Reactive Waste or Not Revisited

Q. In your November 2006 column, “Reactive Waste or Not,” C.W. stated they use an organic sulfide precipitant. I would like more information on this wastewater precipitant containing an organic sulfide. What brand names is it sold under? Who sells it? Does it really work better than conventional neutralization/precipitation using acid/caustic and a flocculant? R.K.

 

A. As far as your first two questions, PRODUCTS FINISHING editorial policy prevents me from mentioning a specific brand product or supplier. I would be very surprised if your current wastewater treatment chemical supplier does not offer an organic sulfide precipitant, but if not, refer to the 2007 PRODUCTS FINISHING DIRECTORY & TECHNOLOGY GUIDE under several titles: Flocculants, Polyelectrolytes; Pollution-Control chemicals, heavy-metal removal; and Pollution-Control chemicals, sulfide precipitation. You can also find the listings online at www.pfonline.com/suppliers.html. Basically, organic sulfides are any compound that contains sulfide (S=) and carbon, although for wastewater treatment these are typically a carbamate compound, such as dithiocarbamate (DTC).

To answer your last question, I would say generally sulfides will “work better” than conventional hydroxide precipitation, but it depends and there are drawbacks.

The main reason why sulfides can work better than hydroxides is because of its much lower solubility for heavy metals typically found in metal finishing wastewaters, except for chrome. For example, the theoretical solubility of zinc hydroxide is 11.0 x 10-1 mg/L and zinc sulfide is 2.3 x 10-7 mg/L; as you can see, sulfides can typically produce wastewaters with much lower metal concentrations.

The advantages of sulfide precipitation are

• Excellent removal of metals in presence of che-
  lating and complexing compounds,
• Depending upon existing chemistry and waste-
  water characteristic, can reduce filter cake volume
  by 20  - 70%,
• Solids typically dewater easier in the filter press
  since the floc particle is more compact and less
  gelatinous as compared to hydroxide,
• Since most metal finishing wastewaters are
  acidic, may be able to reduce caustic usage by
  reducing pH control setpoint, and
• Will reduce hex chrome to trivalent chrome.                                                                   

 

The disadvantages of sulfide precipitation are

• Rotten egg odor; while the liquid precipitant
  will likely have a masking agent or perfume
  added, there is still a noticeable odor,
• Unless you can achieve significant filter press
  cake volume reduction and reduce caustic use,
  sulfide precipitation is likely more expensive,
• Organic sulfide reduction of hex chrome takes
  a very high dose and lots of time,
• Since organic sulfides are typically pesticides,
  you need to carefully watch dosage so as to prevent
  excessive compound to the sewer and possibly
  interfere with sewage treatment plant,
• Controlling dose can be tricky; we have seen
  some pretreatment systems successfully use ORP
  controllers while others not,
• If wastewater's pH falls below two, carbon disul-
  fide, a toxic gas is released; while plant venti-
  lation will likely not make this an issue, if carbon
  disulfide deposits on parts before plating, you
  will experience a very high reject rate,
• If you wastewater contains high organics, your
  required dose will increase since organic sul-
  fides prefer other organics rather than heavy
  metals.  and
• Sulfide released to the air will react with any ex-
  posed copper, turning its surface black.

Typically, we have used organic sulfides in those situations where we needed to achieve compliance that hydroxide precipitation was not achieving. In most of these cases, we utilized it as a metal scavenger after hydroxide precipitation, adding it into the floc tank before the clarifier or between the clarifier and polishing filter. Only one of our clients which had a hydroxide based system achieving compliance found that, after a one month trial, the organic sulfide precipitant saved money, on net.

R.K., this will give you good guidance if you decide to pursue organic sulfide precipitation.