Nanofiltration is much like reverse osmosis, but it is different from RO. Nanofilters are normally found in a spiral-wound arrangement. Spiral-wound membranes roll up many layers of flat membrane sheets around a central pipe. Water that is filled with particulates or organic materials can clog membranes. Membranes are classified according to the size of the molecules that they are able to filter. Nanofilters have an MWCO of approximately 1000 Daltons or less. Nanofiltration is used to remove hardness, natural organic matter, and synthetic organic chemicals from water. Source water must always be treated prior to nanofiltration, so that particulates do not foul the membrane and limit its efficiency. Water with high iron, chlorine, and manganese may also require pretreatment. Even under ideal conditions, nanofiltration systems, like reverse osmosis systems, require regular membrane cleaning and periodic replacement.