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5 Things You Should Know Before Designing a Biogas Plant

We sat down and wrote this article on “5 things you should know before designing a Biogas Plant” to help our readers to create the best biogas plant design in 2015. Six years later, we looked back, and we think this information is still relevant and useful. (But, we have added some additional hints and tips which we think you might find interesting!)

The list below contains our top 5 things that we think new anaerobic digestion plant developers should know about before designing a biogas Plant:

1. Feed Material Choice

So, you have a potential site for a biogas plant, and as in most cases, as the promoter of a new facility, you will have at least some organic matter to use as the feedstock.  When the project plan identifies a need for additional feed material (known as the “substrate”) a decision must be made.

Anaerobic digestion plants can usually accept a wide variety of feed materials, but they may not be adaptable enough for all types of feed material, so this is important. The feed material choice will from the start of the detailed biogas plant design, determine how the biogas process is designed.

So, the thing you should know is that there is growing competition from biogas plant owners to compete for the best types of waste to “treat” and “dispose of” in their plants. If the promoter of a biogas plant facility chooses these, as the feed source, they must know the following fact.

That fact is that, what may now be considered to be a waste, and a waste stream for which the biogas plant company can charge a “disposal” fee, may eventually come to be seen as a saleable material and need paying for.

The answer to this is to negotiate a long term contract with the biowaste provider of say 10+ years duration, after which the original investment will most likely have been paid back.

Image text: "Designing a Biogas Plant 5 Things You Should Know".

Some wastes like food wastes are highly calorific (making them high gas-yielding and highly desirable for digestion) and may come without any need to comply with the Animal By-products Regulations (UK).

But, before assuming that the wastes of this type will always be freely available and bring in a gate fee, the promoter should note that this value may well quite soon be appreciated by the producer. When that happens the producer may start to charge a fee and not the other way around!

Once there is adequate anaerobic digestion capacity in any region it is common for a seller’s market to develop, and for the producer to start charging the AD Company for the honour of digesting their waste product!

For this reason, always probe deeper and find maybe less high gas-yielding feedstock wastes which are less than ideal as a biogas plant feed material, but at the same time, such feedstocks can be much more secure as long-term economic digester feed sources.

2021 Update Tip: Co-digestion of Manure with Kerbside Collected Household and Commercial Food Waste

Many large dairy farms in the US are now finding that they can obtain financial backing for their biogas digester installations by signing up with their local authority to accept the local town's household and commercial food waste.

This provides a guaranteed gate fee income with a long-term contract. The addition of the food waste provides a big increase in the calorific value of the combined manure/ food waste feed material. The larger the calorific value the more biogas is produced.

The combination of the gate fee and higher biogas output makes a big difference to the profitability of a dairy digester on farms with 1,000 or more cows. So much so that specialist financiers are keen to invest.

2. Design Life of Plant

All biogas plant promoters should think very carefully about the design-life of their biogas plant. Many poor-quality biogas plants are being built which will suffer long-term problems and will close a long while before better quality AD plants, built to a longer “design life”. This can make “cutting corners” very bad value.

The majority of biogas plants are built to a budget as a necessity of funding, nevertheless, as the industry matures those buying biogas plants will have to stop buying the lowest priced tender and develop an in-depth understanding of value for money, and “lifetime maintenance” costs. It is ONLY by doing this and specifying the design life of biogas plants from the start, that better value can be obtained.

An example is the use of a cheap mild-steel plate-based digester tank and mild steel ancillary tanks.

A reasonable design life to specify for AD plants is 15 to 20 years, maybe longer. However, few if any tank suppliers will provide a warranty for the continued corrosion-free performance of glass coated steel tanks beyond 10 years.

10 years is too short a design life for anaerobic digestion plants.

Hint: If you decide to specify a concrete-walled CSTR digester tank go for an in-situ cast reinforced tank which is cast all in one pour to avoid construction joints between the base/floor joint and the top of the wall.

Precast concrete tanks which are usually multi-jointed and post-stressed look good on paper but the construction method is truly difficult to achieve.

The wall units are imported to the site ready-made but they must be connected together with multiple water-tight vertical construction joints. Many such tanks have failed structurally well before the end of their design life due to invisible corrosion of the circumferential steel tendons.

3. Need for Mixing when Designing a Biogas Plant

Biogas plant substrates need mixing.

As offered by the cheapest AD Plant contractors, on-farm plants are frequently not supplied with any mixing equipment.

This is more often than not a mistake soon regretted and will shorten the life of the plant in between costly maintenance work.

Hint: We think that the best mixing systems for large CSTRs are those that are externally mounted and include a sparge cycle. In other words, in addition to providing a water jet to stir the contents of the digester, they also draw down some of the stored biogas and inject biogas at the jet nozzle. This is particularly good for getting any surface crust moving. Injecting gas into tanks for mixing is traditionally known as “sparging” in the biogas industry.

4. Avoidance of Pump and Pipe Blockages

Novice designers of biogas plants can offer very low-cost AD plants, which work on paper, but not successfully when constructed.

Designing AD plant pipework is truly the domain of experienced pipe flow engineers only. To avoid problems later with pump and pipe blockages needs a designer who understands every aspect of designing-out blockages.

Blockage avoidance measures range from pump model selection to choice of pipe diameters, bends and specials.
Image introducing the article: 5 Things You Should Know too build the best biogas plant design.

5. Build Up of Grit

Often overlooked is the propensity for any biogas plant design which accepts waste material to become blocked-up due to the presence of grit that enters (wet AD) biogas digesters.

Once grit settles it won’t come out until the whole tank is dug out with a Tomcat type excavator, or similar!

Always ensure that any AD plant designer has made adequate provision for removing any grit build up.

Hint: Better still use one of the new grit separator units to remove it before it enters the digester. The grit takes up space that should be actively in use by the microorganisms, reducing efficiency. There are some innovative new products now available to perform this function. Some can also depackage food waste and remove the plastic.

Conclusion to Designing a Biogas Plant and the 5 Things You Should Know First

This list of tips doesn’t cover all the problems that can occur. Nevertheless, these are at least some of those that keep occurring and that we thought that our readers would benefit from knowing about.

See also our article: 5 Most Critical Factors in the Design of Every Biogas Plant

[Article first published in August 2015. Updated August 2021.]

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  1. Reply

    Hi! Very useful info you give for me to use to design my biogas plant.

    • Snehal
    • August 11, 2018

    Hi I am trying to design my own biogas plan and stuck with the use of two separate tanks or putting tanks on each other . Which will be more sustainable design and how to introduce a stirrer ?

  2. Reply

    nice post and Very useful.

    • Investigator in Oakville
    • November 7, 2018

    Easy to follow, readable… good tips on designing AD plant. Heck. That’s one mighty good article. Sir

    • Theresa East
    • May 21, 2021

    Great post. Yes. Things you should know but sometimes you just need to jump in and get started and learn as you go.

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