Method for controlling water purification, and a water purification apparatus

A water purification apparatus configured to provide a stable permeate conductivity, the apparatus comprising:

• a fluid path;
• a feed pump configured to receive inlet water, and to provide feed water;
• a reverse osmosis membrane, wherein the reverse osmosis membrane is configured to receive feed water from the feed pump, and to produce permeate water and reject water;
• a quality sensor configured to measure an inlet water conductivity;
• a flow rate sensor configured to measure a product water flow rate, wherein the product water is permeate water that is delivered for consumption;
• a recirculation path configured to recirculate a first portion of the reject water to an inlet path or to a feed path;
• a drain valve configured to regulate a drain flow rate to a drain, from a second portion of the reject water;
• and
• a control arrangement configured to:
• receive, from the quality sensor, the inlet water conductivity;
• determine a target recovery for water purification based on the inlet water conductivity, wherein the target recovery defines a target ratio for product water flow rate to inlet water flow rate, wherein the inlet water flow rate is determined as the sum of product water flow rate and drain flow rate;
• control, based on the inlet water conductivity, a feed pump speed of the feed pump to either (i) a predetermined speed at which the feed pump works most efficiently, or (ii) a speed determined based on a relation between the feed pump speed, the inlet water conductivity and a target permeate water conductivity, and wherein the feed pump speed is controlled such that the target permeate water conductivity is achieved for the determined target recovery;
• and
• control the drain flow rate of the drain valve to drain, such that the target recovery is achieved based on the product water flow rate.