This editorial originally appeared in the October 2019 issue of iWWD as "Analyzing TOC"
To protect water quality, governments and international organizations worldwide continue to implement protective water regulations that require new industrial water management technologies to meet them. These regulations are intended to reduce over-consumption by maximizing industrial reuse where possible and assuring industrial effluent discharged from process and manufacturing plants meets stringent regulatory standards designed to protect surface and ground water resources, including the oceans and marine life.
As we move forward with the latest era of water regulations and new industrial water-saving technologies, the need to monitor total organic carbon (TOC) has become more apparent and critical. While a relatively new technology, TOC water analysis is highly accurate, dependable and cost-effective, which makes it one of the most economical and fastest solutions to determine the quality and cleanliness of water.
What Is Total Organic Carbon (TOC)?
TOC is the amount of organic contaminants within a water body. Because TOC can come from anything organic, such as decay from natural organic matter (NOM), petroleum, fat, alcohol, sugar, protein and many other sources, the need to monitor TOC is helpful in a wide range of industries, including petrochemical and electric power generation industries.
All water contains some carbon regardless of that water’s purity level. Organic carbon can form many combinations of chain-like molecules. For this reason, it is difficult to pin-point a specific compound unless the end-user is specifically looking for it.
TOC, on the other hand, is the sum of all organic carbon within the compounds. It is found by subtracting total inorganic carbon from total carbon (TC). Within the TOC analysis also is the measurement of purgeable organic carbon (POC), dissolved organic carbon (DOC) and nonpurgeable organic carbon (NPOC).
Industrial Water & Wastewater Applications
Water is an essential component in diverse process and manufacturing industries including: automotive, chemical, electronics, food/beverage, oil/gas, metals/mining, pharmaceuticals, power generation, pulp/paper and steel to name a few.
Water performs a diverse number of industrial tasks, including acting as a basic ingredient for cooling, cooking, heating, rinsing, washing, steaming, sterilizing and more.
In petrochemical and power generation plants, there are several areas where plant engineers should consider TOC measurement, including:
• Incoming plant water quality;
• Boiler make-up water;
• Evaporative cooling water;
• Storm water runoff;
• Plant water reuse; and
• Effluent discharge monitoring.
Municipal Water & Wastewater Applications
Similar to industrial water and wastewater, TOC measurement in municipal water and wastewater treatment plants also is suitable for a wide range of application measurement tasks that include:
• Surface water supply monitoring;
• Ground water supply monitoring;
• Drinking water disinfection;
• Finished water monitoring;
• Drinking water storage;
• Wastewater treatment plants; and
• Sewer system storm water.
Comparing TOC to BOD & COD Measurement
Historically, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) have been used to measure the loading of the influent on the treatment plant and to set limits for the effluent. These methods, however, are slower to produce results, and in the case of COD, they use hazardous chemicals.
Like COD and BOD, TOC is a sum parameter and measures the amount of carbon present as organic molecules, regardless of their origin. Constant monitoring of the influent with an online TOC analyzer allows plant operators and managers to protect their plants against overloading, which helps to optimize its capacity and performance.
As long as the sample being measured remains similar in composition, TOC results can be correlated to BOD and COD. When TOC is used to monitor effluent, it provides a constant surveillance that the effluent is within the discharge limit.
Industrial cooling water extraction processes also use TOC measurement to monitor the effluent returned to the environment. In several developed countries, cooling water extracted from the natural environment accounts for more than 50% of national water withdrawals.
Much of this cooling water is used in power generation plants producing electricity for domestic and industrial consumption. Many industrial processes in petrochemical refining and other industrial process plants also produce a lot of heat.
Cooling towers or heat exchangers that rely on water are used to dissipate the heat produced through evaporative cooling. These plant systems require a plentiful supply of clean water to operate efficiently and reduce equipment maintenance cycles.
Continuous online monitoring of the surface water intake and the outlet can ensure legal requirements are met and costly fines are avoided. TOC is one parameter that can be used as a primary indicator of water quality in this case. By monitoring TOC, pollution events can be quickly identified and alarms can be sent to a control room where corrective actions can be rapidly instigated.
Methods To Determine TOC
There are several methods to determine TOC. The two main components required to measure TOC are to convert the organic carbon into carbon dioxide (CO2) and the means to detect the CO2.
The three primary oxidation methods most commonly used are: chemical agents, high temperature combustion and photocatalytic. All three methods have provided acceptable results.
The chemical agent method is known as the persulfate or nondispersive infrared (NDIR) sensor process, which can be accomplished through either persulfate oxidation with ultraviolet (UV) light and irradiation activation, or through the alternative process, which is known as heated persulfate oxidation.
The second method to determine TOC is known as the high temperature combustion (catalytic) process. This process measures TOC by heating a sample in a high temperature furnace with a cobalt or platinum catalyst.
The third method to determine TOC is photocatalytic or UV light. The UV light process oxidizes the carbon into CO2 gas. There are multiple manufacturers of TOC analyzers using all three of these measurement methods. All of the products have their advantages and disadvantages depending on the specific application, location of the instrument, etc.
A TOC analyzer like the one shown on page 14, for example, utilizes the UV persulfate oxidation method, which detects generated CO2 using its NDIR detector for analysis. This method and the analyzer conform to standards set by U.S. EPA, DIN, CE, ASTM and NAMUR regulations, as well as ISO.
To use this type of TOC analyzer, the water sample first is acidified and then sparged to remove inorganic carbon. The remaining liquid is mixed with sodium persulfate and digested by two high-performance reactors. The resulting CO2 then is stripped from the liquid and, after drying, its concentration is measured by the NDIR analyzer. The analyzer measures TOC ranging from 0-5 mg/L to 20,000 mg/L.
TOC measurement is accurate, dependable and cost-effective in a wide range of industrial and municipal applications requiring the treatment of water and wastewater streams. There are no harsh chemicals requiring special treatment, employee training and disposal restrictions. For these reasons, TOC analyzers can offer cost savings over conventional water quality measurement technologies.