ABSTRACT
The purpose of wastewater treatment is to remove pollutants that can harm the aquatic environment if they are discharged into it. Because of the deleterious effects of low dissolved oxygen (DO) concentrations on aquatic life, wastewater treatment engineers historically focused on the removal of pollutants that would deplete the DO in receiving waters. These so-called oxygen-demanding materials exert their effects by serving as a food source for aquatic microorganisms, which use oxygen in their metabolism and are capable of surviving at lower DO levels than higher life forms. Most oxygen-demanding pollutants are organic compounds, but ammonia nitrogen is an important inorganic one. Thus, early wastewater treatment systems were designed to remove organic matter and sometimes to oxidize ammonia nitrogen to nitrate nitrogen, and this is still the goal of many systems being built today. As industrialization and population growth continued, another problem was recognized, eutrophication, which is the accelerated aging of lakes and estuaries, etc., due to excessive plant and algal growth. This is the result of the discharge of nutrients such as nitrogen and phosphorus. Hence, engineers became concerned with the design of wastewater treatment systems that could remove these pollutants in an efficient and cost effective manner, and much research during the past two decades has focused on processes for doing that. Most recently, we have become concerned about the discharge of toxic organic chemicals to the environment. Many of them are organic, and thus the processes used to remove oxygen-demanding materials are effective against them as well. Consequently, much current research is directed toward a better understanding of the fate and effects of toxic organic chemicals in those processes.
