ADBA National Conference December 2022 BANNER AD.
There's no Net Zero without Biogas: ending waste, delivering the circular economy, tackling the climate crisis.

Anaerobic Digestion in Wastewater Treatment – UASBs EGSBs and Staging

Anaerobic digestion in wastewater treatment differs from Traditional Aerobic Wastewater Treatment
Traditional Aerobic Wastewater Treatment CC BY by fernando butcher

A well-proven Anaerobic Digestion Wastewater Treatment method is the process of decomposing organic matter of municipal sewage sludge, anaerobically under operational control.

Wastewaters which will Benefit from Anaerobic Digestion Treatment

Anaerobic digestion is an ideal process for treating some polluted wastewaters. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatment. By “digesting” the organic material, they provide a natural way to purify polluted wastewater and are especially useful in treating high BOD effluents, and sewage sludge.

Wastewaters which will be improved by anaerobic digestion in wastewater treatment are those which contain a high percentage of their contamination in the form of recently created waste water, such as those from food processing and fresh manures. The reason for that is that older polluted waters very often will already have been subjected to bacteria, and archaea populations , which will already have carried out much of what a period spent in a digester will also provide. This is one of the few disadvantages of anaerobic digestion.

Distribution in a UASB used in anaerobic digestion in wastewater
A UASB CC BY by Sustainable sanitation

There is not much benefit in applying anaerobic digestion wastewater treatment to, for example, the leachate from an old landfill, because that will have been subjected to anaerobic digestion already, while in a sanitary landfill.

Also, the use of anaerobic digestion wastewater treatment is seldom if ever the final stage of wastewater treatment, if the treated water is to be discharged to a watercourse or stream. In such cases a further stage of treatment such as aeration by extended aeration methods or Reverse Osmosis Membranes will provide.

The reason for this is that the wastewater exiting an anaerobic digestion facility will typically have elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD).

These measures of the reactivity of the effluent indicate an ability to pollute.

Some of this material is termed ‘hard COD', meaning it cannot be accessed by the anaerobic bacteria for conversion into biogas.

If this effluent were put directly into watercourses, it would negatively affect them by causing eutrophication.

As such, further treatment of the wastewater is often required. This treatment will typically be an oxidation stage wherein air is passed through the water in a sequencing batch reactors or reverse osmosis unit. via Anaerobic digestion – Wikipedia

Anaerobic Digestion of Wastewater Sludge

Sludge maturation drying after anaerobic digestion of wastewater.
Sludge maturation drying after anaerobic digestion of wastewater.

In the anaerobic digestion of wastewater the goal is to reduce the amount of sludge that needs to be disposed.

Nowadays, the most widely employed method for sludge treatment is anaerobic digestion.

In this process, a large fraction of the organic matter (cells) is broken down into carbon dioxide (CO2) and methane (CH4), and this is accomplished in the absence of oxygen.

About half of the amount is then converted into gases, while the remainder is dried and becomes a residual soil-like material.

The tank is capped during anaerobic digestion wastewater treatment, to prevent oxygen from coming in, and to capture the methane produced.

This methane, a fuel, can be used to meet some of the energy requirements of the wastewater treatment facility (co-generation).

After anaerobic digestion and subsequent drying. It is rich in nitrates and performs well as a fertilizer. via Anaerobic Digestion of Wastewater Sludge

Upflow Anaerobic Sludge Bed (UASB) wastewater (pre-)treatment systems represent a proven sustainable technology for a wide range of very different industrial … via Anaerobic digestion and wastewater treatment systems …

Upflow Anaerobic Sludge Bed (UASB) wastewater (pre-)treatment systems

UASB for domestic wastewater treatment at University UNITRAR in Lima, Peru (max flow 860 m3 per day)
UASB for domestic wastewater treatment at University UNITRAR in Lima, Peru (max flow 860 m3 per day) CC BY by Sustainable sanitation (

For many years a technique for the anaerobic digestion of the more dilute wastwaters, using the anaerobic digestion process has been used. It is named for what it is, as the Upflow Anaerobic Sludge Bed (UASB) wastewater (pre-)treatment system. It has been used for the best part of 100 years, and as such is a well-proven technique.

UASBs represent a proven sustainable technology for a wide range of very different industrial effluents, including those containing toxic/inhibitory compounds. The process is also feasible for treatment of domestic wastewater with temperatures as low as 14–16° C and likely even lower. Compared to conventional aerobic treatment systems the anaerobic digestion treatment process plant merely offers advantages.

This especially is true for the rate of start-up. The available insight in anaerobic sludge immobilization (i.e. granulation) and growth of granular anaerobic sludge in many respects suffices for practice. In anaerobic digestion wastewater treatment the immobilization of balanced microbial communities is essential, because the concentration of intermediates then can be kept sufficiently low.

So far ignored factors like the death and decay rate of organisms are of eminent importance for the quality of immobilized anaerobic sludge. Taking these factors into account, it can be shown that there does not exist any need for ‘phase separation’ when treating neutral (non-acidified) or slightly acidified wastewaters.

Phase separation even is detrimental in case the acidogenic organisms are not removed from the effluent of the acidogenic reactor, because they deteriorate the settleability of granular sludge and also negatively affect the formation and growth of granular sludge.

The growing insight in the role of factors like nutrients and trace elements, the effect of metabolic intermediates and end products opens excellent prospects for process control, e.g. for the anaerobic treatment of wastewaters containing mainly methanol.

Anaerobic digestion wastewater treatment can also profitably be applied in the thermophilic and psychrophilic temperature range. Moreover, thermophilic anaerobic sludge can be used under mesophilic digestion conditions.

The Expanded Granular Sludge Bed (EGSB) system

Another anaerobic digestion based treatment technique is the The Expanded Granular Sludge Bed (EGSB) system.

The EGSB system offers big practical potential for wastewater treatment, for example, for very low strength wastewaters (COD ≪ 1 g/l), and at temperatures as low as 10° C. In EGSB-systems virtually all the retained sludge is employed. If this is compared to UASB-systems.

Also a substantially bigger fraction of the immobilized organisms (inside the granules) participates in the process. This is due to the fact that an extraordinary high substrate affinity prevails in these systems.

From the manner in which these EGSB systems perform, process scientists have said that it is necessary to reconsider theories for mass transfer in their immobilized anaerobic biomass.

Staging of Anaerobic Reactors

Instead of phasing the digestion process, staging of the anaerobic reactors is best applied. Separate treatment stages may entail separate digester tanks and that entails additional compexity of design.

However, although staging usually improves the cost effective performance of the plants, this needs to assessed alongside the additional cost of operating multiple stage treatment systems.

Nevertheless, in multi-stage AD plants mixing up of the sludge can be significantly reduced, and this form of a “plug flow” is promoted.

A staged process will provide a higher treatment efficiency and a higher process stability. This especially applies for thermophilic systems. via Anaerobic digestion and wastewater treatment systems …

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.

ADI Systems offers several anaerobic digestion technologies for reliable wastewater treatment worldwide. via Anaerobic Digestion | ADI Systems

Anaerobic Digestion in Wastewater Treatment – A Summary

In this article we have provided a basic grounding in the subject of Anaerobic Digestion in Wastewater Treatment. We have discussed when to use AD threatment for the purification/ treatment of wastwaters, the basic processes which exist, and how adding multiple stages can be an effective way to raise the degree of treatment provided.

In recent years there has been considerable innovation in the way anaerobic digestion processes are done, and many of the patented and “branded” processes offered by specialist AD plant desgners and contruction contractors, can achieve better results than the old standard ‘text-book” designs.

One such example of the patented and branded anaerobic digestion wastewater treatment system providers is ADI Systems. ADI offers several anaerobic digestion technologies for reliable wastewater treatment worldwide. Their technologies include:

  • ADI-BVF® Reactor
  • External Circulation Sludge Bed (ADI-ECSB)
  • Anaerobic Membrane Bioreactor (ADI-AnMBR)
  • CH4 Generating Reactor (ADI-CGR®)
  • Continuously Stirred Tank Reactor (ADI-CSTR)
  • Hybrid Reactor (ADI-Hybrid)

This list is just one example from one provider and there are many more.


So, to close this article we will conclude that:

Anaerobic processes are not always useful for wastewater treatment, depending upon such factors as the age of the wastewater, but where they are suitable anaerobic processes are said to:

  1. Use substantially less energy,
  2. require less chemicals,
  3. produce a fraction of the waste sludge generated through aerobic biological processes, and
  4. incur lower sludge handling costs compared to aerobic treatment options.

In addition, the biogas produced in the anaerobic process is a source of renewable energy that can be used to displace fossil fuels such as oil or natural gas, or to generate electricity. via Anaerobic Digestion | ADI Systems

Other Anaerobic Digestion Wastewater Treatment process Provider Articles You May Find Interesting

Sludge Treatment / Anaerobic Digestion

OVIVO: Organic pollutants are converted by micro-organisms into gas. via Sludge Treatment / Anaerobic Digestion – OVIVO

Aerobic versus anaerobic wastewater treatment. … Now that both aerobic and anaerobic wastewater treatment can be … Anaerobic digestion is assumed to … via Chapter six: Aerobic versus anaerobic wastewater treatment

Paper About Commissioning an Upflow Anaerobic Sludge Blanket under Mesophilic Conditions USed for Anaerobic Digestion Wastewater Treatment

In this paper it was demonstrated that an Upflow Anaerobic Sludge Blanket under mesophilic conditions treating a high strength effluent (15,619 – 44,684mg/L COD) from a malted ingredients factory could be started effectively within a period 75 days.

Despite previously reported difficulties with plant start ups, operational performance in terms COD removal efficiency (>81.89% ±4.84) and the percentage methane content of biogas (57.24% ± 4.29) was achieved at a level comparable to a fully established plant.

CAIRNS, Ryland; MEAD, Paul. Start Up of a UASB Treating Malted Ingredient Manufacturing Wastewater. Environmental Management and Sustainable Development, [S.l.], v. 7, n. 2, p. 1-14, jan. 2018. ISSN 2164-7682. Available at: <>. Date accessed: 09 feb. 2018. doi:



Tags: , , , , , , , , ,
Previous Post
Image to refelct the question of "How much biogas"?

How Much Biogas? An Example of High Biogas Performance

Next Post
high rate anaerobic digestion process
Anaerobic Digestion

High Rate Anaerobic Digestion Plant Treats Wastewater Effluent Streams


    • SoftBitch
    • October 12, 2017

    Nothing but BS acronyms. what is this? UASB EGSB? Crap. Use full names or don’t bother.

    • James Broad
    • October 24, 2017

    Thanking. Very broad ranging page about these processes. It is all very positive reading. I would give more weight to the info if there were more downsides listed on this page for anaerobic digestion in wastewater treatment. It is not yet common in my country.

    • Jake Turnbull
    • November 1, 2017

    Oasis in a desert. What is happening to the web these days? There are now so few sites which give independent info on subjects like Anaerobic Digestion in Wastewater Treatment, its even getting so that I almost stopped looking. Thanks.

    • justseegoods
    • February 19, 2018
  1. Reply

    I have checked your website. many time I think it is good, but on other occasion like this page I think not so good. What is UASBs, EGSBS?

    • Sam
    • January 9, 2019

    I really like what you guys are up too. This sort of clever writing provides as great resource for my pupils!
    Keep up the super work guys. Nobody else is applying these true blogging ideas to the Anaerobic Digestion subject
    It is good to see it.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Important: This site makes use of cookies which may contain tracking information about visitors. By continuing to browse this site you agree to our use of cookies.