Q. We are a contract finisher with anodizing, alkaline zinc plating, electrocoat and powder coat capabilities. Our processes are well-established, but a few days ago we suddenly experienced floc rising, rather than settling, in our waste treatment. Our wastewater treatment process is simple neutralization to pH of 8-9.5, then addition of a flocculant, followed by a plate separator and filtration through a filter press. I have looked through the plant and have not identified any glaring changes made to the process that would explain this. As I go through it all more thoroughly, I was wandering if you had any quick ideas of things I should check. C.C.
A. Your question is a fairly common one. Here are some actions we recommend you consider:
- Check to see if the clarifier is level in both planes, front to back and side to side. It is possible that, over time, the floor or the clarifier’s foundation has settled unevenly, causing more wastewater to flow through one pack versus the other. This higher flow means a higher velocity through the plate pack, causing the lighter floc particles to be carried out into the effluent. If you find that it is out of level, we recommend that you re-level the clarifier. Some models have overflow weirs that are adjustable, while others will require the entire clarifier to be leveled.
- When was the last time the settling plates were cleaned? At least monthly, and possibly even more frequently, we recommend that you pump down the clarifier until the wastewater level is below the bottom of the plates. Thoroughly rinse off the plates; if you have access to a high-pressure washer, use it. If the plates are not periodically cleaned, floc can accumulate on the plates and even block some of the passages between them. This forces the wastewater to pass through fewer plates, resulting in higher velocities and more floc carryover.
- When was the last time the entire clarifier was cleaned? At least annually, and possibly even every six months, the clarifier should be entirely pumped out and the bottom section cleaned through the side access hatch that most clarifiers have. Over time, floc can build up and become compacted into the clarifier’s sides below the plates and if not removed will reduce sludge storage volume. Also, floc can build up on the influent distribution structures that slow down the incoming flow and distribute it evenly along the bottom of the plates; this build up will eventually disturb this incoming flow and cause floc carryover into your effluent.
- Have you checked to make sure your flocculant or polymer dosage is not excessive? Excessive polymer can accumulate on your clarifier’s plates, acting as a glue and attracting excessive floc particles, which no longer easily slide down the plates and remove from wastewater. Again, the floc builds up, causing higher velocities up through the plates and causing excessive solids carryover. Rub your fingers over the plates; if they feel really slimy, then check your flocculant dose.
You did not mention whether the floating floc particles contained air bubbles. If this is the case, the problem you described likely has one or both of two causes: creation of gas bubbles due to the neutralization of acids, particularly nitric acid, and the generation of gas bubbles by bacteria growing within accumulated sludge and/or on the clarifier plates.
The neutralization of some acids will generate gas bubbles. (Remember making those “volcanoes” in chemistry class by mixing vinegar [acetic acid] with baking soda [sodium bicarbonate]?) If the time in the flocculation tank(s) between pH adjustment and the clarifier is too short to allow these gas bubbles to detach from the floc particles and escape, they can collect in the clarifier to a sufficient size so as to cause floc or sludge particles to float to the top of the clarifier and out its overflow. The only real solution, if this is the case, is to increase the size of the existing floc tank or install an additional tank. Slow mixing will also help the release of gas bubbles.
The other cause of rising gas bubbles with sludge in the clarifier is bacteria growth within the sludge or on the clarifier plates. Over a period of time, even with good sludge removal, the metal hydroxide sludge will find places to collect within the lower portions of the clarifier. Also, within the sludge and wastewater, you have organic compounds (brighteners, wetting agents, polymers, oils, electro-coat organics, etc.) that act as a food source for the bacteria that enters the water, usually through atmospheric fallout. Over a period of time, the bacteria within the sludge grows enough to generate sufficient gas through their respiration to cause the sludge to break free and float to the surface of the clarifier as a “sludge ball.” The clarifier plates serve as a great place for bacteria to settle and grow. You can check to see if this is the case by pulling out several plates and inspecting both their top and bottom sides. If you see and feel a dark, slimy film, it is likely bacteria growth. Furthermore, if you feel a scaly residue on the plates, this can serve as a great home to bacteria.
Since removal of the clarifier plate packs for cleaning is usually not very feasible for most plants, you can clean them in place. We have found that an effective means to remove the bacteria and any scaly residue is to drain the clarifier and refill it with a 1- to 2-percent solution of hydrochloric (muriatic) acid, being sure to cover the plates. While we have found that the attack on carbon steel is negligible, if you have concerns, try using a hydrochloric acid that contains an inhibitor. After allowing this acidic solution to work for at least 12 hours, drain the acidic solution to the wastewater treatment holding tank, and then rinse the plates with a hose followed by a high-pressure water spray to all accessible areas. If the clarifier is equipped with an inspection plate below the plates, remove it and thoroughly clean the bottom of the clarifier. After this cleaning, you should find the plates to be very smooth and without the dark, slimy film. In addition to regular and periodic water spray down of the plates, be prepared to perform this aggressive cleaning once every year or two during one of your regular shutdown periods.
In order to minimize bacteria growth, look at reducing its food sources, such as removing oils at the cleaner tanks, maintaining brighteners and wetters at the lower end of their operating concentrations in plating tanks, and dosing wastewater treatment polymers at the lowest concentration in order to achieve desired flocculation and settling. Finally, I have seen a facility have some success by “drip” feeding bleach solution into the wastewater stream just before the clarifier in order to maintain 1 ppm free-residual chlorine in the clarifier effluent. And “they” say that we generate a “toxic” wastewater.