If you are using polymer in your wastewater treatment process, you may want to read this
Polyacrylamide-based polymers (sometimes referred to as flocculants) are used extensively in industrial water and wastewater treatment — and in food processing wastewaters, cationic polymers are the norm.
Choosing a suitable polymer for your wastewater plant is only half the battle. Assuming you have a suitable water treatment plant (DAF, for instance), and assuming the myriad of important operational parameters are well managed (flow rate, pH, coagulant type and dose rate, dose locations, etc), your plant may still not be performing as expected. The reason for this may be as simple as having a poor polymer make-up system that is providing poor-quality polymer solution.
Of all the chemicals used on a water treatment plant, the polymer can be the trickiest one to manage properly. Polymers are usually supplied as a product that needs further ‘manufacturing’ before use. For instance, granular polymers (sometimes called powder polymers) need to be added to water, that is, they need to be wetted, hydrated and uncoiled before use — a process that may take up to an hour to complete — while emulsion polymers need to be inverted first, then hydrated and uncoiled before use — and this may take as little as 5–10 minutes if appropriately managed.
Getting this ‘manufacturing’ step right requires a highly evolved understanding of the physico-chemical processes taking place at this initial step where the granular or emulsion polymer is added to the water. This understanding allows the correct and suitable choice of equipment to help initially wet or invert the polymer before hydration so that uncoiling can occur.
As an example, for granular polymers poor wetting can lead to polymer clumping where the dry polymer is surrounded by a wet polymer sheath created when only the outside of the clump of polymer is wetted. The gel-like sheath protects the dry polymer inside so it will never get wetted and uncoil; hence it won’t become a useful part of the polymer solution. No amount of mixing will undo this initial clumping, so the polymer solution is now weaker than expected. Not only is this a costly waste of polymer, but these lumps and clumps can be caught in and block pumps and pipework and this just compounds the problem of low plant performance.
For emulsion polymers, the initial ‘flip’ or inversion is critical. If the polymer is not inverted correctly at the correct polymer to water ratio, whilst using a suitable amount of energy, then the polymer may either be unavailable for use (wastage) or it may be degraded by over-shear, that is, the long polymer chain is broken up or sheared into smaller, less useful polymer lengths.
The next part of the ‘manufacturing’ process is the hydration step where at the molecular level the water molecules start to adsorb to the polymer chain. This process happens almost in tandem with the third step of uncoiling, where the polymer chain itself starts to unwind (uncoil) and extend out in length. Depending on:
- the polymer form (granular or emulsion),
- the polymer type (cationic, non-ionic or anionic),
- the degree of polymer branching,
- the polymer mol-charge (low, medium, high),
- the polymer chain length (molecular weight) and
- the strength of make-up (polymer to water ratio),
the most suitable equipment and operating method for hydrating and uncoiling your particular polymer will vary. Design engineers and operators must play close attention to the type of mixing energy (stirrer or pump for instance), the amount of mixing energy and applied shear, and the mixing time.
Furthermore, make-up water quality will also play a role in determining the final polymer solution quality — parameters such as pH, salt level (TDS), iron content, chlorine levels, temperature, etc.
So next time your plant is not performing as expected, have a look at your polymer make-up and dosing system and then ask Hydroflux to come along and provide simple solutions to complex problems.
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