  We propose a simple model of landfill and study a minimal time control
problem where the re-circulation leachate is the manipulated variable. We
propose a scheme to construct the optimal strategy by dividing the state space
into three subsets E0, Z1 and the complementary. On E0 and Z1, the optimal
control is constant until reaching target, while it can exhibit a singular arc
outside these two subsets. Moreover, the singular arc could have a barrier. In
this case, we prove the existence of a switching curve that passes through a
point of prior saturation under the assumption that the set E0 intersects the
singular arc. Numerical computations allow then to determine the switching
curve and depict the optimal synthesis.
