or a suspension comprising oh ions 3 is added to the reaction tank the solution suspension comprise for example sodium calcium or magnesium hydroxide the most efficient hydroxide source being magnesium hydroxide to increase the basicity on a monomeric aluminium product by the addition of hydroxide ions as sodium or calcium hydroxide may be difficult due to that these are rather reactive and precipitation of al oh 3 easily occurs magnesium hydroxide k sp 1 5 u00d7 10 u2212 11 is less reactive and is thus much easier to use as the hydroxide source a suspension of magnesium hydroxide may be used in the invention alternatively magnesium oxide which upon being mixed with water is hydrolysed to magnesium hydroxide the added amount of oh ions is controlled such that it corresponds to the desired basicity of the hydroxide complexes that are formed when the solution 1 and suspension 3 are mixed and allowed to react with each other the mixing of the solution 1 and the suspension 3 occurs by the aid of an extensive mixer 4 placed within the reaction tank the final solution 5 comprising polymeric aluminium complexes are transferred to the treatment plant where the final solution is mixed with the water 6 which needs to be purified by chemical precipitation this occurs as.