We (my husband, my daughter and I) are living in a village named Shivanandi in Rudraprayag District in Uttarakhand.  We are building a kayaking center close to the banks fo the Alaknanda River.

In the long run. we're expecting to have around 30 people occupying this place (for around 6 months of the year).  We are going to separate our gray water from the sweage.   The gray water will be purified through a wetland and reused for our ornamental garden.

Treatment of sewage will be done with a septic system.  The system will include a separation chamber from which a pipe will conduct the water into a perforated septic pit.

we are located about 15 meters above the river water level (these days... much closer in the monsoon).

I have a few questions

1.  will 15 meters of very sandy soil (i.e. river bank sand) be a large enough filter in order to prevent contamination of the river?  after 15 meters the water will reach the river bank level and will continue filtering until it leaks into the river.  The river is our life, we would not want to harm it.

2.  If the answer to 1 is positive, what size should we build?  As I understand, 48 hours of retention time is good enough in the seperation chamber.  Assuming 50 liters per person per day, we need a 3 cubic meter seperation chamber (50x30x2=3).  We have a much larger area for the chamber (around 12 cubic meters).  Is a longer retnetion time a hazard or a blessing for the system?

3.  How big a pit should we dig for the septic pit?  Should it be empty with a perforated brick wall to hold it or should we fill it with rocks?

 Thank you very much, Alice Shalabh and little Maya.