A method of treating water, which includes: directing, by means of a nozzle which is connected to a water inlet pipe, a jet of water towards a lateral side of a water treatment substance body which comprises a bed of water treatment substance tablets contained within a holder, the holder comprising: a cylindrical sleeve inside which the bed of water treatment substance tablets is partly located, and a transverse trough at an operatively lower end of the cylindrical sleeve, the trough providing a water treatment zone and having a water access opening provided at one end of the trough to direct water towards a lateral side of the water treatment substance body, and a plurality of dosed water openings provided in side walls of the trough, wherein the holder is located vertically, with the cylindrical sleeve located above the transverse trough and with the water treatment substance body resting in the trough and extending up the cylindrical sleeve such that the trough contains the lowermost water treatment substance tablets of the water treatment substance body; wherein the nozzle is located outside of the holder and is directed towards the access opening such that the water jet is directed from outside the holder towards and through the water access opening, or inside the holder, being mounted to the holder by extending through the water access opening, such and the water jet is directed from a lateral side of the holder; wherein with respect to the vertical location of the holder, the water jet enters the trough more-or-less horizontally through the water access opening and directly and actively abrades the lowermost of the water treatment substance tablets in the water treatment zone such that the lowermost water treatment substance tablets are consumed cleanly, with exposed surfaces thereof being stripped off by direct hydrodynamic force and with the dosed water openings continually being blasted open by the water jet, and with water treatment substance thereby being released into the water such that dosed water is formed, and wherein the consumed water treatment substance tablets are replaced by fresh water treatment substance tablets that move downwardly into the trough; and the method further including allowing the dosed water to pass through the plurality of dosed water openings in the trough. directing, by means of a nozzle which is connected to a water inlet pipe, a jet of water towards a lateral side of a water treatment substance body which comprises a bed of water treatment substance tablets contained within a holder, the holder comprising: a cylindrical sleeve inside which the bed of water treatment substance tablets is partly located, and a transverse trough at an operatively lower end of the cylindrical sleeve, the trough providing a water treatment zone and having a water access opening provided at one end of the trough to direct water towards a lateral side of the water treatment substance body, and a plurality of dosed water openings provided in side walls of the trough, a cylindrical sleeve inside which the bed of water treatment substance tablets is partly located, and a transverse trough at an operatively lower end of the cylindrical sleeve, the trough providing a water treatment zone and having a water access opening provided at one end of the trough to direct water towards a lateral side of the water treatment substance body, and a plurality of dosed water openings provided in side walls of the trough, a water access opening provided at one end of the trough to direct water towards a lateral side of the water treatment substance body, and a plurality of dosed water openings provided in side walls of the trough, wherein the holder is located vertically, with the cylindrical sleeve located above the transverse trough and with the water treatment substance body resting in the trough and extending up the cylindrical sleeve such that the trough contains the lowermost water treatment substance tablets of the water treatment substance body; wherein the nozzle is located outside of the holder and is directed towards the access opening such that the water jet is directed from outside the holder towards and through the water access opening, or inside the holder, being mounted to the holder by extending through the water access opening, such and the water jet is directed from a lateral side of the holder; outside of the holder and is directed towards the access opening such that the water jet is directed from outside the holder towards and through the water access opening, or inside the holder, being mounted to the holder by extending through the water access opening, such and the water jet is directed from a lateral side of the holder; wherein with respect to the vertical location of the holder, the water jet enters the trough more-or-less horizontally through the water access opening and directly and actively abrades the lowermost of the water treatment substance tablets in the water treatment zone such that the lowermost water treatment substance tablets are consumed cleanly, with exposed surfaces thereof being stripped off by direct hydrodynamic force and with the dosed water openings continually being blasted open by the water jet, and with water treatment substance thereby being released into the water such that dosed water is formed, and wherein the consumed water treatment substance tablets are replaced by fresh water treatment substance tablets that move downwardly into the trough; and the method further including allowing the dosed water to pass through the plurality of dosed water openings in the trough.