Ozone in Cooling Towers : Hygiene for us

Transcription

1 Ozone in Cooling Towers : Hygiene for us Dr. Matthias Hoffmann BWT Wassertechnik GmbH, Industriestrasse 7, D Schriesheim, Germany Keywords: Ozone, Cooling Tower, Legionella, Hygiene, Air conditioning, Biocide Introduction Cooling towers may have serious output of aerosols into the environment. The latent danger for the health of human being caused by these aerosols is scarely taken into account as the presence of cooling towers is only very few perceived. But there are many cooling towers installed near to the daily presence of many people, there are many installations on office buildings, shopping centres, community halls, etc in order to remove the heat of the building s air conditioning. The cooling towers on top of these buildings are mostly installed completely invisible from outside, so that they are normally not noted. Every day a lot of people are passing by or spend many hours inside such buildings without having any idea about the existence and the risks of those towers and the aerosols. The press regularly reports about people being infected or killed by Legionella. But the daily individual risk of many people keeps far underestimated. Background The risk of infection comes from the aerosols being exhausted together with the operation air when the cooling tower is working. Small and smallest drops of the cooling tower circulation water are blown out into the ambient air, so that we inhale these fully contaminated drops as aerosols. The use of water in a cooling loop changes its chemical composition compared to the make up water: Due to the evaporation of CO 2, the ph-value is shifted to the alkaline range between 8.5 and 9.5, so that disinfection with Chlorine can not be successful. As the cooling effect works by the evaporation of circulation water itself, there is a increase in concentration of the total dissolved solids (TDS). If the make up water has TDS of 500mg/L and the cooling tower works with three cycles of concentration, the circulation water has 1500mg/L of TDS. Furthermore a cooling tower has the property to work as a perfect gas washer, even for high operation air flows, so that all the dust of the operation air is washed out and admixed in the loop water. The gas washing effect leads also to a permanent admixing of all kinds of biological germs from the operation air into the water, so that a constant contamination of the water cannot be avoided during the operation of the cooling tower. The growth of biology, especially bacteria and viruses, in the cooling system is supported by the increase of TDS and admixed dust, because both increase the offer of food. The growth of micro organisms is supported by the ideal ambient temperature of typical C in the cooling system. Without water treatment the cooling tower can be seen as a perfect biotope for micro organisms living in the water or as biofilm on all the wet surfaces of the cooling loop s components. Biofilm The biofilm can be found in the tubes, the tower basin and heat exchanger. Biofilm has a heat conducting factor that is 4 times lower than that of the dreaded limescale deposits in the cooler. Biofilm can grow to a thickness of several millimetres and impair heat transmission to such an extent that the cooling tower is practically incapable of functioning. A biofilm only 1 mm thick results in heat losses of more than 30%. Wasser Berlin

2 The formation of biofilm is furthermore a risk for corrosion. Bacteria produce acids which may come into contact to the metal surface of the cooling system. So corrosion generated by acids can occur although the water in the cooling system is of alkali ph. This is the so called microbiologically influenced corrosion (MIC). Conventional treatment The conventional cooling water treatment to control the biological growth is dosing chemical biocides, mostly Isothiazolones. These chemicals are poisonous for human being but many bacteria are more or less resistant against them. Their application therefore requires permanently repeated shock dosing at high concentrations. In the time between the dosing cycles, the bacteria can grow up again so that the aerosols of the cooling tower may either contain notable concentrations of poisonous biocides or infecting bacteria. Fig. 1: The Progression of Possible Environmental Contamination Example For example consider a cooling tower with cooling capacity of 1 MW. Due to a working temperature difference of 6 C between water input and output, it has a turnover rate of the cooling water of 140m 3 /h. The air flow rate may be in the range of 10,000-50,000m 3 /h. Typical spray losses are % of the turnover rate, so the cooling tower looses L/h of water drops. These drops are of the same composition and so of the same contamination as the loop water. They rain down from the top of tower to area around the tower building. They can be inhaled as aerosols containing up to millions of CFU (Colony Founding Units) per m 3, this is extremely much more than normal ambient air that has only some hundred CFU per m 3. Wasser Berlin

3 Spray losses also cause the contamination of the ambient air with the applied biocide. Typical concentrations for shock dosing are mg/L in loop water. So spray losses of L/h causes 7-28g/h output of the biocide in the aerosols. Ozone treatment Ozone has proved its value in the disinfection and oxidation of water in many areas of water treatment for more than 50 years. In the past few years, ozone has been used increasingly and successfully in the treatment of cooling water. Ozone is a powerful biocide, which compared to the frequently used organochlorine-based biocides, do not only combat microbial growth processes but is also continuously reducing organic impurities in the system. There are several important advantages of ozone compared to chemical biocides. The most important difference in the treatment with ozone is that the micro organisms are not resistant against ozone. This allows continuous dosing of ozone into the water and thus achieving a steady disinfection rate. The high efficiency of ozone allows dosing at very low concentrations much below 0,1g of ozone per rated m 3 of the turnover water. The output of ozone in the operation air is normally much below the natural concentration of ozone in the ambient air, the output of bacteria is at a constant low level. So ozone is the hygienic solution for cooling water treatment. In contrast to chemical biocides which release bromine or chlorine, no chlorinated compounds are formed during the treatment of water with ozone, which means that the AOX value remains unchanged. Ozone oxidises the oxidisable substances which enter the cooling water, thus reducing the chemical oxygen demand (COD) of the circulating water. Coolzon treatment The BWT cooling water treatment with ozone is shown on the following diagram. Fig. 2: The Coolzon Process Wasser Berlin

4 The key idea of this special treatment is to separate the cooling loop from the added treatment loop in order to be able to continue the treatment even when the cooling tower is off, because the absence of cooling operation does not stop the growth of the micro organisms. The second, but not less important property of the separate treatment loop is the layout of the treatment as a two step process. The first step is a filtration followed by the ozonisation as second step. The filtration has several aims: it keeps the water clean, it reduces the food supply for the micro organisms and thus the undesired consumption of ozone is reduced. Fig. 3: The Coolzon 4, Stuttgart A Coolzon 4 unit of 10m 3 /h filtration rate and 4g/h ozone capacity is shown in Figure 3. All components required for the treatment are preinstalled on a rack, so that the installation at site is very easy. The capacity of a Coolzon 4 unit is sufficient for the treatment of a 1MW cooling tower with a cooling water turnover rate of 140m 3 /h and an operation air flow up to 50,000m 3 /h. The results of two installations, each with two cooling towers, are shown in fig. 4: Wasser Berlin

5 Fig. 4: Results from Stuttgart and Karlsruhe Test results of cooling tower system Stuttgart, capacity 2 x 430 kw Parameter unit make up water water from tower VDI 3803 Colony-forming units 20 C - 44h 1/1ml - < 10 < Colony-forming units 36 C - 44h 1/1ml - < 10 < C 1/1ml - 0 < 10 ph-value - 7,9 9,28 - Chemical Oxigen Demand COD mg/l 14,1 8 - Test results of cooling tower system Karlsruhe, capacity 2 x 480 kw Parameter unit make up water water from tower VDI 3803 Colony-forming units 20 C - 44h 1/1ml - < 100 < Colony-forming units 36 C - 44h 1/1ml - < 1000 < C 1/1ml - 0 < 10 ph-value - 7,5 9,2 - Chemical Oxigen Demand COD mg/l 2,8 8,1 - The microbiological requirements valid in Germany due to the VDI 3803 are much more than fulfilled. The measured values of Colony forming units (CFU) are far away from the limits, especially Legionella where not found although just those had contaminated the system of Stuttgart before starting up. The biofilm that was present at the start up in both systems vanished during a few weeks almost completely. Even when the cooling tower is switched off for several hours or whole days, the disinfection quality can be maintained by the interval treatment mode of the Coolzon system. In the interval mode the ozone treatment loop is repeatedly switched on for a distinct time after the cooling tower is switched off. So both operations lead to a really satisfying result. Both cooling tower systems shown in figure 4 are very clean and show no corrosion. The necessary cleaning cycles of the system could be extended from 3 month to 12 month by means of the Coolzon treatment, especially the heat exchangers remains clean. Neither biofilm nor limescale could be found. So shut down time of the cooling tower and maintenance costs are strongly reduced, furthermore the electricity consumption is reduced thanks to the heat exchangers working with their maximum efficiency. Conclusion Ozone is the state of the art solution for the treatment of cooling tower water. Its application solves the problem of conventional chemical treatment systems, producing poisonous chemical and biological contaminated aerosols into the environment. The Coolzon treatment applies ozone in a separate treatment loop, consisting first of a filtration step followed by the ozonisation of the cleaned water. The treatment degrades biofilm in the cooling loop and keeps the water clean. The high disinfection rate of ozone strongly reduces the latent danger for the health of human being inhaling the aerosols. The Coolzon treatment furthermore reduces the operational costs of the whole cooling tower system, while the availability becomes higher. So Coolzon is really a sustainable and hygienic solution for cooling towers. Wasser Berlin