The gas removed from biogas digestion or fermentation units can be heavily contaminated with foam particles, various particulate matter and water vapour. A pre-filtration of the gas (often adequate enough) can be achieved with the use of an ennox gravel filter. The gas flows through the ennox gravel filter layer where large particulate matter and water vapour is left behind. The water collects at the bottom of the filter where it can be drained with a siphon assembly. Following heavy contamination of the gravel filter the spraying unit can be used for rinsing, avoiding the need for opening the digester unit. As an option a gravel removal opening can be installed.