Method for controlling scale in an aqueous system using a synergistic combination

A method of inhibiting the formation, deposition and adherence of scale forming salts, including calcium carbonate and calcium phosphate in an aqueous system having a pH of at least 8.5 and a calcite saturation level of at least 100 times the solubility limit of calcium as calcite, comprising adding to said aqueous system an effective synergistic amount of a combination of (A) an amount to establish a concentration of at least about 1.0 mg/L, of a polyether polyamino methylene phosphonate of the formula:

• ##STR11## where n is an integer or fractional integer which is, or on average is, from about 2 to about 12, inclusive;
• M is hydrogen or an alkali metal;
• and each R may be the same or different and is independently selected from hydrogen and methyl;
• (B) an amount sufficient to establish a concentration of at least about 2.0 mg/L of a terpolymer comprising the monomers of acrylic acid, sulfophenoxymethallyl ether and maleic acid, wherein said terpolymer has a weight average molecular weight in the range from about 4,000 to 10,000;
• (C) an amount sufficient to establish a concentration of at least about 0.20 mg/L of a hydroxyphosphonoacetic acid;
• and (D) an amount sufficient to establish a concentration of at least about 0.75 mg/L of a phosphonocarboxylic acid copolymer comprising the monomers of acrylic acid, (1-hydroxy-1-methylethyl) phosphinic acid monosodium salt and 2-acrylamido 2 methyl propane sulfonic acid, to inhibit calcium carbonate and calcium phosphate scale formation.