Phosphate monitoring

phosphate monitoring Phosphate monitoring will become more important in the latest ‘AMP’ phase being introduced through 2015-2016

This brief article will look at why Phosphates need to be measured, and how we can measure phosphates in wastewater applications.

Phosphorus is present in water bodies via animal, human, domestic and industrial waste, soil and rock erosion and fertilisers. This can be measured as Total Phosphate or Orthophosphate. Where excess phosphates are present, algal growth can be promoted. Initially this may help to provide a food source for the fish population, but over-loading of Phosphates can result in eutrophication. This can cause over-population and limiting of resources for plant and animal life alike.

Phosphate is generally measured through monitoring absorbance.

Molybdenum blue

Ammonium molybdate reacts with orthophosphate ions to form phosphomolybdic acid. This is reduced to molybdenum blue. For the determination of total phosphate, an acidic oxidation at 100 – 120 °C must precede to detect poly- and organic phosphates. (Reaction basis according to DIN EN ISO 6878-D11).


In a dilute orthophosphate solution, ammonium molybdate reacts under acid conditions to form a heteropoly acid, molybdophosphoric acid. In the presence of vanadium, yellow vanadomolybdophosphoric acid is formed. The intensity of the yellow colour is proportional to phosphate concentration.

phosphate measurement

As in the above diagram, tests can be take manually and compared visually or via a spectrophotometer.

For constant monitoring however, on-line continual instrumentation will be required. For the development of AMP projects, increasingly accurate monitors will be demanded specifically aimed at measuring low concentrations of Phosphate in our wastewater. This instrumentation will be almost inevitably based on the methods listed above. Making the correlation between Total Phosphate and Orthophosphate negates the need to heat the sample to over 100 °C, and thus makes the measurement that much simpler to perform in a remote analyser.

Waste Water resource centre