I am responsible for the operation of the wastewater pretreatment facility at our finishing operation. Until recently, our metals had been consistently in compliance. Now, we are experiencing periodic non-compliance with zinc and chrome. We have checked our chrome reduction and found that it is reducing hexavalent chrome to less than 0.05 mg/l. Recently, we did notice small air bubbles bringing sludge particles to the top of our plate clarifier. We dropped the clarifier and used a hose to spray "down the plates as best we could. This seemed to solve the problem for only a "few days. Do you have any further guidance for us? D.B.
The problem that you are describing is becoming increasingly common, especially with those clarifiers that have been in service for a number of years. The problem you describe 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, etc.), which 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 2% 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 spraydown 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 the growth of bacteria, look at reducing its food source such as the removal of 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.