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What are the three stages of wastewater treatment?

  • What are the three stages of wastewater treatment?

The three stages of wastewater treatment are known as primary, secondary and tertiary. Each stage purifies water to a higher level. In some applications, only one or two stages are necessary. The level of treatment necessary depends on the water’s intended use case, and what environment it will be discharged into.

Primary treatment (stage 1)

Primary wastewater treatment involves sedimentation. This is when wastewater is temporarily held in large sedimentation tanks to remove settleable solids.

With gravity, heavier solids sink to the bottom while lighter solids rise to the top. Chemicals can also be added as coagulants to remove more solids.

Sedimentation creates sewage sludge as a by-product at the bottom of the tank. Once this has settled, the treated water is released for secondary treatment. The sludge, meanwhile, may find use in anaerobic digestion or as a liming material.

Secondary treatment (stage 2)

Secondary treatment involves removing soluble organic matter solids that escaped primary treatment. It also deals with smaller suspended solids.

Secondary treatment methods include:

  • Bioreactors
  • Filter beds
  • Aerated ponds
  • Activated sludge
  • Rotating biological contactors

Removal of organic contaminants is best achieved using a membrane bio reactor, where a biological process takes place in the reactor where microorganisms consume the organic matter for energy. The microorganisms replicate to create solids. The solids are then filtered by the membrane to produce effluent.

The key benefit to this technology is it eliminates the need for a secondary clarifier or filtration system for solids. The biological process in the reactor and the physical separation of solids takes place efficiently in one system.

Other methods of secondary treatment include filter beds, aerated ponds, biofilters, activated sludge and rotating biological contactors. However, membrane bioreactors are the most efficient technology and the most modern.

Water that has gone through secondary treatment can be released into the environment without damaging aquatic life and ecosystems.

Tertiary treatment (stage 3)

Tertiary treatment, also known as polishing, disinfects water to the highest standards. This stage is necessary to produce water to specification, such as technical waters, and to treat wastewater for public water systems.

Tertiary treatment methods include:

  • UV disinfection
  • Chemical disinfection

UV disinfection requires no chemicals, and it can be used in place of chemicals. This means no extra filtering stage for additives. UV does not affect the pH, appearance, taste or smell of water, but it does destroy microorganisms.

UV light wastewater treatment is typically used for bacterial control, ensuring organisms are unable to replicate. It can also remove chlorine, chloramines, ozone, and bromine, making it ideal for producing extremely pure technical waters.

Chemical disinfection requires adding a chemical to the water. The most common is chlorine. Chlorination kills bacteria and viruses, but this treatment has the disadvantage of requiring a stage for dichlorination before discharge into the environment.

Another chemical used to disinfect water is iodine. This is equally as effective as chlorine at killing viruses and bacteria.

All things being equal, the best water disinfection technique is Ultraviolet Disinfection. A correctly sized UV system can be used to de-chlorinate or de-ozonate process water and it can control microorganisms equal to chemicals.

To help with your water treatment problems, we offer a wide selection of water treatment systems, as well as finance, rental and servicing solutions. Contact us on 01902 458501 for a chat with one of our experts today.