Phosphate Corrosion Control in Drinking Water


The metals in the pipe tend to rust when they come into contact with dissolved oxygen in the water. This fact results in the formation of stable metal oxides. Corrosion in water distribution systems can affect the health of consumers, the costs and the aesthetics of treated water. Older systems may include lead-based solders that must be protected to prevent high concentrations of lead. In fact, this is how it is regulated by Directive (EU) 2020/2184 of the European Parliament at 5 ug/l, since it is very harmful to health.

Corrosion inhibitors such as inorganic phosphates in the form of polyphosphates, orthophosphates, glassy phosphates and bimetallic phosphates are often used. The addition of phosphates can form a protective layer on the pipe and contain corrosion. To guarantee a correct dosage of phosphate that allows adequate corrosion control, monitoring with a reliable analyzer is required. This also achieves a reduction in operating costs.


Corrosion control with Phosphate.

Phosphate is commonly used to minimize corrosion in drinking water distribution systems. We refer here to residential, commercial and industrial drinking water pipes. Phosphate is also used to reduce lead corrosion and to meet government standards. To achieve passivation of new or previously untreated systems, phosphate is typically dosed at > 3.0 mg/L as orthophosphate. The typical maintenance dose to ensure corrosion control is 0.5 to 1.5 mg/L.


Monitoring treatment with phosphate.

It is necessary to monitor the phosphate concentration to ensure adequate corrosion control. Manual monitoring is possible. However, to avoid underfeeding or overfeeding during periods of fluctuating system flow, continuous monitoring is advantageous. Phosphate dosage control with an online analyzer allows:

  • Minimize corrosion to meet regulatory requirements
  • Reduce costs of added chemicals


The graph above shows underdosing of phosphate (blue line) while the flow rate increases with the manual feed system. The results are an increased risk of corrosion in the distribution network and a higher concentration of lead in drinking water. A continuous system with automatic feed allows maintenance of a desired set point regardless of flow rate fluctuation.

An automatic phosphate measurement provides all the necessary information. This makes it easy to keep proper control of the corrosion inhibitor to minimize corrosion and cost.


OUR SOLUTION: AMI Phosphate II Analyzer

Online analyzer, from the Swiss company SWAN ANALYTICAL INSTRUMENTS, for the automatic and continuous measurement of dissolved orthophosphates. Ideal for monitoring and controlling the phosphate dosing process. Measurement range: 0.01 to 10 ppm PO4

Its overflow chamber and continuous flow photometer eliminate coating and clogging problems. Optional cleaning module reduces maintenance efforts.

Built-in monitoring functions generate alarms if the measurement is invalid. Typical problems are lack of flow, empty reagent reservoirs, valve and photometer functionality.

No data loss after power failure. All data is saved in permanent memory. It has protection against overvoltages of inputs and outputs, and galvanic separation of measurement inputs and analog outputs. The analyzer is factory tested and ready to be installed and run.

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