What happens to wastewater when it arrives at a municipal wastewater treatment plant will depend on the type of treatment plant it is. Below is the general process for a conventional municipal wastewater treatment plant:
The first step in wastewater treatment is preliminary treatment. The aim of this step is to remove coarse solids from the wastewater. This step typically involves screening and grit removal.
Following preliminary treatment, the wastewater would typically undergo primary treatment. Primary treatment is used to remove settlable material, and material that will float. Settlable material is usually removed by sedimentation - this is where wastewater is put into a clarifier and the solids settle to the bottom. The material that floats is removed by skimming. One final process that may be used in primary treatment (in combination with sedimentation) is coagulation/flocculation. Coagulation/flocculation is where compounds are added to the wastewater to join settlable material together so that it settles quicker.
Secondary treatment is often the next step for wastewater. This step is performed to remove organic and biodegradable material and suspended solids. Processes involved in secondary treatment include activated sludge processes, aerated systems, trickling filters, biological reactors and lagoons. In these systems microbes break down components of the wastewater and convert them to water and non-toxic gases.
A large number of municipal wastewater treatment plants then use tertiary treatment. Tertiary treatment is undertaken to remove wastewater constituents that are not easily removed by secondary treatment. Examples of constituents that may be removed by tertiary treatment are nitrogen, phosphorous, suspended solids, heavy metals and dissolved solids. Technologies that are commonly used in tertiary treatment include percolation, filtration, adsorption, advanced biological process configuration and chemically aided removal.
Finally, some municipal wastewater treatment plants disinfect the wastewater. This process can be performed using chlorination, ozonation, ultraviolet treatment or membrane filtration. These techniques remove or kill pathogens.
