Natural Pathogen Control Chemistry to Replace Toxic Treatment of Microbes and Biofilm in Cooling Towers

Abstract

Application of toxic antibacterial agents is considered necessary to control prevalent fresh water microorganisms that grow in evaporative cooling water systems, but can adversely affect the environment and human health. However, natural antibacterial water chemistry has been applied in industrial cooling water systems for over 10 years to inhibit microorganisms with excellent results. The water chemistry method concentrates natural minerals in highly-softened water to produce elevated pH and dissolved solids, while maintaining low calcium and magnesium content. The method provides further benefits in water conservation, and generates a small volume of non-toxic natural salt concentrate for cost efficient separation and disposal if required. This report describes the antimicrobial effects of these chemistry modifications in the cooling water environment and the resultant collective inhibition of microbes, biofilm, and pathogen growth. This article also presents a novel perspective of parasitic microbiome functional relationships, including "Trojan Protozoans" and biofilms, and the function of polyvalent metal ions in the formation and inhibition of biofilms. Reducing global dependence on toxic antibacterial agents discharged to the environment is an emerging concern due to their impact on the natural microbiome, plants, animals and humans. Concurrently, scientists have concluded that discharge of antibacterial agents plays a key role in development of pathogen resistance to antimicrobials as well as antibiotics. Use of natural antibacterial chemistry can play a key role in managing the cooling water environment in a more ecologically sustainable manner.

Keywords: Legionella; amoeba; antibacterial; antimicrobial; biocides; biofilm; cooling towers; pathogens; polyvalent metals; protozoa.

Figure 1

pH values with associated foods, chemicals, and natural environments [5].

Figure 2

Antimicrobial Effects of shifting pH on neutrophile survival. Neutral pH levels (± 1.5 pH units) are conducive to neutrophile microorganism proliferation. As pH rises, in the 9.0 to 10.0 range, stress increases on neutrophile metabolic processes. (Figure by Lon Brouse).

Figure 3

Effects of NaCl concentration on water balance in cells. (a) A typical bacterial cell with a semi-permeable membrane in isotonic NaCl solution. The water movement is at equilibrium into and out of the cell. (b) A plasmolyzed cell in a higher concentration, hypertonic NaCl solution. The water movement is unbalanced, resulting in cell volume shrinkage and disrupted metabolic activity [14].

Figure 4

Cooling tower water planktonic bacterial RLU (ATP) values vs. pH and TDS. (a) Shows RLU vs. pH for CT-A. (b) Shows the RLU vs. TDS for CT-A. (c) Shows RLU vs. pH for CT-B. (d) Shows RLU vs. TDS for CT-B. The linear inhibitory effects of increased pH and TDS vs. ATP (RLU) values measured in two industrial cooling tower systems [1], the green acceptable control limit line at 300 RLU and the red out-of-control limit line at 800 RLU, demonstrate reduced RLU (ATP) values and therefore the reduced microbiological masses in both systems as the pH and TDS values increase [1].

Figure 5

Molofsky and Swanson [26]; the life cycle of L. pneumophila. Studies of broth and phagocyte laboratory cultures support the following model for the persistence of L. pneumophila in aquatic reservoirs. The notes below match the numbered life stages in the modified Figure 5 above. These descriptions were also modified from the original [26] for clarity. 1. Free-swimming, planktonic L. pneumophila that are consumed by phagocytic cells (amoebae or alveolar macrophages) generate vacuoles that protect against lysosomal digestion. 2. When nutrients are present and the internal environment is favorable, intracellular bacteria activate pathways that promote replication. 3. As the environment conditions in the vacuole deteriorate, the offspring stop dividing and develop traits that improve survival in the environment and ingestion by a new phagocytic host. 4. After an extended period, the progeny may develop into a more mature intracellular form (MIF). This cell type is resistant and infectious. 5. The host cell is lysed, and the progeny microbes are released into the water environment. 6. L. pneumophila that are not immediately engulfed by a new phagocyte likely initiate biofilms in anthropogenic or natural water systems, where they are resistant to biocides. 7. When free-swimming microbes are engulfed by a new host, the cycle begins again.

Figure 6

As calcium (Ca²⁺) is added to Tryptone Soya Broth (TSB) inoculated with biofilm forming bacterial pathogen, S. paucimobilis, the CFU/ml count goes up significantly for sessile organisms on the growth plates but the free-living bacteria are not significantly affected, indicating a positive correlation between calcium concentration in the water and biofilm formation. (Graph generated with data from [11]).

Figure 7

As 10, 40, 200, and 400 mg/L of calcium (Ca²⁺) is added to minimum marine medium (MMM) inoculated with biofilm forming bacteria, the total cellular protein of Pseudoalteromonas spp. biofilms associated with (A) glass and (B) Teflon surfaces increased with time, directly with the increasing concentration of calcium ion. (Graph generated with data from [12]).

Figure 8

As the biofilm becomes more established, the microbial diversity increases which is a good environment for Acanthamoeba spp. Acanathamoeba can survive in the trophozoite and amoeboid cyst forms inside and outside biofilm. (Figure 8 generated by Daniel Brouse).

Figure 9

Biofilm formation-Legionella parasitic infestation of protozoa and effects of water flow on sloughing and redistribution of planktonic L. pneumophila in cooling water systems. (Image modified from [9]).

Figure 10

A horrible synergy—Control of scale and corrosion is part of microbial control. (Figure 10 generated by Daniel Brouse).