Can the Septic Tank accept rainwater?

Why rainwater must not enter the septic tank.

Septic tank systems and Sewage Treatment Plants cannot accept ANY rainwater, either from downpipes or surface drains.

This is because the septic tank is sized to deal with the volume of effluent produced by a certain number of people.  This sizing ensures that the sewage remains in the septic tank long enough for full settlement to occur.  If rainwater is allowed to enter the tank, then the flow rate through the septic tank increases and adequate settlement does not occur.

In the case of sewage treatment plants, if rainwater is allowed to enter the system then the sewage does not remain in the plant long enough for digestion to take place and the sewage leaves the plant untreated.

Septic tank  and sewage systems must ONLY be connected to grey and black water drains, i.e. sinks, baths, washing machines, dishwashers, basins, showers and toilets. Consider installing a rainwater harvesting system if you don't have enough land for surface water and foul soakaways.

Soakaways are expensive - design your septic tank system correctly.