Bouzidi half-way bounce back

Hi All
I’m trying to use the Bouzidi half-way bounce back to solve diffusion inside a circular domain, but I’m confused about which
grid points to use for the BB scheme. Are they the last (outer) layer of my domain or the first layer outside my domain.
as far as I understood from Bouzidi’s paper, the BB is applied to the last fluid node and the boundary will be some where between this node and the following solid one, but the results doesn’t look reasonable and the is not mass conservative.

thanks for your help