As water treatment facilities are faced with ever-increasing challenges, it is important for utility staff to proactively evaluate treatment plant performance and implement actions to improve operations, energy efficiency, and treated water quality. The jar test is recognized throughout the water industry as a valuable and proven tool for treatment process optimization. Jar tests are routinely conducted by water treatment plant operators, laboratory staff, consultants, and chemical suppliers. The jar test is conducted in the laboratory and is used to simulate full-scale conventional treatment processes. Laboratory staff can work together with operators in conducting these tests, particularly in the preparation of stock solutions, in which operators may have little or no experience.

 

Jar testing may be carried out for a variety of reasons including but not limited to the following: 1) to optimize chemical dosages and / or points of application, 2) to determine the effectiveness of alternative coagulants or coagulant aids, 3) to optimize mixing times and intensities, or 4) to evaluate the impact of other changes in water chemistry and conditions. A typical jar test apparatus (Figure 1) consists of four to six jars with sample ports and paddle mixers,   

which can be programmed to stir at particular speeds for particular amounts of time, to simulate the coagulation, flocculation, and sedimentation processes. Stock solutions of treatment chemicals are prepared ahead of time and added to the jars in the same sequence and dosage as they are added in the full-scale plant (additionally, the plant dosage is typically bracketed by higher and lower dosages during the jar tests to determine the optimal dosage).

Figure 1: Northglenn Jar Test Apparatus

It is critical that the conditions used in the jar test accurately simulate the full-scale plant. This requires knowledge of the hydraulic characteristics of the plant as well as the properties and dosages of any chemical additions. However, even when theoretical conditions (e.g., velocity gradient, detention times, and surface loading rates) are matched closely, there is often a need to empirically tweak the parameters to make the jar test results match the full-scale results. Therefore, customizing a jar testing procedure so it can yield results indicative of plant performance is iterative and can be time consuming. Facilities with successful jar testing procedures have often used the theoretical parameters as a starting point and then made minor adjustments by trial and error until the full-scale plant results are accurately simulated by the jar test¹.

The City of Northglenn Water Treatment Facility (WTF) is in the process of calibrating their jar test procedure. Once the jar testing procedure has been successfully dialed in, the tests will be used to optimize the dosage of chemical coagulants (i.e., alum and polymer) and oxidants (i.e., sodium permanganate). Optimal coagulant dosages are critical to proper floc formation and filter performance. Higher coagulant dosages do not necessarily provide more effective removal of contaminants, such as particles and organic compounds. In fact, a law of diminishing returns often applies with the addition of coagulants (Figure 2), where further increases in coagulant dosage can reduce total organic carbon (TOC) removal. In addition, coagulant dosages that are too high can result in the production of excess sludge, which increases the cost of sludge disposal. It should also be noted that the coagulant dosage which results in the best turbidity removal does not always correspond to the best TOC removal (Figure 3). Therefore, jar tests can provide comprehensive insight into the relationship between particle and organics removal, and how it relates to chemical dosage, cost, and sludge production.

Figure 2: Law of Diminishing Returns with Coagulant Dosage²

Figure 3: Turbidity and TOC Removal by Coagulant Dosage²

Once successful, the jar testing procedure will be used to optimize the addition of other treatment chemicals used at the Northglenn WTF, such as sodium permanganate. Sodium permanganate is a chemical oxidant used to remove iron and manganese, and to control taste and odor compounds. It can also help to reduce the formation of disinfection byproducts (DBPs) by oxidizing precursors and reducing the demand for disinfectants, such as chlorine. High dosages of sodium permanganate can result in pink water, which can be avoided through dosage optimization with jar tests.

The processes downstream of pretreatment will also operate more efficiently when pretreatment processes are optimized. For example, filters succeeding an optimized pretreatment process will have longer filter run times between backwashes, which results in less energy and finished water use. Further, enhanced TOC removal due to pretreatment optimization helps water plants meet DBP regulations, since organic material is a precursor to DBP formation.

In addition to the potential to dramatically improve treatment process operations, energy efficiency, and treated water quality, the total cost savings associated with optimized treatment can be significant. The City of Englewood saved greater than $100,000 in one year in chemical and sludge disposal costs by optimizing their pretreatment process with jar testing results². Armed with the knowledge gained from conducting jar tests, water plants can ensure the plant is optimized to the fullest extent possible and deliver the highest quality water to customers.

References

[1] American Water Works Association. 2011. Operational Control of Coagulation and Filtration Processes.

[2] American Water Works Association California-Nevada Section. Improved Jar Testing Optimization with TOC Analysis. https://ca-nv-awwa.org/CANV/downloads/2015/afc15presentations/ImprovedJarTesting.pdf. Accessed: January 19, 2022.

Emily von Hagen is the Laboratory Technician at the Northglenn Water Treatment Facility. She has a Master’s Degree in Environmental Engineering and a Class C Water Treatment Plant Operator license in Colorado. She is passionate about everything water and lives in Denver with her parrot, Zappa.