Sewage is water that is discharged after residences, institutions, hospitals, industrial and commercial use.
Sewage treatment refers to the process of removing contaminants, micro-organisms and other types of pollutants from wastewater influent. The main objective of sewage treatment is to produce an effluent (treated waste water) and a solid waste/sludge suitable for discharge into the natural environment.
For what purpose sewage treatment is required?
Treatment of sewage or wastewater protects public health and prevents pollution, disease as well as hazards from sewage contaminants. Hence, sewage treatment is required.
It is one type of wastewater treatment methods which is designed to hold polluted water in specially designed compartments. This plant goes through series of stages that involves extracting waste from the waste water influent.
Sewage/wastewater undergoes several stages in its treatment process and is shown below:
1. Preliminary Treatment: This is the first stage of sewage treatment plant process and its main objective is the removal of coarse solids and other large materials often found in raw wastewater. Preliminary treatment operations typically include large filtering screens, grit removal and, in some cases, breaking of large objects. Excess grit cause severe pump blockages thereby affecting a range of subsequent treatment pumps. Flow measurement devices, often standing-wave flumes, are always included at the preliminary treatment stage.
2. Primary Treatment: The main purpose of this treatment is to reduce any heavy solids (organic & inorganic) that settle to the bottom by sedimentation while oil, grease & lighter solids float to the surface by skimming. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to the next stage i.e. secondary treatment. Primary treatment removes about 60% of suspended solids from wastewater.
3. Secondary Treatment: The prime objective is the further treatment of the effluent from primary treatment to remove dissolved and suspended biological matter. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment. Secondary treatment removes more than 90% of suspended solids.
4. Tertiary/Advanced Treatment: Tertiary treatment generally follows secondary treatment and aids the removal of those wastewater constituents which cannot be removed in secondary treatment. Treated wastewater is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior its discharge into the receiving environment (sea, river, lake, wet lands, ground, etc.)