Cooling Tower Chlorination Plant

Introduction

The proliferation of microorganisms and the resultant formation of slime is a problem which commonly occurs in aqueous systems. Problematic slime producing microbes may include bacteria, fungi and/or algae. Slime deposits typically occur in many industrial aqueous systems including cooling water systems, pulp and paper mill systems, petroleum operations, clay and pigment slurries, recreational water systems, air washer systems, decorative fountains, food, beverage, and industrial process pasteurizers, sweetwater systems, gas scrubber systems, latex systems, industrial lubricants, cutting fluids, etc.

Growth of these organisms is a serious problem in municipal and industrial water systems such as once-through or re- circulating cooling water systems, cooling ponds, intake pipes, ballast water tanks and ship reservoirs that draw water from infested bodies of water. Microbes may cause many problems including pressure loss from fouling of piping and heat exchanger, loss of heat exchanger efficiency due to coating of the heat exchange surfaces, promotion and acceleration of corrosion on metal surfaces, increased down time, or breaks in paper sheets in pulp and paper systems, and deterioration of cooling tower components.

This growth can cause severe plugging and damage to the systems they colonize, resulting in system down time and costly cleanings and repairs. As a consequence of the deleterious effects of uncontrolled biological growth and contamination in many industrial processes, different biocides and antimicrobials have been developed to aid in eliminating and controlling biological growth. Biocides and antimicrobials are used to control microbial growth in a number of different aqueous media.

Activity and Measurement of Chlorine

Below a pH of 1.9, chlorine exists as a diatomic molecule (Cl2) in water. As the pH increases above 1.9, chlorine oxidizes-water to produce HOCl, which further dissociates into OCl above a pH of 7.3. Elemental chlorine is not as effective in killing organisms as HOCl and OCl-. Thus, for optimum effectiveness it is desirable to control the pH of the cooling tower between 7-8 pH. It is also necessary to control the quantity of chlorine in the tower to insure there is enough to control the biological growth, but not so much that it causes corrosion of equipment or creates an excessive load on the water treatment facility. An ORP measurement system can be used to indicate both the quantity and activity of the chlorine in cooling water.

Since addition of chlorine increases the oxidizing capability of water, measurement of the ORP provides a useful indicator of the quantity and effectiveness of the chlorine present in the water. However, as we have seen, the pH affects the oxidizing potential of the available chlorine so the ORP will vary with pH changes as well as changes in the chlorine levels. To provide an indication of chlorine level we must compensate the ORP measurement for the effects of varying pH. One way to do this is to replace the Ag/AgCl reference electrode normally used with the ORP electrode with a pH measuring electrode.

Measurement System

If the total amount of chlorine in the system remains constant, but the pH changes, there will be a corresponding change in the measured ORP reading. Therefore to us ORP to control chlorine addition we must compensate the measurement for changes in pH. The simplest way to do this is substituting a pH electrode for the reference electrode used with a FLXA21 or PH450G configured for ORP service. This technique is only valid over a narrow range pH 6.5-8.0, and should only be used in simple systems that operate at stable temperatures. (The measurement is not compensated for temperature changes.) It should not be used in situations where there may be a large change in background composition such as wastewater streams or treatment ponds. Figure 1 shows typical ppm chlorine levels verses mV reading values.

Product Recommendations

PH450G four-wire pH/ORP Converter or FLXA21 two-wire pH/ORP Transmitter

SM21-AG4 pH glass sensor