Kolmetz.com Cooling Tower Monitoring 10 Page 5
treatment chemicals, engineering concepts, disposal methods, and testing and monitoring
programs.
During cooling tower operation, pH of the cooling water overtime will increase due to the
addition of water treatment chemicals, increase of alkalinity while operating at higher cycles, and
adding CO
2
from
scrubbing ambient air. Therefore, in most of the cases, acid is added to the
tower basin to reduce the pH to an acceptable discharge limit range at the blowdown to between
6 and 9. Residual halogens can be a problem due to the use of the non-oxidizing and oxidizing
biocides for microbial controls. Most permits have a residual halogen limit.
Temperature of the cooling tower blowdown is also an environmental concern for its effects on
the aquatic life in the body of water. The discharge temperature limit at the mixing zone is
usually set by the Government Agency in the wastewater permit and can not be exceeded.
Therefore most of the cooling water systems discharge their blowdown from the cold supply side
of the cooling water system. Water plume dispersion may need to be performed to evaluate the
mixing zone temperature at the cooling tower blowdown discharge.
In some countries, the total dissolved solid (TDS) concentration is also limited at the wastewater
discharge and thus restrict the number of cooling tower operating cycles depending on the TDS
concentration in the makeup water.
3.
History of Treatment Programs
Over time, some cooling water treatment chemicals have been identified to have an adverse
impact on the environment. Since then the treatment chemicals have been continuously under
technological improvement to have less impact on the environment. The earliest chemical for
treating recirculating cooling waters were inorganic polyphosphates and natural organic
materials. This concept was to add a small amount of acid to control the stability index to a
slightly scale forming value. Organic corrosion inhibitors include organic phosphorus
compounds; specific synthetic polymers, organic nitrogen compounds, and long chain carboxylic
acids.
The next cooling water treatment was chromate, an exceptionally reliable corrosion inhibitor.
Acid was added to the system to lower the pH preventing solids from precipitating. Although
chromate has done an outstanding job for years, increasing environmental concerns have brought
pressure on research into new corrosion inhibitors with potentially less environmental impact.
By the Mid 1960s it was apparent that even the low chromate residuals employed in most
chromate programs could be harmful to fish and other aquatic life. Federal, state and local
regulations brought increasing pressure against the use of heavy metals like chromium and zinc.
In 1971 Congress approved the National Pollution Discharge Elimination System (NPDES)
which called for plants discharging into interstate water ways to file for permits with the U S
Environmental Protection Agency (EPA).
