New European waste management regulations posed a major problem for one central Scotland local authority, as Adrian Greeman discovered.
Relentlessly accumulating household refuse is a major headache for any local authority.
Stirling Council, which covers various rural areas, as well as the historic city, has a special problem.
Its long-used Lower Polmaise dump sited on a small plateau in an elbow of land alongside the Forth river has another decade of potential disposal life, but the rather sensitive location, by an important watercourse, means it is unlikely to get European level approval.
'Two years ago, the Scottish Environment Protection Agency decided it was not worth putting Lower Polmaise in for one of the new pollution control, waste disposal licences, ' says Keith Rogers, head of waste management at consultant Atkins.
Atkins has been working on the problem with Stirling, in a framework agreement.
The dump will have to cease operations as soon as its existing permit runs out, which it does shortly.
For the long term, Atkins and the council are exploring options for a new integrated waste management system, taking in household collection to processing and disposal.
This would probably be set up as a private finance initiative, co-ordinated with two surrounding councils, Falkirk and Clackmannanshire, under a regional grouping strategy being pushed by the Scottish Executive. An initial outline is due in January.
Meantime, the old pit had to be dealt with, in just 20 months, with the waste moved elsewhere and the old pit sealed and shut down.
Finding a new landfill location was the first part of the solution.
Unfortunately, this is some 15km away, which is uneconomic for the city collection vehicles. So a temporary transfer station has been built at Polmaise, where the street collectors can dump their loads and larger 90m 3 haulers can be loaded up instead to cut costs for the longer journey.
'They are filled using a front loader working between two 'push-walls', which give them something to react against and thus fill the buckets, ' says Rogers.
The facility, which was completed in August 2004, uses a steel frame 'shed' similar to buildings on industrial estates, which means it can be dismantled and reused for something else.
Work is also under way to complete the capping of the 25ha unlined pit, which dates from the 1980s. 'Capping means sealing it against rain-water ingress to prevent seepage and the production of toxic leachate, ' says Rogers.
A 1mm thick high-density polyethylene (HDPE) membrane laid over a clay bedding layer is used for sealing; the 10m wide rolls are fusion welded, once laid out, with a double strip weld, which means the joints can be tested by blowing air between them.
'You have to protect it with a capping layer of loose material 1m thick, ' says Rogers. For early parts of the site, this was done with a 200mm thick gravel layer at the base, which provided drainage above the membrane.
But Atkins has switched to using a geogrid laid above the membrane. 'It has an 8mm thick layer like the pattern in an egg box, which forms drainage channels for the water coming through the layer, ' explains Rogers.
Topsoil is used for the upper 200mm of the cap. Rather than be responsible for the possible stripping of central Scottish fields somewhere to supply the 500,000t required, the council is using its own waste to manufacture this.
'The council collects green garden waste from households every fortnight and this is composted on a purpose-built treatment area made in 2003.'
Collected material is shredded and laid in 1m high, 3m wide 'windrows' up to 100m long on a concrete platform. A specialist windrowing machine runs along the row every week, turning the material to aerate it, until after three months is can be left alone for final rotting down.
'We can mix that with a bulking agent such as sand and it makes a usable topsoil, ' says Rogers.
The council is also installing leachate drainage into the pit, and a gas pressure relief system.
'A modern site would have been divided into cells for gas collection, but this is all one pit, ' says Rogers. Even so, it should produce substantial gas, and the council is contemplating power production from the expected 2500m 3 output of mixed methane and carbon dioxide.
Currently it has been creating a gas collection network, with holes drilled into the compacted waste at around 40m centres to insert perforated HDPE pipes to be connected. For the moment, gas will be flared as the council looks for the £0.5M required for each of the gas engines and more money for the grid connection to sell the 1MW to 1.5MW output.
Overall costs for the project have been around £4M over the four years of its life.