The joining of the two ventilation ducts, the septic tank fermentation gas exhaust duct and the treatment unit ventilation duct is not permitted.
Paragraph 4 of the installation instructions on page 9 of the guide as well as the diagrams on pages 17 and 18 of the same guide (supplied with each system) is quite clear on the ventilation design. Both ventilation ducts must be separated and installed in accordance with the installation manual.
For the system in question (5EH), BIOROCK allows for the joining of two ventilation ducts in one conduit from the point where there are as few bends as possible (especially not 90° bends) and where the 110mm diameter section is homogenous. It is the installer's responsibility to check the effectiveness of the ventilation system upon activation of the sewage treatment system. The air flow can be improved by installing a wind extractor instead of a static extractor.
Electrical ventilation is only necessary in the event of problems in ensuring effective air flow (lingering odours, unsatisfactory treatment efficiency).
Nine times out of ten, the presence of a foul odour indicates a lack of ventilation or a poor exhaust duct layout.
Talk to your local product product specialist.