The cosmetics, pharmaceuticals and food industries all require large quantities of water that is free from pathogenic bacteria and viruses. To preclude any risk of contamination, manufacturers normally disinfect the water on the factory premises. For these applications, ozone and ultraviolet (UV) techniques are preferred. “It is common practice”, explains Françoise Petitpain, implementation engineer at Degrémont, “to propose incorporating a UV reactor in the recycling circuit for water in the packaging tanks of bottling plants”. The aim is to maintain water quality when a part of the bottling line is shut down.
In such cases UV radiation is particularly effective. Its germicide and virucide properties are well known. Ultraviolet radiation destroys microorganisms by breaking down the cell wall. This process leaves neither residues nor toxic by-products and avoids the need for chemical products. These characteristics make UV the technology of choice for treating wash water for raw materials, preparing lye peeling solutions, or processing 4th or 5th grade foods which are either raw or pre-cooked.
Ultraviolet techniques could for example be
[Photo: Ozone oxidises dissolved matter partially or totally, and promotes precipitation. The gas destabilises colloidal matter and disinfects.]
used in the manufacture of these new foods. As they point out at Actini-Aqua, “the bacterial flora encountered consists of microorganisms associated with deterioration such as pseudomonas, lactic bacteria, and yeasts”. Yet vegetables delivered by the producer in the natural state are still being rinsed and disinfected using a chlorine solution. However, the use of such products is becoming increasingly regulated, and UV technology seems set for a bright future.
Before it can take its rightful place, however, UV disinfection must be shown to be mature. Manufacturers go to great lengths to prove that their equipment is safe and their latest designs include many ingenious features. Saur and Stereau, for example, have just brought out a unit featuring new design geometry. The lamps are positioned vertically, parallel to the hydraulic flow. This approach has also been adopted by Messer, who presented at Pollutec 97 a UV unit with the flow running parallel to the lamps. The UV generation lamps used in this unit were flattened for a better distribution of the energy output.
For some applications, however, with too high a flow or the presence of hard molecules UV treatment is insufficient, and ozone must be used.
[Encart: Ultrapure water: multibarrier treatment
Ozone and UV are ideally suited to the manufacture of ultrapure water. A series of barriers, in addition to a reverse osmosis step, allows the production of water that is ideal for the manufacture of the latest generation of electronic components.
The advantage of a multibarrier system stems from the complementary nature of the two processes: ozone destroys microorganisms and treats minerals, while irradiation with UV removes excess ozone.
At Thomson Electronics, the number of barriers has been increased to obtain water of the very highest quality. It is used in the manufacture of 8ʺ silicon wafers. Degrémont, who supplied the installation, set up a series of barriers. Treatment begins with UV oxidation and filtration by reverse osmosis, followed by ozonation and further UV irradiation at the exit from the ozone tower. The water is then filtered and again irradiated with UV. “The more barriers you use, the more ultrapure the water”, explains M. Jacquard.]
[Photo: UV techniques could be used to disinfect wash water in manufacturing processes for new foods.]
Ozone
Today, ozone is chiefly used in tertiary treatment to break down hard molecules. According to M. Jacquard, in charge of industrial trade at Degrémont, “this accounts for 95% of all applications”. For the remaining five percent, it is used for improving water quality by disinfection. Procter & Gamble, for example, ozonise water in the network to rid it of unwanted microorganisms, before using it in the manufacture of cosmetics.
As M. Jacquard points out, “ozone, being more powerful than UV, allows a greater volume of water to be treated”. The process has also aroused very great interest in farming sectors requiring large quantities of water of impeccable quality, namely shellfish production and fish farming.
Shellfish farms are already taking an active interest in ozone. In both Tunisia and France ozone generators have been installed in farm grow-out units. They disinfect the water entering the tanks and treat the discharges. In fish farming, Air Liquide and Ozonia have equipped one of the largest Italian fish farms with an ozone unit. The generator, placed on the water supply canal, is capable of producing ozone at the rate of 5 kg/h. It is being used on a trial basis to treat the water for a trout pond.
Ozonia conducted its first fish farm trials in Switzerland in the Canton of Zurich several years ago. The results presented at Lille (France) by Bruce Stanley, during the 12th International Congress on Ozone, demonstrate the value of this technique for the treatment of natural waters which vary in quality over time.
“The ozone oxidises the dissolved matter either partially or totally, and promotes precipitation. The gas acts on microflocs of organic matter, destabilises colloidal matter and disinfects”. Followed by an activated carbon filter to remove residual ozone, this treatment provides a solution to problems of water in fish farming.
While ozone treatment is now being applied to the disinfection of industrial water, it should not be forgotten that the first applica-
[Photo : The Pays d’Olmes (France) industrial and municipal wastewater treatment plant is equipped with an ozonisation process supplied by Degrémont. It allows oxygen recovery from the ozone tower.]
tions of the gas in molecular oxidation to treat COD were in paper mills and to treat colour in the textile industry.
Using the oxidising capacity
Still fresh in the memory, it was just five years ago that Ozonia announced the commissioning of an ozone-based unit for the treatment of surfactants and colours at Prato near Florence. The installation is substantial since it treats effluent at the rate of 5,000 m³/h with ozone at 30 g/m³.
Several months later, Trailigaz tested an ozone process for the decolorisation of textile industry effluents at a site at Troyes (France). Since then, they have been joined by other manufacturers such as Wedeco, which has equipped the Belgian factory of Levi Jeans, using technology that allows 75 % of the water to be recycled.
Today, the technology is still progressing. The firm De Cathalo has equipped its Labastide-Rouairoux (France) site with a biological process combined with an ozonation step to reduce carbonaceous pollution and effluent colour. The system, supplied by Degrémont, consists of a water channel and a sludge channel. The ozone generator is placed at the polishing stage, just before the outflow metering channel.
Ozone is produced from air. The process incorporates a compressor, drier and ozone generator capable of producing ozone at the rate of 3 kg/h. Ozone in the off-gas is destroyed using a thermal ozone decomposer. The installation treats a mean flow of 1,350 m³/day.
The industrial (85 %) and municipal (15 %) wastewater treatment plant in the Pays d’Olmes region in the south of France has likewise adopted ozone for the treatment of residual COD and colour. The treatment circuit was engineered by coupling the two stages of biological treatment and ozonation. So as to comply with the latest standards on discharges (125 mg/l for COD and 100 mg/l of platinum cobalt for colour) this plant designed by Degrémont has adopted the appropriate solutions: aeration of activated sludge tanks with pure oxygen, filtration followed by ozonation of effluents before they are discharged, and heat-treatment of sludge.
Engineering studies on the water channel led to the coupling of two processing stages, biological purification and final ozonation. Interestingly, recovering and reusing the oxygen needed for ozonation instead of discharging it into the atmosphere has proved to be economically viable.
The air space in the ozone tower is rich in oxygen, and is therefore extracted and reinjected into the aeration tanks. This equipment has been in service since summer 1998. It has a treatment capacity of 25,000 m³/day and 13,500 kg/h of COD.
