Filtrate: Filtrate is the water that has passed through the membrane. , i.e., the water passing through the membrane, per unit area of membrane: Equation 1. Where: J = flux, L/hr/m 2 (gal/d/ft 2) or Lmh (gfd) Qp = filtrate flow rate through membrane, L/hr (gal/d) Am = surface area of membrane, m 2 (ft 2) Note that the flux unit of L/hr/m 2 is