around about last Easter after extended delays, and not least the problems caused by the credit crunch last Autumn. Under this Contract, Enpure is the EPC Contractor which will provide processing facilities for the PPP Consortium Laing Viridor to operate two MBT Facilities to be built at Reliance Street and Bredbury, under a £57 million Contract.

(In MBT there is always a choice to be made between composting to reduce the activity of the organic material before it goes to landfill, which is of dubious merit, or Anaerobic Digestion which does a better job but requires more investment and some risk of not repaying that investment. - Added by your Blogmaster)

"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 [anaerobic digestion] is what we have on our two MBT projects," says Peter Harvey.

The process begins at the materials recovery facility (MRF) whereby black bag [mixed residual] waste (MSW) 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) 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".

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 impeller 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 digesters," 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.

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 [for incineration].

The fraction above 80mm is then put through a density separation process so that we can eliminate any hardcore 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.

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.

Of course with plants handling so much waste material and especially the Anaerobic Digestion present, there has clearly and quite understandably been much concern about odours.

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.