Heavy Metals in Waste Water

 

 

 

Heavy metals are a major concern in the treatment of water due to the toxic and other detrimental effects these materials can produce. In general, heavy metals are considered to be the following elements: Copper, Silver, Zinc, Cadmium, Gold, Mercury, Lead, Chromium, Iron, Nickel, Tin, Arsenic, Selenium, Molybdenum, Cobalt, Manganese, and Aluminum.

Heavy metals can be found in varying concentrations in any natural source of water, but the main treatment problems exist in the process water of the following industries:

While not all heavy metals are involved, research has shown that certain metals such as mercury, lead and chromium are toxic to aquatic life in relatively low concentrations. For example, 20 ppm of chromium is fatal to trout after 8 days exposure. If 100 gallons of normal chromium planting solution is discharged into a waterway it will be toxic to all microorganisms in the food cycle, even if diluted by 100,000,000 gallons of water.

In larger concentrations these metals may have detrimental health effects on man. Heavy metals are a cumulative toxin that the body cannot dispose of and they accumulate to harmful levels with repeated exposure.

The presence of heavy metals in a waste stream can interfere and even destroy the effectiveness or normal waste treatment operations. Activated sludge secondary treatment plants are especially affected since heavy metals can kill the necessary bacteria.

Heavy metals in water can make it unsuitable for many uses such as drinking, boiler feed, or process uses where high degree purities are required.

Discharge Limits
Federal, state, and local discharge limits for heavy metals vary depending upon the metal involved, but generally range from 0 to 10 ppm when discharged directly into a waterway. A plant discharging to a municipal waste treatment system will normally encounter surcharges if concentrations exceed the municipality’s influent regulations.

Pretreatment by the industry before discharge to the sewer may be required if the municipality is not equipped to deal adequately with large volumes of heavy metals.

Testing for Heavy Metals
The most common and accurate test for heavy metals concentration in a waste stream or effluent is the atomic absorption technique. Since many facilities do not have an atomic absorption spectrophotometer on site, samples often must be forwarded to an outside agency for analysis. It is important that the samples be properly collected to assure accuracy for test results. The sample should be collected in a clean and chemically resistant glass container. It should be preserved with concentrated nitric acid at a dosage of 5 ml HNO3 to every gallon sample. The container should be tightly closed with a metal cap, which is protected from sample contact by plastic wrap or Para film.

Treatment Methods
The most common method for removal of heavy metals is to form an insoluble metal hydroxide or sulfide compound and allow gravity sedimentation to occur. To produce the insoluble metal hydroxide, the pH of the solution must be elevated to the point where maximum insolubility occurs. Each specific metal will have its own ideal pH value. Therefore, when treating a wastewater containing a combination of metals, experiments must be performed to determine the pH at which the removal of all metals can be maximized. Care must be taken, as too high a pH can cause a redissolving of the heavy metals. The pH of the solution can be elevated through the addition of the correct amount of caustic soda or line. Following the formation of the hydroxides, the addition of a small amount of anionic polyelectrolytes will generally help floc formation and improve the setting of the metal hydroxide in the clarifier.

The sulfide process uses either sodium sulfide or ferrous sulfide to generate the insoluble metal sulfide compound. It is useful where the metals have been complexed with ammonia, cyanide or similar compounds. Also, for most metals, the sulfide has a greater insolubility than the hydroxide, which can lead to higher theoretical removals. After the sulfide formation, a small dose of anionic polyelectrolytes will improve setting.

A difficulty of the sulfide process is that use of excessive sodium sulfide may generate hydrogen sulfide (H2S), which in itself is a pollutant. If ferrous sulfide is used, iron is being added to the solution, increasing the amount of sludge to be removed.

Other types of treatment used for primary removal and finishing/polishing operations are carbon absorption, ultra filtration, reverse osmosis, and ion exchange.