This physical/chemical treatment process uses ferrous sulfate and sodium sulfide to remove heavy metals from wastewater through precipitation at normal or alkaline pH. Ferrous and sulfide ions act together to reduce hexavalent chromium to trivalent chromium to form a precipitate. The ferrous ion acts with the sulfide, aiding in the reduction of chromium at neutral or alkaline conditions. The ferrous and sulfide precipitate further removes suspended and dissolved metals through coagulation and flocculation.

The coagulation and flocculation system used by sodium sulfide and ferrous sulfate is similar to other chemical-physical treatment methods used to remove metals from wastewater. The system consists of two mixing tanks with chemical feeds and a clarifier, followed by an activated sludge basin and final clarifier. The influent must be kept above a pH of 7.0 to prevent offgassing of hydrogen sulfide from the addition of sodium sulfide. The wastewater must also be kept between a pH of 7.2 and 7.5 when the ferrous sulfate is added to ensure the proper coagulation and reduction of chromium. External recycled sludge and anionic polymer are added to the chemical tank effluent to aid in the flocculation process. This mixture enters the clarifier for settling and clarification.

The clarifier is used to settle flocculant from the wastestream. The clarifying chamber contains internal sludge recycling to aid in settling. The effluent from the clarifier must pass through a sludge blanket, which aids in the removal of fine particles. The floc produced from this process is light in weight and volume, and has a tendency to form very fine particles. The process must operate at optimum conditions to ensure discharge levels below regulatory standards.

The compliance benefits listed here are only meant to be used as a general guideline and are not meant to be strictly interpreted.  Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Consult your local Industrial Health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.

• This method decreases the amount of sludge generated and chemicals used while producing lower metal levels in treated effluent. Sludge generation is reduced 30% by volume (when compared to sulfuric acid/sulfur dioxide/lime treatment method), resulting in significant savings in sludge disposal costs for most wastewater treatment plants.
• Chemical usage is cut by 40% (when compared to sulfuric acid/sulfur dioxide/lime treatment method).
• No pretreatment or post treatment (i.e., water softening) is needed, which results in operational cost savings.
• This process is highly efficient at removing suspended and dissolved metals from a wastewater stream. It can effectively reduce hexavalent to trivalent chromium, allowing it to be removed by flocculation.
• This process can also remove cadmium, copper, lead, and zinc, among other contaminants. It has an application in treatment of metal-bearing wastewater generated by operations such as plating, paint stripping, and metal cleaning.  

• If the batch becomes acidic (pH less than 7), then there is the possibility of hydrogen sulfide generation. However, this should not occur because the process is not designed to operate in acidic conditions.  

*There are multiple MSDSs for most NSNs.
The MSDS (if shown above) is only meant to serve as an example.

National Concrete Products
P.O. Box F
Greenwood, DE 19950
Phone: (302) 349-5528
FAX: (302) 349-9435
Contact: Mr. Mac McCombs