Oxidation reduction potential cuts wastewater chemicals

Automatic chemical feed modulation by ORP also meets stringent disinfection requirements and chlorine discharge limits.

Directly measuring total chlorine residual has been widely practiced as a way to monitor water chlorination systems' effectiveness. Unfortunately, showing a correlation between residual concentration and degree of disinfection is difficult, especially with wastewater. As a result, total chlorine residual may not be the best measure for establishing water quality criteria or disinfection efficacy.

Using total chlorine residual is based on the assumption that each component is equally effective on microorganisms. This assumptions validity has been increasingly challenged as additional research has been performed on disinfection by-products and the formation of organic chloramines.

The reactivity of many important chemical elements in water/wastewater depends on redox conditions. In 1933, EC. Schmelkes (The oxidation potential concept of chlorination. J. AWWA 25:695) suggested that measurement of oxidation reduction potential (redox potential, ORP) of chlorinated water could be an alternative way to determine disinfection effectiveness. This article presents an overview of the drawbacks of total chlorine residual in wastewater disinfection and a discussion of ORP as a measure of disinfection efficacy. It also reports on a successful ORP application at a municipal wastewater treatment plant.

Chlorine widely used

Most U.S. wastewater treatment plants disinfect the final effluent before discharge. More than 90% of the plants use chlorine for this purpose, either in gaseous chlorine or hypochlorite form. When chlorine is added to the water system, hypochlorous acid and hypochlorite ion are produced, and they are defined as free chlorine.

In dilute aqueous solutions (1 to 50 mg/L), chlorine reacts with ammonia to produce three different inorganic chloramines. These chloramines also have biocidal properties and are referred to as combined chlorine. Chloramines are generally slower acting than free residual chlorine, but have the advantage of being more persistent in water. The sum of free available chlorine and combined available chlorine is called total chlorine.

Free chlorine can react with a variety of organic nitrogen compounds (R-NH2) to form organic chloramines, R-NHCl. While much is known about ammonia's reactions with aqueous chlorine, comparatively little is known of the organic chloramines. Reaction pathways to some N-chlorinated amino acids have been identified, however, and it has been found that N-chloroaldimines respond to residual chlorine in the same way as monochloramine.

The reaction of chlorine with organic nitrogen compounds is complex and often much faster than the reaction with ammonia. For instance, peptides are chlorinated two to three times faster, and amino acids react 10 to 40 times faster than does ammonia. Although these organic chloramines are not effective disinfectants, they still contribute to the total chlorine residual as if they were. Conventional analytical methods such as amperometric titration and the DPD (diethyl phenylenediamine) methods cannot differentiate inorganic monochloramine from organic chloramines.

The pH dependence of chlorine toxicity poses an additional problem to the residual method. Since ionization of HOCl to OCI- is a reversible pH-dependent reaction, an increase in pH decreases the proportion of HOCl and decreases the disinfecting power of chlorine solution. The proportion of elemental chlorine, hypochlorous acid, and hypochlorite ion at various pH values is illustrated in Figure 2. As the system pH increases from 7 to 8, the toxicity of total residual chlorine on, for example, the mosquitofish is reduced by 50%.

ORP offers alternative

As mentioned, measuring residual chlorine does not effectively determine the disinfecting efficacy of chlorine compounds, and ORP measurement has been suggested as an alternative method. Such measurements provide an indication of oxidizing agents' effectiveness. When oxidative disinfectants such as chlorine are added to a water system, their function is to oxidize and destroy unwanted materials. Oxidation is broadly defined as an increase in positive valence and therefore as a loss of electrons. That is, when an oxidant is fed to organic materials, it takes electrons from the species that are inactivated or killed.

Solution ORP can be measured by using an oxidation-reduction-type platinum electrode in conjunction with a reference electrode. These are immersed in the solution to measure the electrical potential, displayed in millivolts, of the oxidizers present. Higher voltage readings correspond to higher concentrations of chlorine in solution. The system responds directly to the balance between the oxidizers and reducers.

ORP measures the net potential from the aqueous system composed of oxidants and reductants. This gives ORP the unique ability to detect whether chlorine present at any given time is sufficient to meet demand, or a sulfite addition is sufficient to neutralize the chlorine for complete dechlorination. As shown in the equation, ORP responses are logarithmic, making them most sensitive at extremely low levels of chlorine or sulfite content.

Figure 3 illustrates ORP as a function of concentrations of free chlorine, chloramine, and sulfite. This clearly shows the dramatic change in ORP when the residual is near zero, a change that makes detection and control of extremely low levels of chlorine or sulfite a practical matter.

ORP measurement accurately predicts virusinactivation rate regardless of the oxidizers used in an experiment. Once a minimum ORP has been exceeded, the inactivation rate progresses proportionally with respect to increasing ORP. In a study on E. coli kill with many different disinfectants, ORP was found to be better correlated with solution-disinfecting power than was chlorine-residuals amount.

As illustrated in Figure 4, free chlorine has much higher oxidative power than monochloramine. An organic chloramine (chloramine-T) generates the lowest ORP, indicating the lowest disinfecting power.

Yong H. Kim is research director at Stranco, Inc., Bradley, Ill. His research interests include polymer dissolution behavior, colloidal suspension surface phenomena, and water disinfection. He has a Ph.D. in chemical engineering from Kansas State University and is a member of AIChE, the American Chemical Society, and the Water Environment Federation.

Roger L Strand is vice president of Stranco, Inc., Bradley, Ill. He has a B.S. in meteorology from Northern Illinois University and has more than 20 years of experience in design, development, marketing, and implementation of chlorine and pH control systems for commercial and industrial applications. He holds two patents on the use of ORP in water treatment systems.

Copyright Instrument Society of America Mar 1997
Provided by ProQuest Information and Learning Company. All rights Reserved