The diverter hose with pre-filter assembly houses the sediment pre-filter for the AQ-4000 countertop water filter systems. This hose replaces the standard diverter hose that comes with the countertop unit, and filters out higher than average levels of dirt and sediment particles, reducing the amount of sediment that enters the main filter unit, and therefore reduces the risk of clogging. How do I know if I need a pre-filter? If your water flow slows down significantly or stops completely before your filter cartridges are six months old, you likely have a sediment problem. You should be getting.4 gallons per minute from a correctly flowing filter set, and if your filters flow significantly less than that, we recommend you use a pre-filter. What cities typically have high sediment levels? Several places in North America have water with higher-than-average levels of sediment, for which we recommend you purchase a pre-filter: New York, NY; Portland, OR; Seattle, WA; and Toronto, Canada.For areas with heavy sediment, switch to a Diverter Hose with Pre-Filter for the Claryum® Countertop water filter.