How to Size a Biodigester for Your Farm
Step-by-step method to size a farm biodigester: feedstock, volatile solids, biogas yield and reactor volume, with a worked example and a sizing table.
To size a biodigester for a farm you follow a four-step chain: (1) quantify the feedstock available per day (manure, slurry, wash water), (2) calculate the volatile solids (VS) that feedstock provides, (3) estimate the biogas yield per kg of VS based on the manure type, and (4) set the reactor volume from the hydraulic retention time (HRT) and check the organic loading rate. These four numbers give you the daily biogas, the available energy and the digester size. Below we work through it with a solved example for a pig farm.
Step 1: quantify the feedstock
It all starts by measuring how much manure or effluent the farm generates per day. The animal headcount alone is not enough: you must estimate the collectable fraction (what actually reaches the channels or pit), because part stays in the field or in uncollected pens.
- Pigs: on the order of 4 to 7 kg of slurry per head per day in fattening.
- Dairy cattle: 30 to 55 kg of manure per cow per day, but only the milking and holding-area fraction is collected; add the wash water.
- Poultry: 0.10 to 0.15 kg of litter/manure per bird per day, with high solids content.
The output of this step is a daily flow (m³/day or kg/day) and its moisture content.
Step 2: from feedstock to volatile solids (VS)
Biogas is not produced from the total mass but from the degradable organic matter, measured as volatile solids (VS):
- Total solids (TS): pig slurry is around 5–8% TS; cattle manure 8–14%; poultry manure is much drier.
- Volatile solids: typically 75–85% of TS is volatile (organic matter). The rest is inert material.
Calculation: VS/day = flow × %TS × %VS. This is the real feed to the digester.
Step 3: biogas yield per kg of VS
Each manure type has a different biogas potential. Typical ranges are:
| Feedstock | Biogas per kg VS | % methane (CH₄) |
|---|---|---|
| Pig slurry | 0.30 – 0.50 m³/kg VS | 60 – 65% |
| Cattle manure | 0.20 – 0.30 m³/kg VS | 55 – 60% |
| Poultry manure | 0.30 – 0.45 m³/kg VS | 60% |
| Crop residue / co-substrate | 0.40 – 0.60 m³/kg VS | 50 – 55% |
Multiplying VS/day × yield gives the daily biogas. As an energy reference, 1 m³ of methane ≈ 10 kWh (calorific value), and a genset converts about 38–40% into electricity and a similar share into recoverable heat.
Step 4: reactor volume and HRT
Digester volume is set by the hydraulic retention time (HRT): how many days the feedstock stays inside the reactor. In mesophilic digestion (≈35–38 °C) with animal manure, typical HRT is 20 to 30 days.
Working reactor volume = daily flow × HRT
Then you check the organic loading rate (OLR) = daily VS ÷ reactor volume. For manure, keep it between 1 and 4 kg VS/m³·day. If the OLR is too high, increase volume or dilute; if it is too low, the digester is oversized.
Worked example: a 1,000-head pig farm
- Feedstock: 1,000 pigs × 5 kg slurry/day = 5,000 kg/day ≈ 5 m³/day.
- Volatile solids: 5,000 kg × 6% TS × 80% VS = 240 kg VS/day.
- Biogas: 240 kg VS × 0.35 m³/kg = 84 m³ biogas/day; at 60% CH₄ ≈ 50 m³ CH₄/day ≈ 500 kWh thermal/day (~190 kWh electric/day in a genset).
- Reactor: with a 25-day HRT → 5 m³/day × 25 = 125 m³ working volume. OLR = 240 ÷ 125 = 1.9 kg VS/m³·day (within the healthy range).
On top of the working volume you add the gas headspace and the gas holder to store the biogas. To understand how that gas is stored, see how a double-membrane gas holder works.
Sizing table by scenario
| Scenario | Flow | VS/day | Biogas/day | HRT | Reactor volume |
|---|---|---|---|---|---|
| 1,000 fattening pigs | 5 m³/day | 240 kg | ~84 m³ | 25 d | ~125 m³ |
| 2,500 fattening pigs | 12.5 m³/day | 600 kg | ~210 m³ | 25 d | ~310 m³ |
| 200 dairy cows + wash water | ~10 m³/day | ~400 kg | ~100 m³ | 28 d | ~280 m³ |
These values are indicative; the final design requires characterizing the real feedstock (TS, VS, COD) and accounting for temperature, mixing and digester heating.
Factors that adjust the result
- Temperature: thermophilic digestion (≈55 °C) allows shorter HRT but is more sensitive; most farms use mesophilic.
- Co-digestion: adding crop residue, whey or fats raises biogas per m³ of reactor, but requires controlling the mix.
- Wash water: dilutes the feedstock and lowers %TS; subtract it when computing VS and add it to the flow for HRT.
- Inhibitors: excess ammonia (poultry, concentrated slurry) or disinfectants can slow the biology.
Sizing correctly avoids the most expensive mistake: an undersized digester gets overloaded and loses yield, while an oversized one ties up capital. To place this calculation within the whole system, review what an industrial biodigester is and how it works, and if your operation is a dairy, the specific case of a biodigester for dairy farms.
At Tech Tank we design the biogas and biodigester solution turnkey, from the feedstock balance to the bolted steel reactor, the gas holder and energy valorization. If you want a preliminary sizing for your farm, send us your animal inventory and effluent flow and we will return a first-pass calculation.
Frequently Asked Questions
What data do I need to start sizing?
The starting point is the collectable feedstock per day: how many animals, how much manure or slurry actually reaches the system, and how much wash water is added. From that you estimate total and volatile solids, which are the true feed to the biodigester.
What is hydraulic retention time (HRT)?
It is the number of days the feedstock stays inside the reactor before leaving as digestate. In mesophilic digestion with animal manure it is usually 20 to 30 days. The working reactor volume equals the daily flow multiplied by the HRT.
How much biogas does a 1,000-head pig farm produce?
With typical slurry, on the order of 80 to 90 m³ of biogas per day, equal to about 500 kWh thermal or close to 190 kWh electric per day in a genset. The exact figure depends on the volatile-solids content and the real yield of the feedstock.
What organic loading rate is appropriate?
For animal manure, keep the organic loading between 1 and 4 kg of volatile solids per m³ of reactor per day. Above that range the digester gets overloaded and loses yield; well below it means the digester is oversized.
Is co-digestion with other waste worth it?
Yes, adding co-substrates such as crop residue, whey or fats increases biogas per m³ of reactor and improves the economics. It requires controlling the mix, the carbon/nitrogen ratio and avoiding inhibitors, so the design should leave margin in mixing and capacity.
Do I need an environmental permit in Paraguay?
A farm biodigester normally requires an Environmental Impact Assessment under Law 294/93 and a MADES license, plus consideration of Law 3239/07 on Water Resources if there is discharge. We recommend verifying current regulations and the license with MADES before building.