Adding total organic carbon analysis to jar testing can help water treatment plants more reliably meet limits for disinfection byproducts
Jar testing is a useful tool that helps water plant operators determine the most effective chemical source-water treatment. By simulating coagulation and flocculation that occurs at full scale in the plant, jar testing can inform quick and effective treatment process adjustments.
In jar testing, two common parameters used for making decisions are turbidity and UV 254 absorbance. Both are effective, but when only these measurements are applied, there is limited capacity to fully detect the effectiveness of chemical dosing for removing organics.
The addition of total organic carbon (TOC) analysis can overcome this limitation by telling operators exactly how much organic content is removed, not only by chemical treatment but by each additional type of treatment or during each treatment step. TOC analysis can also play an integral role in helping water plants meet disinfection byproduct (DBP) regulations, since organic material is a precursor to DBP formation.
Why jar testing?
Jar testing can help minimize operating costs by helping enhance chemical dosing accuracy. Operators can use jar testing to adjust chemical additions, try different chemical types, or alter the dosing sequence to achieve optimal contaminant removal. This is much easier than experimenting with entire plant processes.
Jar testing enhanced by real-time monitoring also helps operators overcome challenges related to overdosing or blindly dosing coagulant. Higher doses do not always translate to better TOC removal, and overdosing coagulant can lead to excess sludge production, increasing the cost of sludge removal.
Jar testing methods
Jar testing is simple and should be conducted periodically throughout the year to accommodate source water fluctuations. Useful times to run jar tests include during seasonal changes, after large temperature swings, when a new chemical is being added, or when new treatment equipment is introduced to the system.
To begin, jars are filled with raw water and dosed with varying amounts of chemicals or different chemical sequences. Then the water is stirred with a flocculator to encourage floc formation. After settling, the quality of the treated water is tested to determine the best coagulant and chemical dosages that can be applied to the full-scale treatment process. While most operators will attest to the value of jar testing, the extent of actual application can vary significantly from plant to plant.
Jar testing limitations
While turbidity and UV 254 absorbance testing can prove effective, there are inherent challenges to using only those indicators. Turbidity doesn’t distinguish between inorganic matter, organic matter and particulates — it only measures the amount of light passing through the sample.
Meanwhile, UV 254 absorbance measures the aromatic content of the organic material in the water by testing the UV absorbance at 254 nm with a spectrometer or UV probe. However, this test does not capture all organics because some organics do not absorb at 254 nm. In addition, multiple interferences, including ferrate compounds, nitrate and high turbidity, can occur at that wavelength.
Adding TOC analysis
By adding TOC analysis to jar testing, operators can gain a more comprehensive understanding of organics removal in each test jar and so better determine the most effective chemical treatment to achieve compliance with regulations.
Under the Disinfectants and Disinfection Byproducts Rule (DBPR), all plants must comply with maximum contaminant levels (MCLs) for several DBPs. To help meet those limits, the percent of influent TOC that must be removed during treatment is regulated in conventional water treatment plants using surface water, or groundwater under the direct influence of surface water.
While some plants struggle to meet TOC removal requirements from source waters, other plants need to remove more TOC than is required to achieve DBP limits. Each plant’s source water is different in terms the amount and characteristics of the organics, which determine how easily the water can be treated.
Case in point
To investigate the value of adding TOC analysis to jar testing measurements, GE’s Analytical Instruments performed jar tests with several surface waters from across the U.S. using two common coagulants: ferric chloride (ferric) and aluminum sulfate (alum). Investigators took samples independently from the source water but not from the water plant intakes.
Central Arizona Project Canal
The Central Arizona Project (CAP) canal is a 336-mile open conveyance that transports Colorado River water to central and southern Arizona from Lake Havasu. Phoenix-area water treatment plants regularly use CAP water, which can experience wide swings in quality.
Jar testing of CAP canal water was conducted using turbidity, TOC, and UV 254 measurements under increasing alum and ferric dosages. Based on raw water TOC and alkalinity, a treated water quality target below 3 ppm TOC was used for the water samples collected. Results demonstrated that TOC removal only met this requirement at 30 ppm alum dosing. The test also suggested that using turbidity alone to optimize chemical treatment could have led the plant to choose a chemical dosage with very little TOC removal, since turbidity dropped at a lower alum dose than did TOC.
The UV 254 data and TOC data were also decoupled: UV 254 went up at 30 ppm alum where TOC went down. This demonstrates that UV 254 data is not always consistent with TOC results. Compared to 30 ppm ferric dosing, which achieved only 21 percent TOC removal, a 30 ppm alum dose achieved 27 percent TOC removal. These results show that alum alone is sufficient to meet TOC removal requirements of 15 percent, but might not be reliable enough throughout the year or to meet all DBP limits.
Horsetooth Reservoir is a water source for cities in Northern Colorado, including Fort Collins, Greeley and Loveland. The reservoir receives water from the Colorado-Big Thompson and Windy Gap projects through a complex system of pipelines, tunnels and canals. In the last several years, population and industrial growth, natural sources and the occurrence of wildfires have led to an increase in TOC.
Results from jar testing with Horsetooth Reservoir water under increasing alum dosages demonstrated a reduction in turbidity, TOC and UV 254. While lower turbidity results were achieved with ferric dosing, alum treatment demonstrated higher TOC removal. This reveals that basing treatment off turbidity results would not lead to the best compliance under the DBPR.
Furthermore, jar testing results showed very little difference in TOC removal between 20 ppm and 30 ppm alum additions. As such, adding more chemical dosing above 20 ppm alum does not bring additional treatment benefits. Armed with this knowledge, water plants can realize more cost-effective chemical treatment.
San Gabriel River
The San Gabriel River in Texas feeds into Lake Georgetown and serves the city of Georgetown, just outside Austin. Jar testing results for San Gabriel River water revealed better treatment performance with ferric dosing than with alum. With a required TOC removal of 25 percent based on raw TOC and alkalinity, a ferric dose at 30 ppm removed over 40 percent of influent TOC, while alum dosing at the same concentration removed only 28 percent.
Pine Brook Reservoir
Pine Brook Reservoir serves a small water treatment plant in the hills just west of Boulder, Colorado. In recent years, the area faced drought, flooding, wildfires and extreme seasonal temperature changes, which together have created significant water treatment challenges.
Jar testing with Pine Brook Reservoir water showed a decline in turbidity, TOC and UV 254 at 10 ppm alum dosing. With increasing alum dosing, TOC continued to decrease. A 30 ppm alum dosing achieved 42 percent TOC removal, well above the 25 percent TOC removal requirement.
However, operators learned that even at 42 percent TOC removal they had not removed enough organics to meet DBP limits. As such, the plant now uses a chemical blend and ultrafiltration membranes to remove additional organics. Recent plant upgrades have increased the size of chemical coagulation and flocculation to optimize pretreatment before the membranes.
These examples demonstrate that adding TOC analysis to jar testing can help operators more easily optimize water treatment processes. The results provide insights into treatment recommendations based on raw-water quality and the effects of different chemicals at varying doses. By incorporating TOC analysis with jar testing, plants can better determine the most effective and cost-efficient chemical treatment solution for meeting DBP limits and TOC removal requirements.
About the author
Amanda Scott is municipal applications manager with GE’s Analytical Instruments. She can be reached at firstname.lastname@example.org.