1. Chemical Clarification Precipitation Methods
Prepared By
Michigan Department of Environmental Quality
Operator Training and Certification Unit

2. Some things (chemists use term compounds) dissolve in water – these are said to be soluble.
Some things (compounds) don’t dissolve in water – these are said to be insoluble.

3. A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate.

4. Precipitate Formation
soluble insoluble

5. Precipitation of Metals from Wastewater
Metals Precipitation
Transforms dissolved contaminants into an insoluble solid, facilitating the contaminant's subsequent removal from the liquid phase by sedimentation and / or filtration.
Usually accomplished by:
pH Adjustment
Chemical Coagulation
Flocculation
Separation

6. Hydroxide Precipitation
Most Soluble Heavy Metals Will Precipitate
When pH is Raised to a Given Point
Cu(NO3)2(aq) NaOH(aq) Cu(OH)2(s)
+ 2 NaNO3(aq)
pH is Raised by Addition of Hydroxide

7. Hydroxide Precipitation
Sodium Hydroxide,
NaOH
Caustic Soda
Calcium Hydroxide,
Ca(OH)2
Hydrated Lime

8. Sodium Hydroxide Caustic Soda, Soda Lye
Available in Liquid or Dry Form
Liquid:
Commercial Strength
50 % , 73 % NaOH
Often Stored at 20 %
50 % Crystallizes at 53 Deg. F
Dry:
99.9 % NaOH
Bag, Drum, or Bulk
Safety
Caustic Poison
Dangerous to Handle, Corrosive
Prevent All Body Contact
Protect Eyes

9. Calcium Hydroxide Ca(OH)2 Hydrated Lime, Slaked Lime
Available in Dry or Liquid Form
Dry Form Available in Bags or Bulk
Commercial Strength %
Safety
Caustic, Irritant, Dusty
Avoid Contact with Eyes, Nose,
Respiratory System

10. Hydroxide Precipitation
Sodium Hydroxide VS
Hydrated Lime
Less Expensive
Solids Settle Faster
Higher Sludge Solids Conc
Sludge Easier to Dewater
Takes Longer to Neutralize
More Complicated Feed System
A Lot More Sludge

11. Chemical Treatment Process Water .. Equalization Tank Treatment Tank
Treated Water
Clarifier
Filter Press
Back To
Equalization Tank
Sludge Tank
Sludge Bin

12. Hydroxide Precipitation of Soluble Metals
Cu(NO3)2(aq) + 2 NaOH(aq) --> Cu(OH)2(s) + 2 NaNO3(aq)
Ni(NO3)2(aq) + 2 NaOH(aq) --> Ni(OH)2(s) + 2 NaNO3(aq)
2 AgNO3(aq) + 2 NaOH(aq) --> Ag2O(s) + 2 NaNO3(aq) + H2O(l)

13. Precipitation of Metals from Wastewater
The Solubility of Metals is Dependent on pH

14. Hydroxide Precipitation
From Chart (“Book Value”)
Minimum Solubility (Best Removal) Value for Some Metals
Cadmium
pH 11.0
Copper
pH 8.1
Chromium
pH 7.5
Nickel
pH 10.8
Zinc
pH 10.1

15. Hydroxide Precipitation
Need to Determine Optimum pH for Waste
May or May Not be Close to Book Value
Metals Precipitate at Various pH Levels
Depending On:
The Metal Being Precipitated
The Chemistry of the Wastewater
Other Metals Present
Presence of Chelates

16. Metal Precipitation
Precipitation of more than one metal from the same solution:
Adjust for Optimum pH for One Metal
(With Lowest Discharge Permit Limit)
Adjust for Compromise pH
Operate in Stages
Adjust pH for Each Metal in Stages
Draw Off Sludge, or Transfer
Supernatant Between Stages

17. Hydroxide Precipitation
Expected Effluent Quality (mg/L) for Total Metals
Precip + Precip +
Settling Settling +
Filtration
Cr 0.1 to to 0.5
Cu 0.2 to to 0.5
Ni 0.5 to to 1.5
Cd 0.2 to to 1.0
Zn 0.2 to to 1.0
NaOH
Mix Tank
Clarifier
Filter

18. Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Many Metals are Amphoteric
Metal hydroxides that re-solubilize if the pH increases or decreases from the optimum precipitation value

19. Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Many Metals are Amphoteric
Double Solubility Curve
pH Must be
Carefully Controlled

20. Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Many Metals are Amphoteric
Double Solubility Curve
pH Must be Carefully Controlled
Presence of Chelating Agents will Interfere
with Ability to Precipitate Metals

21. Chelates Can’t Live With ‘em, Can’t Do Without ‘em
An agent that will form a compound with a heavy metal ion, with the intended purpose of keeping the metal in solution.
Highly Desired in Plating Solutions
A Big Problem in Wastewater Treatment

22. Commonly Used Chelating Agents
Ammonia, NH3
Phosphate
EDTA
Citrate
Cyanide

23. Chelates Methods of Addressing the Problem 1. Precipitate at high pH
pH > 10 (may also add CaCl2)
2. Add a material which the chelate prefers
over the metal to be precipitated.
Ferrous Sulfate
3. Use a reducing agent to convert the
heavy metal from the ion back to the
metal
Sodium Borohydride
Use a precipitating chemical that steals
the metal from the chelate
Sulfide
Sodium Dimethyldithiocarbamate (DTC)

24. Sulfide Precipitation of Soluble Metals
Extremely Low Metal Solubilities
Often Preferred for Wastewaters
Containing Chelating Agents
More Negative Environmental Impacts
More Costly (About Twice $ as for OH)
May Produce H2S in Lethal Concentrations

25. Sulfide Precipitation of Soluble Metals
Cu(NO3)2(aq) + Na2S(aq) --> CuS(s) + 2 NaNO3(aq)
CdCl2(aq) + Na2S(aq) --> CdS(s) + 2 NaCl(aq)
Ni(NO3)2(aq) + Na2S(aq) --> NiS(s) + 2 NaNO3(aq)

27. Filtration Filtration Cr 0.05 to 0.5 0.01 to 0.20
Ca(OH)2
FeS
Mix Tank
Mix Tank
Clarifier
Filter
OH OH + S
Precip + Precip +
Settling + Settling +
Filtration Filtration
Cr 0.05 to to 0.20
Cu 0.05 to to 0.25
Ni 0.25 to to 0.50
Cd 0.05 to to 0.25
Zn 0.05 to to 0.25

28. Sodium Dimethyldithiocarbamate
(DTC or SDDC)
Organic sulfur compound
Controls chelated waste streams
Not pH sensitive –
any pH above 3
Non amphoteric
Very toxic
Agricultural insecticide
Industrial biocide

29. Sodium polythiocarbonate (PTC)
Thio-Red
Much less toxic than DTC
50% less sludge than DTC, lime or iron chemistries;
Requires lower dosing of flocculent chemistry for liquid/solid separation
Treated waters pass toxicity tests for National Pollutant Discharge Elimination System (NPDES) discharges

30. Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Metals are Amphoteric
Double Solubility Curve
pH Must be Carefully Controlled
Presence of Chelating Agents will Interfere
with Ability to Precipitate Metals
Hydroxide Sludges May be Difficult to Dewater
Metal Finishing Sludge is Classified as a Hazardous Waste by Federal & State Law (RCRA)

31. Batch Treatment Usually Preferred for Small Flows
Either a Holding Tank and Reaction Tank,
or Two Reaction Tanks
Reaction Tanks Serve Also as Clarifiers
Allows for Maximum Control of Effluent Quality
Influent Wastewater
NaOH
NaOH
Effluent
Effluent
Sludge
Sludge

32. Continuous (Flow-Through) Treatment
Usually Used For Larger Flows
Addition of OH Controlled by pH Meter / Controller
NaOH pH
React
Storage
Clarifier
Flocculation

33. Sludge Recirculation Batch Treatment
Precipitated Solids Act as Seed Structure for Forming Precipitate
Batch Treatment
Portion of Settled Sludge Left on Tank Bottom that Re-suspends When Mixing For New Batch Begins

34. Sludge Recirculation Continuous Treatment
Sludge is Recirculated from Sludge Hopper of Clarifier to the Influent or Reaction Tank

35. Review – Metal Precipitation
1 Sodium Hydroxide is hazardous to handle in that it is both _____________ and ______________.
toxic
corrosive
2. List three options for precipitating two metals with different optimum pH. a. b. c.
Optimize for One Metal
Find a Compromise pH
Operate as a Two-Stage System

36. 3. Metal hydroxides that resolubilize if the pH increases or decreases from the optimum precipitation value are said to be __________________.
amphoteric
4. Ammonia, phosphates, and EDTA are _______________agents used in plating solutions.
5. Describe the problem that may occur when precipitating metals from wastewater where the materials described in #4 are present.
chelating
Chelates hold metals in solution, not allowing them to precipitate.

37. 6. List 3 ways a wastewater treatment plant operator may be able to overcome the problem in #5 that is caused by the materials listed in #4. a. b. c.
7. Recirculation of _________________ may be beneficial in that it provides “seed” material for the precipitation process.
Precipitate at high pH
Add a metal to tie up chelate
Add a chemical that steals metal
from the chelate (Sulfide)
precipitated sludge

38. Chemical Clarification Precipitation Methods
Prepared By
Michigan Department of Environmental Quality
Operator Training and Certification Unit