Biofouling Control in Cooling Towers with Halogen Stabilizer

Written by: Ludensky, M., Sweeny, P., Lammering, D., A. Zakarian, and E. Meyer


Biofouling in cooling towers is undesirable because it can reduce heat transfer efficiency, restrict water flow, and accelerate corrosion rates. Of even greater concern is the fact that pathogen growth in cooling towers can lead to disease transmission. Given the favorable growth environment of a cooling tower, these microorganisms can reproduce, proliferate and form complex biofilm communities. Legionella bacteria, which cause Legionnaires’ disease, are one of the greatest concerns from a public health standpoint because infections are often lethal and cooling towers are the most frequently reported non-potable water source of Legionnaires’ disease outbreaks (Llewellyn 2017).

Planktonic (free-floating) Legionella are relatively easy to kill, with CT values reported as low as 4 ppm-min (Kuchta 1983). In a diverse biofilm community, Legionella can be much harder to treat. Legionella can act as a parasite and multiply inside amoebae and ciliated protozoa. Living inside other organisms provides another layer of protection from biocides, beyond that already provided by the biofilm (EPA 2016). This means that in order to successfully control Legionella in cooling water systems the selected biocide program must be effective against a variety of both planktonic and sessile populations.

Chlorine is often the biocide of choice in cooling towers due to its high efficiency, low persistence (resulting in low environmental toxicity), broad efficacy spectrum, and ability to oxidize organic material. Chlorine can be added with a number of different products, including chlorine gas, sodium hypochlorite, chlorinated hydantoins and chlorinated isocyanurates (Puckorius, 1998a; Puckorius, 1998b; Nalepa, 2000; Ludensky, 2004). For all of these chlorinated products, when they are added to water, the active biocide that is released is hypochlorous acid (HOCl).

Because HOCl is so reactive, it can be depleted quickly by reaction with contaminants in the water. Public utilities often use monochloramine for drinking water disinfection because it is easier to maintain a residual through the distribution system (EPA 1999). Monochloramine has also been shown to have greater efficacy against biofilm than free chlorine (LeChevalllier 1988). A disadvantage of chloramines is their volatility, where not only is the sanitizer lost more quickly, but corrosion of structures in contact with the gases may occur (Holzwarth 1984).


Chlorine stabilizers such as 5,5-dimethylhydantoin (DMH) offer the best of both worlds for cooling tower applications.

The mechanism of stabilization is shown in Figure 1.

Dantobrum Halogen Release
Figure 1. Aphelion Chemistry

In the DMH/chlorine system, HOCl is still available for killing a broad range of pathogens. The DMH/chlorine system is also like chloramine in that it is easier to maintain a residual and enhanced biofilm activity is seen. The efficiency of hypochlorite biocidal efficacy has been shown to be dramatically increased in high organic demand papermaking environments in the presence of DMH (Sweeny, 1995; Sweeny, 1996).

There are two ways that DMH can be added to a cooling water system. It can be added in the form of solid halohydantoin products such as 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) or 1,3-dichloro-5,5-dimethylhydantoin. Alternatively, chlorine and DMH can be fed separately. By using separate addition, the advantages of high efficacy and low cost of sodium hypochlorite, coupled with the stabilizing effect of the DMH can be optimized. To facilitate the application of DMH to water systems a stable, concentrated aqueous solution was developed. The properties of this solution (designated Aphelion) are shown in Table 2.

Table 2. Aphelion Typical Properties

Aqueous solution
Clear water white liquid
DMH (%)
Crystallization point
< 4o C
Flash point
> 100° C
Density (g/cm3)
2.5cps@ 22oC
DOT Shipping Category

DMH Stabilization Efficacy

The mechanism of stabilization is shown in Figure 1.

Table 3. Stabilization effect of DMH in high halogen demand water conditions

Total Halogen, as ppm Cl2
% Remaining Halogen
Residual After 3 Hrs.
NaOCl/ DMH (1:1)
NaOCl/ DMH (1:1)

Aphelion dramatically increases the photostability of hypochlorite as shown in Figure 2. The photostability of hypochlorite is increased 100 fold when combined with Aphelion at a 1:1 molar ratio. Enhanced photostability is especially important in systems having open cooling tower decks, retention ponds or other areas of high sun exposure.

Figure 2. Photostabilization
Figure 2. Photostabilization

Chlorine efficacy with DMH- Planktonic organisms in low demand water

Free chlorine excels in killing planktonic organisms in water with low chlorine demand. Due to the dynamic equilibrium with DMH as shown in Figure 1, increasing NaOCl:DMH ratios will lead to increasing kill rates as shown in the following Figure. This result shows the ability to vary the mole ratio of NaOCl:Aphelion to achieve the desired efficacy level.

Figure3 Panktonic Chart
Figure 3. Planktonic P. aeruginosa Efficacy, pH = 9.0 low demand synthetic cooling water

Chlorine efficacy with DMH- Planktonic organisms in high demand water

The efficacy of sodium hypochlorite is reduced in high demand waters against a common biofilm organism, the sheathed filamentous species S. natans. In Figure 4 sodium hypochlorite additions of 20 ppm were required to initiate efficacy. In contrast in the presence of chlorine stabilizing Aphelion, hypochlorite addition levels of only 5 ppm are required. See Figure 5.

Figure 4. Naocl Planktonic Efficacy Vs. A Typical Biofilm Organism (S. Natans, High Demand)
Figure 4. NaOCl Planktonic Efficacy vs. a Typical Biofilm Organism (S. natans, high demand)
Figure 5. Naocl: Aphelion Planktonic Efficacy Vs. A Typical Biofilm Organism (S. Natans, High Demand)
Figure 5. NaOCl: Aphelion Planktonic Efficacy vs. a Typical Biofilm Organism (S. natans, high demand)

Chlorine efficacy with DMH- Planktonic organisms in high demand water

The planktonic efficacy of Aphelion stabilized hypochlorite was compared to sulfamic acid stabilized bromine systems in a high halogen demand environment. As shown in Figure 6 Aphelion stabilized hypochlorite provided enhanced efficacy compared to the stabilized bromine systems.

Figure 6. Planktonic Bacteria Efficacy- Field Inoculum, Ph 8.6 High Demand Synthetic Cooling Water
Figure 6. Planktonic Bacteria Efficacy- Field Inoculum, pH 8.6 high demand synthetic cooling water

Chlorine efficacy with DMH- Sessile organisms in high demand water

The ability of Aphelion to facilitate hypochlorite biofouling control is shown in Figure 7. Clearly indicated is the reduced expression of biofilm in the presence of Aphelion stabilized hypochlorite. The Aphelion stabilized sample produced only 3% growth, compared to 33 % growth of the unstabilized sample at equivalent hypochlorite addition levels. The amount of biofilm generated in the presence of Aphelion was 11 times less than that in its absence.

Contamination Chart Aphelion E1602795152861
Figure 7. Efficacy of Aphelion on Biofilm Control

Iron and Copper Corrosion

Aphelion reduces hypochlorite corrosion rates of both iron and copper. As shown in Figure 8 Aphelion reduces iron corrosion rates by 40% and copper by 70%.

Figure8 Corrosion Rate Chart
Figure 8. NaOCl and NaOCl:Aphelion Corrosion Rates (T = 22 °C, pH = 9.0, 2 ppm total halogen)

Field Trial

The performance of NaOCl in the presence and absence of Aphelion stabilization was evaluated in a medium sized, 12,000 gallon, Midwest cooling tower. This tower had a ~8 cycles of concentration, and a high halogen demand, requiring ~15 times the hypochlorite addition calculated from the make-up rate, to achieve a halogen residual.

Hypochlorite was initially fed in the absence of stabilizer after which Aphelion co-application was initiated utilizing the same hypochlorite feed rate. As anticipated the total residual halogen concentration increased substantially, from ~0.3 to ~0.6 ppm with concurrent reductions in the measurable planktonic population.

Figure 9. Cooling Trial Midwest Chemical Facility
Figure 9. Cooling Trial Midwest Chemical Facility

Water Testing

For monitoring residuals, DPD methodologies will yield a free halogen response proportional to the HOCl concentration while a DPD total measurement (less the observed free concentration) will yield the Aphelion stabilized concentration of the system.  

Generally, the ORP reading reflects the level of free oxidant in the system and so ORP values for chlorine will be lower in the presence of DMH.

Application Recommendations

For field applications it is most convenient to directly mix Aphelion 15 and 12% NaOCl through the use of a static mixer as shown in Figure 10. The mixed solution should not be put through pumps or any other mechanical devices. The mixed material when prepared this way increases to pH 13.5, heat generation is minimal (6°C increase). This solution is ideally injected without dilution directly into the process stream immediately after mixing (less than 1 minute and preferably less than 10 seconds). Due to the high pH of the mixed solution, side stream addition, if selected, should take into account the potential for calcium carbonate precipitation. The materials of construction after mixing, the static mixer and outlet piping should be either Teflon® or Kynar® (PVDF).

The application rate should be that to give the optimized weight ratio of Aphelion 15 and 12% NaOCl for the specific tower, this ratio will usually be between 2:1 – 5:1 NaOCl:DMH.

Figure 10. Aphelion Application Methodology
Figure 10. Aphelion Application Methodology


As shown in the data presented here, Aphelion is efficient in stabilizing chlorine residuals.  Aphelion provides excellent planktonic and sessile control in cooling water systems with high chlorine demand.  This technology has the flexibility of being applied at various ratios allowing optimization of microbiological control.  Aphelion stabilized programs can be applied to all chlorine treated cooling water systems, especially where biofilm control is critical. 


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