Anoxic vs anaerobic vs aerobic treatment of wastewater

Anoxic vs Anaerobic vs Aerobic Wastewater Treatment

Before diving into anoxic vs anaerobic vs aerobic processes of wastewater treatment; let’s have a background on water pollution. Water pollution occurs when one or more pollutants alter the properties of water, rendering it useless. When released into the environment, it disrupts the habitat. Water pollution comes from variety of factors, including industrial waste, mining activities, sewage, pesticides and chemical fertilizers, energy use, radioactive waste, urban development, and so on. These factors are classified as point or non-point sources. And, various wastewater treatment processes are classified as preliminary, primary, secondary, and tertiary or advanced. Anoxic, anaerobic and aerobic processes forms the socondary treatment steps alongwith several other technologies.

Anoxic Treatment

In general, anoxic refers to a lack of molecular oxygen. However, oxygen can also be found in other chemical forms, such as nitrates and phosphates. The abundance of these molecules encourages nutrient enrichment in bodies of water, which promotes the growth of algae and macrophytes. Eutrophication refers to the abundance growth of algae and macrophytes as a result of nutrient enrichment. Eutrophication causes increased biomass production in the coastal zone, disrupting the natural ecological balance and having serious consequences for biodiversity, ecosystem resilience, recreational activities, and fisheries. Nitrates must be removed to avoid such problems. Denitrification reactions occur during anoxic treatment.

Nitrogen can be found in wastewater as ammonia (NH3), ammonium ion (NH4 +), nitrite (NO2), nitrate (NO3), or organic nitrogen. Similarly, in wastewater, phosphorus exists as phosphate (Orthophosphate), polyphosphate, or organically bound phosphorus.

Facultative heterotrophic bacteria such as nitrobacter degrades the nitrogen/ phosphorous molecules mentioned above to release molecular oxygen which then utilized by bacteria for their growth. They metabolize the readily degradable substrate under anoxic conditions using nitrate as the electron acceptor. In presence of oxygen they will use oxygen first. Therefore dissolve oxygen condition should be minimized for denitrification to occur.

anoxic treatment
Denitrification reaction occur during anoxic treatment

Anaerobic Treatment Process

The word “anaerobic” means “absence of oxygen”. Thus as evident from the name anerbic treatment process lack of oxygen in free and bound form. The condition is different from anoxic condition where bound oxygen is present. The biological degradation is achieved by no involvement of molecular oxygen. Anaerobic digestion produce methane, which can be trapped and utilized as fuel source thus producing energy.

The ultimate goal of anaerobic wastewater treatment is to create anaerobic conditions in which bacteria can thrive. This is accomplished in a closed tank with no oxygen present. The formation of a sludge blanket during the process aids in the stabilisation of the conditions. Almost 95% of organic matter is decomposed during the anaerobic treatment process.

The anaerobic treatment furthur divided into two stages: acidification and methane production. Anaerobes convert complex organic compounds into acid or short-chain volatile acids during acidification. Methane production happen in two steps: acetogenesis (the synthesis of acetate, carbon dioxide, and hydrogen gas), and methanogenesis (the action of anaerobes on newly formed molecules to produce carbon dioxide and methane gas).

anoxic vs anaerobic
Anaerobic Treatment Scheme. (Pic: emis vito)

Aerobic Treatment Process

The word “aerobic” means “persence of oxygen”. Thus aerobic treatment process occur in presence of oxygen. The aerobic treatment process furthur purifies the treated wastewater from anoxic and anaerobic conditions. However depending upon the wastewater characteristics the aerobic treatment may act as fully independent treatment process. Aerobic treatment processes are suitable for a range of industries such as food & beverage, chemical and municipal.

Aerobic wastewater treatment is a stable, simple, and efficient process that yields high-quality secondary effluent. The resulting sludge is odourless and can be sold as high-quality agricultural fertiliser. Aerobic treatment systems, when combined with anaerobic treatment, ensure complete contaminant and nutrient removal.

After reading this I hope you will have clear idea on Anoxic vs Anaerobic vs Aerobic Processes in Wastewater Treatment.

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  1. Grégorio Crini, Eric Lichtfouse; Advantages and disadvantages of techniques used for wastewater treatment.
  2. B. Yorkor, Y. O. L. Momoh; A Review of Anoxic Wastewater Treatment.
  3. Aesarabia.
  4. Handbook of Water and Wastewater Microbiology by Duncan Mara and Nigel Horan.
  5. Hynds Wastewater.
  6. Veolia Water.

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