Burning and burying – dealing with sludge

While land application is generally recognised as the Best Practical Environmental Option for utilising the properties of sludge, enabling its nutrient and organic matter content to be used in supplying crop nutrient requirements and maintaining soil fertility, it only accounts for about 50 per cent of sludge production.

In 1990/91 total UK sludge production was approximately 1.11 million tds/year, by the year 2006 sludge production is predicted to double to 2.16 million tds/year. Reuse to agricultural land is predicted to remain the principal outlet, with the estimated quantity reused doubling to 926,000 tds/year by 2006. However, in 1990/91 only 0.3 per cent of agricultural land received sludge.

Southern Water’s strategy to manage an increase in bioproduct production, expected to double to 140,000 tds/yr over the next five to ten years, is to recycle it all to agriculture, explained Peter Soulsby, recycling manager of Southern Water.

Southern Water currently disposes of 65 per cent of sludge to agriculture, 19 per cent disposed of to sea, nine per cent landfilled and seven per cent incinerated.

Sixteen sludge treatment centres are to be constructed in the region. The basic level of treatment chosen for these is anaerobic digestion with dewatering along with on-site storage for six months production.

“The future of sludge recycling to agriculture will depend on meeting the demands of all the stakeholders involved,” said Mr Soulsby. “The successful companies will be those who predict those demands, invest in the processes and services demanded and maintain a flexibility that can respond to market changes.”

Southern Water will be continuing to use its incinerator for disposing of eight per cent of sludge produced while after the end of the year, 92 per cent of sludge is going to be sent to agriculture. Landfilling will only be used in emergency, for contaminated products. Because of the very large rural area Wessex Water covers, it was a natural decision to return the sludge to land, said Mr Soulsby. Southern Water is also actively seeking non-agricultural land outlets for its dried products such as golf courses.

Disposal to sea has never been an option for land-locked Severn Trent Water. It currently disposes of 52 per cent of its sludge to farmland, with 16 per cent incinerated and 16 per cent landfilled.

Recent adverse publicity has heightened the company’s concern at being so dependent upon third parties and inevitably made some of the virtues of incineration much more attractive to companies – namely the reduction in volumes of material to be taken off-site and in the in-house management of a greater part of the overall process chain, explained Chris Rowlands, Severn Trent Water’s sludge planning manager.

Incineration is a process which eliminates pathogens, destroys organic contaminants, and reduces the bulk to be disposed, producing a harmless ash. However one of the major hurdles for incineration is planning permission, often resulting in the classic NIMBY – not in my back yard – situation.

The North East of England produces approximately 660,000m³ of sewage sludge a year, of which 60 per cent is disposed at sea and 30 per cent in landfill, with the remaining ten per cent going to agriculture.

Rather than incinerating the sludge, Northumbrian Water, which believes the best practicable environmental option is to produce a reusable end product – biosolids, plans to use sludge to produce a fuel, soil conditioner or a carbon replacement, for example in the manufacture of bricks or steel.

The biopellets can also be used to produce electricity through gasification, which could effectively make the sludge drying process self sufficient, with any the surplus electricity going to the National grid.

Gasification is the production of gaseous fuels by reacting hot carbonaceous materials with air, steam or oxygen. The biopellets produced by Northumbrian Water’s Regional Sludge Treatment Centre are a rich carbon source.

The Regional Sludge Treatment Centre brings sludge into direct and intimate contact with hot air at temperatures up to 400oC, drying it to 90-95 per cent dry solids. After cooling the dry sludge is formed into pellets which can be used as a fuel, soil conditioner or a carbon replacement, for example in the manufacture of bricks or steel.

Phase One of its regional sludge treatment centre, completed in April 1998 and situated at Bran Sands, provides a capacity of 50,000 tds/year with phase two, to be commissioned in 2002, providing a further 40,000 tds/year.

West of Scotland Water is another company planning to use sludge as an energy source. Currently is disposes of 91.1 per cent of its sludge untreated to sea, with the remainder 8.9 per cent sent to agriculture – 4.1 per cent of this is treated by anaerobic digestion. From 1999, 93.3 per cent will be incinerated until 2000, when it will be used an a thermal energy source.

Both Wessex and Yorkshire Water have already stopped disposing of sludge at sea. Majority of Wessex’s sludge is currently going to land – 84 per cent to agriculture, 11 per cent land remediation and five per cent landfilled – and this is planned to continue, with a slight increase (four per cent) used for land remediation.

Yorkshire however, only disposes of just over one fifth of its sludge to agriculture, 50 per cent of its sludge is incinerated, 12 per cent landfilled and 15 per cent used for reclamation. After 2002, it intends to use 27 per cent of sludge produced for reclamation, reducing its agriculture, landfill and incineration levels down to 21, five and 47 per cent respectively.

With a large proportion of sludge produced round the country being used in agriculture after various treatment processes, recent debates have focussed on what treatment is safe, especially after the Government announced that the use of untreated sludge on agricultural land is to be phased out and the discussions with the British Retail Consortium and Water UK on pasteurisation.

“If sludge recycling to agriculture land is to remain a sustainable option, it is essential that land application is viewed as safe and acceptable by legislators, farmers and consumers as a whole,” said Brian Chambers, principal research consultant for ADAS.

” I believe sludge can be recycled in a safe and acceptable manner for legislators, farmers and consumers.

“I believe that all contractors doing the job should be verified, have to be independently audited.” Mr Chambers believes that this could be one of the outcomes of the safe sludge matrix being produced by Water UK and the British Retail Federation.

The discussions between Water UK and the British Retail Consortium on a safe sludge matrix are at an advanced stage and a result is expected shortly.

“Farmers need to be persuaded to accept sewage sludge, it is not something they have to do,” said Michael Payne, pollution consultant for the National Farmers Union. “Farmers’ loyalty to their customers is greater than to the water company as without the customer farmers have no business, without sludge they are still in business,”

Farmers’ representatives have warned that if retailers decided not to buy food grown on land treated with sewage sludge, farmers would stop using it as a fertiliser.

With the use of untreated sludge on agricultural land to be phased out and the risk widely used digestion process may be deemed unacceptable, the treatment required for disposal to land could prove to be expensive.

With retailing groups calling for pasteurisation of sludge – a very expensive option – companies are showing interest in new treatment options, such as the Simon-Hartley Cambi thermal hydrolysis treatment, which achieves similar end results. Thermal hydrolysis combined with anaerobic treatment produces a cake free of pathogens and can be dewatered up to 37 per cent dry solids. Digested cake from a Cambi plant in Norway is now a preferred product of the local farmers for application to agricultural land.

The costs of improvements in environmental quality are a major component to be taken into account by Ian Byatt in setting new price limits.

“It may cost around £0.5 billion to implement the metals limit in the EC Sludge to Land Directive while the costs of meeting the Royal Commission on Environmental Pollution recommendations for the disposal of sewage sludge could cost slightly less than £0.5 billion, said Sheila Reiter, chairman of Ofwat National Customer Council.

“The costs of meeting the Environment Sub-Committee recommendation that by the year 2002 all sludge recycled to land should be subject to stabilisation and pasteurisation is unknown but the costs are likely to remain very high.”

The ONCC is urging the Government to examine very closely whether there is any sound scientific justification for such measures.”

Another method of disposal is to landfill the sludge, but with increasing landfill taxes this could turn into an expensive option. The landfill tax rate for active waste will increase from £7/tonne to £10/tonne from April 1999 and further increases are to be expected. While the landfill tax can be seen as a significant deterrent to landfilling of sewage sludge, it also acts as a stimulus for the dewatering of sludges which are to be landfilled.

With concerns over sludge’s chemical and microbiological properties regarding application to land and planning difficulties for incinerators it is likely that landfill disposal of sewage sludge will be a necessary option over the short to medium term.

Water companies have had a while to plan for the stopping of disposal of sludge to sea and to choose other disposal options. While some of the treatments chosen are stop gap options – chosen to fill the gap until the preferred treatment comes on line. From January 1999, no sludge will be dumped at sea, however, due to high levels of nutrients in sludge, only time will tell if this has any effect on local flora and fauna.