7.2 Plant operation

The operation of a simple biogas plant is relatively uncomplicated. The user must be given all the information and practical assistance he needs before and during the early phases of plant operation.

Collecting substrate

The collection of substrate is a simple matter when combined with work that has to be done anyway, e.g. cleaning the stables. It can be made even easier by arranging for the manure to flow directly into the mixing pit. Experience shows that it is not a good idea to gather dung from fields, roads, etc. or to go to the trouble of elaborately chopping up or otherwise preprocessing plant material for use as substrate. The work involved is usually underestimated, while the motivation is overestimated.

Filling the plant

Filling means: mixing the substrate with water, removing bouyant materials, allowing the fill material to warm up, flushing it into the digester, and removing sand and stones. The simple mixing pit shown in figure 5.16 can handle a daily fill quantity of up to 500 1.

Digested-slurry storage/utilization

The further processing of digested slurry is a critical point in that it can be quite toilsome (cf. chapter 3.4).

In designing the plant, care must be taken to ensure that the slurry store will be large enough. Fixed-dome plants in particular should be equipped with an overflow, so that the digested slurry does not have to be hauled away every day.

Table 7.2: Checklist for the daily operation and regular maintenance of biogas plants (Source: OEKOTOP)

Daily activities:
- fill the plant
- clean the mixing pit
- agitate the digester contents
- check the gas pressure
- check the gasholder contents
- check the appearance and odor of the digested slurry

Weekly/monthly activities:
- remove/use the digested slurry
- clean and inspect the gas appliances
- check the gas valves, fittings and appliances for leaks
- inspect the water trap

Annual activities:
- inspect the digester for scum formation and remove as necessary by opening the plant
- inspect the plant for water tightness and gas tightness
- pressure-test the gas valves, fittings and pipes
- check the gasholder for rust and repaint as necessary

Monitoring the process

If the plant is properly started before being handed over to the user, it may be assumed to be in proper working order. The user will have become familiar with what optimum plant operation involves. This is very important, because from then on he himself will have to watch for any appreciable changes in how the plant functions; the main indication of a beginning malfunction is a change in the daily gas output.