To age, or not to age, comes down to a question of economics. The additional capital costs to provide the aging tank(s), solution metering pumps, secondary dilution, controls, and the installation costs has to be paid for by the additional polymer savings achieved by adding this equipment. This can be done by comparing the cost of an in-line system to a batching system, estimating the polymer savings per year, and dividing the cost difference by the savings. As a result, applications requiring low polymer usage may not be able to justify the additional costs, which can be 3 to 5 times, or more, than the cost of an in-line system. Other considerations should include simplicity of an in-line system, floor space, and overall maintenance costs. Whether in-line or aging is being considered, specifying a polymer activation technology capable of effectively optimizing the activation of all polymers available, not just the simple-toactivate polymers, is paramount (see graph below).
Instances where aging is detrimental:
- when make-down water quality is poor (e.g. contains a high chlorine content or has a high level of suspended solids);
- polymer is over mixed with a tank mixer, or;
- when the polymer is over-aged (typically 24+ hrs)
How much benefit aging provides depends largely on:
- the effectiveness of the activation technology used to initially blend and activate the polymer prior to the aging tank (see the chart above);
- the polymer being used;
- water temperature and water quality (solids, chlorine content, etc.); the use of a tank mixer, and;
- the solution concentration prepared.
Tank Mixers: Tank mixers typically provide little or no benefit when a high-performing polymer activation technology is used to initially blend and activate the polymer prior to the aging tank. Aging tanks with mixers inherently have areas of excessive shear; near the tip of the impeller, and laminar flow around the tank wall. An aging tank system requiring a tank mixer typically
means that the polymer was ineffectively activated in the initial blending process. In this case, optimizing polymer performance is likely unattainable.
Tank System Design – Single Tank Vs. Flip Flop Tank Systems: When processing dry polymer, consider that the polymer being made-down is 0% activated as it enters the tank, requiring total isolation of newly made-down polymer from aged polymer. However, with a high performing emulsion polymer system – a system capable of efficiently inducing ultra-high, nondamaging mixing energy at all flow rates - can reach 90% - 95% activation in the mixing chamber, even with difficult-to-activate polymers. Because of this, there is little to no benefit of isolating newly made-down polymer from aged polymer in a two-tank system, such as a flip-flop batch process. The cost to include this equipment is very high; this means that the investment made in the second tank, automated valves, and more complex control systems has close to a zero ROI. With a single tank system, it is recommended that the low or “refill” tank level be set
at 50% of the tank volume to increase the mean aging time of the polymer solution.
For more information please contact VeloDyne at 303-530-3298 or visit our website at www.velodynesystems.com.