Regional regulatory bodies also regulate mercury. In the Great Lakes basin, for example, the International Joint Commission recommended zero discharge as the goal for Lake Superior.
Local wastewater authorities also are imposing increasingly strict mercury standards to protect the quality of their wastewater treatment plants' sludge, which treatment plants often use or sell in a processed form as fertilizer. If mercury levels are too high, the wastewater treatment plant might need to pay for disposal of its otherwise-useful sludge as solid waste.
Advances in laboratory technique also are responsible for the current interest in mercury. Until recently, the lowest detection limit for mercury using an EPA-approved method was 0.2 g/l, using Method 245.1, which was issued in 1974. Its detection limit is one-fifth of ppb; therefore, any regulatory concern expressed as ppt was a moot point because it simply was not measurable.
The situation changed with the issuance of Method 1631 in 1999. Under Method 1631, a minimum level of 0.5 ng/l is achievable ," 400 times lower than the previous lowest detection limit. As a result, it is now possible, in a practical sense, to assess compliance with regulatory standards set at the ppt level.
Although most pretreatment permits currently held by industry are written at the ppb level and rely on Method 245.1 for measurement, the situation could change gradually. First guidance, then ordinances and, finally, pretreatment permits could be reissued to reflect the new lower level of achievable resolution for at least some of the mercury measurements.
Revealing hidden mercury
We expect to see mercury in products such as batteries, fluorescent bulbs, silent switches, thermostats, dental amalgams, preservatives such as thimerosal, preserved solutions and old fungicide stockpiles. In each of those products, the mercury plays a clearly defined role, so its presence is not remarkable.
With the arrival of better laboratory techniques, however, comes a troubling revelation: Traces of mercury are widespread in other industrial chemicals and commercial products. In fact, many products with no apparent connection to mercury can contain significant traces of the metal.
Commercial-grade chemicals produced in the mercury-cell process are particularly noteworthy sources of inadvertent mercury. Sodium hydroxide (caustic soda), potassium hydroxide, chlorine and hydrochloric acid (muriatic acid) all can be produced in the mercury-cell process.
Another example of a commercial-grade chemical frequently containing inadvertent mercury is sulfuric acid produced at lead and copper smelters.
Direct users of these mercury-contaminated industrial chemicals can be using and discharging mercury inadvertently. The same is true for indirect users of these chemicals because the chemicals are incorporated into commercial products.
Many commercial products contain traces of mercury. Soaps, chlorine bleaches, detergents, fixatives and reagents all potentially can be contaminated with traces of mercury.
A Massachusetts database ," the MASCO database ," listing 5,508 products used in hospitals states that 781 contain measurable traces of mercury. Of those products, 469 have concentrations above 10 g/l, meaning almost 10 percent of the tested hospital products contain significant mercury concentrations.
Fuels such as coal and, to a lesser extent, oil generally contain traces of mercury. And wet scrubbers can convert mercury air emissions into mercury-laden wastewater.
Considering all the potential sources of hidden mercury, it is not surprising that more and more facilities are discovering they have a mercury problem. Fortunately, it is relatively easy to diagnose whether or not your plant has a mercury problem and to take protective measures to address the issue.
Making the diagnosis
A review of historic wastewater analyses can provide a rough initial mercury assessment, but it is only a starting point because of the limitations associated with these analyses.
A review of Material Safety Data Sheets (MSDSs) for every chemical product coming into the plant can provide some additional information. The trace-elemental composition of the fuels used at the plant also should be considered.
A significant limitation of MSDSs is they generally do not identify impurities composing less than 1 percent of the product. As a result, a product could contain as much as 10,000 milligrams/liter (mg/l) of mercury and still not disclose the presence of mercury in its associated MSDS.
A more reliable way to learn the mercury content of a chemical is to ask the supplier for a certificate of mercury analysis. A certified report of mercury concentration should be expressed in ppb. If the supplier's certified report claims "no mercury," then the report should reveal the detection limit of the test that was used to establish the claim.
By asking for a certified report of mercury concentration, plant personnel will be able to develop data vastly better than what are available in the MSDSs. Of course, it also is possible to send samples of your plant's incoming chemicals for analysis, but such analyses quickly can become quite expensive if more than a few chemicals are involved.
Remember: All mercury entering a plant must exit somehow. Therefore, as a first cut, it is safe to assume the mercury leaves the site in the wastewater. Depending on the complexity of your plant and the number of mercury-containing materials, it might be useful to track the pathways of mercury in the plant using a flow chart of the processes.
An ounce of prevention
The best approach to mercury problems is usually one that emphasizes prevention instead of treatment. As an element, mercury cannot be destroyed, only shifted from one medium to another.