Hydraulic retention time (HRT) is the digester volume divided by the daily feed, so a 20-40 day mesophilic design at 30-40°C takes in 2.5-5% of its volume a day, not the 5-8% the parent also states. Warming each day’s feed from 10°C to 35°C costs about 11-35% of a small digester’s gas, which is why cold-climate digesters are insulated or built partly underground.
Part of Biogas Digesters
Retention time is how long feed stays in the digester; temperature sets how fast the microbes work, and together they decide the digester’s size and heat budget.
The Parent’s Three Temperature Bands
| Band | Temperature | Retention | Character |
|---|---|---|---|
| Psychrophilic | below 20°C | 60-90 days | very slow; little gas in cold climates |
| Mesophilic | 30-40°C | 20-40 days | robust; the target for post-collapse use |
| Thermophilic | 50-60°C | 10-15 days | fastest but fragile; needs precise heat |
The parent says to insulate heavily or place the digester partly underground where the ground stays above 10°C all year.
Retention Time Is A Ratio
HRT (days) = digester volume ÷ daily slurry added. A volume of V litres fed Q litres a day has HRT = V ÷ Q, so the daily fraction replaced is Q ÷ V = 1 ÷ HRT.
- HRT 20 days: 1 ÷ 20 = 5% of the volume a day.
- HRT 40 days: 1 ÷ 40 = 2.5%. So a 20-40 day design replaces 2.5-5% of its volume a day.
- The parent’s “daily loading should be 5-8% of the total digester volume” is HRT 1 ÷ 0.05 = 20 days down to 1 ÷ 0.08 = 12.5 days. That is shorter than the 20-40 days it also recommends. Use the HRT figure for design and treat the 5-8% as the upper end only. (The sizing consequences are on Sizing the Digester.)
The microbes that make methane grow slowly. If retention is shorter than they take to multiply, they are washed out faster than they reproduce and the gas collapses (recalled, not a corpus figure). That is the reason for the lower limit, not just completeness of digestion.
The Heat Bill
A digester has to be kept warm. The simplest part of the cost is warming the incoming slurry. From the parent’s table, 2 cows give 20-30 kg of manure a day and, mixed with equal water, 40-60 litres of slurry (assuming 1 kg is about 1 litre). Water takes 4.18 kJ per kg per K:
- Warm from 10°C to 35°C, a rise of 25 K: 40 L × 4.18 × 25 = 4.2 MJ and 60 L × 4.18 × 25 = 6.3 MJ a day.
- The gas from the same digester: 1.0-1.5 m³ a day (parent) at 18-25 MJ/m³ is 18-37.5 MJ.
- Share: 4.2 ÷ 37.5 = 11% at best and 6.3 ÷ 18 = 35% at worst, so the feed alone takes 11-35% of the gas.
That is only the feed. Heat lost through the walls, roof and any exposed pipe is on top and is not computed here. In a cold climate a digester that is not insulated may use more heat than it makes; a digester that uses its own gas for heating is no gain at all. For thermophilic operation (55°C), warming the same feed from 10°C is 45 K, 1.8 times the 25 K case.
What You Need
- A thermometer reading the slurry (a dial thermometer in a sealed well, or a probe in the outlet)
- Insulation: straw, earth cover, or a half-buried pit; see Insulation Materials Ranked and Natural Insulation Materials
- A log: daily temperature, feed, gas
Method
Step 1: Choose HRT from the climate: 20-40 days at 30-40°C, longer if colder.
Step 2: Compute the daily feed: volume ÷ HRT.
Step 3: Insulate and bury as far as site allows; shade a plastic digester in summer and insulate in winter.
Step 4: Warm feed with solar-heated water or by mixing with warm water, which also dilutes it (Batch Water Heaters).
Step 5: Log temperature; a swing of several degrees in a day harms the methanogens.
Common Mistakes
- Using the 5-8% daily loading for a 30-day design. It is a 12.5-20 day design.
- Feeding cold manure straight in during winter.
- Heating with a naked flame under the digester.
Heating a gas vessel
Never heat a digester with an open fire or a stove next to it: it holds flammable gas and leaks. Warm the feed in a separate vessel away from the digester. See Gas Hazards (Methane, CO) and Anaerobic Digestion Science for the gas dangers.
What’s Next
- Learn next: Sizing the Digester
- Combine with: Microbiology of Methane Production