Storm Water And Septic Tanks Systems
Drainage from guttering should be kept entirely separate from waste water from the domestic plumbing. Rainwater harvesting systems are the most efficient use of this water, but just giving it sufficient run off is far preferable to allowing it to enter the septic system. Storm water flowing into the septic tank will not only affect the action of the septic tank in separating the waste, but will flush solid waste further through the tank. This often results in the filter media or soakaway from becoming clogged with solids that prevent the system from draining properly.
Although it is unlikely that your rainwater drainage mixes into the septic system, if you have moved into a new home (or better yet, before you do), have your drainage system inspected to ensure that there won't be any nasty surprises.
Septic Tank Filter Systems
In a well constructed septic system, waste water discharged from the first two holding tanks would be passed through a final filter tank before being released through a soak-away into the environment. This is typically a seperate third chamber or separate tank that is filled with clinker. The water drains through to an outlet.
If the system is not well made the filter tank can become clogged. In even well made systems this is possible, but this is far less likely as it is in poorly constructed. Generally it is silt or sediment causing problems, but if the rainwater system flows into the septic system this can be the cause of such problems. It will occasionally require attention. Unfortunately, this entails digging out the tank to either rinse out the existing filter media or replacing it with new material. Clinker was traditionally used as it was cheap and freely available, this is no longer the case and washed lime stone or similar material is fine as an alternative.
Other causes for problems in the filtration tank include the wrong sized material being used as filter media. For instance using coarse gravel or smaller grade clinker means that the system is far too likely to clog up. Another common problem is failing to use a proper filtration sheet to disperse the discharge from the settlement tanks and all of the waste water being concentrated on a single spot. The other common problem arising with this types of septic system is that there is no real access point allowing maintenance of the outlet pipe. This pipe is as susceptible to problems as other waste pipes, most commonly tree roots and with no access it is very difficult to correct without excavating the entire tank.