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People who collect rainwater in a tank usually want to use it for irrigation, and the question is always the same: which pump, with what pressure and flow [9]. The choice depends on where the tank is and on what the pump has to supply.

Three families of pumps

• Submersible tank pump, lowered into the tank. Many models have a float switch, which switches the pump off when the water drops too low (for example Gardena 4000/1 [26] and Einhell GE-SP 3546 RB [29]). It does not need priming.
• Surface pump, outside the tank, drawing water through a hose. It works if the water level stays within the declared suction depth. The guide on the 10-metre well explains the limit.
• Clean-water drainage pump, designed for emptying: high flow at low head. The DAB Nova 300 M-A reaches at most 7.2 m [37], the Pedrollo TOP 2 8.5 m [38]. They are fine for transferring water, not for supplying sprinklers that need pressure.

An example with manufacturers’ data

Underground tank with the water level 2 m below ground, outlet at 0.5 m, 20 m of 19 mm hose. Estimates from the calculator based on the published curves:

Open this example in the calculator and change the values to those of your tank.

With an open outlet, for filling watering cans or another tank, almost all of them will do. As soon as you need pressure for sprinklers, pumps with a low maximum head and drainage pumps fall away. The table leaves out pumps that declare only maximum values: their estimate is indicative and the calculator shows it separately.

Water and filters

Every pump states its maximum particle size, that is the size of the solids it can take in: for the Einhell GE-SP 3546 RB it is 2.5 mm [29]. Rainwater carries leaves and sediment: keeping the pump raised off the bottom and filtering the water as it enters the tank helps stop it clogging.

What the standard says

In Italy, the UNI/TS 11445:2012 standard covers rainwater harvesting systems for non-drinking uses: irrigation, washing, toilet cisterns. It requires the recovered water never to mix in any way with mains water [4]. Connecting the tank to your household plumbing is a job for a plumber, not a DIY project.