At least three methods can be used to control the addition of chlorine to a process, and each measures a different characteristic. How do you determine which is the best method for your process?

Figure 1. In flow-proportional control, a volume of chlorine is injected relative to the flow rate of water through the system.

Methods for controlling the addition of chlorine to a process include flow-proportional injection, oxidation-reduction potential (ORP) and residual-chlorine analysis. Each measures a different process characteristic. Depending on the specific process, there generally is a best choice for controlling the injection of either chlorine gas or sodium hypochlorite, which is liquid bleach (NaOCl). Knowing the exact measurement for each method, the desired control situation, the limitations and the required routine maintenance will help users determine the preferred method for their applications.

Table 1. The chart lists the volumes required per gallon to achieve a 1-ppm level of chlorine. To reach a concentration of 5 ppm, for example, simply multiply the values by five.

1. Flow-Proportional Injection

In flow-proportional control, a volume of chlorine is injected relative to the flow rate of water through the system (figure 1). In most applications, the system is configured to enable the dosing of the chlorine (generally, bleach through a metering pump) to achieve the desired final chlorine concentration, for example, 5 ppm. Table 1 lists the volumes required per gallon to achieve a 1-ppm level of chlorine. To reach a concentration of 5 ppm, for example, simply multiply the values by five.

The flow-proportional control system uses a flow sensor to determine the flow rate of the carrier solution, which most often is water. Many flow sensors produce a pulse output per volume of flow directly from the sensor. The signal may be taken through an intermediate flow controller -- normally required when the pulse rate off the sensor is not adjustable -- or directly into a metering pump. The function of the intermediate meter is to ratio a pulse output to that which is received from the flow sensor. Other functions such as flow-rate monitoring and flow aggregation also may be achieved.

Many metering pumps have an on-board microprocessor-based controller that can multiply or divide the pulse signal directly. This eliminates the need for an intermediate controller for flow sensors with non-adjustable pulse outputs, which simplifies the overall situation.

Determining Injection Rate. The pulse rate required for the application is determined by the amount of bleach required per gallon (generally shown as milliliters) and output per stroke of the selected metering pump. Adjusting the pump's stroke length can enable the output per stroke to be trimmed to exactly the dose per stroke needed. For example, if 5 ml of sodium hypochlorite is required for each gallon of water, a pump with an output of 6 ml per stroke can be set at an 80 percent stroke length and programmed to stroke once for each pulse received from a flow meter generating 1 pulse per gallon (ppg). As the flow rate increases, the pulse output from the flow meter increases proportionally, achieving a consistent dosing per volume of flow through the system.

Using a smaller metering pump (determined by ml/stroke) pulsing at a higher rate -- for example, operating at 10 strokes/gal vs. 1 stroke/gal -- can reduce "slugging" of the chemical into the flow stream. Metering pumps can stroke at 300 to 360 per min, virtually eliminating slug feed of the hypochlorite.

Selection of the flow meter/sensor as the pulse-generating device can affect the overall reliability and consistency of the chlorine injection. Hall Effect sensors generally are preferred over mechanical reed switches, which usually should be avoided at rates greater than 100 ppm. Reed switches also have a short overall life. The disadvantage of the Hall Effect sensor is that it requires external power, generally in the range of 6 to 24 VDC. Sensor installation should follow the manufacturer's guidelines for an upstream straight run to ensure linearity across the flow range.

The flow-proportional system for controlling chlorine injection is best used for inline (one-pass) applications such as chlorination of final rinsewater for washing/disinfection of produce or poultry. In other systems where chlorinated water is recirculated, it is not safe to assume that all chlorine added previously was consumed and requires replacement. For such processes, either an oxidation-reduction potential or residual chlorine system should be used. The advantage of the flow-proportional system is that it has the lowest maintenance and lowest cost of all three options. Once installed and configured correctly, minimal attention is necessary.

Figure 2. The ORP measurement is able to monitor a drop in oxidation activity, indicating the amount of oxidizer that has been consumed, and control the addition of replacement chlorine or hypochlorite.

2. Oxidation-Reduction Potential

Oxidation-reduction potential (ORP) measures the oxidizing, or reducing, potential or activity of a solution. Chlorine gas and sodium hypochlorite (bleach) are strong oxidizers that can be monitored effectively with an ORP system. In many applications, the control system is intended specifically to control the oxidation of a species such as cyanide or hydrogen sulflide (H2S). In other cases, the oxidation is the disinfection of bacteria such as E. coli. ORP generally is the best measurement for processes of oxidation or disinfection because it measures the solution's true oxidizing activity.

The primary limitation of an ORP system is that it does not provide a direct, repeatable correlation to an exact concentration of chlorine. Two key factors affect this correlation: the pH of the solution and the chloride (salt) concentration. Changes in pH cause a shift in the form of chlorine in water. With increasing pH, chlorine in water changes from hypochlorous acid (HOCl) to hypochlorite ion (OCl-). The hypochlorous acid is much more active as an oxidizer (80 to 300 times stronger) than the hypochlorite ion. The result is that as pH increases, the overall oxidizing activity of the solution decreases.

True Oxidation Potential. The advantage of using an ORP system is that it measures the true oxidation potential, or capability, that a solution has. If the chlorine or hypochlorite is being added for the specific purpose oxidizing or disinfecting, ORP is the best method because it measures the variable that is most important: the solution's actual ability to perform the oxidation. A minimum ORP value of 650 to 700 mV can guarantee immediate destruction of E. coli bacteria. However, a given concentration of chlorine may have reduced activity because of high pH or high chloride concentration, and therefore be ineffective at destroying E. coli.

ORP is a good choice for recirculated systems because it maintains an oxidizing activity by replacing the oxidizer that has been consumed (figure 2). Water used for spraying foods often is recirculated in the initial washing stages and must maintain an oxidation capability to ensure against bacteria in the system or on the food. Pasteurization of canned food and bottles also maintains a constant disinfecting (oxidizing) capability to protect against containers that leak and could contaminate the process. Cooling towers utilize oxidizers to prevent biological growth in the system, and finally, air scrubbers will use chlorine to oxidize odors in an airstream. The ORP measurement is able to monitor a drop in oxidation activity, indicating the amount of oxidizer that has been consumed, and control the addition of replacement chlorine or hypochlorite.

ORP systems require regular maintenance. Generally, the system must be calibrated on a monthly basis using standard solutions, and the electrode must be replaced every one to two years. With proper maintenance, ORP can provide rapid online measurement of a process and reliable oxidizing/disinfecting control.

Some cooling tower controllers or pH/ORP controllers can be integrated with the metering pumps to provide a complete chlorine-injection system.

3. Residual Chlorine

Several technologies are available for monitoring residual chlorine, including polarographic, amperometric and colorimetric. In general, each system may be set up to monitor and control either free residual chlorine that is generally in the form of HOCl or OCl-, or total chlorine that consists of both the free residual chlorine and any combined chlorine that may be present. Combined chlorine usually is in the form of chloramines that are very weak oxidizers and ineffective for oxidation.

The advantage of a residual chlorine system is that it can provide an exact readout of the actual chlorine present in a solution. If actual chlorine concentration is the key variable for the process and not its oxidizing capability, or if the presence of chlorine can be damaging to a process, then a residual chlorine analyzer should be used. Membranes used for water treatment are often damaged by chlorine. A residual chlorine analyzer can more accurately detect chlorine breakthrough from an activated carbon bed or other chlorine removal process, and alarm for unsafe levels before expensive membranes are damaged. In other applications, a regulating agency may require a specific chlorine concentration in a process; a residual chlorine monitor is best suited to accurately report these values.

An automatic air-vent valve ensures that this metering pump stays primed. The flow sensor activates an alarm and/or a backup pump in case of unit failure.

Unfortunately, no one method is accepted as a standard for residual chlorine measurement. All residual chlorine systems are higher in cost than a flow or ORP system and require significantly more maintenance. Depending on the system selected, maintenance may involve membrane replacement on the sensor every four to eight weeks; replacement of the membrane and electrolyte solution on the sensor every four to eight weeks; or reagent replacement every seven days.

With multiple options available for monitoring and controlling chlorine, selecting the best one requires users to review the application, determine the desired result and understand the advantages and limitations for each type of control. PCE