Friday, November 19, 2010

Rainwater Harvesting Cistern

So I finished installing the PEX plumbing, and started on a different water supply:

The decision to try a rainwater harvesting system came about for two reasons. One: Solar power and and a 2500+ watt well pump don't exactly play well together. Two: The cost of a water treatment system required to soften, remove iron, and remove sulfur is ridiculous. Plus, this alternative stuff is addicting :) The over simplified principle is to catch the water, remove large debris, and transfer it to the tank.

The more complex version requires a first flush device, a completely dark tank, a way to control sediment in the tank, a way to pump the "best" water from the tank, a filter system, and a purification system if the water is used as drinking water.

A "first flush" is an automated way avoid the first 10 or 20 gallons from the roof. It allows the dirt, leaves, bugs, bird shit, etc.. to get washed off the roof.

A rough filter in/on the gutters keeps leaves and/or sticks out of the water. (I'm actually pre-filtering the water down to 50 microns before it even hits the tank)

The tank, in my case, is a modified precast "pump house"(basically a septic tank) sealed with Thoroseal, a potable water safe concrete sealer. The rainwater enters the tank through a "calming inlet". This stops any rush of incoming fresh water from stirring up any sediment they may have made it into the tank. The complete lack of light in the makes impossible for any algae to grow.

The pump hose pickup is designed to pull water from about 8 inches below the surface of the water. This way it is above any sediment on the bottom, and below any surface debris floating on the surface. There is also a float switch to make sure the pump stops when the water level is too low.

The water is pumped into a pressure tank. From there it goes through a series of sediment and charcoal filters. In my case I use a 25 micron sediment filter into a 10 micron, and then a 1 micron charcoal filter. At this point water can be used for just about everything. For drinking water I plan to install a point of use UV purifier in the kitchen.

So after that long winded explanation: I'll have better water, and I only need 100 watt pump. Oh, and 1" of rain will produce about 600 gallons of water per 1000 sq. ft. of roof surface area. So with my 2000 sq. ft. of roof surface, and living in central new york, I should be able to completely cycle the cistern every couple weeks... on average.( almost 50,000 gallons a year from one roof!)