Control methods for microorganisms in circulating cooling water systems

1

Sources of Microorganisms in Recirculating Cooling Water

Microorganisms in recirculating cooling water come from two sources: firstly, a large amount of air is introduced during the evaporation of water in the cooling tower, and microorganisms are also brought into the cooling water with the air; secondly, the makeup water in the cooling water system will more or less contain microorganisms, which also enter the cooling water system with the makeup water.

2

Why are the microbial hazards in recirculating water much more serious than in straight-through water?

The temperature of the recirculating water, pH and nutrient content are all conducive to microbial reproduction, and the abundant sunlight on the cooling tower is an ideal place for algae growth. Straight-through water systems, however, lack the air-cooling evaporation process, only having microorganisms carried in with the water flow, and the conditions for microbial reproduction in straight-through water systems are not as favorable as in recirculating water (suitable water temperature, pH, and nutrient content). Most importantly, wastewater discharged from recirculating water returns to the system, creating a vicious cycle, while microorganisms that proliferate in straight-through water are immediately discharged. Therefore, the microbial hazards in recirculating water are much more serious than in straight-through water, and the higher the concentration factor, the more serious it becomes. For example, in some recirculating water systems, the total number of bacteria in the makeup water is only 100-1000, but the total number of bacteria in the recirculating water can be as high as 100,000, which causes microbial hazards in the system.

3

Commonly Used Microbial Treatment Methods in China

1. Strengthen raw water pretreatment and improve makeup water quality

Suspended particles, chromogenic substances, and other organic substances in raw water contain nutrients needed by microorganisms, especially when surface water is used as makeup water, the content of pollutants is higher, and pretreatment is needed for removal. Coagulation and filtration are commonly used as pretreatment processes.

2. Add biocides

Adding biocides is one of the more mature and effective methods for controlling microorganisms in recirculating water. Biocides are also called bactericides, algicides, inhibitors, or disinfectants. They are divided into two categories: oxidizing and non-oxidizing biocides.

（ 1) Commonly used oxidizing biocides.

① Chlorine and hypochlorites. Chlorine is the most commonly used biocide, with the characteristics of high efficiency, rapid action, broad spectrum, economy, and ease of use. It is particularly effective against animal plankton and bacteria. Generally, maintaining 0.5-1 mg/L residual chlorine can inhibit most microorganisms in cooling water.

② Chlorine dioxide (ClO2). Using chlorine dioxide as a biocide has the advantages of small dosage, fast action, and good effect. It is a strong oxidant, and its oxidation and killing ability is 25-26 times that of chlorine.

③ Chlorinated isocyanuric acid. Commonly used are sodium dichloroisocyanurate (DCCNa, trade name: 优氯净), potassium dichloroisocyanurate (DCCK), and trichloroisocyanuric acid (TCCA, trade name: 强氯精). A concentration of 30-40 mg/L has a better killing effect.

④ Bromine and bromides. Bromine is a brownish-red fuming liquid, similar in properties to chlorine. After dissolving in water, it reacts to produce hypobromous acid, which has an oxidizing and killing effect. Hypobromous acid can also ionize, generating HOBr and OBr-, and under high pH conditions, the bactericidal ability of bromine is stronger than that of chlorine. ⑤ Ozone. The killing speed is 300-600 times faster than chlorine, and it decomposes into oxygen after killing, causing no pollution to the environment.

（ 2) Commonly used non-oxidizing biocides.

① Chlorophenols. Chlorophenol compounds are a class of biocides that have been used for a long time, mainly including chlorophenol, dichlorophenol, trichlorophenol, and their sodium salts.

② Quaternary ammonium salts. Quaternary ammonium salts are a type of cationic surfactant widely used in the control of microorganisms in cooling water. It has a strong bactericidal effect, killing most microorganisms, has good permeability and dispersibility, is compatible with most water stabilizers and treatment agents, can be used in a wide pH range, has low toxicity to fish, is easily degraded, and causes little pollution.

③ Methyl isothiocyanate. Methyl isothiocyanate, also known as methyl dithiocyanate, is a widely used organic sulfur biocide, which is in the form of yellow or nearly colorless needle-like crystals. Methyl isothiocyanate is a broad-spectrum bactericide with good killing effects on bacteria, fungi, algae, and protozoa, especially on sulfate-reducing bacteria.

④ Isothiazolinones. Isothiazolinones are a class of newer broad-spectrum biocides. The preparation and separation process of pure isothiazolinone derivatives is relatively complex. For water treatment biocides, pure products do not need to be separated; only one step is needed to synthesize a mixture of these two isothiazolinone derivatives.

⑤ Glutaraldehyde. Glutaraldehyde is a highly effective, rapid, and broad-spectrum bactericide. It is water-soluble, can be dissolved in water at any ratio, is colorless, odorless, and non-corrosive when added to water, has a wide applicable pH range, is heat-resistant, is a special agent for sulfate-reducing bacteria, and can be biodegraded. The bactericidal action of glutaraldehyde is mainly due to the cross-linking effect on proteins in microbial cells.

⑥ Amides. Certain amide compounds have a strong killing ability against microorganisms. It is a highly effective broad-spectrum biocide, characterized by easy hydrolysis, and high pH, heating, and ultraviolet irradiation can all accelerate degradation. It is easily debrominated by reducing agents to become non-toxic ammonium cyanoacetate and loses its bactericidal activity. It is unstable under alkaline conditions and is generally limited to systems with a pH value <7.5 or 8.0.

（ 3) Precautions for the use of biocides.

① Biocides should be used in conjunction with dispersants. The purpose of using dispersants is that dispersants can disperse the bodies of dead microorganisms in the recirculating water system, peel off the sludge from the metal surface, expose the microorganisms that have not been killed underneath, and facilitate the exertion of the biocide's medicinal properties. The combined use of biocides and dispersants can significantly improve the effect of biocides, and the medicinal properties can be fully exerted.

②To avoid microbial resistance, biocides should be used alternately. Due to the strong resistance of microorganisms, long-term use of a single biocide will lead to microbial resistance. Therefore, oxidizing and non-oxidizing biocides should be used alternately.

③The influence of temperature and pH value on the effect of biocides. When the temperature of the circulating water system increases, the effect of biocides decreases, such as quaternary ammonium salts. When the temperature increases, the bactericidal effect of quaternary ammonium salts decreases. pH has a great influence on the effect of biocides, so the pH value of the circulating water system should be kept stable.

④Dosage method. Generally, impact dosing is considered. A one-time addition of a large dose of biocides can drastically reduce the number of microorganisms, making it difficult for microorganisms to recover to their original state.

⑤The influence of concentration factor on the dosage of biocides. With a low concentration factor, the retention time of the agent in the circulating water system is short, and the effect of the biocide cannot be better volatilized, which wastes both the agent and water resources. Generally, the concentration factor of the circulating water system is controlled at 3-5.

3. Protection of Cooling Towers

Protecting the cooling tower and changing the growth conditions of microorganisms can also inhibit the growth and reproduction of microorganisms.

（ 1) Prevent sunlight exposure.

Algae growth and reproduction require sunlight. In the absence of sunlight, they generally cannot survive. As long as the cooling tower is shaded in appropriate areas to avoid direct sunlight, algae reproduction will slow down or even stop. Generally speaking, covering the cooling tower water area and pool area to prevent sunlight exposure; installing louvers at the air intake of the cooling tower, both are very effective.

（ 2) Use biocidal coatings.

In order to prevent algae from spreading in concrete cooling towers, antibacterial coatings can be added to the coating (anti-algae coating) or biocides for protection. The coating is composed of modified sodium silicate, cuprous oxide, zinc oxide, etc.

（ 3) Protection of wooden structures.

The components of wooden cooling towers must be treated to prevent fungal corrosion. Supplementary anti-corrosion treatment can also be carried out on the wooden structure during the annual shutdown overhaul.

4. Regularly change water temperature, flow rate, and salinity

As mentioned above, microorganisms have different adaptability to temperature. Psychrophilic microorganisms are most suitable for growth at a temperature of around 10 ℃, mesophilic microorganisms are most suitable for growth at 20-35 ℃, and thermophilic microorganisms are most suitable for growth at around 45 ℃. Therefore, periodically increasing the water temperature is a good way to kill microorganisms without using reagents. This method is to appropriately reduce the circulating water passing through the heat exchanger, but the heat product flow rate on the process side remains unchanged, so as to increase the circulating water temperature. When the circulating water temperature is high enough and exceeds the lethal temperature of a certain microorganism, it will lead to its death. Sometimes, washing the system with hot water or steam is effective in controlling microbial growth.

5. System design and operation measures

（ 1) Design considerations.

Correct system design is the first step. To prevent the generation of microbial slime and corrosion, the system design and installation should avoid stagnant water areas and low flow rates. Therefore, careful consideration should be given to the selection of pipeline routes and valve positions.

（ 2) Operation and maintenance.

Correct maintenance and operation of the system and regular cleaning.

（ 3) Selection of corrosion-resistant materials.

Corrosion-resistant materials are best used where microbial corrosion is likely to occur. The microbial corrosion resistance of metallic materials is generally titanium > stainless steel > brass > pure copper > hard aluminum > carbon steel.

（ 4) Cathodic protection.

This method is similar to general cathodic protection, that is, a more easily oxidized metal is connected to the metal to be protected as the anode, making the protected metal the cathode to form a primary battery or an external power supply with a certain current for electrolysis.

4

Microbiological Control Technology

Experimental research and theoretical analysis show that microbial control is the primary task of the water quality control scheme for circulating water systems.

In the case of rapid microbial reproduction, effective methods must be taken to control microbial reproduction as soon as possible, such as using chlorine, peroxides and other oxidizing biocides, algicides and oxygen-free quaternary ammonium salts. When the ambient temperature is high in summer, the concentration of biocides and algicides is increased to restore control over microorganisms, and the amount of wastewater discharged is increased to reduce the content of pollutants, suspended solids and total soluble solids in the system.

Strengthen the intensity of biocides and algicides, reduce the total amount of microorganisms in the circulating water as much as possible, select appropriate agents, and for systems with a large amount of microbial slime, it is difficult to achieve good treatment results with biocides alone. The system should be sterilized, cleaned, and peeled off, and high-dose treatment using the synergistic effect of biocides, dispersants, and cleaning agents can achieve satisfactory results.