Fluid dynamics/Air pressure physics question. Poop related.

[Zephyr]

I don't know what kind of septic tanks you have over there, but my parents had the same problem few years ago.

Since their house was built a while ago, the septic tank is basically a tank with a pipe coming out and travelling some distance beneath the ground (you can't build septic tanks like these anymore here, they tightened the environmental laws since then). Anyhow, the idea is simple, the poo sinks to the bottom of the tank and just water goes into the pipe. The pipe is full of small holes from through which the water gets absorbed into the soil.

The problem is that there is always some light particles in the water that during the years clog the small holes in the pipe. Usually they first clog the holes on the bottom of the pipe and then gradually all the holes higher up. After a while, the excess water doesn't get absorbed to the soil anymore, and the tank starts filling up and eventually overflows.

So the solution for the problem was to change the pipe. That means digging up the old one and putting in new one. We also put some rubble around the new pipe so that the soil isn't directly connected to the pipe. Hopefully this will make it last longer until the next time. From what I understand, you need to change the pipe in these roughly every 10-15 years or so.

Yes, they were called leech fields. No most places dont allow them anymore. Thankfully they definitely dont allow them when you have a house on the lake anymore.
(It was disgusting in Maine when we would get a heavy rain and poopy water was streaming from the leech field into the lake)

Still perfectly acceptable in most parts of Colorado.  I just designed a system for some multi-million dollar "cabins" up in the mountains.  As long as you have good soils and are conservative with your calculations for their influence area, they are fairly safe.  Plus, most smaller towns and rural areas do not have sewer treatment systems and this is the cheaper option.  Although with all this ARRA money floating around, we have 5 new plants being designed for some smaller communities.  Unfortunately, the design process has been reduced to one month in order to meet the funding deadline.

[nurtsi]

Still perfectly acceptable in most parts of Colorado.  I just designed a system for some multi-million dollar "cabins" up in the mountains.  As long as you have good soils and are conservative with your calculations for their influence area, they are fairly safe.  Plus, most smaller towns and rural areas do not have sewer treatment systems and this is the cheaper option.  Although with all this ARRA money floating around, we have 5 new plants being designed for some smaller communities.  Unfortunately, the design process has been reduced to one month in order to meet the funding deadline.

My parents live on top of a hill in a rural area where there is no public sewer treatment. The tank is like 10 metres from the house with the pipe going under the lawn away form it. There's never been any problems with smell or the ground getting soft. I can see the problems if it was built near a body of water though.