| You might want a small hinged hatch to access a vertical extension handle to operate the flush valve and larger two-man hatches to access the vortex for major cleaning, to clear a fouled drain line, etc. If the deck is built of two-inch lumber, a hatch gets pretty heavy if its more than a couple of square feet.
Since the top of the vortex has to be slightly above the maximum pond water level, the elevation of that concrete pad for the vortex becomes very important. I don't know if you can shim a vortex up with hollow tile (concrete block) and 2x4's. Remember, regardless of how well the system is engineered, they will invent some new and vastly improved way of filtering water as soon construction is complete.
steve |