To trade a mercury-contaminated wastewater for a mercury-contaminated sludge, ash or air emission is not really satisfactory, even if it satisfies a wastewater regulation. In addition, prevention usually costs less than treatment. So if it is at all possible, address the mercury problem at the source.
Consider adding a mercury-content criterion to feedstock specifications. A mercury criterion can disqualify contaminated materials and encourage your vendors to adjust their formulations to reduce or eliminate mercury. If the mercury content in the plant's fuels is significant, consider a mercury-content fuel specification ," or the substitution of alternative energy sources.
To address the problem of mercury that is "deliberately" present in certain feedstocks, more research and creativity are required because the properties of mercury are presumably vital to the product's performance. For deliberate mercury formulations, it is worthwhile to consider whether mercury-free alternatives are available.
Sometimes a literature review will reveal mercury-free alternatives that others have identified. Other times it will be necessary to be a leader and develop new substitutes for the offending material. The research and development required for alternatives takes time, so it is better to begin the work long before a regulation or pretreatment standard forces the issue.
Sources and Paths of Mercury in the Environment
Source: EPA, "Mercury Transport and Fate in Watersheds," 2000
Resorting to treatment
If mercury cannot be eliminated satisfactorily from the process, it might be necessary to resort to wastewater treatment. A variety of wastewater treatment processes currently are available.
Filtration can be considered if the mercury is attached to suspended solids or colloids. A well-designed simple filtration unit can remove particles down to 5 microns. If necessary, membrane filtration can remove mercury to the sub-micron size.
Reverse osmosis (RO), like membrane filtration, also can provide submicron removal of colloidal mercury. RO systems are fouled easily; therefore, the RO system usually is reserved for a polishing step at the tail end of a treatment train in a waste stream of any complexity.
Ion exchange can be considered as a treatment option if the mercury is present as dissolved ions in the wastewater. Ion-exchange resins can achieve near total deionization of wastewater. In wastewater applications, the expensive ion-exchange resins are fouled easily. If the waste stream is complex, therefore, it usually is necessary to treat the wastewater's gross contaminants first and to use the ion exchange as a final polishing step.
Selective polymeric adsorption also can be considered if the mercury is present in ionic form. Selective adsorption is attractive if, for example, mercury (II) is present in the wastewater with other +2 charged ions. Some selective polymers can be reused, minimizing the production of secondary sludge.
Activated carbon adsorption can provide effective removal of mercury at the ppb level. Organic mercury is adsorbed readily, and various specialty activated carbons are available for removing metallic and ionic mercury as well.
Oxidation/reduction followed by precipitation also is an option. However, the redox reactions are not selective for mercury, so they will result in a sludge that contains a variety of metals, not just mercury.
As long as the appropriate climate, space and geology are available, constructed wetlands can remove mercury from wastewater. Care must be taken in the design and testing of the wetlands to ensure the mercury is captured in the wetlands. It also is important to verify that the mercury is not converted into methylmercury, which would be counterproductive.
Any one of these treatment technologies can be expected to produce better results at a lower cost if it is placed close to the point of mercury generation. Mercury treatment near the source minimizes the volume and complexity of the wastewater stream.
As ordinances and laboratory practices in the field catch up with the state-of-the-art laboratory detection methods and the national movement to restrict mercury, mercury-containing wastewater is destined to become a more important issue. The current period of transition provides a fine opportunity to diagnose and solve your own plant's latent mercury problem before it becomes a regulatory problem. CP
Gilbertsen is a program manager for ENSR International, Warrenville, Ill. Contact him at (630) 836-1700.