Monitoring Nitrates in Drinking Water and Wastewater
Monitoring Aromatic Organics for Optimizing Coagulation
Disinfection By-Products (DBP) Precursor Monitoring
Water Distribution System Security Monitoring
Importance of Measuring UV Transmittance (UVT) for UV Disinfection
Benefits of UV Transmittance (UVT) Field Testing for Selling & Servicing UV Disinfection Systems
Organics Monitoring (TOC)
Total organic carbon (TOC) testing is the traditional method for determining organic matter in water. However there is a far more practical, affordable and often more useful way to measure organic matter. UV absorbance testing (UVA) is rapidly becoming the preferred method of measuring organics even when the levels of organics being measured are very small.
Organic Matter
Organic matter in water is the source of many water quality challenges. Some of the problems organics can cause include: providing food for unwanted microbiological growth; causing the formation of disinfection by-products when mixed with chlorine and other disinfectants; consuming coagulant chemicals and generally complicating many of the chemical processes involved in water treatment; clogging membranes and filters increasing backwashing and carbon regeneration requirements; and many more. Organic compounds are also key components of many industrial processes such as food processing and pharmaceutical manufacturing. Given all the challenges caused by organics in water and the prevalence of organics in our industrial processes it is no surprise that the measurement and control of organics is becoming more and more important.
Common Organics Test Parameters
To measure organic matter in water, there are several common organics parameters available to choose from including UV absorbance (UVA), TOC, DOC, BOD, and COD. All of them can be performed readily in a laboratory. However, testing grab samples in the lab generally does not provide enough information for proper control of organics. For proper control, organics are best monitored in real-time. Due to the complexity of the common lab-based organic test parameters, only UVA and TOC instruments have really had any success in real-time industrial environments.
Real-Time TOC Instruments
TOC is an effective and fully accepted test for organic matter, but while TOC instruments have their place in certain specific applications, they are not the right choice for monitoring many of the most common organics monitoring applications.
The measurement process that goes on inside a TOC instrument actually involves converting all the organic matter in the water to CO2, and there are a couple of methods to accomplish this. The quantity of CO2 produced is then measured inside the CO2 capture chamber. The amount of CO2 indicates the amount of carbon which in turn indicates the amount of organic matter. It's complicated but it does work and extremely low concentrations of organics can be measured with high end TOC instruments.
The only problem is the complexity of the test. This complexity results in TOC instruments that are expensive to buy, are complicated to operate, and are difficult and expensive to maintain. Of course, these facts are often overlooked in a laboratory setting where operating conditions are close to ideal and servicing is relatively convenient.
TOC instruments were originally designed for lab use and it was only after realizing there is a need for monitoring organic matter in real-time industrial type environments that some TOC manufacturers tried to re-tool their lab TOC instruments for continuous sampling, with limited success. Industrial applications are a long way from the laboratory.
Real-Time UV Absorbance (UVA) Instruments
There is a better method to measure organics for municipal and industrial organics monitoring applications. Organic matter is extremely absorbent of UV light, and this is the basis for the UVA parameter. UVA measuring instruments are much simpler in design and operation than TOC instruments and the UVA parameter can be much more efficiently monitored in real-time than TOC, negating all the maintenance costs and headaches associated with TOC instruments.
In the past, UVA has not had the sensitivity that TOC instruments do, although it should be noted that many applications do not require the sensitivity that many TOC instruments provide. However, with Real Tech's UV absorbance instrumentation you can finally measure on par with mid-range TOC instrument sensitivity at 10's of parts per billion if that is required.
Interference from turbidity is often not an issue due to the fact that the levels of turbidity found in most applications are below 5 NTU and therefore do not affect the UVA reading in which case Real Tech's Real UV254 series instruments can be used. In higher turbidity waters a simple 5 micron filter to remove the bulk of excess turbidity are sometimes recommended. Alternatively, Real Tech's Real UV Spectrum series instruments can be used, which allow compensation for interferences to the UVA measurement such as turbidity. TOC instruments on the other hand are well known to have clogging issues due to turbidity. In a side by side test at an independent test site, an online TOC instrument from a leading manufacturer was constantly down after getting clogged by undissolved particles, while Real Tech's Real UV254 M3000 instrument had no trouble at all.
Correlating UVA to TOC
There are applications which require correlation of UVA to TOC. It is first important to assess why you want to correlate to TOC. An example application might be a municipal plant that has to perform quarterly TOC removal measurements across the plant for regulatory purposes. Although TOC may be required for reporting, this most definitely does not mean you should be using TOC for your organics removal process optimization. UVA is actually a better parameter in many cases for process optimization than TOC because of the bias of UVA to the more reactive component of organic matter.
If a direct correlation from UVA to TOC is required this can easily be performed. Correlations between UVA and TOC are very linear for a given site or application. Many industrial applications can have correlations that approach 1.0 r2 and even raw surface water going into a drinking water plant can be expected to have correlations well over 0.9 r2. Since the correlations are so good, Real Tech's Real UV254 series instruments can often be used with a simple scaling factor and the resulting TOC can even be displayed in mg/L TOC if that is desired.
For improved correlations to TOC especially in applications where there can be significant changes in the matrix of organic matter over time, Real Tech generally recommended its Real UV Spectrum series instruments. The Real UV Spectrum series instruments have more data available resulting in better correlations in more difficult applications in addition to compensation for potential interferences such as turbidity.
UVA and Reactive Organics
One of the most important advantages of UVA instruments over TOC instruments for organics monitoring applications is the bias of UVA towards aromatic (reactive) organics. TOC instruments fundamentally measure the quantity of carbon and so all types of organics are measured equally, whereas UVA instruments give more weight to the organics that cause all the problems. Aromatic organic molecules have more functional groups and therefore tend to be more reactive with other molecules. For example, disinfection by-products, such as THM's and HAA's, will form when aromatic organics and a disinfectant are combined. It is the bias of UVA toward aromatic organics that makes monitoring UVA actually the more appropriate parameter to test for many organic monitoring applications.
The Future of Organics Monitoring
Real Tech's Real UV254 series and Real Spectrum series instruments are enabling plants to properly and cost effectively monitor organics in real-time, allowing improved water quality and substantial cost savings from process optimization.