Frequently Asked Questions

What is the UV254 water quality test parameter?

UV254, also known as the Spectral Absorption Coefficient (SAC), is a water quality test parameter which utilizes light at the UV 254nm wavelength to be able to detect organic matter in water and wastewater. This is due to the fact that most organic compounds absorb light at the UV 254nm wavelength.

Unlike other organic test parameters, UV254 has a bias towards reactive or aromatic organic matter which has double bonded ring structures and is typically the most problematic form of organics in water.

UV254 is typically represented as a calculation of UV absorbance (UVA) or UV transmittance (UVT).


What is UV Transmittance (UVT)?

UVT is a relative measurement. It is essentially a measure of how much light passes through a water sample compared with how much light passes through a pure water sample. The sample water measurement is actually divided by the pure water measurement and then converted to a percentage. Therefore, any unit of measure of the light itself is cancelled in the division. An example of a UVT measurement would be 96.4% UVT. Pure water (DI water) will read 100% UVT, and completely opaque water will read 0% UVT.

Typically UVT is only used to refer to the transmittance of light at the 254nm wavelength since its application is usually for UV disinfection system dosing.

UVT will be different for different path lengths. Because the UVT varies with path length and the concentration of matter, the UVT is usually expressed with the assumption that a 1 cm path length is used so that the UVT provides a measure of matter in the water independent of path length.

What is UV Absorbance (UVA)?

The UV absorbance is calculated as a relative measure of the amount of light absorbed by a water sample compared with the amount of light absorbed by a pure water sample. The sample water measurement is actually divided by the pure water measurement before a logarithm is calculated. Therefore, any unit of measure of the light itself is cancelled in the division. An example of an absorbance measurement would be 0.1 A/cm. Pure water (DI water) will read 0.0 A, and completely opaque water will theoretically read infinity A, due to the logarithm.

However, practical instrument limits require maximum absorbance ranges far less than infinity. Real Tech’s highest organics ranges are 8.0 A, which corresponds to much higher organics levels than are present in even Real Tech’s highest organics laden applications.


What should I measure, UVT or UVA?

Since UVT and UVA are related mathematically, no actual information is lost in choosing one parameter over the other. UVT is a measure of how much UV light is able to penetrate through a water sample. The less UV light that is transmitted through the water, the lower the UVT. UVA is a measure of the absorbance of UV light by matter in the water. The less UV light transmitted through the water, the higher the UVA.

Generally speaking, UVT is used with UV disinfection systems to aid in the calculation of UV dose at the 254nm wavelength. UVA is directly proportional to the concentration of matter in the water and so therefore UVA is generally the measurement of choice for most applications.


How is UV Absorbance (UVA) related to UV Transmittance (UVT)?

UVA and UVT are essentially just different representations of the same physical phenomenon. They can be related by a simple mathematical expression as follows:
UVA = 2 – log10 UVT
which indicates an inverse logarithmic relationship between UVA and UVT.

How is it that UV Absorbance can refer to both spectral analysis and UV254 analysis?

Although UVA can be used to discuss the absorbance of a substance across the whole spectrum of UV light, it is also commonly used as the measurement term for the UV254 water quality test parameter. In these circumstances, the term UVA is essentially referencing just the UV absorbance of one specific wavelength, which is the UV 254nm wavelength. For example, the Specific UV Absorbance (SUVA) measurement commonly referenced by the U.S. EPA refers to the UVA measurement as being the UV254 water quality test parameter. It really comes down to the context in which it is being used.

What is SUVA and how is it related to UV Absorbance?

The Specific UV Absorbance (SUVA) calculation is typically performed for the purpose of determining disinfection by-product (DBP) formation potential.

SUVA = UV 254 nm (m-1) / DOC (mg/L)

SUVA is simply the UV absorbance at the 254nm wavelength (UVA) divided by the DOC of a water sample. This allows the aromatic biased UV254 measurement to be normalized over the overall organic load in the water. A characterization of the aromaticity of the water independent from the general level of organics in the water can then be obtained.


A high SUVA indicates that a large portion of the organics present in the water are aromatic. Since aromatic organics have a greater tendency to react with disinfectants to create DBPs, a high SUVA indicates there is a high potential for the formation of DBP’s.

For further details about measuring SUVA for regulatory purposes click here to link to EPA Method 415.3


What is the relationship between UV Absorbance (UVA) and the concentration of substances in water?

UVA is directly proportional to the concentration of matter in the water. UVA is given by the following equation:
UVA = ε•b•c
where ε is the molar absorbtivity of the particular type of matter in the water sample, b is the path length of the water sample, and c is the concentration of matter in the water sample.

Therefore, as an example, if the concentration of organic material in the water were to double, then the UVA would also double.


How is the UV Absorbance (UVA) at a 25 cm path length related to the UVA at a 1 cm path length through the water?

UVA is directly proportional to the path length of the UV light through the water sample. UVA is given by the following equation:
UVA = ε•b•c
where ε is the molar absorbtivity of the particular type of matter in the water sample, b is the path length of the water sample, and c is the concentration of matter in the water sample.

From the equation above, it is clear that if the path length is altered by a certain factor the UVA measurement will be altered by the same factor. Therefore, a UVA reading measured at a 25cm path length can easily be converted to the UVA at a 1cm path length simply by dividing the 25cm UVA measurement by a factor of 25.

Real Tech takes care of this for you however. All Real Tech instruments display the UVA in terms of a 1cm path length except for the Real UV254 M4000 which displays the UV absorbance in terms of a 1m path length. Both 1cm and 1m path lengths are industry standard for absorbance measurements.

Can't I just measure organics with a colour instrument instead of UV Absorbance?

Another water quality parameter that is sometimes used as a surrogate to measuring organics is colour. Since humic acids can sometimes give a visible colour to the water, the visible light used to test colour can give an indication of some organics. However, colour is a far inferior method to performing a UV absorbance test and generally it is not recognized as an accurate organic test parameter.

As with turbidity, it is possible to have a water supply which has a low amount of colour without having corresponding low amounts of organics. An example of this would be detecting the presence of pesticides in water which will not show up with a colour instrument but will be detected in a UV254 or UV spectral analysis instrument.

Also, colour instruments are not able to detect the low levels of organics that UV absorbance based instruments can, since UV light is so readily absorbed by organics compared with other wavelengths.

Can I use Real Tech's instruments to correlate to colour in Hazens?

Yes. Because UV absorbance and colour are both able to detect organics it is possible to correlate UV absorbance measurement to Hazens. The correlations will be dependent on the particular matrix of organics at that site but they are often very linear. Real Tech even allows a scaling factor to be used in its Real UV254 series so that the measured level of organics can be displayed in units of Hazen.


Can't I just measure turbidity (NTU) instead of UV254?

Turbidity testing measures scattered visible light which results from various types of matter in the water, mostly providing an indication of the amount of larger particles and colloidal solids. UV254 measures the amount of UV light absorbed or transmitted through the water, providing an indication of the amount of natural organic matter (NOM) in the water.

Although turbidity is not an organic test parameter, it has long been used as a measure of general water quality and is often used instead of performing an actual measure of organic content. This could also be due to the fact that until the more recent use of the UV254 organic test parameter, real-time organic testing has not been easily attainable given the expense, time and complexity of measuring the organic test parameters. Therefore turbidity testing used to be the only real option for real-time water quality analysis.

This reliance upon turbidity tests alone without analysis of organics can result in the ineffective optimization of a wide range of treatment methods. Problems that occur include inadequate chemical dosing of coagulants and disinfectants, poor filter performance optimization, increased DBP formation and more.

What types of matter absorb UV light?

Many different substances absorb and scatter UV light including: organic matter, nitrates, nitrites, colloidal solids, certain metals, pesticides, diesel fuel and more. Different substances absorb at certain wavelengths of UV light more than others. Some substances such as colloidal solids tend to scatter UV light more than they absorb it. Knowing how different substances absorb or scatter UV light allows Real Tech to measure the levels of these different substances in the water.

Which different types of organics absorb UV light?

There are many different types of organics found in water. The organics can be classified into two main categories – aromatic and aliphatic. Both categories of organic matter are commonly found in water. The structure of aliphatic organics is usually defined by long molecular chains of carbon atoms, whereas aromatic organics are usually ringed structures. Highly aromatic organic molecules tend to contain a high number of double carbon bonds whereas aliphatic organic molecules form mostly single carbon bonds. The double carbon bonds combined with a ringed molecular structure causes aromatic molecules to chemically absorb photons of UV light at wavelengths in the upper 200 nm range exceptionally well. Therefore, if an organic molecule has a good number of double bonded carbon atoms or has some degree of aromaticity (contains some aromatic ring structures), then UV light will be absorbed by it.

The Real UV254 series of instruments is useful for detecting the general level of organic matter in the water. If it is necessary to further characterize the organics or to compensate for some interfering substances, then the Real Spectrum series is recommended.

How can monitoring UV Absorbance at multiple wavelengths of light allow detection of different substances in the water at the same time?

Different substances in the water tend to absorb different wavelengths of light more than others. Therefore, different substances will have their own specific spectral absorbance. This specific spectral absorbance can be used as a signature or finger print to identify the substance. When multiple substances are present in the water, the spectral absorbance of each substance will add together at each wavelength to form a resulting spectral absorbance pattern where the net absorbance at each wavelength is the sum of the absorbances at that wavelength of each substance. Because each substance’s spectral absorbance is different, it is mathematically possible to extract these individual spectral absorbances from the resulting summed spectral absorbance using statistical analysis techniques. This not only allows detection of the different substances, but also the quantification of the different substances.

What are the general biases of all the different organic test parameters?

Although UV254 is an indicator of the total organic level in water, it does have a bias towards a specific type of organics known as aromatic organics. Other water quality parameters that measure organics have their own biases. Other organic water quality parameters include total organic carbon (TOC), dissolved organic carbon (DOC), chemical oxygen demand (COD), and biological oxygen demand (BOD). TOC indicates the level of organics in water by measuring the total carbon content and so gives a good overall level of all organics although inorganic carbon can be a concern. DOC is similar to TOC except all organic particles larger than 0.45 microns are removed prior to testing. COD indicates the level of organics by detecting substances that are susceptible to chemical oxidation. BOD is similar to COD except BOD detects substances that are susceptible to biological oxidation which indicates biologically active organics.

UV254, TOC, DOC, BOD and COD all provide measures of different aspects of the organic content of the water. Each of these parameters possesses a certain bias due to the nature of the particular parameter. However, correlations between the different organic water quality parameters can be made.

UV spectral analysis provides information specific to each of the above organics parameters. Using complex statistical analysis UV absorbance can correlate very well to all parameters. For example, organic matter detected by BOD testing has a spectral absorbance peak at a higher wavelength than organic matter detected by TOC instruments. These types of spectral absorbance signatures allow instruments like the Real Tech’s Real Spectrum series to measure and characterize the nature of the organics better than the other individual organics parameters.

Can I correlate UV254 or UV Absorbance to other common organics parameters such as TOC?

There are several water quality parameters other than UV absorbance that are commonly used to measure organic matter for different water and wastewater treatment applications as well as industrial effluent monitoring and process control. These include Total Organic Carbon (TOC), Dissolved Organic Carbon (DOC), and Biological Oxygen Demand (BOD).

Many of these parameters are complicated, time consuming and expensive to test. This makes them impractical to use for real-time monitoring which is can be vital to the success of many water and wastewater treatment methods and for process control applications.

Fortunately, UV absorbance and often UV254 can be used as supplements or surrogates to these other parameters. Correlations between UV absorbance and UV254 and these other organic test parameters can easily be made for specific sites.

Given their affordability, ease of use and accuracy, Real Tech’s Real Spectrum series and Real UV254 series instrumentation provide the most practical organic testing solution. In addition, in many cases after being used as a supplement to other testing Real Tech’s products end up replacing the parameters they are supplementing altogether.

Why is UV Absorbance analysis useful for disinfection by-product (DBP) control?

The aromaticity of organic molecules is often expressed as an indication of reactivity. The reactivity of aromatic organics causes them to react particularly well with chemicals used in water treatment, such as chlorine, to form various other molecules. For this reason, when chlorine is added to water as a disinfection method, the organics already present in the water tend to react with the chlorine. Recently, it was determined that the disinfection by-products (DBPs) produced by the reaction between the aromatic organics and the chlorine are carcinogenic for humans. This has lead to great emphasis now being placed on monitoring and reducing the level of organics in the water before the chlorine is added.

Real Tech’s products are excellent for monitoring organics in water and have a bias towards the more aromatic organics. Recent studies show the very strong correlation between UV absorbance at the 254nm wavelength (UV254) and resulting levels of DBP’s. It is this correlation that makes Real Tech’s products much more useful for DBP reduction than TOC instruments that do not have the bias towards the problematic aromatic organics.

What is the purpose of calibration samples when using spectral analysis?

Spectral analysis allows detection and quantification of several substances present in the water. However, the instrument being used must first know what the spectral absorbance of the substance should look like. There are some substances that are just simple compounds and therefore have well known spectral absorbances. These compounds are relatively easy to identify with spectral analysis. An example of this would be benzene. This type of compound and its spectral absorbance can easily be added to a spectral absorbance library and then used as part of a global calibration.

However, there are other substances whose molecular structure is so complex and varied from one molecule to the next that it does not have a precise spectral absorbance. Humic acid, commonly found in surface water sources, is an example of such a substance. That means it is much more difficult to use a one size fits all spectral absorbance, which in turn makes global calibration more challenging. Because of this, it is sometimes recommended that a local calibration be performed at the installation site. This is usually done after installation of the instrument. The instrument runs for a few days to a couple of weeks, logging all the spectral data as it goes. At the same time manual grab samples are taken from the sample water and tested for the substances of interest, as well as any expected background interference source, such as turbidity. The results of the grab sample tests along with a memory dump from the spectral instrument are then compared and a local calibration is made. Generally the determination of the local calibration is done by Real Tech off-site and the local calibration is simply emailed and installed on the spectral instrument.

What is the difference between a bypass style instrument and a probe style instrument?

Probe style instruments are designed to be fully immersed in the liquid they are testing. They do not tend to be well suited to applications where the water to be tested is under pressure as in a water treatment plant or a municipal water distribution system. They tend to be installed in wastewater type applications and are often hung from a chain or a rope or are attached to a metal arm that extends down into an open tank or channel containing the water to be tested. The sensor data from the probe is then transmitted along a cable to a special operator control panel that displays the probe sensor data and then outputs the data to the local PLC or SCADA system.

Bypass style instruments are generally mounted on a wall and a bypass stream of water to be tested flows continuously through the instrument. This type of instrument is very well suited to pressurized systems common to municipal water treatment plants and distribution systems as well as many industrial applications.

For non-pressurized wastewater applications some bypass style instruments have had a history of clogging with the larger concentrations of suspended solids present in wastewater type applications. However, with Real Tech’s Pump System in conjunction with its Real Clean automatic chemical cleaning system, Real Tech’s bypass style instruments have no problem at all being installed in wastewater applications. Real Tech took its several years of experience installing its bypass instruments in wastewater applications and put that knowledge into its Pump and Clean systems. The Pump and Clean systems proprietary design prevents any potential for clogging by removing all potential blockage points from the flowing test water.

Customer feedback about the Pump and Clean system has been excellent. Customers are now able to get the significant benefits of bypass style instrumentation for wastewater applications. No longer having to spray down filthy probes covered in sewage whenever routine servicing is required is a major benefit over the probe style instruments.