Once used, water consumed in homes, businesses and industries is carried by the sewerage system and waste water pumping stations for processing and recycling.
Waste water treatment plants (WWTPs)
Waste water treatment plants (WWTPs) play a fundamental role in the water cycle. They process and recycle water before returning it to nature. The building, operating and maintaining of these systems are complex and very expensive but the quality of our environment depends on them.
WWTPs remove physical, chemical and biological pollutants and are classified according to the processes they use – primary, secondary and tertiary treatments.
The removal of pollution in WWTPs with primary treatment is mainly based on clarifying.
As well as the above, those with secondary treatment add a biological stage.
Stage 1. Solids screening.
Coarse matter screens: remove large solids such as sticks, wood, etc.
Fine matter screens: remove small solids and waste such as cloth, paper, wood, etc.
Stage 2. Grit and grease removal.
Grit removers: remove grit and heavy solids.
Grease removers: remove grease and floating matter.
Stage 3. Primary clarifying.
Clarifying removes suspended matter. This process consists of sedimenting tanks in which the speed at which the water moves is reduced to allow suspended matter to sediment by gravity. The sludge generated is removed from the bottom of the clarifier and the foam and floating matter are removed from the surface.
Stage 4. Secondary (or biological) treatment.
The purpose of biological treatment is to remove the organic pollution from the waste water.
This process is carried out in aerated tanks where activated sludge is formed with microorganisms that assimilate the biodegradable organic matter.
Stage 5. Secondary clarifying.
The purpose of secondary clarifying is to separate the activated sludge from the water.
Stage 6. Tertiary treatment.
If the treated water is to be discharged to a water course classified as sensitive and/or protected or is to be re-used in agriculture (irrigation), it is given a refining treatment to filter and disinfect it.
Stage 7. Treatment of sludge.
The sludge is a by-product of the treatment of waste water. It is generated by the primary treatment (primary sludge) and secondary treatment (biological sludge). The sludge is dried to facilitate its transport to a tip or for processing as organic fertiliser for agricultural use.