Two scenarios are possible: 1- If you have just acquired your Berkey water purifier and the filters are new, the problem is usually due to high water tension. This prevents air from being purged from the micropores of your Black Berkey water filters. Your filtration elements must be purged of this air to allow water to pass through them. A beige priming washer/blue valve and priming instructions are provided with the Black Berkey elements. Remove your filters, prime them, then reinstall them in your Berkey. This should resolve the slow flow rate issue. 2- If the flow rate slows considerably during the use of your Berkey system, the problem can usually be resolved by cleaning the elements. Turbidity and clogging of the micropores by sediment are usually the cause of this issue. Just remove the elements from your system and scrub the exterior of each element with the abrasive side of a ScotchBrite sponge. Scrub a section of the element until you see some black on the sponge, then move to another section until you have scrubbed around each element. This process should only take a few minutes and should only be done under running water. Re-prime each element, then reinstall them in tank. Your flow rate should be back to normal.
CAUTION: Never use soap, vinegar, detergent, or any other cleaning product.