How to Manage Bacteria and Pathogen Growth in your Cooling Tower
Cooling towers give bacteria and pathogens optimal sunlight, temperature, nutrients, and oxygen levels to thrive. When not actively managed, microbial activity can lead to a host of undesirable outcomes that can both shorten the asset life and drive excessive operational costs. Specifically, microbial growth can accelerate corrosion, reduce heat exchange efficiency (biofouling), and lead to increased health risks from waterborne pathogens such as Legionella. While many operators are familiar with how “Chemical” treatment controls biological growth, we have found that it is equally important to consider two other “legs” to the stool: “Mechanical” and “Operational”. If all three legs are not employed in-concert within a Water Management Plan, the overall risk of having an undesirable outcome rises.
Mechanical refers to your cooling system’s design, materials, and layout. Architects of these systems are considering many different factors in their design choices for piping, pumps, heat exchangers, with biological mitigation historically taking a back seat to design choices that optimize heat transfer efficiency. Therefore, it is important to note that certain cooling systems inherently need more active management than others.
The first step in evaluating mechanical implications for biological growth in an existing system is to identify “dead legs”. These are areas within a system that experience little to no flow. Common examples include: equalizing lines, pipes running to “future” equipment (towers, chillers, pumps), and pipe extensions that are put in place to span structural supports. In particular, equalizing lines are an important place to start, as they are a staple in multi-cooling tower configurations. Their function is to allow the towers to keep a common water level, but are largely stagnant and therefore optimal places for biological growth.
Once dead legs have been identified, decisions are made on how to address these risks. If they are not needed, removal is optimal, but often not feasible. More often, strategies are applied with the intent of continuously moving water through these areas. For example, placing properly sized taps in strategic locations in these areas can stimulate flow and enable treatment to work as intended.
Onsite teams have to make regular adjustments to operate their cooling systems to meet load demands at maximum efficiency. These decisions include which units (pumps, towers, heat exchangers, piping) to turn on, for what duration, and in what combination or lead/lag cycle. While this is challenging enough on a daily basis, seasonal load changes further complicate this dance. It is therefore completely understandable that the impacts that these decisions can have on biological conditions is not often top of mind.
To keep these decisions simple, EAI recommends that operators set a goal of providing at least one hour of full flow, through all areas of the system, every day. Assuming the system has properly treated water, this allows stagnant areas to be flushed daily and replaced with fresh treatment. It is important to understand this does not necessarily mean an offline chiller, for example, needs to be turned on, but rather just supply full flow through the condenser. Depending on your system controls, this process can be automated in many cases.
Additionally, cooling tower cleaning and online disinfection are important maintenance activities that should not be skipped. OSHA recommends full tower cleaning and disinfection two times per year. At an absolute minimum, all systems should be properly cleaned and disinfected at least once per year. An example of an online disinfection includes flowing water to all parts of the system with a free chlorine level of at least five parts per million (ppm), for a minimum of six hours.
While it is no secret that chemical treatment is key to controlling pathogen growth, the nuances of chemical selection and dosing is less commonly understood. EAI Water subscribes to framework from the Cooling Technology Institute (CTI) recommendations for chemical treatment, which include:
- Oxidizing Biocide: these include bromine and / or chlorine chemistries. At a minimum, this should be applied once per day. This can be effective in single dosing applications; however, the best practice is a low-level continuous dosing as to maintain free halogen residual in the system. For continuous dosing, control equipment with real-time halogen reading can be very helpful for not over or under feeding oxidant.
- Non-Oxidizing Biocide – Intermittent feed of a non-oxidizing biocide is often recommended. While the frequency is application dependent, EAI often finds that a weekly feed in many HVAC and process cooling systems is successful. The important components to feeding a non-oxidizing biocide are understanding the volume of the cooling system AND ensuring the system has flow to all areas when the intermittent feed is applied.
- Bio Detergent – Another option is to feed a bio detergent to the system to aid in sloughing off sessile (surface) bacteria. These are effectively applied as an intermittent slug feed or, ideally, as a lower-level continuous feed.
If you have further questions on how chemical selection drives the budget and performance of your water treatment program, check out our previous blogpost here.
Along with our Three-Legged Stool approach, here are the tools your facility maintenance staff can employ to monitor bacteria buildup, or detect some of its side effects, in your evaporative cooling system:
- Heterotrophic Dip Slides: This sampling method allows you to gauge aerobic bacteria levels in cooling towers. A common sampling frequency is once per month, although weekly onsite monitoring is best practice. The goal should be to achieve target populations below 10,000 CFU/mL in open loop cooling systems. The process includes gathering a sample and incubating for roughly 36 hours before measuring the results.
- Handheld ATP Detector (adenosine triphosphate): A faster method to gauge heterotrophic counts in your system. Though it gives quick results, there is no fixed control range. Ideally, trends detected through an ATP device should be validated with dip slide analysis.
- Legionella Testing: While the items above can be “indicators” to the microbial activity in the system, the direct test for Legionella is also an option. These are traditionally done by gathering a sample and sending it to a CDC Elite Certified Laboratory. Results are typically available in 1 to 2 weeks.
In addition to these biological measurements, other key metrics should be monitored to indicate whether there may be a microbial problem, or whether interventions to control biological fouling are working. These include staying on top of your corrosion coupon monitoring, as well as the trends in your “approach temperature”, which is a primary indicator of heat exchange efficiency.
While it takes initiative, investing attention in each of the three “legs” to manage biological risk will pay dividends through reduced OpEx, longer asset life, and less emergency maintenance. Contact us if your team would benefit from further consultation on your cooling system.