DUPONT FilmTec NF90-400/34i NF nanofiltration membrane element 8in 40x7.9inch area 400ft²(37m²) spacer 34-LDP flow 10000gpd(38m³/d) min salt rej 98.7% test 2000ppm MgSO4 70psi 25°C 15% rec apps municipal drinking water reuse industrial demineralization