In Industrial Cooling Towers (CTs), biofouling is a major concern as they can have a crippling effect on cooling water circuits. Bio films are layers of bacterial colonies, which form saccharide or ‘sugar’ linkages between these colonies. This results in the formation of a film or slime layer on the substrate or any surface in the water circuit.
They utilise nutrients and other microbes present in water to propagate and increase in thickness. Biofilms are the result of evolution by microorganisms to survive in fast moving streams of water. They use the linkages to strongly adhere to any surface or substrate. The structure of these biofilms is referred to as “Woven Matrix Colony”. This makes their removal a major challenge in CTs.
The matrix is basically water. The microbes are dispersed in this matrix of water. Biofilms are made up of 85 to 95% water. The temperature and free nutrient availability in cooling towers make them ideal for biofilm growth. The protection afforded to the microbes by the slime allows for rapid multiplication in the numbers of bacterial colonies – causing the biofilm to increase in thickness, and it results in various problems in the cooling circuit.
Reduction of heat transfer efficiency
Biofilms have one-fifth the conductivity value of carbonate scale. Hence, 1mm of bioslime has the same effect as a 5mm carbonate scale layer. Carbonate scale of around 5 mm thickness reduces the heat transfer efficiency by at least 27%.
Since biofilms have one fifth the conductivity of a carbonate scale, it can be inferred that a 1mm layer of bio slime causes the same decrease in heat transfer efficiency.
Hence, for the same circulation rate ΔT will be reduced by the same amount. Conversely, for the same ΔT, the circulation rate required will increase by an equivalent amount.
Increase in Corrosion: Sulfate and Iron Reducing Bacteria in the circuit can greatly increase the corrosion rate in the pipelines. If unchecked this can even result in the need for changing of the entire pipeline.
Friction Loss: Biofilms function as stagnant films of Water. This reduces the effective pipe diameter, resulting in the increase of friction losses leading to higher head requirements. For power plant and Industrial cooling towers, the circulation rates are so high that even a marginal increase in head loss or pump head requirement leads to a huge increase in BKW and energy costs.
Shut Down for Cleaning: If biofouling reaches an alarming level where shut down and cleaning of the cooling towers is required, then the process shutdown costs in the industries or the zero power generation costs in power plants are enormous. If not properly controlled, this can occur with greater frequency causing huge losses.
Complex Biocide Programs: If disinfection is not satisfactory, complicated biocide programs with frequent change in biocides become necessary. These result in high chemical costs.
Use of Surfactants, Cleaning Agents: The need for surfactants, cleaning agents etc., are increased with increase in fouling and this also leads to higher chemical costs.
Circulating water systems operate mostly between 30 to 40OC. These are ideal breeding grounds for various pathogenic and resistant bacteria. Bacteria like Legionella residing in the biofilm can reach critical pathogenic concentrations and result in outbreaks of Legionnaires disease. It is essential to keep biofilms under check to prevent any such outbreaks.
Conventional treatment and its problems
Chlorine is most commonly used for the treatment of biofouling. Disinfection effect of chlorine is highly pH dependent – almost nil beyond pH > 8. Chlorine has limited action against biofilm and Blue Green Algae. It is ineffective against certain deadly pathogens like Legionella, which are the most common problems in CTs. If sodium hypochlorite is used then the degradation of its concentration is much faster. In case of chlorine gas, there is a high risk in storing and handling. Also, statutory regulations are high in storing chlorine tonners. When organic biocides are used, certain bacteria and pathogens become resistant over the period of time, and hence, the treatment programme needs to be continuously revised. Biocides are generally proprietary, and hence expensive.
Biofouling treatment with ClO2
Chlorine dioxide (ClO2) is a highly effective, environmentally friendly microbicide. It is a selective oxidant that eliminates both planktonic and sessile bacteria, disinfects surfaces, and destroys biofilms very rapidly. Chlorine dioxide is a stable, dissolved gas that is a strong bactericide and virucide at concentrations as low as 0.1 ppm. With minimal contact time, it is highly effective against many pathogenic organisms – such as Legionella, amoebal cysts, Giardia cysts, E. Coli, and Cryptosporidium. ClO2 destroys biofilms, and therefore bacterial regrowth is significantly impeded.
Chlorine dioxide is becoming increasingly popular in the field of water treatment as the disinfectant of choice for many water treatment applications. The chemical formula for Chlorine Dioxide is ClO2. The properties of ClO2 seem to be an ideal mix of the salient properties of ozone and chlorine.
Chlorine dioxide is environment friendly, and in fact, is a pollution free technology, that assists in protecting the environment and human health, from bacteria and by-products formed from other disinfection methods.
Effective even at wide range of pH
Unlike chlorine and bromine, it does not form weak acids in aqueous solution. For example, in case of chlorine it forms hypochlorous acid (HOCl) in aqueous solution which actually is the disinfectant and not the chlorine as such. This reaction is highly pH dependent. The formation of HOCl is highly pH dependent, at lower pH (<4) value it remains as chlorine. When the pH goes beyond 7 it starts forming hypochlorite ion.
Hypochlorite ion is a weak disinfectant. Especially, for CT applications where the pH is maintained between 7.8 and 8.2, only 65 to 85% of chlorine dosed will remain as hypochlorous acid (HOCl) to do the disinfection – rest will be as weak hypochlorite ion. In other words, to have desired result, we may end up dosing 3 to 4 times of the actual demand. Since chlorine dioxide is a dissolved gas, this allows ClO2 to be effective over a wide pH range. ClO2 being a neutral species with rapid disinfection kinetics, is 100% available for disinfection in hard or soft water. So, in case of ClO2, we just need to dose what exactly is required. Fig. 2 is a graphical representation showing how the pH affects the biocidal activity.
Mechanism of disinfection by ClO2
Chlorine dioxide disinfects by disrupting metabolic cycles of microorganisms due to its effect on DNA. This makes it very effective even against virus spores.
This characteristic of ClO2 also ensures perfect disinfection of the water eliminating bacteria, viruses, fungi, algae, protozoa etc. It is approximately 2.5 times more oxidizing than chlorine and hence is able to penetrate tough biofilm layers. This makes chlorine dioxide the ideal disinfectant for biofilm removal.
The mechanism of disinfection coupled with its very strong oxidizing nature allows chlorine dioxide to destroy chlorine resistant biofilms, and many other chlorine resistant germs.
Biofilm is similar to a spider web in design and function. When certain microbes reach a surface, they attach themselves by producing polysaccharides (the web). This material is sticky and very difficult to remove.
Channels are formed in this film, through which water flows. The sticky web catches nutrients and other microbes that pass by, providing food and a quick growth mechanism. Once a biofilm is established it is very difficult to remove, often requiring manual cleaning.
It forms a habitat for pathogenic organisms. Even if all water-borne microorganisms are eliminated, regrowth quickly occurs due to bacterial communities and nutrients in the biofilm. The microorganism in biofilm is often vastly in excess of the quantities of those in the planktonic phase.
Chlorine, by and large, is a good disinfectant (when we use around the pH of 7). However, chlorine cannot be used as a substitute to biocide. Biocidal activity of chlorine is very less. In presence of biofilm, chlorine also loses effectiveness as disinfectant – as it cannot penetrate biofilm cells and kill the pathogens that are underneath the biofilm.
ClO2, like ozone, is a dissolved gas that penetrates biofilm by molecular diffusion. However, unlike ozone ClO2 is stable and soluble, allowing it to travel to the base of the film – where it attacks microorganisms and destroys the biofilm at its point of attachment. Other oxidizers react mostly on the surface of the biofilm to form an oxidized layer, like charring on wood. This precludes further penetration. No biocide has proved to control biofilm better than ClO2.
In terms of stability, ClO2 is much better than Cl2 or ozone. The residual ClO2 remain in a longer time compared to that of chlorine or ozone in a closed environment. Fig. 3 shows how stable ClO2 is in comparison with chlorine.
Unlike chlorine, ClO2 is very selective in its reaction. It does not unnecessarily react. For example, if the water has got ammonia and urea, in case of chlorine it gets consumed, so the dosage goes higher than the demand. In case of chlorine dioxide, it does not react with ammonia and urea, hence, we dose only for the biocidal and disinfection activities, which make it ideal for fertilizer plants.
Also, chlorine dioxide does not react with oils that makes it suitable for applications in petrochemical industries.
In early part of this article, we have seen sulfide and iron reducing bacteria in the circuit can greatly increase the corrosion rate in the pipelines. In addition to this, if we dose more oxidants than the demand – it will worsen the situation and cause more corrosion that happens more often with chlorine.
In case of chlorine dioxide, it can oxidise sulphide and iron reducing bacteria, and prevent corrosion that are caused only by this. Secondly, the dosage rate requirements are very less compared to chlorine, and hence there is no additional corrosion associated with excess dosing. In other words, chlorine and chlorine dioxide have similar effects in causing corrosion, but as the dosage rate of chlorine dioxide is very less compared to that of chlorine, corrosion rate will also be proportionately less.
Though the cooling waters are not in direct contact with humans, there are health issues related to poor treatment of cooling water through Legionnaires.
Legionnaires’ disease is a potentially fatal pneumonia caused by Legionella bacteria. Infection is caused by breathing in small droplets of water contaminated by the bacteria. The disease cannot be transmited by physical contact.
Everyone is potentially susceptible to infection. But some people are at higher risk, e.g., those over 45 years of age, smokers and heavy drinkers, those suffering from chronic respiratory or kidney disease, and people whose immune system is impaired.
Legionella bacteria is common in natural water courses such as rivers and ponds. Since Legionella are widespread in the environment, they may contaminate and grow in other water systems – such as cooling towers and hot and cold water services. They survive in low temperatures and thrive in temperatures between 20-45OC, if the conditions are right, e.g., environments present with nutrients such as rust, sludge, scale, algae and other bacteria. They are killed by high temperatures.
Awareness about Legionella in India is slowly rising, and awareness is needed further. In some of the developed countries, they monitor the deaths due the Legionnaires as we do the same for road accidents. Some of the countries have made some guidelines for the employers to understand the health risks that are associated with Legionella – and how to control them.
Chlorine dioxide is very effective against this deadly bacterium. Secondly, in case of ClO2, there is no hazard of storing deadly chemicals such as chlorine gas, since ClO2 is generated onsite and dosed immediately.
Generation of ClO2
Chlorine dioxide cannot be transported or stored for longer periods, and it needs to be generated on site using safe generation methods with the required safety tips and monitoring systems.
Chlorine Dioxide can be generated by various methods, however for industrial applications, the below methods are used:
Generation from chlorite and hydrochloric acid
This process is most commonly used in the field of drinking water disinfection given the reliability of its operation and no problems associated with chlorine handling.
5 NaClO2 + 4 HCl → 4 ClO2 + 5 NaCl + 2 H2O
Generation from chlorite and chlorine
There are two processes for the production of chlorine dioxide by means of oxidation of sodium chlorite with chlorine: the first uses chlorine in aqueous solution in the form of hypochlorous acid, while the second one uses chlorine in molecular gas form. The first system for the production of chlorine dioxide consists in pumping a sodium chlorite solution into a chlorine aqueous solution.
The two solutions react as follows:
2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
Generation from chlorite, hydrochloric acid and hypochlorite
In this process chlorine dioxide is generated using three chemicals. Because of the volatile nature of hypochlorite, purity and yield of the ClO2 is not so consistent in this method of generation.
2 NaClO2 + NaOCl + 2 HCl → 2 ClO2 + 3 NaCl
Though the chemistry looks very simple, the equipment part of generation is not as simple as the chemistry. Yield and safety are the most important parameter of equipment performance.
Yield is the ratio between theoretical and actual amount of ClO2 generated. On the safety front, ClO2 can be explosive above a concentration of 30 g/l in water, and so the safety interlocks should be fool proof to ensure the maximum concentration in any part of the system is well maintained within this.
There are basically 2 types of generators available: Surface mounted and Underwater.
In this type of generator, the reactor and chemical dosing pumps are mounted on a common skid with all safety interlocks hooked up – to a Programmable Logic Controller (PLC) to ensure safe and efficient generation.
These types of generators are best suited for smaller amount of ClO2 generation say up to 500 g/hr, which are the most common requirements in food, beverage and pharmaceutical water disinfection.
In this type of reactors, the formation of chlorine dioxide takes place only in the water, and it is not present in any other part of the plant.
For capacities higher than 500 g/hr, we can say it is a safe generator – because there is no possibility for ClO2 to be released to the atmosphere. Also, the size of the reactors are much compact and the yield is 95% +/-2 compared to 80 to 85% for the surface mounted reactors.
Dosing pumps used for chemical injection into the reactors play a major role for the high yield. The latest technology-based Grundfos Digital dosing pump, that uses stepper motor technology has got high accuracy of +/-1% with very low pulsation, and is best suited for this application.
The other alternatives such as electromagnetic or mechanical dosing pumps, which are having accuracy level between 4 and 8%, will reduce the yield and thereby increase the operating cost substantially.
In terms of operating costs, with high yield ClO2 generators, the operating cost will be lower than that of conventional systems for cooling tower applications – and offers high degree of environmental safety – and better life for the capital equipments.
Grundfos has got highly reliable, safe and efficient ClO2 generators. The Grundfos team, with its vast experience and expertise, can provide effective solution to your cooling tower biofouling problems.