A sewage treatment plant purifies wastewater and water before returning them to the environment. These facilities remove solids and pollutants, break down organic matter, and restore the oxygen content of treated water.
Four sets of operations are used to achieve these objectives: preliminary, primary, secondary, and sludge treatments. Wastewater and solids from a network of sewers that connect homes, commercial structures, schools, and street grates flow without stopping to a treatment plant’s collection tanks and basins.
During the preliminary treatment phase, wastewater plants remove the “easy pickings.” Bar screens rake away big items like tree limbs, trash, leaves, cans, rags, plastic bottles, diapers, and other waste products.
In certain plants, equalization basins and various sorts of grit chambers regulate the rate of water inflow to ensure that stones, sand, and glass settle out. The basins retain sewage until it can be treated, while overflows due to severe rains are handled by the ponds.
During pre-treatment, some plants skim the water’s surface, removing fats and greases with air blowers. During primary treatment, other plants remove grease.
The wastewater is collected in primary clarifiers, which are large basins and sedimentation tanks, after pre-treatment. Gravity causes small particles to settle out. Solid matter is gathered by manually driven scrapers and transferred to hoppers linked to the sludge treatment equipment.
Oxygen from the atmosphere is introduced through diffusers in the basins to promote microorganism growth, which decomposes and liquefies organic matter. In some cases, aerators use compressed air to mix the wastewater and provide oxygen. After a specified time, usually two or three hours, sludge accumulates on the floor of primary tanks at a depth of about 1 meter (39 inches).
Sludge treatment involves a series of operations that separate it into liquid effluent and dried cake material for disposal. During certain phases, sludge is treated chemically to reduce its volume or weight before final disposal.
In secondary treatment, wastewater moves from storage basins to sedimentation tanks where heavy solids settle out as sludge.
Surface skimmers remove the grease and oil from the plant’s surface in this stage, as it did not do so during pre-treatment. Some plants use equipment to saponify collected fats by combining them with lye, resulting in soaps and glycerol. In more advanced plants, the wastewater is filtered for impurities.
In the following stage, plants aerate and agitate wastewater in secondary basins, adding helpful microorganisms to break down organic material into sludge. Plants can use a variety of different methods to decompose sludge. For example, bacteria may be cultivated in a mass and then passed over the biofilm.
Activated sludge produced from composting is mixed with other waste, giving it activity. The resulting biological floc removes carbon and nitrogen from organic refuse. Carbon and nitrogen can be removed by oxidation on the surface—in lagoons or in filter beds containing coked coal and limestone—or inside the process of activated sludge production
Wetlands and reed beds are constructed at several locations to decompose organic matter. Membrane bioreactors and biological aerated filters are two additional procedures utilized. The produced sewage is collected and settled in a secondary clarifier tank after treatment.
The final stage is to process the remaining water and biosolids, which are known as sludge. Organic waste is separated from heavier grit by gravity, which may be buried in a landfill. The primary sludge is sent to a thickener where it is centrifuged and fed into anaerobic tank digesting tanks teeming with bacteria.
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