How Wastewater is Treated to Become Safe for Release to the Environment?

The United States creates approximately 34 billion gallons of wastewater each and every day both domestically and commercially.  The collection, treatment and release of wastewater is governed by Federal law.  Wastewater is water that contains contaminants, whether physical, chemical, or biological and is unfit for or human consumption.  Every time a citizen flushes a toilet, washes dishes, or does laundry wastewater is created.  Every time a farmer waters crops, provides water to livestock, or washes its products, wastewater is created. Every time a business manufactures a good or provides a service, dependent upon the use of water, wastewater is created.  Every time an industrial plant distills gasoline from crude oil, creates plastic packaging, or processes raw materials into a good, wastewater is created.  Given the finite volume of fresh water in the world, it is critical to treat and clean all this wastewater so it can be reused for possible human consumption.

There are more than 17,000 public wastewater treatment plants, or WWTP's, serving 82% of the population  in the United States.  The remaining 18% of the population relies upon individual septic systems, private WWTP's, or treatment by nature. The principle law governing the treatment and release of wastewater in the United States is the Clean Water Act or CWA,  enacted into federal law in 1977. The goal of the CWA is to monitor, maintain, and restore the quality of water within the United States, by removing biological, physical, and chemical contaminants. The CWA established the National Pollution Discharge Elimination System or NPDES creating federal regulations governing the discharge of contaminants into any waters of the United States. NPDES makes it illegal for any person or entity to discharge contaminants from a point source into US waters unless a permit is granted.  Administration and enforcement of NPDES is entrusted to the Environmental Protection Agency or EPA, including the treatment of wastewaters by WWTP's.  

The treatment of wastewater can contain up to 3 major processes: primary, secondary, and tertiary.  Primary and secondary treatment renders wastewater safe to release to the environment and is mandated by the EPA, while tertiary treatments making wastewater safe to drink is optional by each State.  

Primary treatment is a physical process involving several steps, including screening, grit removal and sedimentation, intended to remove solids from the wastewater.   Screening involves the movement of wastewater through a series of screens intended to remove larger objects and debris to protect equipment in the WWTP.  The wastewater is then retained in a concrete basins or grit chambers for anywhere between 8 and 36 hours, allowing time for sands and other particles heavier than water to settle to the bottom of the basin.  Wastewater then passes slowly through a sedimentation basin for 2-3 hours where suspended and dissolved solids sink to the bottom of the basin.  The material separated from wastewater during the primary stage is known as sludge, is scraped from the surface and bottom of the basins by mechanical means, and stored in hoppers for treatment separate and apart from the wastewater process.

Secondary treatment is principally a biological/chemical process intended to remove 99% of pathogens, including bacteria, germs, and viruses from wastewater.  Treatment typically includes an activated sludge process, in which wastewater flows through sludge heavily laden with bacteria and oxygen in an aeration basin, for 2-6 hours, thereby breaking down all organic matter into harmless byproducts.  The wastewater is then sent to another sedimentation basin for 1-3 hours  where the remaining dissolved and suspended solids can settled to the bottom of the basis.  Wastewater is then doused with chlorine or other chemicals intended to kill 99% of the remaining pathogens, germs and bacteria that escaped prior steps.  Some WWTP's use ultraviolet light or ozone as an alternative to chlorine  to eliminate organic pathogens from wastewater. Based upon NPDES standards wastewater can be released to the environment following the completion of secondary treatment, but remains unfit for human consumption. 

Tertiary treatment uses physical/chemical/biological processes to remove pathogens remaining after Secondary treatment.  Physical treatment involves filtration through multiple micro filters and membranes intended to remove all suspended and dissolved particles and contaminants.  Biological treatment involves the use of biological filters that breakdown remaining organic materials and remove any remaining oil, grease and foam remaining after Secondary Treatment. Chemical treatment involves the introduction of additional chlorine, ozone, or ultraviolet light a to kill any remaining pathogens or contaminants remaining after Secondary treatment.  Treatment using these methods can take anywhere between 2 and 8 hours depending upon the methods used.  While it may sound unappetizing, wastewater is completely safe for human consumption following the completion of tertiary treatment. 

Wastewater in the United States is treated by a minimum of 2-3 processes taking anywhere between 17 and 56 hours prior to release back into the environment.  While wastewater is not fit for human consumption following Secondary Treatment, nature tends to correct this problem through natural biological processes.  Unfortunately, cost is a prohibitive factor with respect to the construction of new tertiary WWTP's or the addition of tertiary treatment to existing WWTP's.  So the treatment of wastewater will remain a joint effort between mankind and nature.