Second wet air oxidation plant enters liquid disposal market

Wet air oxidation technology has made further inroads into the UK hazardous waste disposal business with the announcement of a new unit for Yorkshire Water's Knostrop sewage works near Leeds. But the growing interest in the process is raising questions about whether it should be regulated.

The wet air oxidation process degrades aqueous organic wastes by exposing them to high temperatures and pressures. It is of particular interest for the disposal of toxic or intractable materials which may not respond well to biological treatment.

The £1.2 million Knostrop plant will be the third in the UK. A similar unit is nearing completion at Sterling Organics' site in Dudley, North Tyneside (ENDS Report 192, pp 3-4), as is Leigh Environmental's merchant unit at Small Heath, Birmingham (ENDS Report 197, p 11).

Yorkshire Water's plant will treat 15,000 tonnes of waste per year. The waste will not be completely oxidised, but will be broken down into simpler molecules which can be handled by a secondary, biological treatment stage.

The investment is the latest step in Yorkshire Water's bid to develop the Knostrop works into an integrated waste treatment centre. The site has already been expanded to include a biological treatment plant, physico-chemical treatment, oil/water separation and neutralisation and precipitation facilities. Planning consent is also being sought for the company's fourth sewage sludge incinerator and a £3-4 million clinical waste incinerator on the site (ENDS Report 197, pp 11-12).

Wet air oxidation technology is not directly regulated at present. HM Inspectorate of Pollution (HMIP) says it has no plans to bring it within integrated pollution control, although any units installed on manufacturing sites subject to IPC will be covered by their authorisations. However, it appears that units on merchant waste disposal sites will only be regulated via a disposal licence.

Yorkshire Water's says it has been keeping both HMIP and the National Rivers Authority up to date with developments in the technology. The company is hoping to convince them that wet air oxidation is the best available technology for handling a range of waste materials - and it may need to do so if it is to succeed with its plans to promote the process more widely.

The Knostrop unit is to be supplied by the US corporation Zimpro Passavant, which also supplied Sterling Organics' plant. Zimpro's process is in use at some 200 locations, principally in North America.

Esmil, a Yorkshire Water subsidiary, is now in charge of promoting Zimpro's process in the UK. Likewise, Leigh is offering the Canadian Kenox process used at its Birmingham site for on-site applications.

Both firms are targeting the pharmaceutical and fine chemicals sectors, which are experiencing a squeeze on their disposal options. Waste is currently dumped at sea, discharged to sewer, incinerated or landfilled. Sea dumping will be banned in 1993 - a move which prompted Sterling Organics' investment in wet air oxidation.

Of the other options, landfill is becoming more expensive and the co-disposal of liquid wastes is likely to be curbed by the EC landfill Directive. As for discharge to sewer, water companies are becoming more reluctant to accept intractable wastes and are raising prices accordingly.

Both Esmil and Leigh claim that the wet air oxidation process compares favourably with the cost of landfill - typically £35-55 per tonne for this type of waste. It is also considerably cheaper than incineration, which is an energy-inefficient way of destroying aqueous waste.

Of the two technologies, Zimpro's operates at higher temperature and pressure, typically 280°C and 1700psi. The Kenox process operates at only 250°C and 700psi. Leigh maintains that this does not lead to reduced destruction efficiency as a static mixer in the reactor core and a turbine blade at the base lead to longer retention times. The less severe operating conditions are claimed to reduce capital costs considerably. But in contrast to the track record of the Zimpro system, only one long-running Kenox unit is in operation.

Other processes may also soon enter the market. In Germany, for example, Bayer has developed the Loprox system, which is claimed greatly to reduce disposal costs by using pure oxygen instead of air in the presence of a simple catalyst consisting of hydroquinone and iron salts.

Although the oxygen is an added cost, this is outweighed by savings from the lower pressures and temperatures employed. Typical conditions are 120-200°C and 3-20 bar. Bayer claims that the running costs of the Loprox process are roughly half of those of conventional high pressure oxidation techniques.

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