There are various strategies for keeping the rainwater in your tanks clean, including filters, “roof washers” and so forth.  No matter what though, you find that the water collects debris over time — especially when it’s stored for a  while.  Of course, it will be cleaned by sediment, carbon and ultraviolet filters before use, but the dirtier the water in the storage tank, the more often those filters must be cleaned or replaced.  It sure would be nice to keep the water in the tanks relatively clean.

Borrowing an idea from the swimming pool industry, why not circulate and filter the water from time-to-time?  This seemed pretty easy to do, and I found a canister-style pool filter on eBay that seemed just right.  With the addition of an ultraviolet filter intended for pond service and a small pump used to circulate water in a hot tub, all the elements were present.  How to hook it up?  I knew I would be adding a second tank to my north tank farm, and I also wanted to be able to transfer water from one tank to the other.  Finally, and most importantly, I wanted both tanks to fill in parallel during heavy rain so that my collection capacity was increased.  Waste not, want not!

A scheme involving several valves and pipes evolved.  Five distinct “modes” of operation were identified: “rainwater fill”, ”use old tank”, “use new tank”, “transfer from old to new”, and “transfer from new to old”.  By clicking on the  graphic to the right, a schematic for each of the modes can be seen.  The white disks with an “x” represent closed valves.

In “rainwater fill”, water coming from the roof through a 4-inch pipe fills one tank while two 2-inch pipes connect both tanks together.  As the level in one tank rises, it forces water into the other and they fill together.  “Use old tank” supplies water from just one tank to the house and landscape water system.  “Use new tank” does the same from the other.

“Transfer old to new” and its opposite provides the filtering function.  In each case, water from one tank is forced through a cartridge filter and ultraviolet system by a small pump before flowing into the other tank.  Not only does this clean the water, it allows one to drain a tank for service.  It is also possible to simply circulate water through the filter system and back into the same tank.

The picture at left shows the system as it is today — not quite complete.  The three valves (red handles, two large and one small) will ultimately be connected between the two tanks and the filter system.  The 4-inch pipe coming from the roof is also visible.

When complete, the system will provide a lot of flexibility for capturing, storing, cleaning, and using rainwater.