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topic 18899

Heavy metals concern in solid filter cake from waste water


I'm serving as environmental compliance executive in a dyeing & finishing industry. We have two standard water treatment plants. There is a problem in our treatment operation. In sedimentation tank (may call clarifier)sludge often floats. I seek kind suggestions from experts how we can get rid of this problem.


- Dhaka, Bangladesh


Look for a source of air or gas reaction. From my old days in environmental sales, floaters were a problem and usually caused by induced air.

Jon Quirt
- Fridley, Minnesota


Maybe a little more detail, Jon.

Dye wastewaters are usually treated by chemical oxidation. The reaction produces gas bubbles (I think CO2) that can carry the partially-reacted dye molecules to the surface where they agglomerate, ergo floaters. This can often be handled by a slow-speed agitation of the surface to release the bubbles, which is one reason treatment plants will often have a rotary blade stirring the liquid surface of a cylindrical tank.

Another possible source is excessive residual oxidizer which will attack polymer flocculant and behave as above.

However, first have the floaters analyzed to determine whether harmful. Further oxidation (by stirring and/or additional oxidizer) may be necessary. It is probably worthwhile to install ORP controls (or adjust operation if present) to avoid insufficient or excessive oxidizer.

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


I'm sorry as I kept my letter incomplete, dated 16 january 2003. In our water treatment plants we are encountering two problems. One is sludge floating which I mentioned and the other is heavy metals concern in solid waste(filter cake). I'm seeking experts opinion so that we can detoxify the concern solid waste. Also it'll be very helpful for us if any suggestion is provided to reduce the waste volume. *among heavy metal Chromium is the No. 1 concern *Our plant (each) capacity is 400 cubic meter of waste water per day. Where as produced solid waste is 600 Kg/day.



First, as you may need to modify your plant, I suggest buying an Electroplating Engineering Handbook.

Second, you haven't described your production or wastewater treatment processes are, and it is hard to understand how this wastewater flow is generating 600 KG/day of solid hazardous waste (an equivalent-sized USA plant might take 3 weeks). I suggest reading Wastewater Treatment‹ in the on-line Metal Finishing Guidebook ( Maybe, some high solids, non-heavy metal wastewater only needs neutralization and could be treated separately.

Third, regarding chromium-containing wastewaters: You should oxidize dye wastewaters separately from metal finishing streams requiring Cr(+6) reduction. Since both treatments require a slight excess of reactant to ensure completion, and since dye oxidation may create Cr(+6), you then combine the reacted streams to neutralize residual reactants for a near zero-ORP. Make any necessary adjustments to ensure that no Cr(+6) is present, then do conventional pH adjust, precipitation and removal of solids. The Wastewater Treatment‹ article only briefly describes chromium treatment. A method of reducing Cr(+6) that generates fewer solids is using sodium hydrosulfite powder. This chemical must be handled carefully and requires a solids feeder, but it is especially useful if you have residual Cr(+6) after combining treated wastewaters, as it can work at higher pH than other chemical reducing agents.

Finally, your plants may be complex and require elaborate, detailed advice. Consider hiring a consultant (see link below) and provide wastewater flow charts, concentrations and present treatment procedures, equipment and instrumentation.

Ken Vlach
- Goleta, California

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