Photocatalytic industrial effluent treatment

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.