ARKA myAQUA 1900 or 3800 high-capacity reverse osmosis unit

Both models work on the same principle, with the same pressure range and the same yield of around 1 to 1. The 3800 doubles the instantaneous flow, at 2.63 litres per minute, so a 20 litre jerrycan fills in under 8 minutes and a 200 litre tank in just over an hour. It uses 120 watts during production instead of 70, measures 43.2 x 16.3 x 48.6 cm, which is 7 cm taller, and accepts up to 4.5 bar and water from 5 to 45 °C. The choice therefore comes down to the volume you draw off in one go: the 1900 is enough as long as a tank fills while you get on with something else, whereas the 3800 is justified when several hundred litres need to be ready within the hour, for a large water change or for filling a pond.

The unit is wall-mounted and connects to a cold water supply via a 3/4 to 3/8 inch fitting. The box contains 3 metres of supply pipe, 6 metres for the reverse osmosis water and the waste, a tee with its valve, a shut-off valve, the clip that regulates the waste flow and the power supply unit. Allow for 70 watts during production and 1 to 2 watts on standby, for a unit measuring 43.4 x 16.3 x 41.8 cm. Plan the route of the two pipes before drilling: the waste pipe must reach a drain. Also choose the mounting height with the container that will fill beneath it in mind, and leave enough room in front of the unit to unscrew the filter housings by hand on cartridge-change day. The 6 metres of pipe supplied leave some slack, but a long run kinks easily: secure it along the wall. The water remaining in the cartridges and the membrane must never freeze: in a garage or unheated shed, the unit should be dismantled before winter or placed in a frost-free room. Between production runs, also close the supplied shut-off valve, so the membrane is not left under pressure without producing anything.

The water passes through four stages: a fine pre-filter stops the particles, a first activated carbon (C1) removes the chlorine that would damage the membrane, the membrane retains the dissolved salts, and a second carbon (C2) polishes the water produced. The pre-filter and the two carbons should be replaced at the latest every 6 to 12 months, the membrane every 12 to 24 months depending on the quality of your incoming water and the volume produced. After each change, let the unit produce for 30 to 60 minutes to the drain, long enough to rinse the new elements. All the parts can be found on the page for ARKA myAQUA refills and membranes. When production stops after 12 hours, the indicator lights on the 1900 turn red, and briefly disconnecting the power supply restarts a cycle.

Reverse osmosis water is empty water. It is practically free of minerals, and it needs treating before it goes into a tank: it is remineralised, or blended with tap water, until you reach the conductivity that suits the fish and plants you keep. Poured on its own into a stocked tank, it dilutes what was already there. That is also what makes it useful: you start from zero and know exactly what you are adding. With mains water at 500 µS/cm, a membrane that retains 98 to 99% of the salts gives water at around 5 to 10 µS/cm, and an equal-volume mix with tap water halves the hardness and conductivity of your mains supply. A KH carbonate hardness test checks the result before the first top-up. The guide on pH regulation explains how to adjust the pH of the system afterwards.

An aquaponic circuit loses water through evaporation, and more in summer, when the surface is warm and the plants are transpiring heavily. Topping up with the garden hose means reintroducing, every week, the salts and limescale from the mains supply, which stay in the circuit since only the water evaporates. Topping up with reverse osmosis water restores the level without adding anything. The flow rate per minute makes this task simple, since a container fills in a day while you get on with something else. Take a 500-litre circuit that loses 10 litres a day in midsummer, topped up with mains water at 400 µS/cm: the salts added are enough to raise the circuit's conductivity by about 8 µS/cm per day, or nearly 240 µS/cm in a month, before what the plants take up. The article on pond evaporation in summer quantifies the volumes involved. Bear in mind, however, that reverse osmosis water contributes no carbonates, whereas nitrification uses up the circuit's alkalinity: monitor the KH, and cut the top-up water with a little tap water or remineralise it if the pH drops.