Wastewater is “used” water that is deposited into drains by residents and businesses. Flushing the toilet, taking a shower, and running the washing machine are all examples of everyday activities that produce wastewater. This wastewater drains down the sink or toilet and travels through a network of underground pipes known as the sanitary sewer system.

Your home is being serviced by either 1) a sanitary and a storm sewer or 2) a combined sewer, a single pipe which conveys both sanitary sewage and storm water.   It is 99.94 percent water, with only 0.06 percent of the wastewater being dissolved and suspended solid material. The cloudiness of sewage is caused by suspended particles which, in untreated sewage, range from 100 to 350 mg/l. A measure of the strength of the wastewater is biochemical oxygen demand, or BOD₅. The BOD₅ measures the amount of oxygen microorganisms require in five days to break down sewage. Untreated sewage has a BOD₅ ranging from 100 mg/l to 300 mg/l. Pathogens or disease-causing organisms are present in sewage. Coliform bacteria are used as an indicator of disease-causing organisms. Sewage also contains nutrients (such as ammonia and phosphorus), minerals, and metals. Ammonia can range from 12 to 50 mg/l and phosphorus can range from 6 to 20 mg/l in untreated sewage.

The sewer systems are built to follow the natural slope of the land. This design allows gravity to do most of the work transporting the wastewater to treatment facilities. In some situations, pump stations are constructed in low-lying areas to force the wastewater uphill to the treatment facility.

The average US citizen uses approximately 100 gallons of water per day. The organic and inorganic material found in this wastewater is too abundant to be handled by the environment. Therefore, treatment of our wastewater is an essential process that prevents contamination and destruction of waterways, drinking water sources, and natural water resources.

Sewage treatment is a multi-stage process to renovate wastewater before it reenters a body of water, is applied to the land, or is reused. The goal is to reduce or remove organic matter, solids, nutrients, disease-causing organisms, and other pollutants from wastewater. Each receiving body of water has limits to the amount of pollutants it can receive without degradation. Therefore, each sewage treatment plant must hold a permit from DHEC listing the allowable levels of BOD₅, suspended solids, coliform bacteria, and other pollutants. The discharge permits are called NPDES (National Pollutant Discharge Elimination System) permits. 

1. Preliminary treatment
Upon arrival via the sewer system, the wastewater is sent through a bar screen, which removes large solid objects such as sticks and rags. Leaving the bar screen, the wastewater flow is slowed down entering the grit tank. This allows sand, gravel, and other heavy material that was small enough not to be caught by the bar screen to settle to the bottom. All the collected debris from the grit tank and bar screen is disposed of at a sanitary landfill or recycled.

2. Primary treatment
Primary treatment is the second step in wastewater treatment. It allows for the physical separation of solids and greases from the wastewater. The screened wastewater flows into a primary settling tank where it is held for several hours. This allows solid particles to settle to the bottom of the tank and oils and greases to float to the top.

3. Secondary treatment
Secondary treatment is a biological treatment process that removes dissolved organic material from wastewater. The partially treated wastewater from the settling tank flows by gravity into an aeration tank. Here it is mixed with solids containing micro-organisms that use oxygen to consume the remaining organic matter in the wastewater as their food supply. The aeration tank uses air bubbles to provide the mixing and the oxygen, both of which are needed for the micro-organisms to multiply.

From here the liquid mixture, composed of solids with micro-organisms and water, is sent to the final clarifier. Here the solids settle to the bottom where some of the material is sent to the solids handling process, and some is recirculated to replenish the population of micro-organisms in the aeration tank to treat incoming wastewater.

4. Final treatment
Wastewater that remains is disinfected to kill harmful micro-organisms before being released into receiving waters. Although there are many methods available to kill these micro-organisms, chlorine and ultraviolet disinfection are the most widely used.

Dechlorination occurs in the final wastewater treatment step. A solution of sodium bisulfite is added to the chlorinated effluent to remove residual chlorine.

Following disinfection and dechlorination, the treated wastewater (now called final effluent) can be returned to the receiving waters from which it came.  The flow is conveyed to an outfall and discharged through a series of diffusers into a surface water body or stream.

5. Solids processing
Primary solids from the primary settling tank and secondary solids from the clarifier are sent to the digester. During this process, micro-organisms use the organic material present in the solids as a food source and convert it to by-products such as methane gas and water. Digestion results in a 90% reduction in pathogens and the production of a wet soil-like material called “biosolids” that contain 95-97% water. To remove some of this water and reduce the volume, mechanical equipment such as filter presses or centrifuges are used to squeeze water from the biosolids. The biosolids are then sent to landfills, incinerated, or beneficially used as a fertilizer or soil amendment.