Wastewater is water that is collected in drainage and sewer systems. This may include water from residential and industrial use and rain water. Wastewater may contain harmful bacteria, organisms, solid wastes, and other harmful chemicals and pollutants. A wastewater treatment plant is a facility that receives wastewater and treats and cleans it for redistribution back into nature or for reuse.
In the past, wastewater was simply disposed of in large bodies of water. The large size of a body of water would dilute much of the pollutants present in the wastewater. In addition, natural organisms would, over time, break down the pollutants in the water. Because the process of breaking down harmful organisms takes time to naturally occur, this method of treatment would not adequately nor effectively treat and clean the enormous volume of wastewater that is produced today.
A wastewater treatment plant works by accelerating the natural process of water treatment. Wastewater is directed via a sewage and drainage system to the treatment plant, where the incoming water is known as influent. The first phase of treatment screens out large pieces of debris such as rocks, wood, and litter. This debris is collected and generally taken to a landfill for disposal.
The wastewater is then held in tanks or basins. This allows solid particles to settle to the bottom and for grease or scum to rise to the top. Skimmers on the surface of the water collect the scum on the surface and scrapers collect the solid particles on the bottom, to be further processed as sludge. After processing, about 65% of sludge is reused as fertilizer. The sludge that is not reusable is disposed of in a landfill.
The secondary phase of the process in a wastewater treatment plant involves adding both microorganisms and oxygen to the wastewater. The microorganisms feed on and therefore remove much of the harmful bacteria present in the water. The adding of oxygen into the water is known as aeration and encourages the growth of the helpful microorganisms, which in turn speeds up the treatment process.
The microorganisms that were added to the water are then removed. The water is once again held in basins or tanks to allow the microorganisms to settle to the bottom. Most of these microorganisms are then reused in the treatment process, while a small percentage becomes sludge.
The wastewater is then treated with chlorine or ultraviolet light to kill any remaining harmful bacteria. Because chlorine is in itself a harmful substance, other chemicals may be added to neutralize the effects of chlorine. Once this step is performed the process is complete and the water, known as effluent, is typically discharged from the wastewater treatment plant into a body of water.