Most of Maine’s wastewater treatment plants were constructed after the passage of the clean Water Act of 1972 which was signature legislation of Maine’s own senator Edmund S. Muskie. As of 2012, there were 400 licensed “permitted point source dischargers” in Maine. Of these, 163 were publically owned treatment works and 59 are classified as “major facilities.” Major facilities are generally those licensed to treat an average flow in excess of one million gallons per day.

How do these plants function and why do we do it? First, we must understand that nearly all of the treatment plants in Maine use natural means and methods to clean sewage to the point where the water is healthy for the plants, animals and people that depend on our waterways for life. We treat the wastewater in sewage treatment plants in order to speed up the process and to keep waterways from becoming overloaded.

Treatment generally begins with measuring the flow of the wastewater and screening to remove large objects and stringy materials that could clog pumps or other equipment later on in the wastewater treatment process. The wipes, cleaning pads, sanitary products and other nondispersible items that some people flush need to be removed and sent to a landfill or other solid waste disposal facility. Dangerous and costly repairs to pumps are one good reason not to flush anything except toilet paper.

The next step often involves removing sand and grit so that downstream pipes do not become unnecessarily worn out. The sand and dirt in wastewater acts much like sandpaper when pumped, wearing away pipes, valves, pump impellers and other expensive equipment. To end the preliminary phase of treatment, grease is often skimmed off the wastewater. Grease and fats can clog pipes and foul aeration systems and is often difficult to dispose of once it has been in contact with sewage.

After the preliminary treatment described above, primary treatment is generally employed to remove solids. Simply by letting the wastewater remain quiescent (or nearly still), gravity allows solids to settle to the bottom of primary clarifiers where it can be removed by scrapers and pumped to solids handling machinery. At this point in the wastewater treatment process, approximately half of the solid materials and one third of oxygen demanding pollutants have been removed. Further removal of the oxygen-demanding materials (often referred to as BOD or biochemical oxygen demand) is critical because without removal these materials would rob all the oxygen from the receiving water and kill any fish living there.

The next step, biological treatment, uses naturally occurring microorganisms to eat the remaining wastes and allows us to remove the pollutants in the form of the microorganisms themselves. Aeration basins bubble air (oxygen) into the wastewater, while trickling filters and rotating biological contactors work by getting the microorganisms to grow on plates or disks while air is circulated around them. In either case, the microorganisms are subsequently removed in secondary clarifiers by gravity.

Finally, the clean water (which at this stage is more than 99.97 percent pure water) is disinfected to kill any remaining potentially harmful bacteria and released into a local river or water body where it supports a clean, “fishable and swimmable” resource for all to enjoy. To learn more, call your local wastewater treatment plant and plan for an informative tour.