The Science of Phosphorus | Blog Series | 3 of 5

Phosphorus Monitoring

The two most common measurements of phosphorus in wastewater are Total Phosphorus (TP) and orthophosphate (OP). TP is typically the compliance parameter required by National Pollutant Discharge Elimination System (NPDES) discharge permits; while OP is a useful process control parameter. TP is measured following digestion of the sample with acid and heat but OP is not well defined, analytically speaking. OP is generally assumed to consist of dissolved reactive phosphorus (DRP) which may consist of compounds besides orthophosphate. 

DRP is measured as that portion of the sample that passes a 0.45 um membrane filter and can be measured using direct colorimetry. Lastly, there is non-reactive phosphorus (NRP). NRP occurs at a very low concentration. However, the proportion of NRP increases as the TP in treated effluent becomes lower because it is difficult to remove from wastewater.

Measurement Methods of P

Colorimetry is the most convenient analytical method for measuring phosphorus in wastewater. Multiple colorimetric methods exist but all are the same in principle: phosphate ion combines with ammonium molybdate under acidic conditions to form ammonium phosphomolybdate. In the “yellow” method vanadium in the reagent generates a yellow color in solution, the intensity of which is proportional to the phosphate concentration in the sample. The “yellow” method is the principle of measurement for the YSI IQ SensorNet P700 and most other online phosphate analyzers.


A different method, the “blue” method, is required for compliance-level measurements. In the “blue” method, tin and antimony in the reagent generates a blue color proportional to the phosphate concentration. The “blue” method has a lower method detection limit (MDL) and is generally considered more accurate than the yellow method. However, the chemistry requires the use of more complicated and expensive optics which increases the cost of the analyzer but is not warranted for process monitoring.      

The aforementioned colorimetric methods are applicable only to dissolved phosphate. Sample preparation using chemicals and or heat is required to dissolve particulate phosphate. The digestion step is not a common feature of online phosphate analyzers because of the added complexity and cost. Therefore, compliance monitoring is almost exclusively done in the lab on grab or composited samples. The digestion step also increases the measurement time which increases analyzer response time limiting its application for process control. 

What is the P700 and How Does it Measure?

The IQ SensorNet version of a phosphate analyzer is the P700 IQ orthophosphate monitoring system. The base model (item 8P-000Y) consists of a photometer, peristaltic pump, and reagent tray in a powder-coated IP54 aluminum enclosure which can be mounted on a handrail, on a wall, or be free-standing and runs off 115V power.YSI-P700-Ortho-Mounted-with-Filter-Visible.jpg Most customers purchase the version that includes an integrated electric diaphragm sample pump (item 8P-010Y). A lightweight 0.45 micron filter (item 821 987Y) on a sliderail (item 821 986Y) is available separately. The maximum total suspended solids (TSS) for the filtration system is 6,000 mg/L. Competitive systems without filtration are limited to a maximum of 75 mg/L TSS which limits application to treated effluent. A version with a sample pump and climate package (item 8P-011Y) allows operation at ambient temperatures between -4°F and 104°F.

The P700 IQ features a wide measuring range, low reagent consumption, automatic calibration to a standard, and automatic cleaning. Two measuring ranges, 0.05 to 15 mg P/L, and 1 to 50 mg P/L are available for monitoring untreated wastewater, treated wastewater, or sidestreams. Competitive systems are limited to 15 mg P/L or less or only offer 1 measuring range with a resolution of 1 mg P/L. Automatic calibration to a standard and cleaning at user-programmed intervals provides assurance of analyzer performance and trouble-free operation. A manual 1 or 2-point calibration is also possible. Competitive systems only perform a zero adjustment and manual calibration is not possible. Reagent (vanadomolybdate solution in sulfuric acid) consumption is 1 x 2.5L bottle every 4 months at a 5 minute measurement frequency.



Additional Blog Posts of Interest

The Science of Phosphorus | Blog Series | 1 of 5

The Science of Phosphorus | Blog Series | 2 of 5

Monitoring Orthophosphate for Reduced Chemical Costs in Water Resource Recovery Facilities

6 Steps to Reduce Total Phosphorus in Water Resource Recovery Facilities [Technical Note]

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