Rainwater Collection

Rainwater collection, also referred to as rainwater harvesting, involves the gathering and storage of rain into one or more tanks or cisterns for later reuse.  Some common uses are for landscape irrigation, the flushing of toilets and as a coolant.  Secondary uses may include water for washing laundry and, with additional water treatment, bathing.

There are a few key benefits to rainwater collection.  The first is a significant reduction in rainwater run-off, which causes increased rates of soil erosion and flooding.  The second benefit is a reduction in the need for tap water and thus lower water utility bills.  The third benefit is the saving of energy in avoiding extra water treatment and long-distance pumping of the water being used.  In a climate like ours, where is doesn't rain for more than half the year, rainwater collection is especially valuable for use during the drier months and in drought conditions.

For our residential project, we plan to have the roof drainage be channeled into a large tank for irrigation and toilet flushing.  Because we would like to have sufficient water storage to last us during the dry months, we will likely need an especially large tank.  We also expect to need an initial filtration mechanism to prevent debris from entering the tank, a pump and an outflow filter to prevent sediment from being pumped out with the water.  In rainier parts of the country, a smaller tank can be used to achieve the same ends.  Rainwater collection on an even smaller scale can also be done by hooking up existing roof drains to rainwater harvesting barrels with a valved outlet and an overflow outlet.  Once the barrels have a usable amount of water, one can then simply hook a hose to the valved outlet to get water for low pressure irrigation.

The variations of rainwater collection systems involve the type of drainage system, the type of inflow filter, the storage capacity, the type of outflow filter and the type of pump to be used.  Drainage systems can either be standard gravity drainage or siphon drainage.  The difference has to do with the use of air baffles to create negative pressure in the drains, allowing for more effective horizontal movement of water and narrower pipes to achieve the same throughput.  Inflow filters can be either self-cleaning or require regular clearing of debris.  The storage capacity can range significantly but should generally reflect a combination of the surface area of the roof and/or other collection areas with the amount of rainfall expected and the needed duration of storage.  For rough calculations, 1000 square feet of roof yield over 600 gallons for every inch of rain.  Outflow filters can be fixed or floating, the latter being intended to most effectively reduce the transfer of sediment without cleaning.  Pumps can vary significantly in terms of type, power, energy usage and noise generated.  So, it's best to first determine the desired range of pump output and work backwards from that point to determine the best match for your particular application.