On Wed, 22 Apr 2020 10:17:48 +0100, NY wrote:

"Commander Kinsey" wrote in message
newsp.0jfq8biwwdg98l@glass...
I suppose we use what you would call a two pipe system. There is talk
here of introducing a new third pipe for greywater. That is, water that
comes from baths, sinks, showers, etc. as opposed to sewerage from
toilets. Difficult to retrofit of course but new estates are likely to
be plumbed this way in the near future.

My parents have a holiday cottage in a tiny village that has no mains
drainage. All the houses have their own septic tank (two-chamber sewage
treatment unit that does more than just store the sewage, as happens with a
cesspit). However to reduce the amount of water that goes into the septic
tank, all the grey water is piped to a communal "land drain" that discharges
into a nearby stream. I'm sure that arrangement contravenes almost every
health and safety and environmental law known to man!

Every time I've walked near a septic tank it has stunk to high heaven. They're a disgusting invention.

Do many houses have a rainwater drain in the street?

I thought all streets had those - where do you think the rainwater goes that flows off the road?

I thought the usual arrangement was for there to be a soakaway under the lawn for rainwater,

Soakaways don't work, not in Scotland anyway. The land is already waterlogged, you can't shove more into it.

so the rainwater and grey/sewage water never mixed.

How would a soakaway prevent that? Every house I know of (apart form old ones where it's all one pipe) have to pipes, one goes into the rainwater drains and meets up with water flowing off the road, and the other gets treated. Rain into the rivers, grey and brown into the sewage treatment plant. All a soakaway would do is to put less into the rainwater system which goes straight into a river anyway.

As far as I know, our house
doesn't put rainwater into the septic tank, but pipes it into the same
soakaway under the lawn that is used for the outflow of treated sewage water
from the septic tank.

You must have a huge lawn. I couldn't soak that much water away. The lawn is already full of rainwater that landed directly on it, you can't add more to that.