Environmental
A Tandem Approach to TOC Analysis in Drinking Water Treatment
I
n 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) regu-
lates the levels of disinfectants and disinfection byproducts in drinking
water. Permissible levels of trihalomethanes (THMs) were lowered be- cause 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 dis- infected by either chlorination or ozonation to kill dangerous microbes. Chlorine (CI), chloramines (NH2 effective disinfectants. Ozone (O3
Cl) and chlorine dioxide (ClO2 ) are highly ) 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 prac-
tice for drinking water treatment and distribution system monitoring. Depending on the compliance parameters and system/process require- ments, 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 ana- lyzer 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 AMERICAN LABORATORY • 20 • AUGUST 2015
by John Welsh, Jr.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52