Cold weather chemistry for water treatment plants
During cold weather operations, some water treatment systems require more chemicals than normal which can affect manager’s budget. Facilities using ferric chloride may need to adjust the addition point of the chemistry to allow for more contact time or controlled slowing the flow can aid in better contact time. Hybrid chemistry that combines iron with sulfated aluminum creates a product that can have the benefit of both weight from iron and the cold water performance of aluminum. As most operators who have worked in the drinking water industry know, aluminum products have a distinct advantage in performance during cold weather operations.
In addition to metal salts, polymers may also be slow to react in cold weather. The addition point for polymer may be adjusted by adding it sooner in the process to provide a longer contact time for effectiveness. Also using an aging tank allows the polymer to reach its full potential when mixing the polymer with cold water. Aging tubes can be used in-line to provide wide spots to allow aging prior to feeding at point of use.
Bench testing of proper dosages during challenging operational periods is highly recommended. Samples should be kept as close to the operational temperature as possible and not be allowed to warm to inside lab temperatures. This will ensure that the operator will see how the chemistry responds in as-close-to-normal operations as possible. Bench work can be performed on a gang-style stirring device or in simple one-liter beaker. Bench testing is good practice to test a wide range of application rates by tracking the settling rates and floc formation of each test.
After the operator has selected the best combination of dosages from the bench test, the chemistry selected should be trialed in the treatment system to verify that the bench work will perform well in the operation. Application points should be selected to provide adequate mixing and contact time, but caution should be taken to make sure that excessive aggressive mixing will not cause the formed floc to shear and lose its effectiveness. A sheared floc will not form the bridging necessary to create a controlled settling that has good removal and compaction. Small particles that have not been allowed to floc properly will tend to settle rapidly because of the small surface area of the particle and not provide the clarity and performance required.
After the operator has found a good dosage to achieve steady state operation, the operator can start to adjust his chemical addition to find operational efficiency where the chemistry will provide the best removal rates along with the most efficient costs.
Managers may find that putting time into jar testing and analysis of chemicals will provide efficient operations and keep that budget in line. Facilities can fall into a status quo mode of operation because what worked before should still be effective now. Initiating a chemical review can be very helpful by bringing in a new set of eyes along with strong experience with chemical applications. Many times staff may be too close to a problem to be able to recognize that an issue exists.
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