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Sludge production in the treatment process

 

I'm working in a treatment plant for plating (of all kind Al, Zn, Cr, Ni,...)waste water. We also deal with waste water from the preparation of metallic material to be painted with powder. We are using lime in the treatment so we produce a big quantity of sludge. Are there any techniques or chemicals I can use to decrease the sludge production?

Milton Fontes
- Aveiro, PORTUGAL

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You will generate substantially less sludge if you neutralize with caustic soda instead of lime. But everything has its trade-offs, and caustic soda [affil links] is a bit more hazardous to handle and generally less effective as a precipitant, so you would probably have to add calcium chloride or ferrous sulphate ferrous sulphate [affil links] to maintain good precipitation.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

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Milton,

Using lime is going to obviously precipitate out any of the metal species in solution if you take the pH high enough. The only way to reduce the amount of precipitate sludge is to remove the metal species by another process. Depending on which metals are present it may be possible to plate them out as a valuable product down to very low ppm levels in discharge to the treatment plant. This would cut down precipitate volumes and possibly produce a saleable product depending on flowrates and metal concentrations.

Peter Becker
-Ashmore, Qld, Australia
 

Thank you for answering my letter! Keeping on this subject, I'm thinking on: Stepwise addition of neutralizing chemicals, CAUSTIC SODA in the equalization/neutralization tank and just a little bit of lime on the coagulation tank. The stepwise addition gives me a better control over the pH. The addition of lime besides neutralizing helps on the coagulation. I think that this amount of lime would't be enough, so I'm thinking on recirculating some of the sludge (after sedimentation) back to the coagulation tank. Do you think this is a good aproach to the problem?

Mr. Peter Becker wrote about saleable products. Is there a market for 98%Al(OH)3 dry sludge?

Thank you,

Milton Fontes
- Aveiro, PORTUGAL
 

The most marketable products that I can suggest are those of metal powder or plate product should they be recoverable from the given solutions. There may in fact be a market for intermediate hydroxides however I would presume that selling prices would be significantly reduced over metal.

Peter Becker
- Ashmore, Qld, Australia
 

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There are several types of chemical precipitants that are available that can reduce sludge production in metal finishing waste streams by 70-80 or more. Most of these involve the use of a thiol reduction ion, many use DTC, many use forms of inorganic sulfides and organic sulfides. There is an extreme amount of concern about the use of dimethyl DTC and its toxicity in the breakdown products. It is a significant pollution source in the US coming through municipal treatment facilities, and it is now being found in ever increasing concentrations in groundwater in areas where there were a high quantity of industrial dischargers thatused it.

There is a patented material called Thiol-Red that has some unique capabilities, such as being able to work at near neutral pH values effectively while still dropping metals to the low part per billion range. You would normally need to use some form of a coagulant to ensure good settling, however this is dependent on the presence of other ions in the water. Aluminum, since you asked, really doesn't react with the precipitant, so if it is present in high quantities, you will likely eliminate much of that solids volume. I believe there is a European source for it.

One other point: You could possibly get your metals to a sufficient concentration in the sludge that you might have some nominal value for it. We have a client in Phoenix, Arizona that is using the material, and he has gotten the copper in his sludge up to almost 5% by weight. Previously, it was less than 1%. He is giving it away to a smelter - they pay the freight. Saves him almost US$40,000 annually, and the chemical only costs him around $25,000. His total waste water discharge in 2001 was over 21 million gallons, and his average TOTAL metals in the water was 0.050 mg/L.

Tom Baker
wastewater treatment specialist - Warminster, Pennsylvania
 

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I think crystallisation is the option for you. Instead of water-rich sludge, pellets are produced which are dried to the atmosphere without any sludge treatment. The technique has proven itself for metal removal, fluoride, hardness, phosphate etc.

Robert de Boer
- The Netherlands