Tertiary wastewater treatment is a crucial final stage in purifying effluent, especially in regions with limited water supplies or where treated water is intended for reuse. After primary and secondary treatments, effluent often still contains impurities that prevent its unrestricted use or safe discharge. This advanced treatment aims to remove these remaining contaminants, ensuring the water meets stringent quality standards. It typically involves processes like sand filtration and disinfection, often followed by maturation.
Purpose of Tertiary Wastewater Treatment: Why it's Essential
Even after aerobic secondary treatment processes, effluent might not be pure enough for unrestricted reuse. The primary goal of tertiary wastewater treatment is to further purify the effluent to a standard suitable for various purposes, including agricultural use, industrial applications, discharge into sensitive receiving waters, or even domestic consumption after further processing.
In countries facing water scarcity, the reuse of effluent is a necessity. Tertiary treatment makes this possible by significantly enhancing water quality beyond what secondary systems can achieve.
Impurities Addressed by Tertiary Treatment
Effluent from the best secondary treatment systems still contains several types of impurities that tertiary processes target:
- Finely suspended matter: Such as humus or activated sludge particles.
- Dissolved chemicals: Including chlorides, nitrates, sulphates, phosphates, sodium, and potassium.
- Bacteria: A significant concern, as these can include pathogenic organisms.
Removing these impurities is vital, with the specific level of removal depending on the intended use of the re-purified effluent.
Sand Filtration in Tertiary Treatment
Sand filtration is a key physical and chemical process used in tertiary wastewater treatment. It's highly effective at removing not only suspended and colloidal material but also bacterial contaminants from the liquid.
How Sand Filtration Works
The removal mechanisms within sand filters are complex and include:
- Adsorption: Particles adhere to the surface of the filter media.
- Flocculation: Small particles clump together to form larger, more easily filterable aggregates.
- Sedimentation: Heavier particles settle out within the filter bed.
- Straining: Larger particles are physically blocked by the filter media.
This process, whether with or without the addition of chemicals, provides the means of upgrading biologically treated sewage for critical applications. It allows for reuse in industries and agricultural applications, and helps achieve an effluent quality superior to general standards.
Types and Considerations for Sand Filters
Various types of sand filters are available, including slow, rapid gravity, and pressure filters. The choice depends on several factors:
- Local conditions: Such as the availability of power, land area, and manual labor.
- End use of the effluent: This dictates the acceptable limit of suspended solids in the final water.
It's important to note that sand filtration is not a substitute for adequate biological treatment. For optimal performance, effluent fed to filters should be settled, completely stable, and well nitrified. Biologically poor effluent, containing very fine colloidal material, may not settle readily or filter out effectively.
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Bacteriological Purification in Tertiary Treatment
Raw wastewater contains a vast number of pathogenic and non-pathogenic organisms. While earlier treatment stages reduce these numbers, the final effluent often still exceeds legislative standards for bacterial content, necessitating additional treatment for bacteriological purification.
Two common processes are generally employed: chlorination and maturation ponds.
Chlorination: Disinfecting Wastewater
Chlorine is a highly effective, toxic substance that destroys pathogenic organisms upon contact. However, its efficacy depends on sufficient chlorine presence, the correct form, and adequate contact time. As a highly active oxidant, chlorine also reacts with most organic substances and ammonia.
Therefore, for chlorination to be effective, the effluent must be adequately stabilized beforehand. This ensures the chlorine dose isn't consumed by other reactions before it can attack microorganisms.
Optimizing Chlorine Disinfection
The objective is to introduce chlorine in the correct form and quality, ensure proper mixing with the effluent, and provide a sufficient contact period in a suitable tank. This allows chlorine to react with organisms, aiming for an effluent with only a minimum quantity of free chlorine upon discharge.
Breakpoint chlorination is particularly crucial for the destruction of viruses. This process ensures that all ammonia is fully oxidized and the chlorine demand of the effluent is completely satisfied, maximizing disinfection potential.
Maturation Ponds: Natural Purification and Polishing
Maturation ponds are large earth ponds where the final effluent is discharged for further purification or