5 Must-Have Features in a Waste To Biogas

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 way back in . 10 years later, we look back, and we think this information is still relevant and useful. (And, we also added some additional hints and tips which we think you might find interesting!)

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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 Implications When Designing a Biogas Plant

When starting a biogas plant project, you likely have some organic matter available as feedstock. However, most projects require additional feed materials (also called substrates) to operate efficiently.

While anaerobic digestion plants can process many types of organic materials, each plant has limitations on what it can handle. Your choice of feed materials will fundamentally shape the plant's design and process flow.

Seek a Dependable Long Duration Feed Supply Contract

An important consideration is the growing competition among biogas plant owners for premium waste materials. What starts as a waste stream—where plant operators can charge disposal fees—may become a valuable commodity that operators have to pay for.

To protect against this market shift, we recommend negotiating long-term contracts (10+ years) with biowaste providers. This timeframe typically allows for recovery of the initial investment while securing a stable feedstock supply.

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, when designing a biogas plant 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.

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.

When is a Construction Cost Saving a Long Term Cost?

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 cheap mild-steel plate-based digester tanks that are inadequately corrosion protected later leading to corrosion penetration, leakage and plant commissioning for repair work.

Another common error is to spend too little on feedstock pre-processing for the removal of all non-organic contaminants especially when accepting food waste. A digester tank that fills with plastic, and inert materials can require hugely expensive emptying and specialist cleaning.

Fortunately, the latest low-destructive depackaging and separation equipment is far better than older models.

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.

Avoid the Temptations of Short Lifetime Design

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.

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.

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For more information, please visit Waste To Biogas.

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.

Frequently Asked Questions About Designing a Biogas Plant

See also our article: 5 Most Critical Factors in the Design of Every Biogas Plant [Article first published in August . Updated August and FAQs added January .]

Definition of biogas

Merriam Webster defines biogas as “a mixture of methane and carbon dioxide produced by the bacterial decomposition of organic wastes and used as a fuel.” 

Biogas is a renewable energy source produced from raw materials like food waste, manure, sewage, agricultural waste, and plant material. The process that transforms this organic waste into a mix of gases is called anaerobic digestion and is a natural fermentation process in which bacteria break down the organic matter into its components until all that is left are gases and a residue called digestate. 

The biogas production process

The biogas production process is practically natural fermentation in an oxygen-free environment. In nature, it can occur in marshes and swamps, where water helps to seal natural chambers that lack oxygen. 

Biogas plants are artificial environments, but the process inside the sealed digesters is 100% natural.

At first, biowaste must be crushed into smaller pieces, and, often, some water is added to accelerate the chemical reactions inside the digester. Then, bacteria decompose organic matter to obtain the right mix of gas.  

The process consists of four stages: 

1. Bacteria start breaking down complex carbohydrates, proteins, and fats and turn them into sugars, amino acids, and fatty acids through a process called hydrolysis.
2. Acidogenic bacteria transform these substances into volatile fatty acids (VFAs), alcohols, and gases. This second stage is called acidogenesis. 
3. In the third stage, acids and alcohols become acetic acid, hydrogen, carbon dioxide, and other gases through a process called acetogenesis. 
4. In the last stage, methane is produced from the substances now present inside the digester, and the process is called methanogenesis, as methanogens are responsible for completing the process. 

This process is optimized when the temperature inside the digester stays between 30 to 38 degrees Celsius (86-100 Fahrenheit). The procedure’s success depends on the chemical composition of the waste fed into the digester. If the raw materials are too acid or have too much nitrogen, you risk disturbing the balance inside the digester, resulting in inconsistent production levels. 

Moreover, not all organic wastes have the same fermentation timeline. Bacteria can easily break down food waste, fats, oils, and greases, but livestock waste can be more challenging when fed into the digester. By mixing multiple types of waste inside the digester (co-digestion), you can accelerate the fermentation process and generate consistent amounts of biogas. 

Main chemical composition of biogas 

Biogas composition consists primarily of methane and carbon dioxide, but the mixture of gases also includes a series of other substances and sometimes traces of water. 

The chemical composition of biogas: 

• Methane (CH4, 50-70%)
• Carbon dioxide (CO2, 25-50%)
• Other gases: nitrogen (N2, less than 5%), hydrogen (H2, less than 1%), and oxygen (O2)
• Traces of hydrogen sulfide (H2S, less than 3%), water vapor (H2O, less than 10%), and ammonia (NH3, less than 1%)

[Tabel from: https://www.biogas-renewable-energy.info/biogas_composition.html]

The chemical composition of biogas varies with a series of factors, from the quality of the organic waste put inside the digester to the digester’s feeding rate and the temperature and humidity inside the digester. 

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As it contains hydrogen sulfide, carbon dioxide, and water, biogas is highly corrosive, so all the systems built for producing, transporting, storing, and using it must be made with specific materials. 

Biogas must often be treated to enhance its properties — different chemical reactions remove some components, such as hydrogen sulfide, water, or carbon dioxide, to obtain biomethane. This removal process requires a more complex system for producing biogas, including working with corrosive substances.  

Biogas physical characteristics 

Thanks to its similar properties, biogas can substitute natural gas with a significant impact on the environment. Here’s how biogas composition can change its physical characteristics and how they’re different from natural gas: Table

If you use equal amounts of biogas and natural gas, biogas produces twice fewer calories by combustion. 

Biogas also helps keep manure from landfills and, therefore, reduces the amount of methane released directly into the atmosphere. Methane is a greenhouse gas with effects 34 times more potent than carbon dioxide’s, so by redirecting it towards biogas production, we get to reduce its impact on climate change.

It’s also important to note that when biogas is combusted (when used as cooking gas, for instance), it still generates carbon dioxide. However, it all comes from plants, which had naturally removed this gas from the atmosphere. Therefore, these emissions are considered carbon-neutral, as the amounts of new CO2 added to the atmosphere are minimum. 

Biogas usage and applications

Thanks to its physical characteristics, biogas can be used as an alternative to fossil fuels. It can be used similarly to natural gas in water and space heating, drying, absorption cooling, steam and electricity production, and transportation. The more organizations and governments become aware of fossil fuel’s impact on climate change, the more they’re interested in innovating and finding new ways to shift to clean energy sources, including biogas usage. 

The most popular applications for biogas are:   

1. Cooking gas 

Direct combustion is a more straightforward way to use biogas to generate heat and lightning, being an increasingly popular alternative to fossil fuel energy in developing countries. A gas pipe connects the digester to a biogas cooking stove, so you can produce cooking gas and manage waste with minimum impact on the environment. 

2. Combined heat and power 

A more complex way to use biogas is co-generation or combined heat and power process (CHP), in which case you simultaneously produce electricity and heat with biogas. This application is more suitable for industrial biogas plants that manage large amounts of waste than for domestic use. These systems can supplement energy production in rural areas that otherwise are forced to remain off-grid. 

3. Transportation 

Biogas can also be used as vehicle fuel if purified and treated to match natural gas quality. The extra effort can pay off, as this shift could significantly impact climate change. By switching to biogas, the transport sector can reduce greenhouse gas emissions by 60 to 80%, compared to gasoline and diesel.

Pros and cons of using biogas

Biogas advantages 

• It’s an eco-friendly, renewable source of energy that has a smaller impact on the environment than fossil fuels. 
• Biogas combustion is carbon-neutral and reduces greenhouse gas emissions. 
• Using biogas can reduce the dependence on oil imports in many countries. 
• Biogas production is a cost-effective waste management solution that improves environmental quality by keeping waste away from landfills and water sources. 
• The biogas production process also generates organic fertilizer for plants. 
• Biogas plants benefit local economies by creating new jobs. 

Biogas disadvantages: 

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• Producing and purifying biogas still needs to be improved significantly to make it effective at scale. 
• The biogas production process is suited for specific climates and geographical areas, as it requires consistent supplies of raw materials and constant temperature inside the digester. 
• Treated biogas is still not 100% pure, so more research is necessary before we can use biogas as vehicle fuel on a scale. 
• Biogas is an eco-friendly energy source only as long as it comes from existing waste. If people start producing raw materials only to transform them into biogas, the process no longer positively impacts the environment. 
• Biogas is easy to produce in rural areas where organic materials are easy to procure, but biogas plants are less effective in dense urban areas. 

Conclusion