Water Treatment

Optimizing cooling tower performance

By our Editorial Team - 23/07/2018

Fouling associated with the excessive growth of microorganisms is a common issue in cooling tower systems. We consider solutions to this problem, and review a case study of electrochemical generation for water disinfection that minimizes operator risk and chemical handling, while increasing system performance.
Pesky little microbes
Excessive growth of microorganisms is a common occurrence in cooling water systems. Microbes are grouped into three broad categories: algae, bacteria and fungi. Under the right conditions, they can grow into large colonies that block water flow, impede heat transfer, induce corrosion and cause offensive odours, causing rapid degradation of the cooling system.
  • Algae mats and bacterial slimes often restrict water flow.
  • Slime growths and other microbiological debris can insulate the heat transfer surface, causing a dramatic reduction in heat transfer rates.
  • Bacteria, which are frequently anaerobic, include -reducing (SRB), which contribute to corrosion when metal structures are exposed to sulfate-containing water.
  • An unpleasant by-product of microbiological growths is odour. In particular, SRB emit a rotten-egg smell caused by the hydrogen sulfide liberated as a by-product of their metabolism.
  • Fungi attack the wooden structures of cooling towers by feeding on the cellulose or lignin components of wood. Three types of attack are common; brown rot, white rot, and soft rot.
Conventional solutions
A number of biocides have been used to help control the growth of microorganisms in cooling water systems. Broadly, they fall into two categories: oxidizing and non-oxidizing biocides. Oxidizing biocides include chemicals such as chlorine, chlorine dioxide, sodium hypochlorite, calcium hypochlorite, bromine, bromo-chloro-hydantoins and ozone. Non-oxidizing agents include isothiazolin, quaternary ammonium compounds, organo-tin compounds , organo-sulfur compounds, 2,2-dibromo-3-nitrilopropionamide (DBNPA), decylthioethaneamine (DTEA) or tetrahydro-3,5-dimethyl-2H-1,3,5-thiazine-2-thione (DMTT). These biocides can be added to cooling water systems either intermittently or continuously, as required by the system under use.
Typically, facilities use a variety of disinfection solutions when developing a treatment program. The effectiveness of the program is best determined by periodic checks of microbiological activity. Unfortunately, in some cases, conventional solutions do not control fouling as anticipated, preventing facilities from achieving their operational goals.
When they don’t work…
A South Dakota industrial manufacturer was combining ozone and a non-oxidizing microbiocide to manage microbiological growth in their cooling water system.  Unfortunately, this solution was not controlling fouling as anticipated, preventing the facility from achieving its operational goals. Furthermore, operators were manually feeding additional microbiocide chemicals (including bleach), to the cooling tower sump, multiple times during the week. This required additional handling and the need for appropriate PPE. The facility looked to their water treatment partner, U.S. Water, for a solution that would minimize operator risk and chemical handling, while increasing system performance.
As an alternative to the current program, U.S. Water recommended a mixed oxidant (MIOX) generating system for primary disinfection. The system used salt combined with electricity to generate disinfectant at the point of use, and could be fed directly into the tower sump or injected into the circulation loop.
The automated unit eliminated the delivery, storage and handling of hazardous chemicals. Not only was operator safety increased, chemical dosage could be managed and maintained. The MIOX unit, paired with a cooling tower control system, measured water temperature, pump speed and flow rate to ensure a safe and efficient cooling tower management program.
The unit was effective in protecting the cooling towers from microbiological. Operators noticed visible improvements to the cleanliness of the system, including the sump and filter screens, and a reduction of algae in the basin. Clean cooling tower and heat exchanger surfaces help to maintain necessary temperature differentials and achieve maximum heat transfer efficiency while optimizing water use. The facility reduced its chemical usage by over 2,000 pounds, saving approximately $8,300 annually, while eliminating the chemical handling risks and manual maintenance for the operators. 
A range of chemical solutions exist to manage the build-up of micro-organisms in industrial water systems. This management is a vital part of ensuring the performance and efficiency of the systems in question. When conventional solutions fail to have the desired effect, more advanced solutions, such as electrochemical generation for water disinfection described here, may be beneficial. These advanced solutions frequently come with added benefits such as reduced chemical exposure to operators, enhanced safety and reduced overall costs.
U.S. Water, 12270 43rd Street NE, St Michael, MN 55376, USA
T: +1 866 663 7633