Filter Cake from wastewater treatment sludge is slimy; will filter aid or floc help?

A. I'm not saying that polyelectrolyte floc'ing agents aren't important in wastewater treatment, but I doubt they are the problem in this case. And you can certainly use a diatomaceous earth filter aid, which is quite cheap, and it will help a lot -- but it's bulky and can increase your disposal cost since the sludge/filter cake is hazardous waste.

But I think the problem may be more of either technique or under-capacity. A filter press has to run at a very very slow speed for a full shift and often more before you can empty it without expecting some slime.

A. HI !

DEBRA, I READ YOUR QUESTION ABOUT FILTER CAKE IS NOT DRY ENOUGH, I THINK:
REASON #1 -- FILTER CLOTH IN FILTER PLATE CLOGGED UP; THAT'S WHY NO DEWATERING
REASON #2 -- FILTER PRESS LEAK BECAUSE OF FILTER PLATES GASKET WORN OUT
REASON #3 -- NOT ENOUGH HYDRAULIC PRESSURE FOR FILTER PRESS REASON #4 -- CHECK YOUR POLYMER FEED RATE.

A. Debra,

You're blinding off your media with the sludge. That's why it's slimy. You could try to steam cleaner ⇦ this on eBay or Amazon [affil links] your cloths, precoat your press with D.E. or perlite @ .2#/sq.ft. That will protect your cloths until you figure out what's wrong with your sludge, why you're not building a full cake. And you can body feed your treatment tank with the D.E. What's your pressure doing on the press?

Good Luck,

A. Debra-

I think the answers are good so far. If I assume a feed into the filter press of a "regular" electroplating wastewater treatment sludge that has been thickened in a settling tank, and the press has been packed full, then my guess is that a slimy cake is due to fouled cloths in the press. If the cloths on the plates are a typical polypropylene weave you can effectively clean them with hydrochloric acid, 30% + or - 10%. Don't remove the cloths, soak the plate and all in a tank of the acid solution for a couple of hours. Be careful, rinse thoroughly before reusing. You'll find that the plates in the acid solution have to be oriented just right so that gases formed during the cleaning can escape. Otherwise the trapped gas can pull the cloth away from the plate. This procedure made an enormous long-lasting difference for me. I'll probably do it yearly now.

Good luck.

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Ed. note:
Please see letter 39112, "Cleaning filter press cloths" for additional discussion of filter cloth cleaning.

A. Debra:

You may want to check the filter press feed pressure. Your solution may have some colloids difficult to filter. Some solutions need a lot of pressure to be filtered. 70 to 80 psi should do, but some may need more. If you are not getting the pressure you want from your pump, you may want to check for obstructions in the lines, valves and the condition of the pump's impeller. It could be worn out, loose or clogged.

A. Help me understand why you question cleaning the plates with HCl, Hank? Readers say they do it, and others have said so in other threads, and I've done it. Thanks.

Direct filtration of 80 GPM can indeed be impractical. You may need a clarifier and/or sludge thickener tank to reduce the flow to the filter press in order to get a drier filter cake. Good luck.

A. Paul

I would expect you could drop out more chromium at a pH of 7 - 8.

If your problem is new, I would pressure wash, acid wash, or replace the FP cloths.

If not the equipment, I would suggest running some jar / bench tests. Your target would be big flakes that settle quickly. Ask your polymer supplier for assistance with precipitation aids. The small flakes / fines plug the FP cloths prematurely.

A. Not surprising. Cleaners are notoriously hard to waste treat conventionally.

You might consider, after a pH adjustment, passing the waste through a bed of a chelating ion exchange resin. That will selectively remove transition metals, like Cr3+, Zn, and Cu.

There would be substantial expense. But, in the long run, you might save over all the labor involved in processing all of that sloppy wet glop...forever.