A Tandem Approach to TOC Analysis in Drinking Water Treatment

In 1974 Congress passed the Safe Drinking Water Act (SDWA) to regulate the nation’s drinking water supply. The act empowers the U.S. EPA to establish standards for contaminants in drinking water, as well as to monitor analytical test methods. Standards have been set for 90 chemical, microbiological, radiological and physical contaminants, most of which occur naturally in source water.

TOC analysis as a quality indicator

Total organic carbon (TOC) analysis measures organic contamination levels, and is thus an important indicator of general water quality. TOC is an effective monitoring parameter because it responds to all types of organic carbon, either dissolved or suspended in water, including compounds without a chromophore. Both benchtop systems for highly accurate laboratory analysis of spot samples collected at designated intervals and on-line analyzers that monitor influent and effluent process water streams in real time are available. When used in tandem, these two analyzers provide the most comprehensive approach to the monitoring and optimization of the water treatment process.

Community water systems that serve large populations generally rely on surface water such as rivers, lakes and reservoirs as their “raw” water intake source, and the quality of the raw water from these sources determines the drinking water treatment process that is employed. No matter the process, TOC analysis is critical for the accurate monitoring of organic carbon levels in compliance with water treatment methods.

In the drinking water treatment process, raw source water is pumped through a screen to remove large debris. Potassium permanganate (KMnO4), a substitute preoxidant to chlorine, is a strong agent that may be added to oxidize organic matter and is used to control total organic trihalomethanes (TTHMs). It also imparts undesirable flavors, odors and colors. After the addition of KMnO4, the water is clarified by flocculation and sedimentation. Alum, iron salts or organic polyelectrolyte polymers are added to coagulate small particles into larger ones that settle out as sediment. The clear water is then filtered through a bed of sand and gravel to remove remaining particles and natural organic matter.

TOC analysis as an indicator of NOM and THM levels

The EPA’s Disinfectant and Disinfection Byproduct Rule (D/DBPR) regulates the levels of disinfectants and disinfection byproducts in drinking water. Permissible levels of trihalomethanes (THMs) were lowered because they have been found to cause cancer in lab animals; acceptable levels for haloacetic acids (HAAs), bromate and chlorite in drinking water were lowered as well.

After filtration, water may also be directed through a bed of granular activated carbon (GAC) to adsorb and remove residual natural organic matter (NOM) and disinfection byproduct precursor compounds such as humic and fulvic acids. The efficiency of postfiltration GAC treatment is monitored by TOC analysis. Decreasing the TOC content enables a facility to reduce the formation of THMs and HAAs and stay in compliance.

Before drinking water is released into the distribution system, it is disinfected by either chlorination or ozonation to kill dangerous microbes. Chlorine (CI), chloramines (NH2Cl) and chlorine dioxide (ClO2) are highly effective disinfectants. Ozone (O3) and ultraviolet radiation are useful for treating relatively clean water, but are not relied upon to control microbial contaminants throughout a distribution system.

Limitations of traditional methodology

Spot sampling at predetermined intervals has been the standard practice for drinking water treatment and distribution system monitoring. Depending on the compliance parameters and system/process requirements, sampling may be done once a day, twice a week or once a month at the discretion of the facility.

The process of running spot samples through a benchtop TOC analyzer complies with established regulations, but is limited because the monitoring is not continuously visible. Even if a lab or treatment plant does sampling and runs tests once a week, it may fail to notice a spike in particulate levels caused by a specific weather event. The same would hold true if the samples were taken at the same time of day, which would not account for the impact of fluctuations caused by daily temperature increases or decreases. While benchtop TOCs are very accurate and are required for regulatory compliance, the lag time between sampling and analysis means that the treatment process is not as efficient as it could be and will likely use more chemicals than necessary.

Real-time visibility

Whether it is done at several points during the treatment process or within the distribution/supply infrastructure, on-line TOC monitoring of drinking water systems provides the visibility needed to obtain accurate, ongoing assessments for process optimization.

A key component of on-line monitoring is the ability of a treatment plant to run as many as three or four streams from the process through the analyzer. Parameters and time increments can be set for each stream, as can warning or alarm notifications, as parameter readings approach predetermined levels.

On-line TOC analysis allows authorities to heed the warnings of organic chemical contamination from accidental or intentional incidents. The frequency of analysis provides continuous visibility so that peaks and valleys, or important readings throughout the process, are not missed due to time increments that are too long or intermittent.

On-line TOC analysis

Figure 1 – The 9210p on-line TOC analyzer can be used as an on-line instrument throughout the drinking water treatment process, or in the laboratory, with streams coming off the process for intermittent analysis.

The 9210p on-line TOC analyzer from OI Analytical, a Xylem brand (College Station, Texas) (Figure 1), utilizes a heated persulfate oxidation technique in U.S. EPA-approved Methods 415.3 and SM 5310. The instrument can be used to analyze source/raw water and finished drinking water, or at any step along the way.

Running the 9210p in process, samples are drawn into the analyzer at programmed time intervals from a fill-and-spill sampling system. Phosphoric acid is introduced to the syringe to sparge and remove the total inorganic carbon (TIC) content. The TIC-free sample is then transferred to a reaction chamber to be oxidized by heated sodium persulfate (Na2S2O8) at a programmed temperature of up to 100 °C.

Oxidation of organic molecules requires 2.5–3 sulfate or hydroxyl radicals per carbon atom.1 Nearly all organic compounds dissolved in water can be oxidized by this technique. Concentrated solutions (1 or 1.5 M) can effectively oxidize organic matter present in the form of colloids, macromolecules and suspended solids.2

Organic compounds in the sample are oxidized and converted to CO2, which is measured by a solid-state, nondispersive infrared detector to calculate and report the TOC content. Results for each sample are shown on the touchscreen display and can be output to a SCADA (supervisory control and data acquisition) system, a PC via Ethernet connection, relay/alarm closure or as a 4–20 mA analog signal. The user-friendly analyzer is adaptable to most processes and plant environments.

Results and discussion

Test methods for U.S. EPA regulatory compliance reporting are currently based on laboratory TOC analysis. On-line TOC analyzers are generally used for process control and need not operate under the quality assurance requirements specified in U.S. EPA methods.

The raw and finished drinking water results shown in Table 1 were run on an OI Analytical Aurora 1030W laboratory TOC analyzer and 9210p on-line TOC analyzer employing the heated sodium persulfate oxidation technique in U.S. EPA-approved methods 415.3 and SM 5310C.

Table 1 – Results of raw and finished drinking water TOC analysis

Use of benchtop and on-line TOC analyzers in tandem is optimal, since on-line analyzers must be calibrated periodically to ensure accuracy, minimize drift and verify oxidation efficiency.

As shown in Figure 2, the correlation of 0.99747 demonstrates that the results between the two instruments are statistically the same.

Figure 2 – Correlation of results obtained on unknown samples using the 9210p on-line TOC analyzer and Aurora 1030W TOC analyzer.

Thus, tandem use of benchtop and on-line TOC analyzers provides water processing facilities with a very efficient, cost-effective tool for the monitoring and optimization of influent and effluent process streams.

References

  1. Peyton, G.R. The free-radical chemistry of persulfate-based total organic carbon analyzers. Marine Chemistry 1993, 41, 91–103.
  2. Hargesheimer, E.; Conio, O. et al. Online monitoring for drinking water utilities. American Water Works Association, 2002; http://www.waterrf.org/ExecutiveSummaryLibrary/ 90829_2545_profile.pdf

John Welsh, Jr., Ph.D., is TOC product manager at OI Analytical, P.O. Box 9010, College Station, Texas 77842, U.S.A.; e-mail: [email protected]www.xyleminc.com

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