Pending technical issues
Several tests have been made by CAMARTEC. The results of these tests have not yet been put into practice, but might already be of interest to the reader.
Position of the Outlet
As reported above, the slurry is found in layers of different TS-content. Until now, the outlet pipe has been placed near the bottom, because it is known that digested slurry is heavier than the fresh substrate. At the same time, liquid slurry which hardly produces any gas remains for a long time in the plant. On the other hand, active sludge is driven out from the plant relatively early. Hence, the retention time of the viscous slurry is reduced. When the outlet pipe is placed higher up in the liquid zone, unproductive liquid overflows and active sludge remains longer in the plant. CAMARTEC has not yet decided if the volume of the plant could be reduced when the outlet pipe will be higher or if two alternative outlet pipes would be provided. For the time being, the present models remain valid.
Standpipe for Gas Release in Case of Scum
In one case, where pigs have been fed with husks, a heavy layer of scum had been developed. Most of the gas escaped through the inlet pipe because it could not penetrate the scum. It has now been proposed to place a perforated plastic pipe vertical in the centre of the digester, to allow the gas to pass the scum. It might still be necessary to take out the scum from time to time. For easier opening and closing of the lid, a rubber sealing and wedges of steel would be advisable.
Pre-Heater for Water
In institutions, or when there is a surplus of gas, pre-heating of water might be a reasonable way of utilizing the gas. The gas burns continuously and heats the water to 30-400C. This water is used for washing or for boiling. A long-lasting and reliable apparatus has not yet been developed.
Slurry Lifting Device (Pressure Booster)
In case of an insufficient natural slope, the slurry could be lifted up with the help of water pressure from a main. Slurry flows in a pressure tank which is connected to a water pipe with sufficient pressure. The slurry inlet valve will be closed when the tank is filled. Then the water cock will be opened. The hydraulic pressure transports the slurry to a higher level from where it flows by gravity to the field. When the slurry becomes watery, the water cock will be closed and water from the pressure tank will be drained off. Then the slurry valve is opened again for re-filling. The system is valid when water is supplied free or at a low price from the main. The volume of the pressure tank depends on the intervals of distributing the slurry.
Fig.42: Stand-pipe for gas release
In case of heavy scum formation, a perforated 4"-plastic pipe ( 1 ) is placed in the centre of the digester to allow the gas to pass through the scum. Thus, removing of scum will be only necessary in longer intervals. It is held at the bottom in a concrete foot (2) and fixed at the top with a timber cross (3). The pipe can be inserted, when necessary, even after the plant is in operation (4).
Gas Pressure Equalizer
When there are several biogas plants scattered. over the compound of an institution or a bigger farm, the pressure will be different because of the friction losses and different gasplant designs. A standardized instrument is to be developed which can be installed in such places.