Photocatalytic industrial effluent treatment

Photocatalysis has been considered for a number of years to be an ideal technique for the treatment of industrial effluents. However, until now the cost of the process has been prohibitive for all but the most intractable problems. Recently Photox Bradford has developed a new approach to the application of the technique based around a new catalyst developed at the University of Bradford and a much simpler reactor design.

Chemists at the University of Bradford have researched the suitability of photocatalysts as an effective water treatment process for the part 20 years. They have accumulated expertise in the treatment of various organic compounds as well as the removal of heavy metals from industrial process water. Examples examined have included the destruction of dyes, pesticides (OPs and SPs) phenols, cyanide, chromium removal, COD reduction, surfactants, greases and oils.

Because the process gradually breaks down the contaminant molecule, no residue of the original material remains and therefore no sludge requiring disposal to landfill is produced. This results in considerable savings and a simpler operation of the equipment involved.

Additionally, because the contaminant is attracted strongly to the surface of the catalyst, the process will continue to work at very low concentrations allowing sub part-per-million consents to be achieved. Taken together, these two advantages mean that the product water may be recycled, resulting in savings in the cost of input water and effluent charges.

Traditionally, in order for photocatalysis to be effective the catalyst has been added to the effluent as an extremely fine dust. This makes the catalyst efficient in its use of the UV light energy but the extremely fine powder is difficult and expensive to recover from the effluent stream. This has meant that approximately 90 per cent of the cost of treating process effluents using this method has been due to the use of sophisticated solid recovery methods ­ usually an expensive membrane and catalyst release system. Quotes from a small US supplier have indicted that the approximate capital cost for a typical UK dyer for example would be in the region of £2-3 million.

As a result of the research conducted by Dr Bickley and Dr Hogg at the University of Bradford, a catalyst with an activity that is comparable to that of a fine dust but has an average particle size that is nearly 100 times larger has been developed. In practice, this means that instead of using a custom-built form of ceramic membrane filtration system to remove the catalyst at a cost of approximately £1 million per 10m3/hr capacity, a standard off-the-shelf separator is able to remove the new, coarser catalyst at a cost of approximately £6,000 per 10m3/hr capacity.

The system itself consists of a series of interconnected tanks and trays above which are mounted standard UV lamps. A single pump is used to reticulate the catalyst slurry via the separator and is the only moving component in the flow system.

Following its formation in 1997, Photox has completed a number of successful trials on a range of process effluents ranging from the removal of chromium ions (15pm to 0ppm) to the complete removal of colour from screen printing effluents.

In the summer of 1998, Photox won an innovation award from the Department of Trade and Industry's Smart Competition. This has been used to fund the development, through a firm of engineering subcontractors, of a full-scale production unit that can be transported to industrial sites to demonstrate the effectiveness of the process on waste streams up to 10m3/hr. The unit is modular and the process takes place at ambient temperatures and pressures.


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