This is one of three article in the District Energy series on water quality and will focus on treating your building’s hydronic cooling loop for biological contaminants. Generally, the treatment of open cooling water systems gets much of the attention of the building owner and chemical consultants. While it is appropriate to give attention to open systems, often the chilled water systems get overlooked or receive minimal attention. Think of these systems as the heart (open system) and arteries (closed system), and you will see that they are equally important and dependent upon each other.
The good thing about closed chilled water loops is that they are closed. The bad thing about closed chilled water loops is that they are closed. This means that anything added to the system either intentionally or unintentionally stays in the system. Things that are intentionally added to the system include: biocides, inhibitors, and glycol. Glycol can be added either as part of a coil lay-up program or as a treatment plan (direct connect customers read more). Things that are unintentionally added to the system include: corrosion by-products, glycol from poor rinsing of laid up coils, mill oils from new piping installation, chlorides and sulfates from city water, dirt and debris from system maintenance, and possibly others.
Bacteria require food and water to live. Food is in the form of organics or inorganics. Because bacteria grow quickly under ideal conditions and then die when the conditions are no longer ideal, dead bacteria can help to feed young bacteria. Bacteria are troublesome because they can cause:
- Poor heat exchange capabilities.
- Localized high corrosion rates.
- Depressed pH and high general corrosion rates.
- Plugging of small openings.
The dollar value of the destruction caused by microbes in the industry is staggering. Accounting for mechanical work and interruptions in business easily make this a significant and relatively silent problem.
There is no single solution to treat for microbes in closed loops. It generally involves a number of steps that need to be done to ensure that these systems are being protected. These steps include:
- Remove any dead legs (areas with no flow) in the system as they are areas that are uncontrolled and generally will work as a site to constantly inoculate the system.
- Pre-operationally clean and passivate new systems to remove mill oils and general debris, which helps to clean out exiting corrosion and prevents future corrosion.
- Flush new and systems until the water is clean.
- Treat with a suitable corrosion inhibitor to control corrosion rates.
- Install a side stream filtration that is capable of constantly filtering 1-2% of the flow. Generally filters should start with 1-5 micron capability and be managed down to .5 micron as the larger particles are removed. Filtration cleaning takes a long time, but is required to remove food, dead bacteria, and corrosion byproducts.
- Monitor aerobic bacteria levels using paddle testers and anaerobic bacteria using a sulfate reducing bacteria test kit on a routine basis. (Samples can also be sent to a lab for analysis.) If sulfate reducing bacteria are found and/or the aerobic populations exceed 1,000 cfu/ml, then it is time to treat the system.
A system will require less treatment for microbes if the operators can prevent foulants or remove foulants through filtration. However, if a system does need to be treated, it can be treated with either oxidizing biocides or non-oxidizing biocides. Some of the oxidizing chemistries that can be used to treat bacteria include: chlorine dioxide, hydrogen peroxide, bromine, chlorine, and ozone. Some of the non-oxidizing biocides that can be used include: isothiazolone, glutaraldehyde, and quaternary amines. All biocides have instructions for use on the labels that dictate dosing that need to be followed. The drawback of the oxidizing biocides is that they can accelerate corrosion. The drawback of the non-oxidizing biocides is that they break down to provide food for future generations of microbes. A qualified chemical consultant can help to administer the proper biocide program.
Despite all of the challenges that biologicals can cause, properly managed systems will last longer and save money and time. Now that’s good news!
Note from your District Energy Customer Service Team: We work with many different water quality and treatment experts to help keep our distribution system and your building systems operating effectively. This article was developed in collaboration with one of our water treatment partners, ChemTreat, Inc. Thank you to Neil Swenson for helping with this article.