Strong Swedish lobby for advanced recycling
Malmo (pop. 270,000) in southern Sweden, is aiming to commission a new domestic sludge disposal plant by 2002 which will provide the city with a stable, reliable and long-term sludge-disposal solution.Malmo Water, owned by the city, has two sewage treatment plants with tertiary treatment for the removal of nitrogen and phosphorus. Both operate to standards below 10mg/L of BOD7, less than 0.3mg/L of total phosphorus and less than 12mg/L of total nitrogen.
Integrated with other waste disposal facilities, it is hoped the new plant will produce saleable products and cost-effectively solve the city's sludge disposal problems.
Sweden has very stringent standards for metals in sludge used on farmland. Malmo Water's technical manager, Bengt Andersson said, 'We've been meeting these standards working with point source discharges to the sewers since the 1970s.
For example, dentists have been reducing their use of mercury, there's been a campaign to reduce chromium waste in chromium plating and several industries have been working to reduce cadmium waste. Most of the minimal heavy metal contamination that's left comes from diffuse sources such as atmospheric fall-out, presence in food etc,' he said.
As metal concentrations are well within 1980s EU limits, the government still requires Sweden's water companies to recycle a proportion of sewage sludge as agricultural fertiliser.
Environment plus people
However, a powerful environmental lobby in Sweden has influenced food producers sensitive to the question of whether recycled sludge should be used as a fertiliser.
The ensuing debate resulted in a voluntary agreement established in 1995 between the Swedish association of water companies, the Swedish Environmental Protection Agency and the national union of farmers.
Key points include that all laws and standards for contamination levels in sludge must be complied with; there must be independent and transparent annual review and monitoring of this; and a progressive reduction in the levels of sludge contamination.
The Swedish public is well informed about possible contaminants, and in late 1999 cited flame-retardant in furniture and the use of silver as new risk items to be monitored.
Bengt Andersson said, 'In principle, the entire Periodic Table of elements could be considered as potential contaminants.' A further claim was that pathogenic bacteria are still present in finished sludge because there is no pasteurisation to ensure their elimination.
As a result of this type of campaigning, last November the Swedish farmers' union decided not to accept sludge from sewage treatment plants as fertilisers. Crop buyers will only buy products for export from land fertilised by sludge. All of this has intensified the search form a more satisfactory way of disposing of, or ideally recycling, sludge.
In Sweden 40% of sludge goes to farmland, the remaining 60% into landfill - permits for which are becoming increasingly rare. In addition, disposing of sludge to landfill will not be permitted after 2005. Incineration is not an option in Sweden at the moment, though co-incineration with other forms of waste has not be ruled out.
After studying the various options available, Malmo Water has opted for Kemwater's KREPRO process, which has been piloted in Helsingborg, for sludge disposal.
At its simplest, KREPRO produces an organic sludge with a high dry solids content which can be used as a biofuel; an inorganic sludge with a high phosphorus content which can be used for agricultural fertiliser; and finally, a liquid containing soluble organic matter and the precipitation chemical used in phosphorus removal. This liquid can be returned to the treatment plant where its organic content provides a carbon source for de-nitrification and its precipitation chemicals can be re-used in the phosphorus removal process.
The energy value of the biofuel produced exceeds all the energy consumed and is cost-effective if the value of surplus biofuel and fertiliser is added into sludge disposal costs.