Q. I recently came across an old column of yours (“Moly Removal From Wastewater,” September, 2006) and wanted to pass along to you and your readers my experience.
I have been working for 15 years in the field of water treatment and the problem with molybdenum in wastewater is getting more and more prevalent. I have one customer that has a molybdenum limit of 109 parts per billion (0.109 mg/L).
We’ve found the following treatment scheme to be successful. First, the wastewater’s pH is lowered with some inorganic acid (sulfuric, muriatic/hydrochloric) to a pH of around 7.0. We then feed a ferric-based coagulant to help precipitate the molybdenum. Normally we have to feed the coagulant at a rate of at least 15× the moly concentration. The ferric-based coagulant can be ferric chloride, ferric sulfate or a combination of ferric with an organic coagulant mix (the organic coagulant seems to reduce the required dosage).
You may have to go further to get a good pinfloc to develop, because the wastewater may contain other contaminants. Once the pinfloc has developed, add a good anionic polymer flocculent in order to form large floc that are easier to settle or filter. L.H.
A. L.H., thanks for your input on this issue. Just like with electroplating, wastewater pretreatment is part science and part art. Just because one chemistry works at one facility does not necessarily mean it will work at another plant.