A continuous-feed digester is fed daily, discharges digestate as it goes and gives steady gas, which is why it is preferred; a batch digester is loaded once, sealed, and digests for 30-60 days with gas that peaks and then fades. Several batch digesters filled at staggered intervals approximate continuous supply, for example three digesters filled 15 days apart on a 45-day cycle, but each unloading means opening a vessel that holds hydrogen sulfide.
Part of Biogas Digesters
Batch and continuous feeding are the two ways of loading a digester: all at once and sealed, or a little every day.
The Parent’s Description
- Continuous flow: fed daily with fresh material, digestate leaves continuously, steady gas production; preferred for most applications.
- Batch: loaded once, sealed, left to digest for 30-60 days; simple, but gas peaks and then declines; staggered batch digesters give continuous supply.
Staggering: Number Of Digesters
If a batch needs T days and you fill one digester every Δ days, you need n = T ÷ Δ digesters. Each holds Δ days of feed. Total volume equals that of a continuous plant with retention time T: n × (Δ × Q) = T × Q, where Q is the daily slurry.
Example: T = 45 days, Δ = 15 days, so n = 3 digesters, each holding 15 days of feed. With 100 L of slurry a day (an assumed figure), each digester is 1,500 L, 4,500 L in all, the same 4.5 m³ that a continuous plant with a 45-day retention would need. A batch plant gives gas as each digester passes its peak, so the three overlap to smooth the supply. A single batch gives gas with a rise and a fall, and the stock for cooking is uneven.
What Suits Which
| Continuous | Batch | |
|---|---|---|
| Feed | slurry daily | whole load at once |
| Gas | steady | peak then decline |
| Labour | daily, small | heavy at loading and unloading |
| Material | slurry, dilute | can take bulkier, drier material such as straw (recalled; not in the corpus) |
| Hazard | pits and tanks | opening the vessel at the end of a batch |
Batch loading needs a starter of active culture, for example a share of the previous batch or fresh cow manure (Microbiology of Methane Production).
What You Need
- For continuous: a daily routine, a mixing pit and a feeding schedule
- For batch: several digesters, a sealed lid on each, a way to unload them, and a plan for the digestate (Digestate as Fertilizer)
Method: Running A Staggered Batch Set
Step 1: Fill digester 1 and seal it; log the date.
Step 2: Fill digester 2 after Δ days, and digester 3 after another Δ.
Step 3: At the end of T days, vent and unload digester 1 with the gas valve closed, outdoors and with the lid opened from a distance; refill it.
Step 4: Repeat.
Common Mistakes
- Treating a batch like a continuous plant and expecting steady gas.
- Unloading in a closed shed.
- Skipping the inoculum, so a batch never starts.
Opening a finished batch
A sealed batch digester at the end of its run holds hydrogen sulfide, carbon dioxide and methane in the headspace and in the slurry. Open it outdoors, from the side, with the gas line closed, and let it air; never put your head over the opening, never enter, and keep flames away (Anaerobic Digestion Science).
What’s Next
- Learn next: Construction
- Combine with: Sizing the Digester