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Anaerobic Digestion in Wastewater Treatment – UASBs EGSBs and Staging

The Role of Anaerobic Digestion in Wastewater Management

Typically, anaerobic digestion (AD) has been used for municipal wastewater sludge treatment, and in the industrial wastewater sector for COD/ BOD reduction as in the Upflow Anaerobic Sludge Bed (UASB). Only very rarely has it been used to treat industrial effluent the vast majority of which continues to be discharged directly to the public sewer system.

In the future, to accomplish more sustainable resource recovery and reduce the overall energy footprint of businesses, wastewater can be regarded as a renewable resource to convert embedded chemical energy to biogas.

A large WWTP AD Plant.  CC BY-NC-ND by nsub1

Anaerobic digestion technology has multiple benefits for the municipal and commercial/ industrial (non-agricultural) wastewater sectors. It enables a more sustainable practice for waste management that also generates renewable power, cuts greenhouse gas emissions, provides security both in energy availability and price for businesses.

Of increasing, importance post COP26 “net zero emissions by 2050”, for any business that operates a wastewater plant based upon the anaerobic digestion process it provides a very useful means by which the company can satisfy its own internal net zero goals plus show investors and would-be partner organisations that it has a viable decarbonisation strategy.

Of course, although not normally mentioned, anaerobic digestion at ambient temperatures also has a hidden role in large parts of the globe both in the humble septic tank of northern climes, and the masonry built household digesters of tropical nations, notably China and India.

What are the Economic Advantages of Wastewater Anaerobic Digestion?

What is undoubtedly highly financially beneficial to the municipal wastewater treatment industry is the anaerobic digestion of sewage works sludge produced as a product of the aerobic treatment as a first stage.

For industrial wastewaters with much higher biodegradable COD and BOD concentrations and elevated temperatures, anaerobic processes can be more economical than aerobic alternatives.

Energy recovery by anaerobic digestion can power an entire sewage works and still leave enough electricity available to supply nearby homes. For businesses in the food processing and catering industries with high organic contamination effluents, the wastewater previously sent to the nearest sewer attracting a volumetric payment for its disposal becomes an asset.

What are the Risks of Biogas Production to Wastewater Facility Operators

Anaerobic digestion is a relatively new technology in many countries and the high degree of expertise present to design aerobic wastewater treatment systems cannot yet be matched. The quality of biogas reactor design is not generally as good as for aerobic systems. This is a risk that is a function of the level of innovation taking place in this rapidly developing technology.

Large conical digester tanks at a Wastewater Treatment Facility
CC BY-NC by Massachusetts Clean Energy Center

Until a body of skilled engineers has become sufficiently mature in any technology the risk of failure, or at least of inefficiencies and lack of compliance with specified economic project goals is highest. While the best of the league in biogas technology are now producing highly efficient AD facilities with high up-time others and consistently high biogas product rates others are doing less well.

Hence, there is a still need to address matters, such as the reliability and maintainability of wastewater treatment plants (WWTP). For existing digesters, these should be addressed by analyzing the plant operational effectiveness, to identify design flaws or improvement opportunities. Quite often for older plants there will be inexpensive adjustments which once made may produce big dividends.

Wastewater AD in its current form was not an active discipline until the middle of the first decade of this century. Japan for example could hardly be considered to be technologically backward, and yet according to the Japan Sewage Works Agency, 280 out of 2,150 WWTPs have an anaerobic digestion system operating.

This low level of adoption of biogas production technology is common globally, but the level of adoption and of expertise in the field is growing rapidly.

Passive Wastewater Treatment

Image text: "The Role of Anaerobic Digestion in Wastewater Management".No article about anaerobic wastewater treatment plants would be complete without a section on passive wastewater treatment. Techniques such as facultative ponds are not common nowadays due to the large land areas required and the risk of odour and flies in the summer.

Facultative ponds mean large, but shallow basins meant for the treatment of the sewage through the application of both anaerobic and aerobic processes (Bougrier et al. , 2008). Biological and physical processes facilitate the complex treatment of wastewater. They also have the potential to handle wastewater flow fluctuations. In many a three-week retention of the sewage has the potential to achieve low microbial load that is recommended by the World Health Organisation (WHO) for unrestricted irrigation. The requirement of a large space is a major disadvantage affecting the treatment method.

In facultative lagoons, methanogens convert these products to methane and carbon dioxide. As in other AD systems, the methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments. A well established facultative pond with a healthy established ecology will not emit significant methane due to the fact that as the methane rises within the lower anaerobic sludges it passes into the top aerobic zone. In the air-mixed zone, the natural organisms very effectively oxidise it and use it as an energy source.

The downside of facultative ponds is that biogas collection is not possible. However, a high-quality effluent suitable for watercourse discharges is achievable.

Biological wastewater treatment (anaerobic and aerobic digestion reactors) takes advantage of the ability of certain microorganisms (including bacteria) to assimilate organic matter and nutrients dissolved in the water for their own growth, thus removing soluble components in the water.

Historically, passive treatments for urban wastewater and most industrial water was common, due to their simplicity and low operating economic cost. In most cases, organic matter constitutes the energy and carbon source that the microorganisms need in order to grow.

Nevertheless, for some WWTPs particularly in rural communities with plenty of available lands such as in rural Australia a prevalent component of many WWTPs in future is likely to be a facultative lagoon. Especially if eligible for Australian carbon credit units.

Why Treat Wastewater Sludges?

NVP Energy LTAD Plant
Low-temperature industrial effluent treatment (c) NVP energy

The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. In many large WWTPs, the raw sludges after aerobic treatment are reduced in volume by the process of digestion. At the same time, the polluting potential of the effluent is reduced and with suitable pasteurisation and control of the nutrient levels placed on the land, the output becomes a natural fertiliser.


These advantages of AD for municipal WWTPs and food processing and livestock companies are being accepted more widely. Waster Utility Companies are and industries are working together to increase biogas utilization by combining sewage sludge, food waste, and manure in anaerobic digesters.

Treatment of Sewage Sludge Using Anaerobic Digestion

Anaerobic digestion is a biological wastewater treatment process that is used for the treatment and reduction of organic wastes such as organic sludge or concentrated organic industrial waste which contain solids. As the quantity of organic solids is decreased in the sludge after this process, the treated sludge is easier to dispose of due to less volume.

The AD stage can lower chemical usage and reduce sludge handling costs. Aerobic treatment after anaerobic treatment can achieve a good treated-water quality for discharge into a watercourse. Veolia Water Technologies is an example of a company using anaerobic digestion in this way.

The biological decomposition of organic materials in the absence of free oxygen is known as anaerobic digestion and its use for WTTW sludge works well if the sludge is pre-treated to break up the cellular structure. Much of the organic waste is transformed into biogas (methane, carbon dioxide, and water) during this process (CPHEEO 2012).

Anaerobic digestion has been used in India for many years with a variety of substrates, including sewage sludge.

The Central Public health and Environmental Engineering Organization's “Manual on sewerage and sewage treatment part a: engineering” (CPHEEO 2012) describes the suggested designs and technical features for the building of anaerobic digesters in the Indian nation. A big Indian WWTP may produce an average of approximately 15,000 m3 of biogas per day, which is entirely utilised as household fuel through 3,500 connections.

Characteristics of Sewage Sludge

The literature on anaerobic co-digestion of sewage sludge, as normally accomplished under mesophilic conditions, shows considerable variation in the results reported as a consequence of its dependence on substrate characteristics and operational conditions (Derbal et al).

Digested sludge is inoffensive, having the appearance and characteristics of rich potting soil.

Anaerobic Co-digestion of Sewage Sludge

The energy recovered from the sewage sludge goes a long way towards making the entire wastewater treatment process energy self-sufficient, producing about half the requirements of the entire treatment works site at bran sands.

Image of a large Cambi Hydrolysis Plant
Large Cambi Hydrolyis Units

When the potential codigestion of food waste is considered, the economics of green gas production becomes even greater. Research at California state university suggests that mixing 25% food waste with 75% municipal wastewater sludge can increase biogas production by as much as 60% compared to digesting sewage alone.

The quick mixing of all substrates allows a higher volumetric loading going together with increased gas yields.

Energy-efficient process for converting organic wastewater matter into biogas

Anaerobic digestion is a process in which microorganisms convert organic matter into biogas in the absence of oxygen. It is an energy-efficient process that is typically utilized to treat high-strength wastewater containing high concentrations of biodegradable organic matter (measured as BOD, COD, and/or TSS).

AD technologies reduce the quantity of sludge and recover biogas

AD digestion is a naturally occurring biological process in which large numbers of anaerobic bacteria convert organic matter into methane and carbon dioxide (a mixture called biogas) in the absence of air. Anaerobic digestion stabilizes the organic matter in wastewater solids, reduces pathogens and odours, and reduces the total solids/sludge quantity by converting part of the volatile solids fraction to biogas.

AD Wastewater Management During Biogas Production

In some cases, in addition to the reduction in resources expended, some wastewater facilities with anaerobic digesters can increase their production of biogas, which has the potential to generate heat, power, or electricity ( WERF, 2014a ).

Both the system inputs (source and transportation of feedstocks) and the system outputs (use of biogas and digestate) need to be thoughtfully considered. Since each digester is part of a larger system of waste management and energy production, there are several guiding principles for creating a viable application of anaerobic digestion.

An example is reported by Tay J, zhang x (2000) for a fast predicting neural fuzzy model for high-rate AD wastewater treatment systems. Water res 34(11):2849–2860. 98. Abu QDAIS h, Bani Hani k, Shatnawi N (2010) “Modelling and optimization of biogas production from a waste digester using artificial neural network and genetic algorithm”.

Anaerobic Digesters & Wastewater

Anaerobic digesters have been widely used to treat wastewaters in livestock farms. With the increasing risk of antibiotic resistance originating from livestock husbandry, the removal of antibiotics and antibiotic resistance genes (ARGs) via anaerobic digesters deserves more attention.

A study group investigated the removal of antibiotics and ARGs in swine wastewater by three on-farm full-scale anaerobic digesters, including buried biogas digester (BBD), up-flow anaerobic sludge blanket (UASB) and high-density polyethylene covered biogas digester (HDPE-BD).

Low-rate anaerobic digestion plant systems are suited for all kinds of biodegradable slurries (e. g. animal manure, excess sludge from municipal wastewater plants, mixed organic solid wastes).

Typical low-rate anaerobic biogas digesters are batch reactors, fed-batch reactors (accumulation systems), Plug-Flow Reactors (PFR) or continuously stirred tank reactors (CSTR).

The requirements of individual wastewater treatment facilities will determine the precise set-up of the digester system, and whether single or multiple units are required.

Maximizing opportunities of AD of wastewater

The use of microalgae in wastewater systems has shown to be an effective strategy for secondary treatment, reducing operation costs and bringing relevant opportunities for resource recovery.

Perhaps most historically associated with its utilization as a conversion system employed within sewage plants and wastewater treatment facilities handling human waste, or others that process animal manure.

Ever-evolving technological advancements have given birth to an even greater smorgasbord of practical and potential biogas conversion opportunities, thus widening the possibilities of both anaerobic digestion’s applications and beneficial results across multiple industries once deemed outside its scope.

Benefits of anaerobic digestion

Benefits of anaerobic digestion. Presents the many benefits of ad compared to traditional manure management systems. Ad system design and technology. Describes the elements of a biogas and digestate recovery system.

In the past, industries that produced effluent with a high organic load usually flushed it away as wastewater. Now with anaerobic digestion, it's possible to find value in what was once wasted. Why choose fluence waste-to-energy (WtE) technologies? the benefits include:.

Anaerobic digestion is a biological process that uses microbes to biodegrade organic matter (feedstock) in the absence of oxygen. It provides a host of benefits in the treatment of challenging wastewater streams while offering the opportunity to produce value in the process.

Anaerobic digestion from wastewater potential

However, choosing suitable microorganisms and wash-out occurrences are potential disadvantages of bioaugmentation. Milan et al. (1997) compared the effect of different homoionic zeolites on piggery wastewater by anaerobic fixed bed reactors (Na, Ca, K and Mg).

Anaerobic wastewater treatment is a type of biological treatment where anaerobic microorganisms are used to break down and remove organic contaminants from wastewater. While anaerobic treatment systems may take a variety of forms, they generally include some form of bioreactor or repository capable of maintaining the oxygen-free environment needed to support the process of anaerobic digestion.

The anaerobic digestion of sludge is perhaps the oldest anaerobic digestion technology. In spite of the many advances made in reactor designs for wastewater treatment, few can be applied in the treatment of sludge and manure, as the high suspended solids content of this waste impedes biomass immobilization.

Delivering cost savings for wastewater treatment

Economic benefits of anaerobic digestion. Aside from numerous environmental benefits, there are several economic benefits associated with utilizing anaerobic digestion technologies. For wastewater treatment facilities that are incorporating food waste into anaerobic digesters, they experience two-fold savings through the reduction of energy costs via onsite power production, as well as through the receipt of tipping fees for accepting the food waste from food processing companies.

By incorporating food waste, wastewater treatment facilities can have significant cost savings due to tipping fees for accepting the food waste and increasing energy production. Wastewater treatment plants are usually located in urban areas which make it cost-effective to transport food waste to the facility.

AD is an attractive wastewater treatment technology, leading to the generation of recoverable biofuel (methane). Most industrial ad applications, carry excessive heating costs, however, as ad reactors are commonly operated at mesophilic temperatures while handling waste streams discharged at ambient or cold temperatures.

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

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

Wastewaters that 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 that 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 wastewater, 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 AD 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 AD 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 reactor 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 of.

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 AD 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 wastewaters, 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 the 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 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 settle-ability of granular sludge and also negatively affect the formation and growth of granular sludge.

The growing insight into 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.

AD 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 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 complexity of the design.

However, although staging usually improves the cost-effective performance of the plants, this needs to be 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 higher process stability. This especially applies to 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.

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 treatment for the purification/ treatment of wastewaters, 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 designers and construction contractors, can achieve better results than the old standard ‘textbook” designs.

One such example of the patented and branded AD 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 fewer 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.

Other AD 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 AD 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 of 75 days.

Despite previously reported difficulties with plant start-ups, operational performance in terms of 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: <http://www.macrothink.org/journal/index.php/emsd/article/view/12411/9993>. Date accessed: 09 feb. 2018. doi:http://dx.doi.org/10.5296/emsd.v7i2.12411.

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    • 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.

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