Here is a good example of a drywell type system. These systems were usually constructed using a concrete treatment tank with concrete blocks making up the effluent disposal area. This particular drywell was made up of cinder blocks positioned around natural boulders, it had a concrete cover and was still in use for the backwash of the water treatment system. These old drywell systems should be inspected semiannually as they tend to deteriorate over time and can become a safety hazard.
