WHY MEASURE DISSOLVED ORGANICS WITH OUR MODEL 140 UV-COLOR ANALYZER

In most water treatment situations the amount and character of dissolved organic carbon is the most important factor in determining the cost, effectiveness and quality of water treatment. Through a combination of improvements in sampling, referencing, and instrument design we have advanced on-line UV-Color measurement technology to a new level that allows for reliable and accurate continuous on-line measurement of the dissolved organics that contribute to charge demand, chlorine demand and disinfection by-product formation. Improved and timely measurement of dissolved organics will reduce costs through improved coagulation control and disinfection by chlorination and UV irradiation.

The Model 140 dual wavelength UV-Color analyzer provides accurate measurements of dissolved organics and colored species in the water treatment process. This instrument has applications for improving water treatment and use in the areas of:

• Wastewater

• Drinking water

• Surface water

• Chemical/industrial process water

Dissolved organic substances in surface water are normally derived from biological substances and processes. These organic substances can reduce the efficiency of water treatment processes and lead to new toxic substances.

• A source of chlorinated disinfection by-products such as THMs (trihalomethanes) and haloacetic acids

• Require removal with coagulation, flocculation and sedimentation.

• Contribute to color and taste issues

• Reduce the effectiveness of UV disinfection processes

Over the past several years we have developed an accurate and stable UV-visible measurement for the drinking water and wastewater industries.

Our instrument design incorporates the following features:

• Stabilized light source

• Direct reading of filtered sample without compensation for turbidity

• Automated instrument zero and referencing with patented process

• Automated cleaning

• Dual wavelength option (254 nm and 460 nm)

• Cross-flow membrane sampling system option

• Improved bubble suppression

254NM MEASUREMENT OF ORGANICS AN ALTERNATIVE FOR TOC MEASUREMENT

In response to the increasing use of UV measurement for dissolved organic substances in water and wastewater, The Standard Methods Organization, authors of Standard Methods for the Examination of Water and Wastewater, adopted Method 5910 in 2000. This method describes the measurement of dissolved organics using absorbance of UV light at 254 nm. This analyzer conforms to the Standard Methods measurement.

Figure 1. Correlation between TOC and UV absorbance.

There have been numerous studies that have characterized the relationship between total organic carbon, or dissolved organic carbon and UV absorbance at 254 nm. Although these two measurements are different measurements, there is generally a correlation between the two measurements. UV measurements are simpler to make and do not require reagents or as much maintenance. The correlation between TOC and UV absorbance for water from an Alberta river system is shown in Figure 1.

MEASUREMENT FOR COLOR

Water color is an important parameter for characterization of wastewater and the visual aesthetics of drinking water. The standard method for analysis of water is Standard Method 2120 C. Spectrophotometric - Single-Wavelength Method, published by the standard method committee.

The absorbance measurement at 460 nm is a measurement of the filtered color of the water sample. This measurement, when combined with the UV254 measurement as a ratio, provides an excellent correlation of specific absorbance. Specific absorbance relates to the amount of humic substances in water and is determined by the ratio of UV absorbance to TOC.

Figure 2. Diagram of the automated referencing and cleaning functions.

When the measured values are very small, the accuracy of a light absorbance system is particularly sensitive to the accuracy of the reference measurement. To improve the accuracy of the referencing system we have an automated instrument zero that uses high purity water that is generated in a water purification unit that is connected with the system. This automated referencing system works with a washing system to clean and rezero the instrument at user set intervals. Figure 2 shows the components of the automated referencing and zeroing system.

UNIQUE SAMPLING SYSTEM FOR RAW WATER AND WASTEWATER

The diagram in Figure 3 shows the concepts behind cross-flow filtration for water sampling. In our cross-flow filter, raw water passes a 0.05 micron membrane that is on a tubular support. The flow across the membrane is about 500 times the flow through the membrane, and helps prevent build up of a filter cake that can selectively adsorb dissolved species. The dissolved organics easily penetrate the membrane and all material that is colloidal size or larger is retained and washed away. Preparation of the sample using this cross-flow filtration has big advantages over measurement of unfiltered water. These advantages include:

• More accurate measurements are possible because measurement at 254 nm does not have to be compensated for using the light attenuance from a long wavelength

• Reduced cell fouling requires increased frequency of instrument zeroing and cleaning

Figure 3. Tubular, cross-flow, ultrafiltration sampling system.

APPLICATIONS

The applications for the UV-Color analyzer in relation to drinking water relate to the removal of dissolved substances or to the interferences of these substances in achieving improved water treatment. Potential application points are shown in Figure 4. On the finished water side the measurement can be used to improve control of UV irradiation or to predict THM formation due to chlorine addition. On the raw water side, the measurement provides a measure of dissolved organics in the water. The degree that the dissolved organics are removed during the water treatment process depends on the nature of the dissolved organics and the treatment processes. Treatments such as enhanced coagulation and the addition of activated carbon may be required for some surface waters.

Figure 4. Diagram showing potential points for the placement of the UV analyzer in drinking water treatment.

In the United States, The Environmental Protection Agency's Disinfectants and Disinfection By-products Rule (DBPR) and The Long Term Enhanced Surface Water Treatment Rule (LT2ESWTR) will be finalized and implemented simultaneously to ensure that drinking water is microbiologically safe at the limits set for disinfectants and DBPs. These regulations require removal of dissolved organics when their occurrence leads to high disinfection by-product levels.

Figure 5. UV absorbence and coagulant addition.

Although coagulation levels are typically controlled to achieve certain settled and filtered turbidity levels, these measurements poorly correlated with required doses and they are often distant from the chemical addition. The importance of the UV organics to the required coagulant dose is shown in Figure 5. In this figure it is clear that the coagulant dose trends very closely to the UV absorbance. The correlation between UV and coagulant dose is much better than the correlation between the raw turbidity values and coagulant dose.

Numerous studies have shown that UV absorbing species are more reactive to chemicals used for adsorption, coagulation, and disinfection. Normally dissolved UV absorbing substances may dominate the water purification chemistry even though these substances are not the target substances. For these reasons UV absorbance is excellent for measurement of dissolved organics in control or addition of the following water treatment chemicals:

• Activated carbon

• Coagulants (alum, PACl, ferric chloride)

• Disinfectants (chlorine, chloroamines)