A process for treating a natural or wastewater containing dissolved Mg or dissolved Al comprising the steps of adding at least one Mg-containing compound or at least one Al-containing compound to the natural or wastewater to thereby form a layered double hydroxide (LDH) containing Mg and Al as a predominant metal species in a lattice of the LDH. The LDH may contain hydrotalcite. The Al-containing compound may be aluminate or aluminium hydroxide derived from the Bayer process or from an alumina refinery.