[3 houses]->[2 chamber septic tank]->[settling tank]->[leach field]
I don't know if I've used the correct words. Rainwater from the houses also runs directly in the "settling tank", which is just a concrete-block lined hole in the corner of their field. The houses are uphill of the "leach field", which helps.
The "leach field" appears to consist an unknown amount of perforated 100mm pipe running underneath their field. The perforated pipe seems to be totally clogged up with something - maybe silt, maybe roots of numerous trees planted since the leach field was installed.
To stop the settling tank from overflowing, my relatives have dug a 30cm x 30cm x 20m trench in their field, which handles all the run-off from the "settling tank". Is this a really bad idea from a public health point of view?
I'd quite like to hire a mini-digger for a few days, and install a new leach field - is that a DIY job?