Even if no one has ever suggested that your plant has a mercury problem, it is worthwhile to think carefully about whether the wastewater's mercury level is truly acceptable.
During the mid-1990s at one large facility, an indirect discharger on one of the Great Lakes, plant effluent mercury levels appeared to be reasonably low. Analytical data showed mercury concentrations of 1 microgram/liter (g/l), or 1 part per billion (ppb), at worst, and usually less.
This effluent concentration would be acceptable in most sanitary districts and is lower than the 2 g/l of mercury allowable in drinking water. However, because the wastewater flow from the plant is so great ," accounting for approximately one-third of the host town's wastewater flow ," and because the Great Lakes ecosystem is so sensitive, the local wastewater authority encouraged an investigation into the root cause of the mercury concentrations.
A careful examination of the plant's feedstocks revealed that the plant's sulfuric acid was obtained from a lead smelter and was contaminated with as much as 10,000 g/l of mercury. By switching to an alternative source of commercial sulfuric acid, the plant achieved a 98 percent reduction in its effluent mercury level. The wastewater authority now holds up the plant as an example of success under the authority's current pretreatment ordinance limit of 0.3 g/l and its policy goal of zero discharge.
This example underscores the importance of fundamentally investigating and understanding the mercury concentrations in a plant's wastewater. Before considering the practical issues at an individual plant, however, it is helpful for chemical and pharmaceutical plant personnel to review current mercury regulations and to develop an understanding of the mercury problem.
With that context in place, it then is appropriate for plant personnel to discuss a proactive approach for addressing the concern before it becomes a regulatory problem. The most appropriate approach usually is one that emphasizes careful materials management, giving secondary attention to end-of-pipe treatment.
Why is mercury in wastewater such a problem, and why is it more likely to arouse regulatory scrutiny now than it did in years past? Appreciation of chemistry and toxicology and improvements in laboratory techniques all are driving mercury to a higher profile and pushing wastewater standards to tougher levels.
The chemical fate of mercury in nature truly is unfortunate. Most other water pollutants become diluted in the receiving water and then become assimilated through natural processes that gradually render the pollutants harmless.
For example, if dissolved hydrocarbons are present at acceptable levels in an industrial effluent, they quickly become diluted to a lower concentration in the receiving water and then become consumed as a food source by microorganisms. Through the action of the microorganisms, the hydrocarbons are broken down into carbon dioxide and water. The combination of dispersion and biological action, therefore, eliminates the hydrocarbons from the environment.
Mercury is different. Unlike most other pollutants, mercury is both persistent and bioaccumulative. Because mercury is an element, it cannot be broken down into innocuous components. In fact, microorganisms convert dissolved mercury into methylmercury, an organic compound that is more active and toxic biologically than is the original metal.
Once methylmercury enters the food chain, it bioaccumulates and bioconcentrates. As a result, the most contaminated fish tissues tend to be in larger, older predator fish ," precisely the fish most valued for human consumption. Game fish tissues easily can contain several million times the mercury levels of their native water. Furthermore, the methylmercury is bound into the flesh of the fish; therefore, gutting, skinning and cooking the fish provide no benefit in reducing the mercury content.
Advances in the understanding of mercury's human toxicological properties also have focused attention on the metal. The general dangers of mercury have been known for centuries. "Mad as a hatter" was a popular expression arising from the odd behavior of hat makers exposed to mercury fumes.
In the early 1950s, mercury first emerged as a wastewater-related health concern when a Japanese chemical plant discharged mercury-laden effluent into Minamata Bay. Fish comprised a large portion of the local diet, so many of the residents of Minamata City became ill with mercury poisoning. They experienced a loss of peripheral vision, a pins-and-needles sensation in their hands and feet, a loss of coordination and difficulty speaking, hearing and walking.
The United States has avoided such notorious scenarios of mass poisoning by mercury, but the nation's waters contain numerous fish that are unsafe to eat without restriction. Fish advisories have steadily increased over the years ," in 2000 alone, 41 states issued fish advisories.
To control mercury in natural waters before the element has an opportunity to contaminate fish tissue, the U.S. Environmental Protection Agency (EPA) established a number of controls such as ambient water criteria. Under the National Toxics Rule, promulgated by EPA in 1992, the ambient criterion for mercury is 12 nanograms per liter (ng/l), or 12 parts per trillion (ppt). In the Great Lakes system, the ambient criterion is even lower ," just 1.3 ng/L under EPA's Great Lakes Water Quality Initiative.
And certain watersheds ," designated by states as impaired waterbodies ," are regulated under EPA's Total Maximum Daily Load (TMDL) program. Established under Section 303(d) of the Clean Water Act, the TMDL program requires states to specify how much a particular pollutant needs to be reduced for a water body to meet water quality standards. The state then allocates pollutant load reductions among a watershed's pollutant sources. In practical effect, the TMDL program easily can dictate ppt discharge concentrations because of mercury's tendency to bioaccululate in fish.