Programs / Sewage Treatment Plants

Challenges of Sewer Plants to Water Quality

Sewer plants can present a number of challenges to water quality, including overflows, low dissolved oxygen levels, nutrient pollution and harmful algal blooms. 


A sewer overflow is the release of untreated or partially treated sewage from a sewer system.

Overflows from sewer systems can contaminate our rivers and lakes, and thus cause serious water quality problems, and can back-up into homes, and cause property damage and threaten public health.  Exposure to bacteria from overflows can cause everything from mild stomach upsets and diarrhea to life-threatening illnesses.

Other problems caused by sewer overflows can include damage to rivers and streams, including having these waters closed to swimming, other water sports, and bans on fish consumption from them.  Tourism and property values can also suffer.

USEPA estimates there are between 23,000 - 75,000 (and perhaps more) overflows from sanitary sewer systems each year.  (This number does not include sewage backups only into buildings.) 

Causes of sewer overflow are many and can include:

  • Failure to properly design and maintain sewer systems;
  • Blockages, which can be caused by disposing into toilets and sewers of things that can cause problems like fats, oils and grease (FOG), and some household products like some baby wipes; 
  • Tree roots growing unchecked into sewer lines;
  • Line breaks;
  • Power failures;
  • Equipment failures, and other causes. 

For more information see: EPA's websites on sanitary sewer overflows and FAQs.

For more information about overflow issues with Franklin's sewer plant, click here.

Low Dissolved Oxygen (DO) Levels

The term “dissolved oxygen” refers to the amount of oxygen that is dissolved into water as a result of its interaction with the atmosphere and with structures it interacts with - think about the oxygen picked up when water flows rapidly over a rock, for example (DO is in addition to the oxygen that makes up “H2O”). DO is crucial for the survival of fish and other animals and microorganisms in a stream. DO levels vary with temperature (summer hot weather can result in low DO levels), and can even vary daily. 

Low levels of DO indicate a stream or lake that is unhealthy, in which fish, and other animals struggle to survive. Water bodies can die (eutrophy) if they have too little DO.

Causes of eutrophication include the die-off of algae, the growth of which is a result of too much phosphorus (and nitrogen) being discharged into the water body. The algae die and decompose, which uses up the available oxygen. Phosphorus is a component of sewer plant discharges, as well as runoff from agricultural areas and urban lawns and streets.

For more information see: USGS's webpage on dissolved oxygenLow Dissolved Oxygen in Water Causes, Impact on Aquatic Life – An Overview, or our Harpeth Conservancy page on the issue.

Nutrient Pollution

USEPA has noted that “[n]utrient pollution [discharges of excessive amounts of nitrogen and phosphorus] is one of America's most widespread, costly and challenging environmental problems, …”

Nutrient pollution can over-fertilize river and streams. Just like putting too much fertilizer (which is composed of nitrogen and phosphorus) on your lawn can burn or kill it, too much nitrogen and phosphorus can cause the growth of algae. Too much algae growth (called an algae bloom) can choke off a water body.  This can result in low dissolved oxygen levels, which can kill fish and endanger other life in and around a stream. In some cases, nitrogen and phosphorus discharges can cause a “harmful algae bloom.”

For more information see: 

Harmful Algal Blooms

One kind of harmful algae bloom is caused by blue-green algae. Blue-green algae, in reality, are not algae at all, but a form of bacteria, cyanobacteria. Some forms of these bacteria can be toxic to people and animals. University of Tennessee experts say that exposure to blue-green algae [cyanobacteria]:

"...can cause irritation of skin, eyes, nose and throat and inflammation of the respiratory tract. Ingestion can cause nausea, vomiting and diarrhea. In humans, toxins have been linked to liver disease and neurological effects….   Toxic cyanobacterial blooms have caused the death of wildlife, livestock and pets. Dogs are especially susceptible…"

-- University of Tennessee report, Cyanobacteria (Blue-Green Algae) Harmful Algal Blooms, available HERE 

For more information see: The Sun Chronicle article "BAMBERG: Blue-green algae a serious threat to dogs"  

Harmful algae blooms are fast becoming a major national, and even international, problem. Harmful algae blooms have plagued areas as diverse as Toledo, Ohio, from California to Florida, all the way to China. Toledo’s water system, which draws from Lake Erie, has been shut down for days at a time due to harmful algae blooms. The problem is growing “exponentially” according to one observer, with more blooms reported every year. 

For more information see: New York Times article "Miles of Algae and a Multitude of Hazards" or Environmental Working Group's "Across U.S., Toxic Algal Blooms Threaten Lakes and Other Waterways"