01 Jul Operation of a water treatment plant
A Wastewater Treatment Plant (WWTP) has the objective of treating used and contaminated water by human beings so that they can be discharged into the natural environment.
A wastewater treatment plant cleans wastewater and water so that they can be returned to the environment. These plants remove solids and pollutants, break down organic matter and restore the oxygen content of the treated water. They achieve these results through four sets of operations: preliminary, primary, secondary and mud treatments. Normally, a network of sewers connected to homes, commercial buildings, schools and street grills delivers wastewater and solids to the collection tanks and basins of a treatment plant in an endless stream.
Wastewater plants eliminate ‘easy pickups’ during the pretreatment phase. A set of bar screens pulls large objects such as tree branches, trash, leaves, cans, rags, plastic bottles, diapers and other waste materials. In many plants, equalization basins and sand chambers of various types regulate the rate of water entry for stones, sand and glass to settle.
Watersheds retain sewage until they are ready for treatment and handle overflows due to heavy rains. Some plants remove grease and fats from the surface of the water during pretreatment, and sometimes use air blowers to beat the oily material to form a foam to facilitate its removal. Other plants eliminate fat during the primary treatment.
After the previous treatment, wastewater accumulates in the primary clarifiers, which are large basins and sedimentation tanks. Gravity allows smaller particles to settle. The mechanically driven scrapers collect the solid matter and direct it to the hoppers connected to the sludge treatment equipment. If the plant did not remove grease and oil during the pretreatment, it does so in this phase using surface skimmers. Some plants use equipment to saponify the fats collected by blending them with bleach, producing soaps and glycerol.
In the next phase, the plants aerate and agitate the wastewater in the secondary basins, adding beneficial microorganisms to decompose the organic matter in sludge. The plants employ a series of alternative strategies to decompose the sludge. For example, plants can grow a mass of microbes and pass the waste material over the biofilm. Other plants mix the biomass with the waste material, creating activated sludge that can be recycled for reuse. The resulting biological flow removes carbon and nitrogen from organic waste. Oxidation can occur on the surface, in lagoons, or in filter beds containing coked coal and limestone. Some facilities build wetlands and cane beds that decompose organic materials. Other technologies used include membrane bioreactors and aerated biological filters. The resulting wastewater accumulates and sits in a secondary clarifier tank.
The final phase is to treat the water and the remaining biosolids, or sludge. Gravity separates the organic waste from the heavier grain, which can be deposited in a sanitary landfill. The remaining primary sludge passes to a thickener, where it is centrifuged and fed to the digestion of the tanks containing anaerobic bacteria. These tanks produce methane that can be used to feed the plant. The final solid product, stabilized sludge, can be partially deodorized and plowed into the soil as fertilizer. The remaining wastewater is treated to remove phosphorus, nitrogen and other nutrients, disinfected with chlorine, ozone or ultraviolet light and then returned to the water supply. All the discharge and equipment used by the wastewater treatment plants must comply with the standards of the Environmental Protection Agency of the country where it has been built.