How Sewer and Septic Systems Work

Measuring the Effectiveness of a Treatment Plant

The effectiveness of wastewater treatment plants is measured on several different scales. Here are some of the most common:
  • pH - This is the measure of the water's acidity once it leaves the plant. Ideally, the water's pH would match the pH of the river or lake that receives the plant's output.
  • BOD (bio-chemical oxygen demand) - BOD is a measure of how much oxygen in the water will be required to finish digesting the organic material left in the effluent. Ideally, the BOD would be zero.
  • Dissolved oxygen - This is the amount of oxygen in the water as it leaves the plant. If the water contains no oxygen, it will kill any aquatic life that comes into contact with it. Dissolved oxygen should be as high as possible and needs to cover the BOD.
  • Suspended solids - This is the measure of the solids remaining in the water after treatment. Ideally, suspended solids would be zero.
  • Total phosphorous and nitrogen - This is the measure of the nutrients remaining in the water.
  • Chlorine - The chlorine used to kill harmful bacteria needs to be removed so it does not kill beneficial bacteria in the environment. Ideally, chlorine should not be detectable.
  • Coliform bacteria count - This is the measure of fecal bacteria remaining in the water. Ideally, this number would be zero. Note that water in the environment is not totally free of fecal bacteria -- birds and other wildlife do introduce some.
The main reason why these indicators need to be watched so closely is because any community produces a huge quantity of wastewater. Discharge levels ranging from 10 million to 100 million gallons per day (38 million to 380 million liters) are common for a wastewater treatment plant.

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