The Downspout...

was designed in such a way that it would block potential contaminants form entering the storage tank. This was accomplished by incorporating a debris screen and a pre-wash system. The pre-wash system is designed to utilize the initial few gallons of rainwater to wash the roof of any potential contaminants that may settle from the air between rainstorms.

The debris screen was formed from a typical downspout scupper -a fitting that passes through the gutter and into the downspout. It was then covered with wire mesh and sealed into the gutter with roofing tar.
   
The entire downspout was constructed from 2" PVC piping. Pipes were connected using 45° elbows, 90° elbows, and T's, and then sealed with PVC pipe cement.
Above: This T junction is where rainwater is diverted to the storage tank. The diversion happens after the initial rainwater from the gutter passes through the downspout and is backed-up at the ball valve below. (the pre-wash) Left: Diverted rainwater heads into the tank through the intake. When the tank is filled, excess water is sent back to the downspout via the overflow.
Right: The ball valve, when half closed, allows for the capture and release of the initial rainwater. As rainwater enters the system it is backed-up at the ball valve, allowing for the pre-wash. As water backs-up to the T junction above, it is diverted to the tank. At the rainstorms end, leftover water in the downspout slowly escapes through the ball valve. Since the ball valve is a spot for potential clogging, a clean-out valve was installed above it.
As the tank reaches its capacity, water is sent through the overflow and back to the downspout. It joins the downspout just below the ball valve, where both the overflow water and the escaping pre-wash water are sent away from the building.  

To Recap:

Rainwater passes through the debris screen, which blocks large debris from entering the system. It flows through the downspout and is drastically slowed down at the ball valve. The water then backs up, resulting in a pre-wash of the roof, and is diverted into the tank. As the tank fills, excess water is returned to the downspout via the overflow. This water connects to the downspout below the ball valve, where both the escaping pre-wash water and the overflow water are sent away from the building.

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tank or deck