(7) Making biogas from waste - the process and generator (digester, fermenter) designs, factors affecting rate of production, advantages and disadvantages of producing biogas

See also Biofuels and alternative fuels, hydrogen, biogas, biodiesel (GCSE chemistry notes)

Introduction to biogas

Biogas (mainly the hydrocarbon gas methane CH₄) is produced naturally in marshes, septic tanks and sewers - oxygen deficient places anywhere anaerobic bacteria thrive!

Biogas (methane) is produced from the chemistry of anaerobic respiration.

Compost consists of decomposed organic material including plant waste from the garden or food waste from the kitchen.

When enclosed in a compost bin it gradually decomposes into a rich organic material that is a really good natural fertiliser - a good example of partially recycling the biomass from photosynthesis.

Air is admitted to the compost 'heap' and little methane gas is made, in fact aerobic bacteria decomposers are producing carbon dioxide.

Its no use for producing useful quantities of biogas.

As an alternative, using specific microorganisms, by enclosing the organic waste in a tank, you can produce biogas.

Sludge waste from a sewage works (animal waste!) or a sugar factory (plant waste) can be used to make gas on a larger scale.

So, microorganisms (anaerobic bacteria) can be used to break down organic waste under anaerobic conditions to produce biogas, which is mainly the hydrocarbon methane gas, CH₄.

Biogas can be produced from variety of waste raw 'organic' materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste.

Biogas is produced by anaerobic digestion with methanogen or anaerobic microorganisms, which digest biodegradable materials.

Specially grown crops of maize are used in some large-scale biogas digesters.

The residue from the digestion process can be used as rich source of fertiliser.

 

Advantages of using biogas-methane

(a) The biogas can be burned like any other fuel to produce heat.

Biogas is a relatively cheap fuel for cooking, heating, vehicle fuels and small scale production of electricity.

The heat can be used to generate steam to drive a turbine and electrical generator.

This is quite handy for small scale electricity production in remote areas far from a national grid supply. It could also power road vehicles too.

(b) Theoretically it is eco-friendly, its a renewable resource and carbon neutral.

The decomposed plants are replaced by new crops, and, with the animal waste from eating plant material, the carbon is recycled by carbon dioxide formation on burning.

The growth of new crops removes and balances the same carbon dioxide by the process of photosynthesis in plant leaves.

(c) The raw materials for biogas are relatively cheap and readily available, mainly from agricultural sources.

(d) Burning biogas is also eco-friendly because it is a relatively clean fuel, although it produces carbon dioxide and water on combustion it does not produce much pollutant gases such as sulfur dioxide, oxides of nitrogen or carbon or hydrocarbon particulates.

(e) The leftover waste after digestion can be used as a fertiliser.

(f) In developing countries biogas generation advantages include (i) reduces soil and water pollution, (ii) its a simple and low-cost technology that encourages a green recycling economy and (iii) a healthier and less polluting cooking alternative.

Animal dung makes a great source of biogas on a small scale for cooking.
 

Disadvantages of using biogas-methane

(a) At the moment biogas cannot be produced on a huge scale.

(b) An unfortunate disadvantage is that the systems used in the production of biogas are not efficient.

The biogas contains Impurities and even after refinement and compression, it still contains impurities.

(c) There have been few recent technological advancements.

(d) Biogas is less suitable for dense Metropolitan Areas.

(e) Unfortunately, by its nature, biogas cannot be readily stored as a liquid - you need a very high pressure and a very low temperature to liquefy it (boiling point of methane is -161^oC at normal atmospheric pressure!).

Therefore the biogas from the digester must be used immediately for cooking, heating, lighting or using the heat from combustion to make steam to drive a turbine-generator to make electricity.

 

The composition of biogas

Typical values are quoted below, but a wide variation depending on the source of the organic material.

 

The design of a biogas generator - also called a digester or fermenter

All biogas generators have the same basic design and are based on a tank of varying size.

The tank has to hold sufficient amount of rotting organic matter to ensure a steady production of biogas.

The starting organic waste material can be animal dung, farm waste like slurry or garden waste.

The waste material is digested in a tank to which the microorganisms are be added.

You need an input pipe to inject the waste organic material into the tank.

You also need an output pipe to extract the residue (waste slurry), which can be used as a fertiliser.

 

Batch biogas generators

You manually fill the biogas generator with a relatively small amount of waste material, the batch to be digested.

The batch is left to digest and when no more gas is produced you have to extract the residue (by-product for fertiliser).

Then the biogas generator is then completely cleaned out and fresh lot of organic waste put in and the process repeated to make the next batch of biogas.

This means you cannot have a continuous stream of biogas.

 

Continuous biogas generators (the diagram above is more like a continuous process)

With a continuous biogas generator, the organic waste is continually fed in through the inlet pipe and, at the same time, the residue of digested waste is continually removed to be used as fertiliser.

This system allows for a continuous supply of biogas, a big advantage over the batch process system and a better design for larger scale production.

 

The best optimum reaction conditions for producing methane

The factors affecting the rate of decay were discussed in detail in Part 5 decomposition, so just a brief summary is repeated here.

To keep the microorganisms continuously anaerobically respiring away as efficiently as possible!

1. Warm conditions e.g. a constant temperature of around 35^oC to 45^oC - an optimum temperature for many enzymes, which actually carry out the biochemical processes of digestion.

A typical enzyme graph of rate of reaction versus temperature.

(i) Initially rate increases with increase in temperature, molecules have more kinetic energy, more forceful fruitful collisions to break bonds and form new products e.g. CH₄.

(ii) However, above ~50^oC, the enzyme starts to be become denatured, the protein structure of the active site is altered and cannot function properly, and the rate dramatically falls with further increase in temperature.

2. Exclusion of oxygen, so anaerobic decomposition takes place via anaerobic respiration.

3. Mix the waste with water to make a sort of slurry to give a better reaction medium.

4. A high concentration of decomposer microorganisms - some will be already present in the animal/plant waste, but you can add more to increase their concentration.

Initially the rate of decay to produce biogas is proportional to the enzyme concentration, which in turns depends on the concentration of anaerobic bacteria.

 

See also Biofuels & alternative fuels, hydrogen, biogas, biodiesel (GCSE chemistry notes)

and factors affecting the rates of chemical reactions (GCSE chemistry revision notes)

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Making Biogas from Waste

Biogas is a renewable energy source produced through the anaerobic digestion of organic waste by microorganisms.

It mainly consists of methane (CH₄), carbon dioxide (CO₂), and small amounts of other gases like hydrogen sulfide (H₂S).

Process of Biogas Production

1. Collection of Organic Waste – Materials like animal manure, plant residues, sewage, and food waste are gathered.

2. Anaerobic Digestion – Microorganisms break down the waste in an oxygen-free environment, producing biogas.

3. Gas Collection – The methane-rich gas is captured and stored.

4. Utilization – Biogas can be used for cooking, heating, or electricity generation.

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Biogas Generator Designs

Biogas generators, also called digesters or fermenters, come in different designs, tailored to efficiency and environmental conditions.

Types of Biogas Generators

1. Fixed Dome Digester

   • Features a fixed gas storage chamber.

   • Simple and durable, suitable for long-term use.

   • Lower initial cost, but gas pressure may fluctuate.

2. Floating Drum Digester

   • Has a moving gas storage drum that rises as gas accumulates.

   • Provides a steady gas pressure, making it easy to extract.

   • Requires more maintenance due to moving parts.

3. Plug Flow Digester

   • Suitable for large-scale agricultural waste processing.

   • Waste is fed in one direction, continuously producing biogas.

   • Efficient but requires regular monitoring.

4. Balloon Digester

   • A flexible plastic bag or balloon that inflates as gas accumulates.

   • Low-cost and simple but prone to damage.

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Factors Affecting the Rate of Biogas Production

Several environmental and biological conditions influence the efficiency of biogas production.

1. Temperature – Optimal microbial activity occurs between 35–40°C (mesophilic conditions). Higher temperatures can lead to thermophilic digestion, increasing gas yield.

2. pH Levels – The ideal pH range for anaerobic digestion is 6.5–7.5. Too acidic or alkaline conditions inhibit microbial activity.

3. Substrate Composition – High carbohydrate, protein, and lipid content improve methane yield. However, excess lignin (found in wood) slows digestion.

4. Retention Time – The time waste remains in the digester affects the amount of biogas produced. Longer retention allows more breakdown of organic matter.

5. Moisture Content – Ensuring a proper balance between solids and liquids improves microbial efficiency.

6. Oxygen Availability – Oxygen must be absent for effective anaerobic digestion. Any oxygen presence can disrupt the process.

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Advantages and Disadvantages of Producing Biogas

It is important to understand the pros and cons to help evaluate its sustainability.

Advantages of making biogas

Renewable Energy – Reduces reliance on fossil fuels.

Waste Management – Converts organic waste into useful energy.

Reduced Greenhouse Gas Emissions – Prevents methane release from decomposing waste.

Cheap & Localized Energy Source – Suitable for rural areas with limited energy access.

Production of Fertilizer – The leftover digestate is rich in nutrients and can be used as compost.

Disadvantages of making biogas

Initial Setup Costs – Construction of digesters can be expensive.

Requires Constant Waste Input – Needs continuous organic material supply.

Methane Leakage – Poorly maintained systems may leak methane, a potent greenhouse gas.

Dependent on Temperature – Cold climates reduce efficiency, requiring additional heating.

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Summary of learning objectives and key words or phrases

Be able to describe the making biogas from waste composition process, biogas, generator, design of digester fermenter.

Be able to describe and explain the factors affecting the rate of digestion.

Be able to describe and discuss the advantages and disadvantages of producing biogas by anaerobic respiration compared to other manufacturing methods.

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Practical work you may have done

• building a simple biogas generator to collect methane and demonstrating how the methane can be burned as a fuel

• investigating and designing a way of measuring the gas output of a biogas generator and compare the amount of gas produced by different materials.

  Biogas (mostly methane CH₄ and some carbon dioxide), is formed by the anaerobic decomposition-fermentation of organic waste by microorganisms (bacteria with right enzymes to decompose organic compounds).

  • Organic waste eg plant or animal from domestic refuse (waste food), farm waste (usually from animals), sewage sludge waste, factory waste (from sugar factories) sources etc. is broken down by microorganism to the simplest organic compound, namely methane.

  • It can be carried in quite simple biogas fermenters, sometimes called biogas digesters or biogas generators.

  • The anaerobic fermentation should carried out in the absence of air with the right bacteria and at a constant temperature appropriate to the optimum rate of catalysis of the enzymes in the bacteria e.g. 30-40^oC.

  • The biogas is easily stored because it isn't easily liquified and is an explosive flammable gas.

  • The gas can be used directly for heating, lighting and cooking.

  • The gas can also be burned to provide heat to make steam to drive a turbine and electrical generator.

  • Biogas generation can be done on a small domestic scale or large scale and the residue (what's left after tapping off the biogas) can be used as fertiliser.

  • All this waste will rot naturally, often under aerobic conditions, so its worth noting, that, disposing of organic waste in a biogas generator in this way, is better than letting the methane diffuse into the atmosphere where its acts as a powerful greenhouse gas.

  • All biogas generators will have features in common ...

    • an inlet for the waste organic material to be fermented,

    • a valve controlled outlet for the biogas formed,

    • an outlet for the waste material left over after the digestion has finished.

  • Biogas generators (biogas digesters or biogas fermenters) are designed to operate in one of two ways ...

• In a batch process, the biogas is made in small amounts or batches i.e. the biogas generator is filled up and left to ferment, the biogas is continually tapped off once the anaerobic fermentation starts and then when gas production falls significantly the residue is cleaned out. The generator is then re-filled new organic waste material and the whole process repeated.

  • Cheaper batch process technology than continuous process because of simpler digester design.

  • Batch processes are not as efficient because after each batch has fermented the process must be stopped and the generator stopped, cleaned and re-filled to restart.

• In a continuous process, the waste organic material is continually fed/pumped into the generator, the gas continually drawn off and the residue continually removed/pumped out.

  • The continuous process generator employs more costly technology because of the extra pumps needed.

  • A continuous process is more efficient and economic than a batch process, no stopping, cleaning out and re-filling required.

• Either production method requires good temperature control of 30-40^oC, so biogas generators may need insulation if too cold or a heating source if too cold.

• Ideally the biogas generator should be sited near the source of organic waste e.g. a small scale batch process on a farm or a large scale continuous process at a sewage farm.

• The use of biofuels has both environmental and economic benefits.

  • Biofuels should be 'carbon neutral', that is the carbon dioxide they release on burning is re-absorbed by the plants the carbon originally came from.

  • On burning they do not release sulfur oxides that cause acid rain.

  • Biogas digesters are a good way of using potentially harmful and polluting organic waste (contain pathogens), rather than just dumping it in the ground, where, it will still break down and release methane - a powerful greenhouse gas contributing to global warming.

  • In poor rural areas biogas from animal dung is a convenient way of providing heat for cooking, especially if wood is scarce and in general the raw waste organic material is cheap and readily available.

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