In most cases, the water entering a reverse osmosis (RO) system is filtered to remove larger particles and prevent premature fouling of the reverse osmosis membranes. The quality of the incoming water determines how much filtration is needed. The diagrams above show water systems for treating raw water, including the filters for removing larger particles (highlighted in the diagram). Systems using municipal water will need fewer of the upstream components.
Particle filters (housings marked “1”) are located after a multi-media filter (sand filter), which is used in systems drawing water directly from a surface source or well. In most cases, water from municipalities does not require the sand filter. However, if used, the sand filter could release small particles that should be removed before they reach downstream components and clog them or otherwise interfere with their operation.
A carbon fines trap (housing “3”) captures fine grains of activated carbon after the activated carbon filter. Activated carbon removes chlorine, chloramine, and other dissolved organic materials from the water supply that can damage RO membranes. However, the carbon granules can break down over time and release small particles into the water that might interfere with downstream processes.