Using Stored Water
While bare-bones systems may draw irrigation water through a gravity-fed hose, most installations require a pump to pressurize the stored water. For nonpotable systems, we ordinarily use a centrifugal pump, sized to meet the flow requirements of the irrigation lines. The pump only runs when the irrigation is on, and can be activated manually or wired to the start circuit of the irrigation controller. A small expansion tank can be installed after the pump to reduce short-cycling.
A potable system must supply pressurized water for immediate use. The conventional way to do this is by installing a pressure tank after the pump, but we prefer to save space and simplify the installation by using an on-demand pump that combines a pump, a controller, and a pressure-tank function into one small package. When a valve in the domestic-water side of the system is opened, a small bladder built into the unit provides pressure until the controller activates the pump’s motor.
Filtration. In a nonpotable system, water is ordinarily drawn from the bottom of the tank, even though it contains sediment and may be somewhat stagnant due to a lack of air exchange with the surface. In a potable system we want to use the best water first, so we use a floating filtered intake that pulls water from about 6 inches below the surface. As the season progresses, the water nearer to the bottom of the tank will eventually get used.
All potable water — unlike nonpotable water, which can be used straight from the tank — must be filtered and disinfected before being used. On our systems, the pump pushes water first through a 10-micron sediment filter, then through a 5-micron carbon filter. We recommend replacing filters every six months, but the actual time between filter changes depends on the amount of water used and the volume of particles in the rainwater. If the filters are not replaced often enough, they will become clogged and restrict the flow.
Disinfection. The filters remove even very fine particles, but not the viruses and bacteria that may be present in the water. To eliminate the risk of waterborne illness, some form of disinfection is also necessary. We prefer ultra-violet (UV) disinfection because it’s the easiest system to install and maintain. The disinfection unit is a tubular chamber with a long UV light inside. The light, which is always on, kills any viruses or bacteria that pass by. The unit must always be installed downstream from the filters, because suspended particles can block UV light. Some models contain a sensor that alerts the homeowner if the bulb isn’t working. We recommend replacing the bulb once a year.
Nonpotable systems vary greatly in price because there so many options in terms of size, type of tank, and whether or not a pump is needed. As an example, the installed cost of a 1,000-gallon nonpotable system is between $2,000 and $3,000. As storage volume increases, the cost per gallon goes down. A 20,000-gallon system, suitable for a household of four, costs about $20,000. In the areas surrounding Austin, this is in line with the cost of a well.
Chris Maxwell-Gaines, P.E., co-owns Innovative Water Solutions in Austin, Texas, with Blake West.