For treatment with UV light to be successful, it is essential that the water should be transparent. UV light has therefore been tested for a long time on drinking water. When it comes to treating sewage effluent, lighting is switched on as an extra stage when the water has passed through the usual processes of the treatment plant.
UV treatment of sewage effluent has been tested in Denmark for a few years, where Hörsholm Municipality wants to enhance the recreational value of the Usseröd River. In the long run, bathing just to the south of Louisiana Museum at Humlebäck should also be improved.
UV as an extra stage. APOP – Advanced Photochemical Oxidation Process, i.e. UV treatment, is sited after the mechanical and biological treatment stages and is switched on in the bathing season.
Manages disinfection
Usseröd treatment plant treats about 7,500 cubic metres of sewage effluent every day, which is the load from the over 35,000 inhabitants of Hörsholm Municipality. The Danish Minister of Environment inaugurated the UV installation at the Usseröd treatment plant in 2005. She talked at that time of her hopes that the UV light would deal with discharges of hormone disrupting substances from e.g. contraceptive pills. But the municipality has not so far focused on this, since there is no formal requirement to remove endocrine substances from sewage effluent.
- It will be some time before UV treatment can deal with this, says Jes la Cour Jansen, professor of water and environmental engineering, Lund University.
- UV treatment works well for disinfection, especially since we are not allowed to use chlorine in Denmark. But for the treatment to be effective against pharmaceutical residues also, we must develop lamps of another wavelength, he continues.
Sensitive lamps. UV lamps may be destroyed or become overgrown by chemical precipitation if the water contains too many particles. They use electricity which also costs money and natural resources.
EU codes. The irradiated water is of good quality for bathing. Photographer: Henrik R Andersen/DTU
Pharmaceutical residues
The UV lamp looks like an ordinary incandescent bulb, but is more advanced in its construction. It radiates light of another wavelength which kills bacteria by destroying their cell walls. A wavelength of 252 nanometres works for the treatment of both drinking water and sewage effluent as regards disinfection.
- In order to remove hormone disrupting substances we need wavelengths of about 190-230 nanometres, he explains.
Even though the municipality only switches on the UV component of the treatment plant in the summer, Jes la Cour Jansen has an agreement that he can come and press the button as soon he and his researcher colleagues need to carry out experiments at the plant. Experiments are for instance in progress on combining UV lighting with ozone. Together they are a very powerful oxidant as has already been experienced in the treatment of drinking water. In a Mistra/Pharma project, Jansen and his colleagues are now studying how pharmaceutical residues in the effluent can be treated with UV light.
- We are looking at the whole spectrum of pharmaceuticals and are compiling a list of the substances that cause the most problems, he says.
In the spring he will also go to France to discuss how he can take part in a large project in Toulouse at a treatment plant that is directly connected to a hospital.
Better than EU codes
The demonstration project at the Usseröd treatment plant in Hörsholm Municipality was carried out over the period 2003-2007 within the EU Programme Life-Environment concerning UV irradiation of treated sewage effluent. The project is finished, but a demonstration plant remains which, with some minor midifications, can be converted into a test installation for traditional UV irradiation of treated sewage effluent, with the focus on destroying pathogenic microorganisms.
Over two years, the treatment plant will carry out a number of tests which are to investigate the economics of UV treatment of effluent.
That the UV installation at Hörsholm is so far only an experiment is emphasised by Torsten Duer, environmental worker at the municipality. It is only part of the treated effluent that is irradiated with UV and is then discharged into the Usseröd River. From there the water runs a few kilometres longer out to the coast, and on the way it is mixed with treated sewage effluent from another plant that has no UV treatment.
- Since the water is affected by so many factors before it reaches the coast, we cannot as yet see any direct effect of the UV treatment on the bathing waters in the sea, says Torsten Duer who also says that the results of analysis by the municipality show that the UV irradiation is very effective in removing bacteria from the treated effluent.
- With a correct dosage of UV light, so many bacteria are removed that the irradiated water amply satisfies the limit values of the EU Directive for bathing water of good quality, he says.
Torsten Duer sees a number of operational challenges in running the plant at the existing scale, especially as regards economics.
- We have no overall grasp of the economics of the project, and are not yet certain just how expensive it is to irradiate the effluent in this way in the plant. We expect to have a final report in a few months which will provide this information, says Torsten Duer.