Let's open the black box.
I don't know what chemical substances your lab work with, but chances are they work with both acid and alkalyne substances. When you throw them into the drain, they're directed to the "processing tank", which of course is made with a corrosion resistant material (most likely FRP or some other plastic). Acid and alkalyne substances neutralize each other, but one of them will almost always prevail, i.e., there's no sufficient acid to neutralize the alkalies and vice-versa.
Therefore, there's a pH sensor in the tank that "reads" whether the pH is acid (below 7) or alkalyne (above 7). In the first case, this starts a dilute alkaly metering pump (not centrifugal pump, please!) and an agitator inside the tank to neutralize the acid. In the second case, what starts is a dilute acid metering pump together with the agitator to neutralize the alkaly. In this manner, the pH within the processing tank is always maintained around 7.
Of course, all materials in contact with the chemical substances must be corrosion proof.
When the processing tank is full, you've got to empty it. But, as William Shakespeare said in one of his comedies, this is another story.
Giovanni S. Crisi
Sao Paulo - Brazil  

A little more of the story: The tank is then full of metalic salts disolved in water. The water is evaporated, and You pay somebody a lot of money to take away the metalic salts and hopefully dispose of them properly.