Clean water is used in houses, governmental buildings, hotels, sport facilities, commercial areas and industries. Clean water becomes dirty and polluted after its usage and thus it is called wastewater. Wastewater contains high levels of different pollutants such as organic materials, nitrogen, phosphorus and sometimes heavy metals and pathogens. The level of these pollutants should be reduced before discharging wastewater to the environment. The process of reducing the level of pollutants up to the levels that do not pollute and harm the environment is called wastewater treatment.
Treatment of wastewater is usually done into different stages namely physical, chemical and biological. Physical treatment includes reducing the pollutants by physical operations such as sedimentation. Pollutants such as suspended solids are removed from wastewater by precipitation or sedimentation. Chemical treatment includes reducing pollutants from wastewater by using chemicals. Examples are the use of disinfectants such as chlorine to disinfect the water and kill the pathogens. Biological treatment is the process of reducing the pollutants in wastewater by using different types of microorganisms. Theses microorganisms use some types of pollutants, such as organics, for food and energy source, contributing therefore to their elimination. Once the microorganisms fulfill their task, they are precipitated by sedimentation and removed from the treatment plant. These microorganisms together with the suspended solids cause what is called sludge. Treatment and disposal of sludge should be done carefully in order to avoid environmental pollution.
Treatment of wastewater is practiced in, what is called, wastewater treatment plant. There are many types of wastewater treatment plants such as activated sludge, trickling filters, rotating biological contactors (RBC), and lagoon systems.
EU Council Directive concerning urban waste-water treatment requires wastewater collecting systems and treatment plants for agglomerations with more than 2000 p.e. while they are not required for agglomerations with less than 2000 p.e. where the common practice is that certain small settlements or single houses are equipped with individual systems (septic tanks, package treatment plants etc.) to address the collection and treatment of waste water.
The septic tank is a buried, watertight container typically made of concrete, fiberglass, or polyethylene. It holds the wastewater long enough to allow solids to settle out (forming sludge) and oil and grease to float to the surface (as scum). It also allows partial decomposition of the solid materials. Compartments and a T-shaped outlet in the septic tank prevent the sludge and scum from leaving the tank and traveling into the drainfield area. Screens are also recommended to keep solids from entering the drainfield.
Figure 7a. A Typical single-compartment septic tank with ground-level inspection risers and screen (EPA, 2012)