When people hear about treated wastewater being discharged into a water way, most envision a cesspool of nasty unmentionable things. When I tell people that the facility I work at discharges 40 to 50 million gallons per day into a major river in Texas, they immediately want to know about the pollution it causes. I have to explain why this release of water actually improves the quality of the river. You see all municipalities have strict regulations they must follow to be allowed to release anything into a body of water or stream here in the USA. It is my belief that 99% of these municipal facilities meet their requirements. The requirements are usually over and above what would be need to simply prevent pollution. (Industrial laws on treatment and discharge or disposal are another matter.) I will try to give you an overview of what happens in the municipality where I am a manager of a wastewater treatment facility.
* Plumbing codes and building permits are written to ensure that home, industry, and business piping systems are designed and constructed such that no wastewater leaks into the ground at any point. During construction, inspectors must approve every single foot of pipe and connection.
* There are actually codes that restrict what you are allowed to put into the drains at your home or business. There are very strict laws and monitoring in place for the industries in my city which prevent them from releasing anything stronger than household waste into the system.
* The municipal collection system is simply a large network of pipes of ever-increasing size which carries the used water from your home to a treatment facility. There may be pumping stations required at varying intervals depending on terrain and distances. This system is designed to strict criteria and constantly maintained to prevent spills of untreated sewage.
* Once the sewage reaches the treatment facility, it goes through many stages of a process that cleans it to a quality better than that in the river itself. Note that sewage is essentially still water. It contains on the average less than 1% solids or human waste.
* First any large trash, like paper, rags, sticks, and plastic, is screened out. This is taken to a landfill and mixed with other trash from your house.
* Then the flow is slowed down enough to allow solids as heavy as, say, sand, to settle out. This sandy material is removed and mixed with the screened out trash.
* The flow is then slowed further so that about 60% of the remaining solids settle out. These solids are sent to a treatment unit that allows microorganisms to process the human waste into safe form. This bio-sludge may be safely used for fertilizer. In my city we mix it with wood chips and allow it to compost. The end product has been through a process that destroys any remaining microorganisms and is safe to use in flower beds, etc.
* The water that remains after removing 60% of the solids flows to a huge basin in which a culture of billions of microorganisms is held. These "bugs" eat and absorb the remaining solids. When the population of "bugs" grows too large for the amount of "food" coming in, a portion of them are sent to the previously mentioned process where they are reduced to safe bio-solids.
* The cleaned water is separated from the microorganism by another settling process called "clarification".
* The water is then disinfected with chlorine. The same substance used in drinking water and you washing machine.
* After that, it is filtered through charcoal and sand to remove the tiniest particles of silt remaining.
* Then the chlorine is neutralized before we release it to the river.
* There are many monitoring devices and lab testing used to keep the process effective. Three main things monitored are Oxygen Demand (a number representing how much oxygen a substance will use if it degrades), Total Suspended Solids, and Ammonia. At my facility we take wastewater that has as much as 200 ppm (parts per million) Oxygen Demand, 200 ppm Solids, and 20 ppm Ammonia and treat it to 2 ppm Oxygen Demand, 2 ppm Solids, and 0.12 ppm Ammonia.
The water discharged into the river is much cleaner than the regulations require and much cleaner than the river itself. Check with your utility and ask about their system.