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Aerobic Digestion vs Anaerobic Digestion in Wastewater Treatment

In conventional wastewater treatment, there are three distinct steps that it must undergo – primary, secondary, and tertiary phases. Primary wastewater treatment involves the removal of solids by sedimentation or floatation. The secondary treatment phase involves the removal of organic matter which can be through microbial decomposition. When the effluent leaves the primary phase, it is introduced into a specially designed bioreactor where the matter is utilized by microorganisms such as bacteria (aerobically or anaerobically), algae and fungi (aerobically). Once the aerobic or anaerobic digestion is carried out, the tertiary treatment occurs which is an additional wastewater treatment used if the water is reused, recycled, or discharged into the environment.

Aerobic Digestion vs Anaerobic Digestion

The selection of the secondary treatment depends on multiple factors such as:

  • Nature of wastewater
  • COD and BOD
  • Energy demands
  • Treatment time
  • Investment
  • Desired effluent quality
  • Microbial concentration

The optimum configuration would be to utilize a combination of these two technologies.

Aerobic Digestion vs Anaerobic Digestion

Typically, aerobic treatment is applied to treat low strength wastewater (COD <1000 mg/L) when the treatment requires the presence of oxygen. However, anaerobic treatment is commonly applied to treat wastewater with higher organic loading (COD > 4000 mg/L). The mass flow meters monitor fast, accurate and stable flows of methane and carbon dioxide in anaerobic processes.

Aerobic treatment employs oxygen and bacterial biomass to assimilate organic matter and other pollutants like nitrogen and phosphorus into carbon dioxide, water, and another biomass. Anaerobic treatment, on the other hand, breaks down organic impurities in the absence of oxygen to produce methane, carbon dioxide and another biomass. Mass flow controllers and meters are critical to achieving fast, accurate and stable flows of air and oxygen in an aerobic process. These can monitor flows of methane and carbon dioxide in an anaerobic process.

Key differences between aerobic and anaerobic processes:



Aerobic Treatment

Anaerobic Treatment


Low to medium strength wastewater (<1000 ppm) eg. Municipal sewage, refinery wastewater, etc.

Medium to high strength wastewater (>4000 ppm) eg. Food and beverage industry wastewater

Capital Investment

Relatively high

Relatively low with pay back*

Energy Consumption

Relatively high

Relatively low


Relatively large

Relatively small and compact

Net Sludge Yield

Relatively high

Relatively low


Typical direct discharge

Required to fulfill wastewater standard discharge requirement

Example Technologies

Activated Sludge Process (ASP), Trickling Filter, and Rotating Biological Contactor (RBC)

Anaerobic Digestors (AD), Continuous Stirred Tank Reactors (CSTR), Sequencing Batch Reactors (SBR), Upflow Anaerobic Sludge Blanket (UASB) Reactors

* CH 4 generated can be used to generate energy


Advantages of Aerobic and Anaerobic Processes:

Aerobic Digestion vs Anaerobic Digestion

There are some distinct advantages of aerobic digestion over the anaerobic digestion process. The advantages include a reduced odour due to the non-production of hydrogen sulphide or methane and better nutrient removal efficacy (facilitating direct discharge into surface waters or disinfection). Despite this, aerobic treatment also has several disadvantages. Oxygenation is an energy-intensive process severely increasing the overall energy consumption, utility, and maintenance costs of the process. Solid waste that microbes are not able to digest also often settle out as biosolids which require appropriate disposal which means additional costs.

Anaerobic treatment processes have many advantages over aerobic treatment processes. The biogas produced during an anaerobic treatment process can be used as a source of renewable energy (natural gas/methane). This process also produces very low sludge that is dewaterable and fully stabilized for disposal. Anaerobic treatment is less expensive, simpler, and more flexible compared to aerobic treatment processes.

Both methods have clear advantages and disadvantages which is why a combination of anaerobic and aerobic treatment processes are employed to achieve the most efficient treatment of wastewater. Wastewater going into an aerobic reactor will typically undergo pre-treatment in an anaerobic reactor to fulfil wastewater standard discharge requirements in an energy efficient and cost-effective manner.


Over the past few decades, bioreactor usage during wastewater treatment has moved from a foreign new technology to a standard process. The key strategy for optimisation of these systems is to measure input and/or output gasses to understand and control the process efficiently. Precise usage of aerobic vs. anaerobic digestion processes depends on factors unique to each facility, and metering of mass flow is essential to understanding how to utilise available tools that can provide an optimal treatment regimen.

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