The 150,000 tonnes of APC residues produced each year are classified as hazardous waste. Since the waste acceptance criteria (WAC) for hazardous waste landfills were introduced in July, APC residues have been dealt with in three different ways: treatment that meets the WAC followed by disposal in a hazardous landfill; treatment that renders them non-hazardous followed by landfill; or storage in Minosus' underground storage facility.
Waste Recycling Group warns that some treatment processes that render APC residues non-hazardous may fail to contain the hazard posed by contaminants such as lead. They may also involve the dilution of the waste with other wastes - normally prohibited as a treatment for hazardous waste.
In contrast, the company treats the APC residues from its only incinerator, in Nottingham, using a process that does not change the residue's classification as hazardous. The material is then sent to WRG's own hazardous waste landfill at Winterton in Lincolnshire.
"The Government must, as a matter of urgency, consult with the European Commission on its interpretation of the Directive's requirements and get confirmation that what is being implemented in the UK is acceptable, and with ministerial support publicly state that this is the case," said a spokesman.
Grundon echoes these views. It has applied for a derogation allowing it to deposit APC residues at its Bishops Cleeve hazardous landfill without having to meet the WAC.
"My concern is that in many cases residues that have been neutralised to get the free lime content below the 10% limit set by the WAC, and are deposited in non-hazardous landfills, are still leaching chloride significantly," said technical director Steve Roscoe. "It would require an extraordinary level of dilution to bring the level down below the permitted level of three times that specified in the WAC."
Mr Roscoe warned that chloride could pose a threat to the groundwater around such landfills. "The recipient landfills should have groundwater risk assessments but I'd be amazed if they did." He added that the Agency is examining whether treatment processes should deal with lead in APC residues, on top of dilution.
Grundon's interest in the issue stems not only from its hazardous landfill business. Through its partnership with Viridor, the firm is due to bring its Colnbrook incinerator on stream next year. It is "in the late stages of developing a novel treatment process" for APC residues.
Where the Agency stands on the issue remains unclear. According to head of hazardous waste policy Roy Watkinson, it is acceptable to classify the output of dilution and neutralisation processes as non-hazardous waste "provided the effect is not exaggerated, for example by using lots of diluted acid". But he added that some treatments for APC residues "continue to raise regulatory and compliance concerns". The Agency is conducting investigations in this area.
The treatment issue is also important for waste firms from a commercial point of view, as Government acceptance of dilution and non-hazardous landfill would put pressure on hazardous landfills and underground storage to lower their prices.
Sita is treating most of its APC residues but is sending those from its Cleveland incinerator to Minosus' underground storage facility in Cheshire. The former salt mine, which does not have to meet the WAC for hazardous waste landfills, started operating at the end of August.
Minosus managing director Roger Shaw said it is talking with other producers of "thermal process residues", such as metal industry firms and operators of clinical and municipal waste incinerators.
Onyx, which has six incinerators, is sending its APC residues to its physico-chemical treatment plant in Walsall for treatment that renders them non-hazardous.