The above links to current regulations for cement plants, and on Page 1008 (only a few pages from the beginning), there is a table showing limits for existing and new sources. MCL means Maximum Contaminent Level. It might be called something similar, but MCL is typical. For mercury, Table 1 shows so many pounds of mercury allowable per million tons of clinker.
Why does the EPA use an amount rather than a percentage? Because amounts are what cause harm to people and the environment. Of course, percentages can be corrolated, but it is a lot of repetitive calculation, and sometimes can be easily manipulated to make things look good to the general public.
For instance, anything above 90% removal sounds great, right? Even higher removal like 99% is even better, right? Anyway, making the math simple, let's say we have 90% removal capability and EPA makes "1" the MCL for something where it is considered "safe" (not really how it works, but stay with me). So, with 90% removal, if you start with a sample of pollutant of "10," all is good.
But, what happens when the sample has more than "10" pollutant to begin with? Even with 90% removal, it is above MCL. People and the environment may get harmed. In the real world, starting pollutant values do not fall nicely into place to reach a safe MCL even with some very high percentage removals. There are some constituents that, even at low levels with 99.99% removal, can still harm people and the environment.
This is why EPA uses numbers. Typically to persuade improved removal technology. Keep in mind, in the case of cement plants, EPA also wants them to find clinker with less initial pollutant to start with. Less initial pollutant, better chance of reaching that MCL.
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