Abstract: The present invention relates to a composition comprising at least one photosensitizer of the tetrapyrrole and/or tetraazapyrrole series and a solid carrier which is swellable in water. Under the action of electromagnetic radiation the photosensitizer develops its antimicrobial action by way of the activation of oxygen and/or the promotion of processes in which free radicals are involved. Said composition is useful for treating water with the aim of freeing it of bacterial germs, algae, yeast and fungi.