About this module
This module trains you to understand small biogas digesters and to help build, feed and look after them. You learn by doing: most of the time is spent at a working digester, and every unit ends with a practical task and homework.
Who it is for. Beginners with no formal certificate. The level is National Proficiency I, where a trainee does routine tasks under close supervision.
What a digester does. A digester is a closed tank where bacteria break down dung, toilet waste or food waste without air. They make biogas, which burns with a blue flame for cooking, and slurry, a good fertiliser.
This is not an accredited CTVET programme. COTVET approved a competency-based biogas standard for artisans in 2016 at National Proficiency II, for fixed-dome digesters; it said it must not be used after December 2021. No current biogas qualification at National Proficiency I was found. This module follows CTVET's published format only. Ghana also has no Ghana Standards Authority standard for biodigesters yet.
How the module works
Each unit has learning outcomes, performance criteria, the range and the evidence your assessor needs. You are judged Competent or Not Yet Competent by being watched at work, answering questions, and showing your logbook. About 70% of the time is practical, following CTVET's Oil Palm programme, which uses about 30% in school and 70% at the workplace.
Unit 1 · Work safely around biogas
Biogas burns and can explode. It can also contain a gas that kills in one breath. Learn the rules before you go near a digester.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 1.1 Know the dangers | names fire, explosion, toxic gas, confined space and germ dangers |
| 1.2 Never enter | states and follows the rule: never enter a digester, pit or manhole, even to rescue |
| 1.3 Check for leaks safely | uses soapy water, never a flame |
| 1.4 Give first aid | shows what to do if someone is overcome by gas, without entering |
Range. Digesters, pits, gas pipes, stoves. Evidence. Observation every session; oral questions.
Fire and explosion
Biogas mixed with air explodes when it meets a flame or spark. No smoking or open flames near a digester, and rooms with gas appliances must have air moving through them. Never store biogas in a room where people live or sleep.
Hydrogen sulphide (H₂S): the silent killer
Biogas can contain up to 3% hydrogen sulphide. It smells of rotten eggs at first, but at higher levels it destroys your sense of smell within minutes; at 700–1,000 ppm it knocks a person down in one or two breaths and kills within minutes. Smell is not a warning.
Never enter a digester or pit
The air inside a manure pit or digester cannot be trusted. Never go in. If someone collapses inside, do not go in after them: call emergency help.
In July 2026, at a pig farm in Vietnam, one worker fell into a biogas pit. Co-workers went in one after another to save him: five died and eight went to hospital. In the UK, gas released while opening a digester roof killed a worker.
Leak checks
Check pipes and joints with soapy water brushed on: bubbles show a leak. Never use a flame.
Germs
Dung and toilet waste carry germs. Wear gloves and wash your hands and tools.
Other dangers
Hot burners can burn, especially children; gas-lamp mantles are slightly radioactive, so change them carefully and wash your hands; water-trap and valve pits need heavy lids.
First aid if someone is overcome by gas
- Protect yourself first: do not enter any pit or tank. Remove flames and sparks.
- If the person is in the open, move them to fresh air and keep them still.
- A trained person gives oxygen, rescue breathing or CPR if needed.
- Get medical help.
What would you do?
(1) A neighbour lights a match to look for a gas leak. (2) Your co-worker drops a tool into the outlet chamber and climbs down to get it. (3) Your co-worker collapses beside an open manhole. (4) The kitchen smells strongly of rotten eggs, then the smell goes away.
Safety walk and leak check
- With your trainer, walk around a digester. Point out: inlet, outlet, gas pipe, water trap, valve, stove, any pit or manhole.
- Say the "never enter" rule out loud at each opening.
- Mix soapy water and brush it on every gas joint from the digester to the stove. Record any bubbles.
- Record the walk in your logbook.
Tell your family
Explain the "never enter" rule and the "soapy water, never a flame" rule to two people at home. Write who you told and what they said.
Unit 2 · How a digester works
Tiny living things do the work. Give them warmth, no air, and the right food, and they make gas.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 2.1 Explain the process | explains in simple words how dung becomes biogas |
| 2.2 Name the types | names fixed-dome, floating-drum and bag digesters, with one good and one bad point each |
| 2.3 Tell them apart | tells a gas-producing digester from a toilet "biodigester" that only treats waste |
Range. Household and farm digesters in Ghana. Evidence. Oral questions; a labelled sketch in the logbook.
The process
Biogas is made when bacteria break down organic matter without air. It is mostly methane (50–70%) and carbon dioxide, with traces of hydrogen sulphide, and burns with a clear blue flame. The bacteria work in three steps: big pieces are broken down, other bacteria make acids, and the methane bacteria turn the acids into gas. The methane bacteria grow slowly and don't like sudden changes.
Warmth helps. Gas is best around 35 °C, good at 25–30 °C, and almost stops at 10 °C. Southern Ghana is warm all year, which suits biogas.
Types of digester
| Type | Good | Bad |
|---|---|---|
| Fixed dome (brick or concrete, underground) | low cost, no moving parts, long life | leaks in the dome are common and hard to find |
| Floating drum (steel drum over the tank) | easy to see how much gas is stored; steady pressure | steel drum costs more and rusts; may last only about 5 years on tropical coasts |
| Bag or tube (plastic) | cheap, simple, easy to move | short life; easily damaged |
From Biogas Digest. The fixed dome is the most common type in Ghana; it works well but costs more and needs skilled builders. In a survey of Ghanaian digesters, broken balloon gas-holders were one cause of failure.
Not every "biodigester" makes gas
Many Ghanaian household biodigester toilets treat waste but do not collect gas: the waste is broken down and the water goes into a soak pit. The Ghanaian Biofil toilet uses earthworms, not a gas tank. Ghana's Ministry of Sanitation published a Bio-digester Manual in 2022 for these toilets. This module is about digesters that make gas.
Sketch and label
Draw a fixed-dome digester from the side. Label: inlet, digester, gas at the top, outlet (expansion) chamber, gas pipe, water trap, stove, slurry out.
Visit and compare
Visit one gas digester and (if possible) one biodigester toilet. For each, write: what goes in, what comes out, does it make gas, and how you can tell.
Find a digester
Ask around your community: does anyone have a biogas digester or biodigester toilet? Is it working? Record what you learn.
Unit 3 · Choose the feed and size the digester
Size the digester to the dung you really have. Digesters that were too big for their dung supply are one of the most common reasons plants fail, in Ghana and elsewhere.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 3.1 Choose the feed | names good feeds and things that must never go in |
| 3.2 Mix the feed | mixes dung and water in the right proportion |
| 3.3 Size it | works out digester size and gas from daily dung |
Range. Cattle, pig and poultry dung; food waste. Evidence. Sums in the logbook; observation of mixing.
How much gas from each kg of fresh dung?
| Dung | Gas per kg (m³) |
|---|---|
| Cattle | 0.023–0.040 |
| Pig | 0.040–0.059 |
| Poultry | 0.065–0.116 |
| Human | 0.020–0.028 |
From FAO Biogas Manual. Food waste works too: 2 kg of household kitchen waste gave about 200 litres of gas, enough for about 45 minutes of cooking.
How many animals? Ghanaian researchers estimate a household digester needs at least 20 kg of dung a day, the output of about 7 local cattle.
Mixing
Mix fresh dung with water 1 to 1 by volume; dry dung needs more water. Remove stones.
Never put in
Soap and detergents, antibiotics and solvents; wash water and wood or straw. Household cleaners like Dettol, Parazone and Mr Muscle killed all the worms in a Biofil toilet in tests. Keep cleaning water out of the digester.
Balance the food
The bacteria like a carbon to nitrogen balance of about 20–30. Cow dung (about 24) is close to perfect; chicken dung (about 10) is rich in nitrogen; rice straw (about 70) is rich in carbon. Mixing them balances the feed.
Sizing
Digester volume = daily feed (dung + water) × days it stays inside.
How many days should feed stay inside? Simple designs need at least 40 days, often 60–80. A Ghana study used 40 days for dung and 60 days for toilet waste. An international factsheet gives at least 15 days in hot climates, but 60 days for toilet waste. Use at least 40 days for dung and 60 for toilet waste.
Worked example
20 kg of cattle dung a day + 20 litres of water, 40 days (calculated from the figures above):
| Step | Sum | Answer |
|---|---|---|
| Daily feed | 20 kg dung + 20 L water | about 0.04 m³ |
| Digester volume | 0.04 × 40 days | about 1.6 m³ |
| Gas per day | 20 × 0.023 to 20 × 0.040 | 0.46–0.80 m³ |
| People it cooks for (at 0.3 m³ each a day) | 0.46 ÷ 0.3 to 0.80 ÷ 0.3 | about 1.5–2.5 people |
A family of 6 needs about 1.8 m³ of gas a day at 0.3 m³ each, which means far more dung: about 45–80 kg of cattle dung a day (calculated). Count the dung first.
Size for pigs
A farmer has 10 pigs that give 25 kg of dung a day. (1) Daily feed volume with equal water? (2) Digester volume at 40 days? (3) Gas per day (use 0.04–0.059 m³/kg)? (4) How many people could it cook for?
Weigh the dung
- For three mornings, help collect all the dung from the farm's animals and weigh it. Record each day.
- Work out the average per day.
- Using the method, size a digester for that amount, and work out the gas.
- Mix one bucket of feed at 1:1 with water and look at it with your trainer: is it like thick soup?
Count at home
If your family or a neighbour keeps animals, estimate how much dung they make each day. Is it enough for a household digester?
Unit 4 · Help build a digester
A digester must be gas-tight and water-tight, in the right place, with a gas pipe that drains. A qualified builder leads; you help.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 4.1 Check the site | checks distance from wells, water table, shade, trees and slope |
| 4.2 Prepare materials | checks cement, sand, gravel and bricks are good; mixes as directed |
| 4.3 Assist the builder | carries, mixes, passes and holds as directed; works safely in the pit only while it is open and being built |
| 4.4 Help lay the gas pipe | helps lay a sloping pipe with a water trap at the low point; leak-tests with soapy water |
Range. Fixed-dome digesters (bag digesters as an option). Evidence. Observation on a build; site checklist in the logbook.
Site
- Sunny, close to the animal shed and water, close to the kitchen so the gas pipe is short.
- Away from trees (roots crack the digester), away from shade, and where roof water won't drain onto it. A high water table can push an empty digester out of the ground.
How far from a well? FAO's training manual says at least 10 m. The Biogas Digest says at least 30 m, and downhill of the groundwater flow. Use 30 m: wells must stay clean.
Materials
Cement not older than 6 months, clean sand, gravel 10–20 mm, clean water, and well-baked bricks soaked before laying.
Build order (fixed dome)
The COTVET artisan standard sets out the order: foundation ring and slab; brick walls with the outlet pipe; outside plaster and backfill; the "weak ring" and inlet pipe; close the dome; plaster inside and out; build the gas neck; then the plumbing. Work inside the pit only while it is open and being built, with the builder present. Once the dome is closed or the digester has been fed, nobody goes in.
The gas pipe
Galvanised iron above ground; PVC only underground or out of the sun. The pipe must slope (at least 1%), with a water trap at the lowest point in a covered chamber, because water condenses in the pipe and blocks the gas.
Leak test
Pressurise the pipe, then brush soapy water on every joint; bubbles show a leak. Or close all valves and check the pressure a day later; no drop means the line is gas-tight.
Site check
Your trainer gives a sketch of a compound: a well, a mango tree, a house, a pig pen, a slope. Mark the best digester site and the gas pipe route. Explain each choice.
Build helper
On a real build (or a bag digester set-up) with the builder:
- Help check the site and mark it out.
- Check and prepare materials; mix mortar as directed.
- Carry, pass and hold as directed, wearing gloves, boots and a hard hat in the pit.
- Help lay and slope the gas pipe; fit the water trap.
- Leak-test every joint with soapy water and record the results.
Price the materials
Find the price of a bag of cement, a trip of sand, a length of galvanised gas pipe, and a gas valve near you.
Unit 5 · Feed and run the digester
A digester is like an animal: feed it the same good food every day and it stays healthy.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 5.1 Start up | helps fill and seed a new digester; waits for gas |
| 5.2 Feed daily | mixes and feeds the right amount every day; removes stones and rubbish |
| 5.3 Do routine care | drains the water trap; cleans the burner; checks pipes and outlet |
Range. Running household and farm digesters. Evidence. Feeding and care record; observation.
Starting up
Start a new digester with cow dung or septic sludge to bring in the right bacteria. The first gas can take 3–4 weeks.
Daily
Feed every day as soon as the dung is available; mix dung with urine or water; remove stones and sand; poke the inlet and outlet with a stick to keep them clear; check the overflow. Clean the burner every day.
Don't overfeed or underfeed. About 6 kg of dung per m³ of digester per day is a guide; overfeeding makes the digester sour, underfeeding gives little gas.
Weekly and yearly
- Weekly: drain the water trap.
- Monthly: check flexible pipes for cracks.
- Yearly: remove scum and pressure-test the plant, done by a qualified person from outside.
Two weeks of care
- Each day, weigh, mix and feed the digester. Record kg of dung, litres of water, time.
- Each day, poke the inlet and outlet, check the overflow and clean the burner.
- Once a week, drain the water trap and record it.
- Each day, note how long the stove burns and how strong the flame is.
A care chart
Make a simple daily care chart a family could pin on the wall: feed, poke, check, clean, and a weekly "drain the trap" box.
Unit 6 · Find and fix problems
Most digester problems have simple causes. Follow the signs.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 6.1 Read the signs | links each common problem to its likely cause |
| 6.2 Fix simple problems | drains traps, clears inlets with a stick, adjusts feeding, under supervision |
| 6.3 Report | reports cracks, leaks and sour digesters to the trainer |
Range. Gas, flame, pipe, inlet and outlet problems. Evidence. Troubleshooting record; oral questions.
Common problems
| Problem | Likely cause | What to do |
|---|---|---|
| No gas after first filling | Still starting | Wait 3–4 weeks |
| Little gas | Too little feed, or a leak | Feed more; check for leaks |
| Gas in the plant but none at the stove | Water blocking the pipe | Drain the water trap |
| Gas won't burn | Poor gas mix | Feed properly mixed dung |
| Flame lifts off the burner | Pressure too high or dirty nozzle | Turn down the valve; clean the nozzle |
| Inlet or outlet blocked | Rubbish in the feed | Filter the feed; clear with a stick |
| Slurry in the gas pipe | Overfilled or overfed | Remove slurry at the outlet; clean the pipe |
| Cracked wall | Weak foundation or poor backfill | Report for repair |
From FAO Biogas Training.
A sour digester
If the slurry smells sour, the digester is too acid. Raise the pH with cattle dung, lime, wood ash or biochar.
What pH is right? FAO says about 6–7, with a stable digester at 7.2–8.2 and above 8.5 toxic. The Biogas Digest says 7–8.5, with below 6.2 toxic to the methane bacteria. A university note says 6.8–7.4. All agree: close to 7 is good, and sour (low pH) is bad.
Diagnose
For each, give the likely cause and the fix: (1) the stove gives a weak flame that dies quickly; (2) there is a strong sour smell at the outlet and little gas; (3) last week someone poured mopping water into the inlet, and now there is little gas.
Problem round
- Test the slurry at the outlet with a pH strip and record it.
- Check the flame: colour, height, how long it lasts.
- Drain the water trap and note how much water came out.
- Write any problem you find, its likely cause, and what you did or reported.
Interview an owner
Ask a digester owner what problems they have had and how they fixed them. Compare with the table.
Unit 7 · Use the gas and the slurry
The payoff: free cooking gas and a fertiliser richer than compost. Use both safely.
Competency standard
| Learning outcome | Performance criteria — the trainee… |
|---|---|
| 7.1 Use gas safely | lights the stove safely; ventilates the kitchen; turns off the valve after use |
| 7.2 Use slurry | dilutes and applies slurry correctly; stores it properly |
| 7.3 Follow toilet-waste rules | applies the rules for slurry from human waste |
Range. Cooking stoves; vegetable gardens and crops. Evidence. Observation; logbook notes.
Cooking with biogas
A person needs about 0.3 m³ of biogas a day for cooking. A household burner uses about 300–450 litres an hour; boiling 1 litre of water takes about 30–40 litres of gas. Keep the kitchen ventilated, light the match before opening the gas, and turn the valve off after use.
Slurry as fertiliser
| Nutrient | Digested slurry | Compost | Farmyard manure |
|---|---|---|---|
| Nitrogen | about 1.6% | 1.0% | 0.8% |
| Phosphorus | about 1.55% | 0.6% | 0.7% |
| Potassium | about 1.0% | 1.2% | 0.7% |
From FAO Biogas Slurry. Slurry keeps more nitrogen than drying fresh dung, and digestion kills weed seeds and plant diseases.
- Dilute at least 1:1 with water before putting it on growing crops, so the ammonia doesn't burn them.
- Use it fresh or compost it with plant material; drying it loses much of its nitrogen.
Slurry from toilet waste
Slurry from human waste is not free of germs; treat it further before use. WHO guidance: leave at least one month between applying it and harvest, don't use it on crops eaten raw, wear gloves and wash.
Ghana's plans
Ghana's Renewable Energy Master Plan aims for about 900 biogas plants by 2030 (institutional, household and agricultural), and names a lack of standards, poor maintenance and no spare parts as problems to fix. A 2011 Ghana study put a 6 m³ digester at GH¢2,200–4,000; in 2025 a biodigester toilet cost GH¢5,000–9,000 a unit. Prices change; ask local builders.
How much slurry water?
You have 20 litres of slurry for a vegetable bed. (1) How much water do you mix in? (2) If the slurry came from a toilet digester, can you use it on lettuce harvested next week? Why?
Gas and garden
- With your trainer, light the stove safely, boil 1 litre of water, and time it. Turn the gas off.
- Collect slurry, dilute 1:1, and apply it to a vegetable bed around (not on) the plants.
- Record what you did and mark the bed so you can compare growth with an untreated bed.
Compare beds
Over the next month, compare the slurry bed with an untreated bed: height, colour, leaves. Record each week.
Glossary
| Term | Meaning |
|---|---|
| Biogas | Gas made by bacteria breaking down waste without air; mostly methane |
| C:N ratio | The balance of carbon and nitrogen in the feed |
| Competent / Not Yet Competent | The two possible results of a CBT assessment |
| Confined space | A tank, pit or manhole where air can be deadly |
| Digester | The closed tank where biogas is made |
| H₂S | Hydrogen sulphide, a deadly gas in biogas |
| Retention time | How many days the feed stays inside the digester |
| Slurry | What comes out of the digester; a fertiliser |
| Water trap | A low point in the gas pipe where water collects and is drained |
Logbook
Copy these pages into an exercise book. Your logbook is part of your assessment evidence.
- Unit 1: safety walk; leak-check results
- Unit 2: labelled digester sketch; visit notes
- Unit 3: dung weights; sizing sums
- Unit 4: site checklist; build notes; leak test
- Unit 5: daily feeding and care record
- Unit 6: troubleshooting record; pH readings
- Unit 7: stove test; slurry application and bed comparison
- Every homework, dated
Self-check quiz
- What is the most important safety rule for digesters and pits?
- Why is smell not a warning for hydrogen sulphide?
- How do you check for gas leaks?
- What two things does a digester make?
- Name three digester types.
- What must never go into a digester?
- How much water do you mix with fresh dung?
- How do you work out digester volume?
- Why size the digester to the dung you have?
- Gas reaches the plant but not the stove. What do you do?
- How do you fix a sour digester?
- How do you use slurry on growing crops?
Further reading and sources
Best next reads: FAO's Biogas Technology: A Training Manual for Extension; the Biogas Digest; the Ministry of Sanitation's 2022 Bio-digester Manual (ask the ministry).
What this module does not cover: large institutional digesters, electricity from biogas, building a digester without a qualified builder, and the full Ministry of Sanitation manual (not publicly available).
Every source used in this manual is listed, with links, in the Teacher Copy.