What does a cooling tower water treatment system typically control?

A cooling tower water treatment system is usually made up of technologies necessary to regulate the level/extent of:

1. alkalinity and hardness: control the potential of calcium carbonate scale. Scale on the heat exchanger of a cooling tower can significantly reduce the efficiency of heat exchange.

2. chlorides: can be corrosive to metals; different levels will be tolerated based on materials of the cooling tower and the auxiliary equipment

3. hardness: contributes to scale in the cooling tower and on heat exchangers

4. iron: when combined with phosphate, iron can foul equipment

5. organic matter: promotes microorganism growth, which can lead to fouling, corrosion, and other system issues such as foaming. It can also lead to fouling of other equipment treatment the blowdown such as Reverse Osmosis.

6. silica: is known for causing hard scale deposits

7. sulfates: like chlorides, can be extremely corrosive to metals and can form scales such as BaSO4 and CaSO4

8. total dissolved solids (TDS): contribute to scaling, foaming, and/or corrosion

9. totals suspended solids (TSS): undissolved contaminants can cause scaling, biofilm, and/or corrosion and can harbor bacteria

10. Bacteria and Algae: bacteria and algae result in biofouling on the cooling system surfaces thus resulting in heat exchange efficiency loss and can lead to the necessity of expensive cooling system component replacements such as pack replacements. In addition, certain bacteria, such as Legionella, if they proliferate in a cooling water system represent a health hazard which is required to be controlled by Law.There is also a risk that under-neath biofilm, a phenomenon called Microbial-Induced Corrosion can occur.

11. Ammonia: certain cooling waters can contain Ammonia which together with Dissolved Oxygen can be very corrosive to yellow metals such as Copper and Brass.

12. pH: this indicator measures the acidity in the water and this parameter is very important in a cooling tower as it affects the corrosive effects of the water

13. LSI: The Langlier Saturation Index is an indicator of whether a cooling water is balanced i.e. neither scaling nor corrosive. 

Technologies used in Cooling Water Treatment:

Makeup water, or the water replacing bleed and evaporated and leaked water from the cooling tower, is first drawn from its source, which could be raw water, city water, city-treated effluent, recycled in-plant wastewater , well water, or any other surface water source.

Depending on the quality of this water, you may or may not need treatment. If a water treatment system is needed at this part of the cooling tower water process, it is usually technology that removes hardness and silica, stabilizes and adjusts the pH and/or disinfects the system to control levels of bacteria and algae.

At this point of the process, the proper treatment optimizes the tower evaporation cycles and minimizes the water bleed rate to drain beyond what might be done with chemicals alone.

Clarification and Softening of Make Up Water

Some cooling towers use make up water from a hard surface source such as a river or lake. In this case it is necessary to treat the make up flow of water through a clarifier to remove suspended solids as well as hardness in the water. Clarifiers normally use a coagulant such as Ferric Chloride and flocculants which are polymers that help suspended coagulated particles to coalesce and settle in the clarifier.

Filtration and Ultrafiltration

The next step is generally running the cooling tower water through some type of filtration to remove any suspended particles such as sediment, turbidity, and certain types of organic matter. It is often useful to do this early on in the process, as the removal of suspended solids upstream can help protect membranes and ion exchange resins from fouling later on in the pretreatment process. Depending on the type of filtration used, suspended particles can be removed down to under one micron.

Ion exchange/water softening

If there’s high hardness in your source/makeup water which is cometimes municipal such as City of London water, there may be need for treatment to remove the hardness. Instead of lime/soda ash softening, a softening resin can be used; a strong acid cation exchange process, whereby resin is charged with a sodium ion, and as the hardness comes through, it has a higher affinity for calcium, magnesium, and iron so it will grab that ion and release the sodium ion into the water. These contaminants, if present, will otherwise cause scale deposits and rust.

Chemical addition

Certain chemicals sometimes need to be added to the cooling water such as:

corrosion inhibitors to protect metal components

algaecides and biocides (e.g., bromine) to reduce the growth of microbes and biofilms

scale inhibitors (e.g., phosphoric acid) to prevent contaminants from forming scale deposits

Thorough treatment prior to this stage can help reduce the amount of chemicals needed to treat water at this point in the process, which is ideal considering many chemical treatments can be expensive.

Ozone is considered an environmentally friendly biocide. 

Side-stream filtration

If the cooling tower water is going to be recirculated throughout the system, a side-stream filtration unit will be helpful in removing any problematic suspended contaminants that can enter through forced draft contamination, process leaks, etc. A good rule of thumb is that, if you cooling tower water treatment system requires side-stream filtration, about 10% of the circulating water will be filter through. It typically consists of a good quality multimedia filtration unit or a cyclone filter.

Blowdown treatment

The last part of treatment required for a cooling tower is the blowdown or bleed from the tower. 

Depending on how much water the cooling plant needs to circulate for proper cooling capacity, plants will choose to recycle and recover the bleed water through some type of post treatment in the form of reverse osmosis or ion exchange, especially in places where water might be scarce. This allows liquid and solid waste to be concentrated and removed while treated water can be returned to the tower and reused. 

If the water from your blowdown needs to be discharged, any discharge your system creates will need to meet all regulatory requirements. In certain areas where water is scarce, there could be large sewer connection fees, and demineralization systems can be a cost-effective solution here, as they can help minimize the cost to connect to water and sewer lines. Also, the discharge of your cooling tower bleed must meet local municipal discharge regulations if your effluent is being returned to the environment or a publicly owned treatment works.

Ultrasonic Sound Generators: have also been used to reduce the bacterial burden in cooling waters. Technologies such as SONOXIDE are in use.