The wastewater treatment process is complex and critical to civilization. In simplest terms, wastewater is water that has been mixed with waste matter. Wastewater can be classified as domestic, municipal, or industrial based upon the source of generation. For instance, domestic wastewater comes from everyday liquid waste, including water used to flush the toilet, to shower, or to wash dishes. After the water is used, it is collected and conveyed to a wastewater treatment plant through a network of sewers, manholes, and lift stations. This water carries contaminants within it that need to be treated and removed before the water can be safely discharged to rivers and lakes or directly recycled and reused. Wastewater treatment is generally a combination of physical, chemical, and biological processes designed to treat the water based on federal, state, and local regulations. Various levels of treatment are used to remove solids, organic material, and nutrients from wastewater.
Preliminary treatment, or pretreatment, removes inorganic matter such as large debris (cans, bottles, tree limbs, etc.) and heavy particles (sand, gravel, etc.) from wastewater. This step is necessary because it prevents damage to the downstream pipelines and reaction basins.
The objective of primary treatment is to allow organic solids to settle to the bottom of the primary clarifier while simultaneously allowing greases and oils to rise to the surface. The sludge is then scraped from the bottom of the basin, and the greases and oils are skimmed off the top.
Secondary treatment aims to substantially remove and degrade the biological content of the wastewater. This is achieved by utilizing processes like attached growth systems or suspended growth systems. Attached growth systems include fixed film systems such as trickling filters, rotating biological contactors, and submerged fixed-film and moving-bed bioreactors. Suspended growth systems include activated sludge systems and sequenced batch reactors.
The tertiary treatment step, often referred to as advanced treatment or polishing, is used to provide a final treatment stage that will further improve the effluent quality of the water before it is discharged into the environment.
Once the wastewater treatment process is complete, the water is most commonly released into a natural body of water or reused for applications such as irrigation. With the advancement of water technology, the water can also be processed more thoroughly to be reused as drinking water.