I think there are two good options for designs going forward.
The first is using an alternating system with two chambers, and the second uses one continuous tank with the finished compost collecting at the bottom and the fresh waste starting at the top and moving down.
Apart from that they both have similar features that need to be considered:
- They both need a ventilation pipe to evaporate liquids and reduce smells
- They both need a soakaway system to remove urine, and this should filter the urine well enough to remove pathogens
- Both septic tanks should be above the water table
I understand that the usual practice in rural Bangladesh is to have buried tanks made out of concrete cylinders, and that changing the design for this construction will be difficult.
However, I believe that separating the tanks from the groundwater will be an important factor in making sure the system is hygienic, and that making the tanks above ground will be worthwhile in the long run.
Let me know how you feel about each of these ideas, and if there is one that would be more appropriate to the community you are working in. I have included some images to represent each design.