Biogas flowing within the pipeline may get saturated by water vapor that condensates in inner wall of pipes. If not regularly drained, the water can clog the pipeline and block gas flow.
If this clogging occurs, biogas flame in stove will appear yellowish in color for quite a long time until the water concentration reaches a certain point and will result in gas not being able to ignite at all. To address this issue, a water drain is installed in the lowest point of the pipeline system.

The primary function of the water drain is to collect and retain water drops and vapor in the pipeline, therefore ensuring a regular flow of gas. After some time, the drain will be full; by that time the water in the drain must be regularly pumped out.