Analysis Municipal Drinking Water: A Comparative Study of TOC and THM

Water quality can be determined at all stages of the water treatment process through its total organic carbon. In order to remove natural organic matter (NOM) and particulate matter, the raw source water is continuously treated.

The remaining NOM in water are disinfected through chlorination and contain fulvic acids and humic acid. These acids react with chlorine to form by-products like haloacetic acids (HAAs) and trihalomethanes (THMs), which are known to cause cancer.

THMs are constantly formed during the distribution of drinking water due to the excess chlorine levels needed to ensure microbial disinfection.

The amount of THM and NOM in source and treated water can be determined by TOC analysis. However, TOC cannot be substituted for THM analysis, which requires significantly more time.

For a water treatment process to function efficiently, the relationship between TOC from natural organic matter (NOM) in source water, reduced TOC levels during the treatment, the level of THMs and TOC in the treated drinking water, and THMs and TOC from different points in the water distribution system, must be established effectively and on time.

Most of the United States Environmental Protection Agency (USEPA) methods to analyze volatile organic compounds (VOCs) require analytes to be extracted through purging with helium for a period of 11 minutes at 40 mL/minute. As a result of this, purge-and-trap (P&T) is one of the biggest consumers of helium in the lab.

A lot of laboratories are searching for cheaper substitutes for helium, and have chose nitrogen as a suitable option. Nitrogen is not only more abundantly available, safer to use and inert, but it only costs a third of helium’s current price, providing a major cost-saving.

The comparative data from an improved method of THM analysis using USEPA Method 524.4. and TOC analysis conducted using heated sodium persulfate oxidation technique in USEPA-approved Standard Method 5310C is presented in this article. It will also show the connection between THM and TOC while exploring the effects of time, chlorine dosing, and other factors on the investigated parameters.

Experimental

Heated Persulfate Oxidation

Heated sodium persulfate (Na2S2O8) oxidation can be used to oxidize almost all dissolved organic compounds in water. Organic matter such as macromolecules, suspended solids, and colloids can be successfully oxidized using concentrated solutions of 1 or 1.5 M.

Sodium persulfate is highly soluble in water:

Na2S2O8 + H2O → Na+ + 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

In order to oxidize organic molecules, 2.5 to 3 hydroxyl or sulfate radicals are required for every carbon atom.

An OI Analytical Aurora 1030W laboratory TOC analyzer fitted with a model 1088 rotary autosampler was used for the study. Table 1 presents the instruments setting used for the research.

Table 1. OI Analytical Aurora 1030W Instrument Settings

Parameter Aurora 1030W
Analysis Mode NPOC
Sample Volume 5.0 mL
Phosphoric Acid Volume 0.5 mL
Persulfate Volume 2.0 mL
TIC Reaction Time 1.5 min
TOC Reaction Time 3.0 min
TOC Detection Time 3.0 min
Calibration Standard KHP – C9H8O4K
Calibration Points 0, 1.0, 5.0, 10, 25 ppmC

Inorganic carbon is internally removed by the 9210p TOC and Aurora 1030W analyzers with the acidification process followed by purging with air (9210p) or nitrogen (1013W). After the removal of the inorganic carbon, the persulfate reagent is added.

A non-dispersive infrared detector (NDIR) is used to quantify the sample and the outcomes are reported as TOC content in both concentration and mass.

Calibration

Reagents for all of the analyzers were created by the same analyst using the same neat material source. The reagents and rinse water were prepared using low TOC laboratory reagent water.

ULTRA Scientific and OI Analytical provided certified calibration standards. Potassium hydrogen phthalate (KHP) is used as the carbon standard to calibrate both instruments. Reagents were prepared using carbon-free water, which was also used as a zero standard.

Purge and Trap – GC/MS

The research is based on volatiles analysis by GC/MS; USEPA Method 524.4. An OI Analytical 4100 Sample Processor, an Agilent 7890A/5975C GC/MS, and an OI Analytical Eclipse 4660 Purge & Trap Sample Concentrator were used for the research.

The thermal energy of the purge volume must be increased as the heat capacity of the nitrogen molecule is higher than that of helium and nitrogen and is capable of withdrawing more thermal energy from the solution through which it is purged.

Due to this, numerous changes including increasing purge temperature to 55°C, fitting a 0.6 mm draw-out plate in the mass spectrometer, and slowing purge flow to 35-37 mL/min were made. Table 2 presents the instrument setting for this research.

Table 2. Purge & Trap, 4100 Sample Processor, and Agilent 7890A/5975C GC/MS Instrument Settings

Purge & Trap Eclipse 4660 P&T Sample Concentrator
Trap #10 trap; Tenax®/Silica gel/CMS
Purge Gas Zero grade Nitrogen at 45 mL/min
Purge Time 8 min
Sparge Mount Temperature 45°C
Sample Temperature 45°C
Desorb Time 0.5 min
Bake Time 4 min
OI #10 Trap Temperature Ambient during purge
180°C during desorb pre-heat
190°C during desorb
210°C during bake
Water Management 120°C during purge
Ambient during desorb
240°C during bake
Transfer Line Temperature 150°C
Six-port Valve Temperature 150°C
Autosampler 4100 Water/Soil Sample Processor
System Gas Zero grade Nitrogen
Purge Gas Zero grade Nitrogen
LV20 Pressure 8.0 psi
Gas Chromatograph Agilent 7890A
Column Restek RTX-VMS 20 meter, 0.18 mm ID, 1 µm film
Carrier Gas Zero grade helium
Inlet Temperature 250°C
Inlet Liner 1.5 mm Direct
Column Flow Rate 0.6 mL/min
Split Ratio 40:1
Oven Program Hold at 40°C for 1.5 min
16°C/minute to 180°C
40°C/minute to 220°C
Hold at 220°C for 1.0 min
Total GC run is 12.25 min
Mass Spectrometer Agilent 5875C
Mode Scan 35 – 300 amu
Scans/Second 5.19
Solvent Delay 1.40 minute
Trasfer Line Temperature 250°C
Source Temperature 300°C
Quadrupole 200°C

A ten-point calibration with a concentration range of 0.5 ppb to 40 ppb was prepared. Zero-grade nitrogen was used to purge the standards for 8 minutes. The standards were desorbed for 0.5 minutes onto a Restek RTX-VMS 20 meter column after the purge. The internal standard approach was used to calculate the response factors. %RSD and response factors of all compounds were checked in line with method 524.4.

Results and Discussion

Most of the samples were gathered by the employees of OI Analytical in College Station, Texas. Most of the samples were obtained from within a 45-mile radius of the OI Analytical office.

Other additional samples were obtained from Concho County, Texas (approximately 250 miles) and Houston, Texas (90 miles). The employees were instructed on proper collection methods, and the samples were collected in regular 40 mL VOA vials.

As per the method, maleic and ascorbic acids were used to preserve the samples obtained for THM analysis. Table 3 presents a summary of the results.

Table 3. Summary of Water Sample Results

Location Source CHCl3 BrCl2CH ClBr2CH CHBr3 Total THM (ppb) TOC (ppm)
Wellborn Tap 7.22 4.51 1.66 0.22 13.61 1.987
Bryan Tap 0.18 0.34 1.00 1.18 2.70 0.480
Wickson Tap 0.48 0.80 1.37 0.73 3.38 0.415
Somerville Tap 1.01 0.77 0.75 0.97 3.50 1.124
Wickson Tap 1.52 1.37 1.54 0.50 4.93 0.685
Wickson Tap 4.67 9.92 12.77 4.74 32.10 0.655
College Station Tap 0.64 1.87 5.52 7.29 15.32 0.384
College Station Tap 0.64 1.60 4.79 6.20 13.23 0.366
Bryan Tap 1.67 5.55 14.69 18.26 40.17 0.439
Wickson Tap 3.98 7.89 11.92 4.10 27.89 0.571
College Station Tap 0.58 1.55 4.58 6.55 13.26 0.378
Kurten Well 0.00 0.00 0.00 0.00 0.00 0.791
College Station Tap 1.69 4.20 12.33 16.02 34.24 0.506
Bryan Tap 1.56 4.06 11.62 14.81 32.05 0.447
Washington County Tap 1.06 2.18 4.49 4.40 12.13 0.407
Bryan Tap 1.34 4.15 11.92 15.18 32.59 0.521
Wellborn Tap 4.80 2.25 0.84 0.00 7.89 1.707
Montgomery County Well 0.00 0.00 0.00 0.00 0.00 0.248
Calvert Well 0.00 0.00 0.00 0.00 0.00 0.485
Calvert Tap 0.66 1.15 3.97 9.53 15.31 0.496
Hearne Tap 0.31 0.53 2.01 6.24 9.09 0.350
Bryan Tap 0.93 2.96 8.73 12.08 24.70 0.642
Bryan Tap 1.66 5.12 13.95 17.44 38.17 0.641
Bryan Tap 1.60 4.90 14.43 18.33 39.26 0.500
Houston Tap 12.68 5.22 1.46 0.27 19.63 3.299
Houston (SE) Tap 13.50 6.34 1.84 0.00 21.68 4.767
Wellborn Tap 4.10 2.43 1.00 0.27 7.80 3.302
Navasota Tap 0.33 1.11 3.44 5.99 10.87 0.443
Concho County Tap 3.54 10.87 22.57 10.26 47.24 1.801
McCullah County Tap 0.74 1.16 1.06 0.44 3.40 0.295
Williamson County Tap 9.18 15.49 16.53 5.16 46.36 2.806
Wellborn Tap 7.50 4.89 1.73 0.14 14.26 3.946

The TOC range of the private well samples was 0.248 to 0.791 ppm. The THM concentration in the samples was below the detectable limit.

Water is supplied to these wells from an unknown aquifer. In addition, the water source for Calvert, Hearne and other places that are not mentioned below are not known. The methods employed for water treatment are also unknown.

Samples taken from the Bryan-College Station are obtained from Simsboro Sand wells that are supplied by the Carrizo-Wilcox aquifer. Chlorine disinfection is performed to the water pumped from an approximately 3000 ft well and distributed in the area, which encompasses Wellborn, College Station, and Bryan.

The concentration of TOC was within the range of 0.384 to 1.987 ppm, with the highest concentration sample from the Wellborn area. The total THM concentration of the samples was within the range of 2.70 to 40.17 ppb. The total THM concentration for the Bryan community was two to three times more than that of the College Station community on an average.

Sparta aquifer supplies the water for Wickson (Northeast of Bryan) from wells that are approximately 550 - 900 ft. Chlorine disinfection is conducted and the treated water is distributed to the community.

Water for the community in Somerville, which is located about 35 miles west of Bryan-College Station, is supplied from Lake Somerville. Coagulation, flocculation, filtration, and disinfection are used to treat the water before it is distributed. The total THM concentration and TOC concentration were 3.50 ppm and 1.12 ppm, respectively. This water sample had one of the lowest total THM concentrations used in this research.

Houston community, located 90 miles to the south of Bryan-College station, gets its water from surface water sources. Conventional water treatment plants treat the water before it is distributed in the city. The total THM concentration was within the range of 19.63 to 21.68 ppb and TOC concentration was within the range of 3.23 to 4.77 ppm.

Interestingly, the total THM concentration of all of the samples taken for the research, the major cause for the total THM concentration, are brominated compounds such as chlorodibromomethane, bromoform and bromodichloromethane. There is a 96 to 37% of brominated compounds in the total THM concentration.

Among the samples collected from the Bryan-College Station area, the lowest brominated compound concentration of about 47% was found in the Wellborn sample. The lowest brominated compound concentration in the Houston water samples was recorded at 37%.

The correlation between total THM and TOC concentrations are shown in Figure 1. No direct correlation can be seen for the total THM concentration. For TOC concentration range of 0 (in sample from private wells) to 0.500 ppm (approx.), the total THM concentration was between 0 to 40 ppb. The presence of brominated compounds is shown to be the major cause for this.

Correlation between TOC and Total THM

Figure 1. Correlation between TOC and Total THM

The correlation between chloroform and TOC is shown in Figure 2. The direct correlation between the chloroform and TOC concentration is apparent from the data but variability in chloroform concentration can be seen at approximately equal TOC concentrations. However, the brominated compounds are a lot higher than the magnitude of the variability.

Correlation between TOC and Chloroform  AnchorConclusion

Figure 2. Correlation between TOC and Chloroform

Conclusion

The study has demonstrated an approved method of THM analysis by employing USEPA Method 524.4. Using this method in tandem with the Model 4100 Sample Processor can provide laboratories with a time-and cost-effective option for analysis.

All of the samples tested in the study demonstrate an acceptable total TOC concentration. The total THM concentration was also within the USEPA set limit of 80 ppb.

All of the samples collected for the study demonstrated a direct correlation between total TOC concentration and chloroform. No direct correlation between TOC and total THM was apparent but, the data obtained shows that THM concentration can increase over a narrow range of TOC concentration due to the presence of brominated THMs.

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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