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Anaerobic Treatment is a process in which microorganisms convert organic matter into bio gas in the absence of oxygen. It is an energy-efficient process that is typically utilized to treat high-strength industrial wastewater that is warm and contains high concentrations of biodegradable organic matter (measured as BOD, COD, and/or TSS). An anaerobic system can be used for pretreatment prior to discharging to a municipal wastewater treatment plant or before polishing in an aerobic process.

Anaerobic treatment is a process where wastewater or material is broken down by microorganisms without the aid of dissolved oxygen. However, anaerobic bacteria can and will use oxygen that is found in the oxides introduced into the system or they can obtain it from organic material within the wastewater. Anaerobic systems are used in many industrial systems including food production and municipal sewage treatment systems.


Anaerobic processes have been used for the treatment of concentrated industrial wastewaters for well over a century. These processes convert organic materials into methane, a fuel that can yield a net energy gain from process operations. Because of recent advances in treatment technology and knowledge of process microbiology, applications are now extensive for treatment of dilute industrial wastewaters as well. The advantages of anaerobic treatment over aerobic treatmnet, current understanding of microbiology and the newer processes have been well handled by Austro’ experts.

It is apparent that bio film processes offer greater stability than dispersed growth processes do for treating dilute wastewater. The reason for this are not entirely obvious, and are probably multiple. One factor that makes bio film processes more favorable is related to the role of diatomic hydrogen (H2) in the control of process dynamics.

Method and installation description
Anaerobic treatments on wastewater are normally implemented when treating more concentrated wastewater. The anaerobic sludge contains various groups of micro organisms that work together to eventually convert organic material to biogas via hydrolysis and acidification. Biogas typically consists of 70% methane (CH4) and 30% carbon dioxide (CO2) with residual fractions of other gases (e.g. H2 and H2S). The methane can be used as an energy source. Anaerobic reactors can be implemented in a variety of ways. The figure shows a contact reactor and an upflow reactor.

The contact reactor is comparable with a conventional active sludge system, but under anaerobic conditions. The sludge is mixed with wastewater in the reactor and is then separated in the sedimentation tank and returned to the reactor.

In the anaerobic upflow reactor, the influent is introduced at the bottom of the vertical reactor. The sludge in the reactor is primarily grain shaped and forms a blanket in the reactor, with the most compact sludge grains at the bottom and the lighter grains and heavier sludge floccules above it. Very light sludge floccules will be released by the upward flow, but can potentially be collected in a sedimentation tank. The biogas is collected and disposed of at the top of the reactor, separately from the partly purified water and the sludge.

In addition to the contact reactor and the upflow reactor, other types are also available:

Conventional digester
This type is primarily implemented for the fermentation of RWZI sludge and liquid organic waste. The system is characterised by very low loads and a large volume in order to achieve the longest possible retention time. This type of reactor does not include recirculation of anaerobic sludge.

Packed anaerobic filter (sludge on carrier)
This reactor is filled with carrier material and is normally used as an upflow reactor.

UASB (upflow anaerobic sludge blanket) or EGSB (expanded granular sludge bed)
Both systems are variations of the upflow reactor. The main difference between the two is the increased recirculation of the EGSB reactor. Together with the prominent sludge grain, this enables higher loads in the EGSB (15-30 kg COD/m³/day).

Anaerobic membrane reactor
This type of application uses membranes for sludge-water separation. To date, little use has been made of this system.

An extra purification phase will often be implemented after anaerobic purification, e.g. for the removal of residual fractions of COD and nutrients N and P. This often involves the use of an aerobic post-purification treatmen


  • Anaerobic treatment is ideal for pretreatment, pretreatment prior to aerobic treatment, and pretreatment of segregated waste streams. Used for standalone pretreatment, an anaerobic system can be used as the sole biological component of a treatment system for wastewater discharged to a POTW.
  • Used prior to aerobic treatment, an anaerobic system can be very effective and economical for removing high concentrations of BOD and COD prior to final treatment by an aerobic process.
  • Many industrial facilities have waste streams that represent a fraction of the total flow, yet contribute a majority of the pollutant load. These high-BOD waste streams can be segregated for treatment by an anaerobic process prior to combining with the total flow. ETS anaerobic systems are very effective in wastewater treatment at a variety of industrial facilities.



  • Low sludge yield – Anaerobic systems typically produce a small fraction of the sludge generated by aerobic systems. This means that there is less sludge to water and dispose of.
  • Lower electrical requirements – Because an anaerobic system does not require oxygen, the high horsepower requirements of surface aerators or blowers are avoided.
  • Higher organic loading – Anaerobic systems are capable of providing high treatment efficiencies at BOD concentrations ranging from 2,000 mg/L to 50,000 mg/L. These systems are also typically more effective than aerobic systems at COD removal.
  • Energy production – A byproduct of anaerobic degradation of pollutants is the production of a methane-rich biogas which can be used to supplement or replace natural gas for fueling plant boilers, engine generators, and other energy systems.
  • Good process stability – The anaerobic process is very stable under varying hydraulic and organic loadings and other conditions that may cause upsets in other types of biological systems.
  • Lower nutrient requirements – Anaerobic systems require a fraction of the nitrogen and phosphorus that an aerobic system does.
  • Lower operating costs – Because anaerobic systems require less nutrients and electrical input and generate less sludge than aerobic systems, they have inherently lower operating costs.


  • Incomplete break-down of organic compounds: Need for post-purification via, for example, aerobic purification;
  • No thorough nutrient removal: Again, later aerobic purification with nutrient removal is often needed;
  • Most efficient purification in the mesophilic range, i.e. between 30-37°C, whereby the influent must be heated in most cases;
  • Less robust system with regards to toxicity and inhibition;
  • Risk of odor problems.
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