Sludge settling problem in clarifier

A. Hi Miguel. In industrial wastewater systems for plating and metal finishing waste, both traditional coagulants/co-precipitants plus polyelectrolytes are usually used. Please try your best to describe your own situation because the endless "ifs, ands & buts" involved in abstract questions tend to scare many of our readers away from even starting :-)

Thanks & Regards,

A. Industrial wastewater treatment involves some reasonably complex issues and it is hard to answer a question like yours, Mehmet. Some causes of poor settling include a leak in the pumping system that lets tiny air bubbles in, too much of a temperature rise in the clarifier so that dissolved oxygen comes out of solution and floats the floc, selection of an inappropriate polymer (anionic polymers should usually be used for industrial waste water), excess chlorination, a slant in the sawtooth weirs which results in a fluid short-circuit, etc. Also, the design may be wrong with too much expected--settling rates for metallic floc are slow, and if the clarifier is too small, having too high a rise rate, it simply won't work.

Some guidance can be found in the Metal Finishing Guidebook, but Clarence Roy's "Operation & Maintenance of Surface Finishing Wastewater Treatment Systems" may be the ideal book for the situation. Good luck.

A. In the past, we used TMT-15 (trimethyl-mercapto-triazine) in silver precipitating because it was non-toxic and had no bad odour. But most of the time settling was difficult. Indeed, every now and then we had to change to a new and different flocculant to get the best results. Since TMT-15 is an expensive product, I would expect you to get help from local dealer.

Our water contains a high level of thiosulphate complex, and TMT-15 never gave lower residual silver than 1.0 ppm. We changed to good old sodium sulfide and achieved a silver level of 0.05 to 0.20 ppm, at a fraction of cost compared to TMT-15. The overdose of sulfide is co-precipitated with ferric sulphate solution, dosed by an ORP controller. We get some more sludge but the water is cleaner. Off-gases are ventilated through a simple activated carbon filter.

A. Mehmet

I am not sure of this solution of course because I have just the information you have listed on this site. BUT maybe you should raise your pH in your equalization tank? 7.5 may be a little low and could cause rx issues down the line? The plants I have input in on design and process, we use a higher pH after the primary clarifier. Good luck I hope you find your solution.

A. Hi, cousin.

The gases of the air, like oxygen, dissolve into the wastewater, and their solubility decreases with temperature. So cold water from the process line enters the clarifier. As it warms up, small oxygen bubbles form, particles of floc agglomerate on them, and they rise to the top of the clarifier instead of settling. The practical point is probably that clarifiers can benefit from a flotsam trap.

Regards,