The tertiary treatment process follows primary and secondary wastewater treatment and generally uses biological, chemical, or physical processes to remove impurities. The tertiary treatment process can be divided into three sub-processes: fine screening, sedimentation, and disinfection.
Fine screening involves the removal of relatively large solids (e.g., grit) that are too large for tertiary treatment (i.e., primary treatment). This is generally done by an automated bar screen that forces the particles through bars or screens with openings approximately 25 mm square. The coarser particles are then transported out of the wastewater collection system via pipeline to a solid waste landfill site on land where they may be deposited directly on the soil surface. Alternatively, depending on local regulations governing the disposal of solid wastes, some coarser particles may be disposed of in an approved wastewater treatment plant where they will be subjected to further processes to reduce their size and make them more environmentally acceptable prior to the final disposal.
Sedimentation is the process whereby small solids in the wastewater (e.g., colloidal particles) are allowed to settle out under the influence of gravity. This is generally done in a large tank called a sedimentation basin. The settled solids accumulate at the bottom of the tank while the clarified wastewater flows over them and is discharged from the tank. A sludge blanket or scum layer forms on the surface of the wastewater in the sedimentation basin and this must be periodically removed (or “dewatered”) using mechanical means before it overflows from the basin.
In some tertiary treatment facilities, ‘floatables’ (e.g., rags, sticks, plastics, and rubber material) may be separated from the wastewater in a fine screening plant prior to its entry into a sedimentation tank.
Disinfection is the process whereby pathogens are reduced by subjecting them to chemical or physical procedures for a sufficient period of time or exposing them to an adequate UV radiation dose that renders them unable to cause disease in humans who come into contact with the tertiary effluent. In all cases, disinfection must be preceded by primary and secondary treatments that produce treated wastewater with microbiological qualities consistent with water reuse standards before it can be discharged back into the receiving body of water or used for other purposes.