Drinking Water Treatment Process with Online TOC Analysis

Public Law 93-523, also known as the Safe Drinking Water Act (SDWA) was passed by the Congress of the United States in 1974. The purpose of the law was to regulate the supply of drinking water and guard drinking water sources to protect public health. The law that first came into effect on June 24, 1977 has been revised on numerous occasions.

USEPA is authorized by the SDWA to establish enforceable standards for contaminants in drinking water and monitor analytical test methods and requirements. The regulating authority has established standards for 90 microbiological, physical, radiological and chemical contaminants.

Total Organic Carbon (TOC) is a key indicator of water quality across the water treatment process analysis and determines organic contamination levels.

Drinking Water Treatment

Two types of drinking water systems - public water systems (PWSs) and community water systems (CWSs) - have been defined by the SDWA. A PWS supplies drinking water to a minimum of 15 service connections or 25 people for a minimum of 60 days per year.

The United States has approximately 161,000 PWSs. A CWS is also a public water system that serves homes throughout the year. The 54,000 CWS supply drinking water to as many as 268 million people in the nation.

Generally, CWSs that supply water to large communities get their water from lakes, reservoirs, rivers, and other surface water sources. The treatment process used to disinfect the water from these sources is determined by the water quality. Figure 1 shows the basic steps involved in the treatment of drinking water.

TOC Analysis in the Drinking Water Treatment Process

Figure 1. TOC Analysis in the Drinking Water Treatment Process

Large debris is removed in the initial step as raw source water is pumped through a screen. In order to oxidize organic matter that provides undesirable odors, colors and tastes, the oxidizing agent potassium permanganate (KMnO4) may be added. Potassium permanganate is listed by the U.S. EPA as an alternative preoxidant for chlorine used for controlling total trihalomethanes (TTHMs).

Flocculation and sedimentation are then used to clear the water. In order to coagulate small particles into large ones that can settle down as sediments iron salts, alum or organic polyelectrolyte polymers are added. Any residual particle and natural organic matter are removed by filtering the clear water through a bed of gravel and sand.

The filtered water may be subjected to another round of filtration through a granular activated carbon (GAC) bed to absorb and extract disinfection by-product precursor compounds like humic and fulvic acids, and remaining natural organic matter (NOM). TOC analysis is used to determine the effectiveness of post-filtration GAC treatment.

By decreasing the TOC concentration, a facility can minimize the formation of haloacetic acids (HAA5) and trihalomethanes (THMs). This is essential to meet the Disinfectants and Disinfection Byproducts Rule (D/DBPR).

Harmful microbes are killed through ozonation or chlorination to disinfect the water before its distribution. For chlorination, highly effective disinfectants such as chlorine dioxide (ClO2), chloramines (NH2Cl) and chlorine (Cl) are used. Though ultraviolet radiation and Ozone (O3) can effectively treat clean water, they cannot be relied on to control microbial contaminants across a distribution system.

Disinfectants and Disinfection Byproducts Rule (D/DBPR)

Two rules for regulating the disinfectants and disinfection by-products levels in drinking water have been laid down by the U.S. EPA. In the year 1998, the Stage 1 Disinfectants and Disinfection Byproducts Rule (D/DBPR) was declared.

The law took effect on 1st January, 2002 and it reduced the permissible THMs level down to 80µg/L. It also set out regulations for the levels of chlorite, bromate, and five haloacetic acids (HAAs) for the very first time.

In 2006, the USEPA Stage 2 D/DBPR was declared and its compliance dates were phased out over time, depending on the strength of the population served by a water system. The compliance date for systems serving a population of more than 100,000 was 1st April, 2012, and 1st October, 2013 was set as the compliance date for systems serving less than 10,000 people and 10,000-49,000 people.

The levels of THM and NOM in both source and treated drinking water can be determined by TOC analysis.

Drinking Water Security

In order to protect the water supply infrastructure of the United States, the country’s government has issued directives and passed legislation. According to Title IV of the Public Health Security and Bioterrorism Preparedness and Response Act, drinking water systems that serve population strength of over 3,300 people must develop response measures to events that could pose serious pubic health concerns or disrupt the supply of drinking water.

The Homeland Security Presidential Directive 9 requires the USEPA to develop a “robust, comprehensive surveillance, and monitoring program to provide early warning in the event of a terrorist attack using biological, chemical, or radiological contaminants.”

Intentional or accidental events resulting in organic chemical contamination can be detected beforehand due to the warning provided by on-line TOC monitoring of drinking water supply systems. As the system responds to all types of suspended or dissolved organic carbon, including compounds that do not contain a chomophore in water, it is an efficient monitoring parameter.

TOC Analysis

Figure 2 shows the OI Analytical 9210p On-line TOC Analyzer, which analyzes both source and treated drinking water using heated persulfate oxidation technique in USEPA- approved methods 415.3 and SM 5310C.

9210p On-line TOC Analyzer  AnchorPrinciple of Operation

Figure 2. 9210p On-line TOC Analyzer

Principle of Operation

During operation, water samples are drawn from a fill and spill sampling system at programmed time intervals into the 9210p On-line analyzer. In order to remove the inorganic carbon (TIC) content from the sample, phosphoric acid is added to the syringe.

After this process, the sample is moved to a reaction chamber to be oxidized at a programmed temperature up to 100°C using heated sodium persulfate (Na2S2O8)

Sodium persulfate is highly soluble in water:

Na2S2O8 + H2O → 2Na+ + S2O8(-2)

Applying heat will result in the formation of hydroxyl and sulfate radicals through the following reactions:

S2O8(-2) + 2H2O → 2SO4(-2) + 2H+ + 2OH

S2O8(-2) + 2H2O → 2SO4(-2) + 2H+ + H2O2

2H2O2 → 2OH + O2

2.5 - 3 hydroxyl or sulfate radicals per carbon atom are essential for oxidation of organic molecules. This technique can be used to oxidize almost all dissolved organic compounds. Organic matter such as macromolecules, suspended solids, and colloids can be successfully oxidized using concentrated solutions of 1 or 1.5 M.

Organic compounds found in the samples are converted to CO2 using oxidization. A solid state nondispersive infrared (SSNDIR) detector is employed to measure the CO2 and to determine and report the TOC content.

The results for all of the samples can be viewed on the touch-screen display and can be transferred to a computer or Supervisory Control and Data Acquisition (SCADA) system through ethernet connection, a 4 - 20 mA analog signal, or relay/alarm closure.

Correlation of Laboratory and On-line TOC Analysis

Currently, laboratory TOC analysis determines the test methods used for regulatory compliance reporting. As online TOC analyzers are often employed for process control, there is no need for these devices to function within the quality assurance standards set in USEPA methods. However, these devices must be periodically calibrated to maintain the standards of minimum drift, accuracy, and to validate oxidation efficiency.

The comparative data derived from raw and treated drinking samples through laboratory and on-line TOC analyzers using the heated sodium persulfate oxidation technique in methods SM 5310C and 415.3 (both approved by USEPA) are presented in OI Analytical Application Note #3945. A summary of the study and its results is given below.

An Aurora 1030 W laboratory TOC analyzer fitted with a 9210p on-line TOC analyzer using a process gas module to deliver CO2-free air and a model 1088 autosampler was used for the study. Table 1 reports the reagent conditions, calibration settings, and instrument operating parameters employed in the research.

Table 1. Instrument Operating Conditions

Parameter 9210p Aurora 1030W
Analysis Mode NPOC NPOC
Sample Volume 2 mL 2 mL
Phosphoric Acid Volume 1.0 mL 0.5 mL
Persulfate Volume 1.2 mL 1.2 mL
TIC Reaction Time 0.5 min 1.30 min
TOC Reaction Temperature 98°C 98°C
TOC Reaction Time 0.5 min 2.30 min
TOC Detection Time 1.45 min 3.0 min
Calibration Standard KHP - C9H8O4K KHP - C9H8O4K
Calibration Points 0, 1, 5, 10, 25 ppmC 0, 1, 5, 10, 25 ppmC
Calibration Blank 0 ppm 0 ppm
Cal 1 1 ppm 1 ppm
Cal 2 5 ppm 5 ppm
Cal 3 10 ppm 10 ppm
Cal 4 25 ppm 25 ppm

Adjustments to the analyzers’ operating conditions were made to achieve the minimum required reporting limit of 0.35 mg/L set by the SDWA, without disrupting the quantitation accuracy of terrorist and storm incidents that are capable of yielding TOC values higher than the average of 2 - 10 ppm values.

After it is calibrated to the above specifications, the 9210 on-line analyzer offers process control capability to remove TOC and reduce DBP formation. In addition, an early warning of organic contamination in the water supply system is also provided by the device.

Table 2 presents the data of a PotableWatr™ Quality Control Sample from Environmental Resource Associates (ERA) that was tested in quadruplicate on all the instruments. The results from both the devices were very close to the certified value (8.53 mg/L) and within the acceptable range of 7.21 - 9.85 mg/L. The 9210p on-line analyzer had higher precision compated to the 1030W laboratory instrument.

Table 2. TOC Data from Analysis of Potable Watr™ Quality Control Standard

9210p (mg TOC/L) 1030W (mg TOC/L)
8.68 (1.03) 7.92 (0.20)
8.75 (0.82) 8.67 (0.06)
8.76 (0.75) 8.56 (0.63)
8.73 (0.86) 8.41 (3.62)

%RSD in parentheses n=3

By employing a 0.2 mg/L TOC standard, the individual method detection limits (MLDs) of all the analyzers were set for the 40 CFR part 136 Appendix B. The MLD of the 1030W was 0.07 mg/L and that of the 9210p was 0.05 mg/L. The accuracy of 9210p MDL standard was 127% and that of 1030W was 75%.

The upper limit of linearity was verified only on the 9210p. A 50 mg/L carbon standard delivered a 49.9 mg/L result and a 98.4 mg/L result was obtained for a 100 mg/L carbon standard.

A Suwannee River NOM reference material (1R101N) with known organic carbon content was acquired from the International Humic Substances Society. In order to prepare a 52 ppm TOC humic acid quality control standard, the humic material obtained was moved to a 1000 mL volumetric flask and diluted.

This solution was again diluted to form solutions with two different TOC concentrations, 1.04 mg/L and 5.2 mg/L, and the solutions were tested on each analyzer.. Better precision and recovery was obtained from the 9210p on-line TOC analyzer at higher humic acid concentration.

Six water samples from surface water sources with unknown TOC concentrations were obtained from Accutest, a local commercial lab located in Bryan, Texas. These were analyzed in both the devices.

One randomly chosen sample was used as a duplicate and seven samples were analyzed in triplicates. The correlation of the samples between the two analyzers is shown in Figure 3. The similarity between the results of the two devices is demonstrated by the correlation of 0.99747.

Correlation of Results Obtained on Unknown Samples Between 9210p On-line and the 1030W Laboratory TOC Analyzers.

Figure 3. Correlation of Results Obtained on Unknown Samples Between 9210p On-line and the 1030W Laboratory TOC Analyzers.


The study results demonstrate that both devices Aurora 1030W Laboratory TOC Analyzer and the 9210p On-line TOC Analyzer produce comparable data of drinking water samples. Continuous real-time TOC data can be obtained from the 9210p On-line TOC Analyzer. The device can also be used for regular DBP reduction and TOC removal monitoring as well as drinking water security monitoring.

The 9210p On-line TOC Analyzer meets the QC verification, oxidation, detection and calibration requirement of the USEPA- approved methods SM 5310C and 415.3. As such, continuous TOC monitoring results can be used for regulatory compliance reporting purposes.

This information has been sourced, reviewed and adapted from materials provided by OI Analytical.

For more information on this source, please visit OI Analytical.


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