An assessor from Syracuse in the US noticed my discussion paper on bottomless sand filters (BSFs) and was asking about filter sand specifications and maximum slope angles for such installations. Richard Mason from Sorell Council has done more work on BSFs than anyone in Tasmania and I asked him to join the on-line discussion.
it moves parallel to the slope in the apron soil.
The first two ought to dominate over the third if the system has been sized and designed properly. But the steeper the slope below the module, the faster the treated effluent moves parallel to the slope through the apron soil, and the less chance it has to evapotranspire and infiltrate. So, there is more chance of leakage from the lower, outer limit of the apron.
So, we should perhaps lengthen the flow path. But then, we are dealing with highly treated effluent so downslope leakage should not be too much of a problem. Perhaps one answer is to install a cut-off drain along the lower edge of the apron.
Read what the US EPA has to say about BSFs and intermittent sand filters.
“Site slopes are not a limitation for on-site soil units. Slope limitations are primarily for construction safety concern. Systems on steep slopes with slowly permeable soils should be long and narrow to reduce the probability of toe leakage.
I’m not sure that a downslope cut-off drain is the answer though; I tend to favour construction a low bund downslope of the land application area (there’s usually plenty of soil on a construction site to do this) in order to catch any toe leakage and protect downslope env/public health values.
Evergreen vegetation plantings on both sides of the bund will also help to mop up any excess moisture too.
Ooops. Pressed submit too soon.
Developing on the theme of “Systems on steep slopes with slowly permeable soils should be long and narrow to reduce the probability of toe leakage”, going back to AS1547.2012, this simply means using a smaller linear loading rate.