New chemical recipe works in Jar Test but fails in Real ETP

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I recently visited a client who wants to switch the set of used chemicals to Ferrous Sulphate + Lime + Polyacrylamide in their Effluent Treatment Plant (ETP). This new Ferrous-based recipe works well in Jar Tests with large flocs and clear liquid, but the problem is, it apparently shows no effect in the Real ETP.

Previously they used to use HCl+PAC+Decolor+Poly which worked somewhat, but was costly and hard to maintain process parameters. The Wastewater to be treated comes from a Knit Dying Textile production factory. The ETP is a Bio-Chemical one which operates in a 45 m3/hr flow for 24 hr/day. Here the Chemical Treatment Unit is followed by the Biological Treatment Unit; and in the Chemical Treatment Unit, there is a 3-staged Coagulation-Flocculation chamber made of concrete that is agitated by separate aerators, the mixture then follows to a Primary Clarifier.

We tried various recipes by increasing and decreasing the doses of different chemicals but the results were the same: They all work in Jar Tests but not in the Real ETP. In the Jar Tests, we used the same sample of wastewater from the Equalization tank as for the Chemical Treatment Unit in the Real ETP; and the mixture product in the plant sometimes formed cloudy appearance but very rarely and it was inseparable. We also patiently waited for whole days to let the old water out which is multiple times longer than the ETPs rated residence time.

Here are the concerns that we tested and their results:
1. Calculations: Rechecked multiple times, no errors.
2. Jar Testing at ETP site: Done from the beginning, so this was not the concern in the first place. Also took the water samples and chemicals from the site that are in action in the operation.
3. Fluctuations in flow and pollutant concentrations: Tested different samples and also flow-weighted volumes in Jar Tests and found the variation had very little effect on the quality of the treated water. The chemical recipe is fairly robust to reasonable fluctuations.
4. Calibration of Flow Meter: Rechecked the accuracy of the Flow Meter by filling an emptied tank of known volume and recording the time with a stopwatch. Also checked for any recirculation channel or leak or short-circuiting of water. And we found no apparent trouble along with an acceptable error of no more than ± 5%.
5. Pumping after coagulation & flocculation: No, the mixture moves under natural gravitational flow through an open channel to the clarifier.
6. Preparation of dosing chemical solution: Both the Jar Tests and the ETP uses fresh water for mixing the dosing chemical solution. And since there is no recirculation in the Chemical Treatment Unit, no treatment chemicals are re-introduced, it’s all pollutants only.
7. Unknown reactions with residue from previous chemicals: Cleaned the tanks thoroughly and ran the plant with new recipe for a long time to wash out the old mixture.
8. Temperature: There is a cooling tower that circulates the raw water from the equalization tank. So the raw water is fairly close to atmospheric temperature so are the samples for the Jar Tests.
9. pH: The pH was found identical between the Jar Tests and in the Plant at the corresponding test points.
10. Oxidation: Yes, it’s likely since the ‘green’ ferrous sulfate solution partially turns yellowish before dosing in the plant.
11. Dissolved Oxygen (DO): Not tested.
12. Agitation with aeration in the Jar Tests: Done. But no significant difference was witnessed.
13. Bach Dosing in the plant: Filled the 3-staged coagulation-flocculation chamber with raw water and stopped the flow and dosed with proportionate amounts of chemicals identical to the Jar Tests with the sequence: Ferrous Sulfate → Lime → Polyacrylamide. Mixing was done with the usual aerating agitators with reduced aeration at the end. This time the outcome of the experiment was positive. It formed large flocks and clear water just like the Jar Tests.
14. Continuous Dosing, Mixing, Agitation & Time: At this point, after the above findings, the only thing to check was the Turbulence/Shear and Stability of the flocks. We suspected that the Linear Velocity of the pump flow was too strong and was breaking the flocks just after it had formed. To test this, we changed the points of chemical mixing by dosing the Ferrous Sulfate and Lime at the 2nd stage and the Polyacrylamideat the 3rd and leaving the first stage to absorb and reduce the initial turbulence caused by the pump flow alone. The residence time in each stage is 5 minutes. This time, we stopped all other agitations and allowed the mixing to be done at the Energy Dissipating points at the inter-stage connections. And yes, our suspicion came out right. Once again, the flocks were successfully formed with satisfyingly clear water.

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