4.2 Scaling of the digester

The size of the digester - the digester volume (VD) - is determined by the length of the retention time (RT) and by the amount of fermentation slurry supplied daily (Sd). The amount of fermentation slurry consists of the feed material (e.g., cattle dung) and the mixing water.

Example:

30 l dung + 30 l water = 60 l fermentation slurry

The digester volume is calculated by the formula

VD(l) = S_d(l/day) x RT (days)

Example:

Daily supply (S_d): 60 l
Retention time (RT): 80 days
Digester volume (VD):
60 l/day x 80 days = 4800 1 (4.8 m³)

For a specific digester volume and a known amount of fermentation slurry, the actual retention time is given by the formula

RT(days) = V_D (l) -:-S_d (l/day)

Example:

Digester volume (VD): 4800 l
Daily supply (Sd): 60 l/day
Retention time (RT):
4800 l -:- 60 l/day = 80 days

If the digester size is given and a specific retention time is required, the daily amount of feed is calculated by the formula

Sd (l/day) = VD (l) . RT(days)

Example:

Digester volume (VD): 4800 l
Retention time (RT): 80 days
Daily fermentation slurry requirement (Sd):
4800 l -:- 80 days = 60 l/day

If a biogas plant is loaded not daily but at relatively long intervals, the daily supply (Sd) decreases although the fermentation slurry proportion (S) remains the same. The retention time is correspondingly prolonged.

Example:

Digester volume (VD): 4800 l
Fermentation slurry proportion (S): 60 l
1. Daily loading, i.e. S_d = S = 60 l/day:
Retention time (RT):
4800l -:- 60 l/day = 80 days
2. Loading every other day, i.e.
S_d=S 2=30Q/day:
Retention time (RT):
4800 l -:- 30 £/day = 160 days
3. Loading twice a week, i.e.
S_d = S x 2/7 = 17.2 l/day:
Retention time (RT):
4800 l -:- 17.2 l/day = 279 days