Bio Digester Technology Level 3

Who it is for. People who have finished Level 2 or can show the same skills. On Ghana's framework, National Certificate I covers complex, varied work with minimal supervision, and National Certificate II covers non-routine work where you may guide others. Level 3 sits near these two; it is not a CTVET curriculum.

Bridge from Level 2: what you already know

Level 2 gave you a one-page service business plan and basic payback. Level 3 goes deeper on design and quality, then on law, accounts and growth. Read the hub's Money Skills Level 2 and Entrepreneurial Thinking Level 2 first; Units 6 and 7 build on them for a biogas trade.

How the module works

Each unit has learning outcomes, performance criteria, the range and the evidence. You are judged Competent or Not Yet Competent. Work is mostly practical, on real or mock sites.

• Methane explodes at 5–15% in air; biogas diluted 10–30% with air is also an explosion hazard.
• H2S: a smell at 0.01–0.3 ppm, eye and lung irritation at 10–150 ppm, death becoming likely at 150–750 ppm and possible in minutes above 750 ppm; at dangerous levels there is no smell warning.

Competency standard

Range. A farm and an institution. Evidence. Sizing sheets with checks; feed plan; type comparison.

Sizing in six steps

1. Weigh the feed for at least three days; do not trust the animal count.
2. Pick retention time: 40 days for dung, 60 for toilet waste (Level 1/2). Add a few spare days to protect the methane bacteria.
3. Volume = daily feed (dung + water) × retention days.
4. Gas = kg of dung × yield: cattle 0.023–0.040, pig 0.040–0.059, poultry 0.065–0.116 m³ per kg.
5. Check it: a fixed dome gives about 0.15–0.6 m³ of gas per m³ of digester per day. If your sum is higher, plan with the lower figure.
6. Gas holder: farm plants usually have a digester-to-gas-holder ratio of 3:1 to 10:1, most often about 5:1 to 6:1. Never build one digester above about 100 m³.

Digestion nearly stops at 10 °C, so size for the coldest likely temperature. Southern Ghana stays warm.

Feed plan

• Solids: fixed domes run at about 5–8% total solids for cow manure and 4–7% for pig manure; inclined tubular digesters below 8%. Thin it with water, not cleaners.
• Carbon to nitrogen about 20–30: cow dung about 24, chicken dung about 10, rice straw about 70. Blend them, and change the mix slowly because methane bacteria dislike sudden changes.
• Never in: soap, detergents, antibiotics, solvents.
• No loading rate in kilograms of volatile solids was found for Ghana feeds. As a guide, the handbook Biogas Plants in Animal Husbandry puts the upper limit for a simple farm digester at roughly 1.5 kg of volatile solids per cubic metre of digester per day. Use the retention-time method plus the gas check above, and ask a lab for advanced co-digestion.

Types compared

Use a qualified designer's drawings above about 10 m³.

Competency standard

Range. One kitchen line and one engine proposal. Evidence. Pipe plan; demand sheet; engine note.

Pipes, water and sulphur

• Half-inch or three-quarter-inch pipe is fine up to about 30 m, and ¾ inch carries about 1.5 m³ an hour. Slope the pipe at least 1% to a water trap. Biogas holds 2–8% water vapour, so water collects.
• Nepal banned thin PVC gas lines because they burst, and the main valve was the most common leak.
• Use an iron-oxide scrubber, ideally two beds; steel wool removed about 95% of H2S in a laboratory study. Sulphur can shorten engine life considerably, and air injection can remove up to 95% of it. Do not add air yourself: a gas and air mix explodes. Leave it to a designer.

Appliances

• Household burners use 150–450 L/h at 75–150 mm water column; stove efficiency is about 55%, lamps only 3%. Industrial burners use 1,000–3,000 L/h.
• Fixed-dome pressure can tear a lamp mantle.
• A converted LPG stove: widen the jet to about 3 mm, holes to 4 mm, add an air flap.

Engines

• A plant should make at least 10 m³ a day to run an engine. One m³ gives about 1.5 kWh; other pages say 1 to 1.7, so ask the engine maker.
• Dual-fuel diesel engines can replace up to 80% of diesel, but another page says pilot engines need about 2% ignition oil. Sources conflict: get the maker's figure in writing.
• Diesel engines last over 10,000 hours, petrol 3,000–4,000. Generators usually pay only above about 50 kW. A 100 L fridge uses about 2,000 L of gas a day, and commercial heaters need 30–80 mbar while biogas is often below 20 mbar. Use a licensed electrician, and see Unit 5 for the Energy Commission.

Competency standard

Range. One real or mock build. Evidence. Hold-point sheet; commissioning log; training record.

Hold points

A weak ring is a flexible joint that stops cracks in the lower part reaching the gas holder. Many Janata-type plants failed on gas-tightness. In Ghana only 44% of 50 plants worked well. Reject work and record the reason; never pass it "to keep the peace".

Commission and maintain

Fill 1:1; seed with slurry from a working plant; vent the first gas outdoors; do not fill to the top. Record dates, volumes, pressure and the owner's training. Maintain: drain the trap weekly, check flexible pipes monthly, and have scum removed and the plant pressure-tested yearly from outside. India's scheme gives a two-year installer warranty and three years of routine maintenance. After-sales service raised East African working plants to 77–94%.

Train masons

The old Ghana artisan standard covered dome construction, plumbing, steel bending and formwork. Use: show, do together, do alone, check. Each mason builds a test section (a ring or a pipe joint) that you check before real work. Keep a record of faults per mason. Nepal trained women first.

Competency standard

Range. At least three plants with a fault. Evidence. Diagnosis sheets; step plan; slurry plan.

Diagnose

Collect the feed log, gas log, pressure readings and pH. Then test:

pH close to 7 is good; sources give ranges from 6.8–7.4 to 7–8.5, with below about 6.2 and above 8.5 harmful. Lab tests of volatile acids and ammonia were not found for Ghana; use a university lab if the cause stays unclear.

Safe operation

Most accidents happen during start-up, venting, shutdown and maintenance. Only qualified people should use gas sensors to decide an area is safe, and airing out does not make it safe because some gases are heavier than air. Use spark-free tools, no flames or smoking. Run a rescue drill each term.

Slurry

• More than half of slurry nitrogen can be lost from uncovered slurry in a month; covering cuts the loss about tenfold. Work it into the soil straight after spreading. Dilute at least 1:1 for growing crops.
• Toilet slurry: leave at least a month before harvest and never on crops eaten raw. Slurry volume out is about the same as feed in, so 0.4 m³ a day of feed gives about 12 m³ a month to store (calculated).
• Selling slurry: sell covered, labelled slurry from dung digesters; keep toilet slurry off the market. No Ghana application rate per crop was found. As a guide, an FAO training manual from Nepal says field trials suggest about 10 tonnes of slurry per hectare on irrigated land and 5 tonnes per hectare in dry farming, used with the normal dose of chemical fertiliser. Ask a local extension officer to adjust this for your crop and soil.

Competency standard

Range. One farm and one institution. Evidence. Rules checklist; operating rules; carbon note.

Ghana programme: a short history

• The first demonstration plant, a 10 m³ Chinese fixed dome, was built in 1986 by the Ministry of Energy; the first large community plant in 1992.
• Early plants were mostly at hospitals and slaughterhouses; the Kumasi Abattoir plant cost €1.28 million. The EPA funded plants at senior high schools. A 2002 pilot was reported as failed.
• Over 50,000 biodigester toilets were built in Greater Accra since 2016.
• The Biogas Association of Ghana (EPA, Cleaner Production Centre, Energy Commission) aims to develop quality standards through training.
• In 2021 the Ministry said the Energy Commission had begun drafting a first biogas framework and urged industry to remove "quacks". Its current status was not verified. The Renewable Energy Master Plan targets about 900 plants by 2030.

Rules checklist

Institutions

A school, hospital or prison needs a named responsible person, written operating rules and a yearly service contract. One institutional plant failed when staff changed and knowledge was lost. Allow at most 3 L per flush, no chlorine cleaners, and post-treat effluent.

Carbon finance, honestly

Ghana's carbon framework lists biodigesters as additional, and farmers' purchases are subsidised by carbon revenue. A programme plans about 20,000 home systems for pig farmers. Standards cover household digesters, but credits have been claimed for broken or unused digesters. In Ethiopia, credits for 4,112 digesters paid US$390,000 in total, so small per plant. Keep dated records of every plant's use and service; never promise carbon income.

Competency standard

Range. An installer and service business in the Western Region. Evidence. Comparison; registration list; budget; plan.

Choose a legal form

The Energy Commission licence is open to citizens, companies and registered partnerships.

Registration step by step

Premises, tools and budget (example)

Base near farms and schools, on a road open in the rains, with room for cement and pipe.

Financing

Own savings (aim for 30%), susu or VSLA, microfinance and banks. A VSLA shows that groups work when rules are written and kept. The Ghana Enterprises Agency helps MSMEs reach credit; a 2023 grant round for 150 persons with disability was applied for online. The Assemblies' disability fund rose to 5% in 2026. Never pay a "processing fee" for a grant or loan. Call 292 if asked.

Hiring and the Labour Act

• A job of six months or more needs a written contract; give a statement of main terms within two months (s.12, s.13).
• Notice: one month for three years or more, two weeks for less, seven days for week-to-week (s.17, s.18(4)). At least 15 working days of paid leave a year (s.20); 8 hours a day and 40 a week (s.33). The employer must provide safe conditions and PPE at no cost.
• Unfair dismissal can go to the National Labour Commission (s.63, s.64).

Employee SSNIT shares conflicted across sources, so ask SSNIT. Self-employed people are urged to contribute for at least 12 months. Train apprentices with a written agreement and a fair allowance. Insurers will not cover premises without a fire certificate; get written quotes for tools, stock and liability.

Market yourself

WhatsApp Business is free with a profile and catalogue. A Google Business Profile is free; never pay anyone to "verify" you. Show photos of working plants (with permission), a price list, vehicle signage, and ask happy owners to refer you.

Accessibility

Ghana's Persons with Disability Act 2006 is reported to give employers a tax rebate for hiring persons with disabilities. Persons with disabilities in agriculture report a lack of land, money and tools. Design, quoting, records and customer care suit many people; use large-print logbooks, colour-coded valves, recorded instructions in Fante or Twi, and visual gas alarms for deaf workers.

One-page plan and pitch

Competency standard

Range. One month of records; one quote; a year's forecast. Evidence. Cash book; month-end set; costing sheet; forecast; panel presentation.

The books you keep

• Cash book: every money in and out with a receipt number. Give a numbered receipt for every payment; send an invoice with a due date for credit jobs.
• Petty cash: a fixed float topped up against slips. Stock record: cement, pipe, valves, stoves, bags in, used, left. Debtors and creditors: who owes you and whom you owe, with dates.
• Owner and business money are separate: pay yourself a salary.

Sample cash book page (example; opening cash GH¢3,000):

Check: 3,000 + 17,100 − 15,100 = 5,000. Count the real money: cash 1,500 + MoMo 900 + bank 2,600 = 5,000. If it differs, find the gap before closing the month.

Month-end routine

1. Total the cash book; reconcile cash, MoMo and bank. 2. List debtors and creditors; chase overdue payments. 3. Count stock. 4. Record depreciation. 5. Prepare the profit statement and balance sheet. 6. File receipts; note tax and SSNIT dates.

Profit statement, balance sheet, depreciation

Revenue minus cost of jobs gives gross profit; minus other costs gives net profit. A balance sheet shows assets = liabilities + equity.

Straight-line depreciation. Tools and detector GH¢12,000 (example), life 5 years, no end value: 2,400 a year, GH¢200 a month. Motorbike GH¢18,000 less GH¢3,000 resale over 5 years: GH¢250 a month.

Worked month (example):

Margin = 10,450 ÷ 42,500 = 24.6% of price. Mark-up = 10,450 ÷ 32,050 = 32.6% of cost. They are different numbers; do not mix them.

Costing and quoting a job

Bill of quantities × local prices, plus labour, transport and a share of overhead; then price for the margin you need. Price = cost ÷ (1 − margin). Example: direct costs 25,300 + overhead share 3,000 = 28,300; for a 25% margin the price is 28,300 ÷ 0.75 = GH¢37,733, quoted as GH¢37,800. A 25% mark-up would give only 35,375, a 20% margin. State what is included, the payment stages and the validity date. Prices in Ghana studies are old (a 10 m³ fixed dome with filter bed was GH¢26,200–28,000 in 2021; digesters cost US$235–446 per m³ in 2011): always get three fresh quotes.

Cash-flow forecast

A forecast has opening cash, money in, money out and closing cash. Put big rare costs in the right month and test a bad case: jobs 20% fewer, materials 10% dearer.

Tax records

The Modified Taxation Scheme began on 1 July 2025: an instalment up to GH¢20,000 turnover, turnover tax of 3% for GH¢20,001–750,000, or the modified cash basis. From 1 January 2026 the VAT threshold for goods is GH¢750,000. For services there is no turnover threshold: a business supplying taxable services must register for VAT within 30 days of starting. Ask GRA whether your service is taxable; a VAT-registered business cannot use the Modified Taxation Scheme. Show parts and installation labour separately on invoices.

Grow and sustain

• Quality system: the hold-point sheets of Unit 3, a standard quote, a standard handover and a yearly service call.
• Customer care and pricing: answer complaints the same day and record the fix; price from cost plus margin, then check the market.
• Contracts and tenders: write quantity, scope, price, payment dates, warranty and what happens if work is late or fails. Schools and hospitals may buy by tender; keep registration, tax and fire papers ready. Tender rules were not verified.
• Partnerships: a group can share a vehicle and bulk-buy materials.
• Scaling: grow when six months of records show profit and you can supervise another crew. Write down buyers, suppliers and loan papers so someone can take over.
• Ethics: never pass poor work, hide a leak, bribe an inspector or promise carbon money. A good name is your most valuable asset.
• Records for lenders: 12 months of cash book, MoMo and bank statements, registrations, forecast and quotes. Never give your Ghana Card or SIM as security to an unlicensed lender.

Capstone: present to a panel

Present your one-page plan and 12-month cash-flow forecast in 7 minutes, then take 5 minutes of questions. The panel checks your numbers (price per job, break-even, lowest cash month), your licences, your safety plan and your risk plan. Say "not known" for what you do not know; never invent a figure.

1. What do you do if someone collapses in a digester pit?
2. Name the six sizing steps in short.
3. What gas per m³ of digester per day is the fixed-dome range?
4. Why is a fixed dome unsuited to engines without a regulator?
5. Why must you never add air to your gas yourself?
6. Name two hold points in a build.
7. What is a weak ring for?
8. A digester smells sour after you doubled the feed. What do you do?
9. Which Act requires an Energy Commission licence for installing biogas facilities?
10. What is the difference between margin and mark-up?
11. Why keep the owner's money and the business's money apart?
12. Why must you never promise carbon income?

Best next reads: the Biogas Digest volume 2; the ICRC prison guide; the Energy Commission licence manual.

What this module does not cover: entering or repairing inside digesters, large industrial plants and grid sales, detailed structural design, and current fees, loan rates and insurance premiums (get written quotes). Items marked "not verified" must be confirmed with the agency named.

Every source used in this manual is listed, with links, in the Teacher Copy.