Process water treated by UV disinfection

British company Hanovia will supply Sasol Technology Ltd with four medium pressure UV disinfection systems for its South African production plant, to enable secondary effluent re-use

The Hanovia systems will be used to treat up to 42MLD (Million litres/day) of secondary effluent for use as process water in polymer production at Sasolburg.

Sasol specialises in converting coal into fuel and other value-added hydrocarbons.

The company ability to manufacture synthetic fuels contributes significantly to the South African energy supply - it currently supplies 41% of South African liquid fuel requirements, saving the country more than R18 billion (GBP1.24 billion) in foreign exchange.

Each of the Hanovia UV systems will be fitted with a patented transmittance monitor to measure changes in the transmittance of the incoming effluent, allowing UV output to be constantly adjusted so as to ensure a consistent dose.

Automatic lamp wiping and power switching technology will also ensure optimum dose at all times.

UV disinfection systems are generally split into two distinct types: low pressure (LP) and medium pressure (MP).

LP systems have a monochromatic UV output (limited to a single wavelength at 254nm), whereas MP systems have a polychromatic UV output (with an output between 240-310nm).

UV works by disrupting the DNA of micro-organisms, rendering them unable to reproduce.

It is generally accepted that DNA absorbs UV most effectively at 265nm, a wavelength that MP lamps produce in abundance.

For LP systems to be effective at destroying DNA, a large number of UV lamps are therefore required.

This has obvious cost and maintenance implications for the operator.

All UV lamps have set life-spans and need replacing after a certain number of hours (normally several thousand).

The more lamps there are, the greater the likelihood that the system will need to be stopped to replace those that fail.

UV monitoring is also more difficult with a large number of lamps - UV monitors are located on the wall of the UV chamber, so if a lamp located away from the chamber wall fails, it may not be detected.

In addition, a large number of lamps impedes fluid flow through the UV chamber, resulting in pressure head-loss and higher pumping costs.

For these reasons more operators are now switching to the newer, more efficient MP technology.

With a wider and more powerful UV output than LP lamps, far fewer MP lamps are required for the same level of disinfection.

Head-loss is significantly reduced and monitoring is far more effective.

Maintenance costs are also reduced as there are physically fewer lamps to replace.

Typically, 10-12 LP lamps are required to produce the same UV output as one MP lamp.

Hanovia MP UV technology was therefore a deciding factor in the choice of treatment system at Sasol, and with more and more operators switching to MP UV wastewater disinfection as a more efficient alternative to LP technology, Hanovia expects to win more such contracts in the near future.

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