A biocidal compositions including a tetrakis(hydroxyalkyl)phosphonium salt biocide, wherein the salt is selected from sulphate, phosphite, bromide, fluoride, chloride, phosphate, carbonate, acetate, formate, citrate, borate, and silicate； a biopenetrant, wherein the biopenetrant includes a polymer of an unsaturated carboxylic acid or a copolymer of an unsaturated carboxylic acid with an unsaturated sulphonic acid, the polymer or copolymer being either terminated by vinylphosphonic acid (VPA) or vinylidene-1, 1-diphosphonic acid (VDPA) or having such monomers incorporated into the polymer backbone； quaternary ammonium cationic surfactant, and an amphoteric surfactant. This composition enhances the biocidal efficacy of the THP salt against both planktonic (free-swimming) and sessile (attached) bacteria and archaea and enhances the efficacy of the THP salt in the dissolution of iron sulphide scale.
