Exploring the Role of Glycol in Water Treatment
Glycol refers to a class of organic compounds within the alcohol family, characterised by the presence of two hydroxyl (-OH) groups attached to different carbon atoms. These compounds are widely utilised in water treatment due to their heat transfer and antifreeze properties.
The primary advantage of glycol in water treatment is its ability to prevent freezing in cold environments through a process known as freezing point depression. This process occurs when a solute, such as glycol, is added to a solvent like water, resulting in a lower freezing point for the solution compared to pure water. This lowered freezing point allows the glycol-water mixture to circulate within the system without freezing under sub-zero conditions, ensuring continuous operation. Negligence or incorrect treatment of these systems can lead to freezing, which in turn, can result in costly mechanical failures of the system pipework and fittings.
In water treatment, the most commonly used types of glycol are monoethylene glycol (MEG) and monopropylene glycol (MPG). While these glycols share similar properties, they are utilised in different applications. MPG, with its lower toxicological profile compared to MEG, is preferred for processes involving food, pharmaceuticals, and other sensitive products exposed to humans. MEG, on the other hand, is typically used in chilled systems where there is no risk of human or sensitive product exposure.
As general guideline, most systems are dosed with glycol at 25% system volume. However, depending on the frost protection required, it is common for the product to be dosed up to 35% system volume. It is important to take care and monitor the system if dosing glycol at higher concentrations as it can lead to pumping problems. At a 25% concentration, you can expect frost protection down to -10°C and -12°C for MPG and MEG, respectively. To ensure you have dosed the correct amount of glycol, you can use a refractometer to measure the glycol concentration in the system.
Although glycol can exhibit biostatic properties (inhibit bacterial growth) at higher concentrations (i.e., >20%), it is important to dose with an appropriate biocide so there is adequate protection if the glycol concentration is to decrease. If the glycol concentration falls below 20%, and no biocide is present, bacteria can proliferate and further degrade the glycol. As the glycol breaks down, it forms acidic byproducts (such as acetic acid and formic acid). Furthermore, this degradation reduces the system's pH, leading to issues like corrosion and biofilm formation.
Glycol can come pre-blended with inhibitors and biocides if approved by the Biocidal Products Regulation (BPR) for specific applications. Since glycol is more corrosive than water, having the corrosion inhibitors pre-blended in the glycol solution simplifies your water treatment regime as you only dose one product rather than multiple separate products. However, if you prefer to have more control over your inhibitors and biocides, dosing separately alongside the glycol is a standard practice. All WTP inhibitors, including WTP M27 and WTP CW25, are compatible with glycol, but it is always best to check with your supplier for chemical compatibility.
References:
Simpson, P., 2nd ed. (2021) BG50/2021: Water Treatment for Closed Heating and Cooling Systems. BSRIA.
