In wastewater treatment, phosphorus (P) is a critical parameter. It is a key nutrient for biomass development, but its excess in discharges is the main cause of eutrophication in receiving water bodies (rivers, lakes, and seas).
To operate a WWTP or interpret an analysis, it is not enough to know “how much phosphorus there is,” but rather what type of phosphorus we have and how it is being measured.
Phosphorus Fractions in WWTPs
Definition
Based on the state of matter aggregation, we can mainly find two large groups:
Based on chemical nature, the Total Phosphorus (P) entering a WWTP is the sum of all phosphorus chemical species present in the sample, both dissolved and in suspension. It is mainly divided into two large groups:
A. Inorganic Phosphorus
This is phosphorus that is not part of carbon structures (organic matter). It occurs in two forms:
- Orthophosphates PO4^-3: This is the reactive and soluble form. It is “free” phosphorus, ready to be consumed directly by activated sludge bacteria… or by algae in a river.
- Polyphosphates: Complex phosphorus chains, very common in industrial detergents, food additives, and anti-scaling products. Over time and through bacterial digestion in the WWTP, they hydrolyze and turn into orthophosphates. It is important to know that polyphosphates have the capacity to complex certain metals;
The precipitation of polyphosphates with aluminum salts is more difficult than the precipitation of a simple phosphate due to this complexing effect, since the polyphosphate stabilizes the metal within the complex, preventing it from precipitating.
B. Organic Phosphorus
This is phosphorus integrated into organic molecules: proteins, nucleic acids (DNA/RNA from bacteria or fecal remains), cell membrane phospholipids, physiological fluids, detergents, organic pesticides….
Ways to Express Phosphorus: Am I Measuring the Atom or the Molecule?
This is where 90% of calculation errors in the plant occur. An analytical result can be expressed in mass of elemental Phosphorus (P) or in mass of the Orthophosphate molecule.
They are two ways of saying the same thing, but the numbers change drastically due to molecular weights.
The Mathematics of Conversion
- Atomic weight of Phosphorus (P): 31 g/mol.
- Molecular weight of Oxygen (O): 16 * 4 = 64 g/mol.
- Molecular weight of Orthophosphate (PO4^-3): 31 + 64 = 95 g/mol.
The conversion factor is obtained by dividing the total weight of the molecule by the weight of the phosphorus:
Conversion factor = 95/31 = approx. 3.068.
What does this mean in practice?
Illustrative example:
If the laboratory photometric kit gives us a value of 3 mg/L of PO4^ {3-}, in reality we only have 0.97 mg/L of elemental P (3 / 3.07). The correct way to express this chemical reality would be “I have 0.97 ppm of P-PO4^3-.”
Be careful! If you confuse the terms, you might think you are exceeding discharge limits by a factor of 3 when you are actually within the law.
Real Life in the Company: How are Discharge Limits Expressed?
In industrial and municipal environments, environmental administration leaves no room for ambiguity.
A. The regulation always refers to Elemental Phosphorus (P)
Integrated Environmental Authorizations (IEA), Municipal Discharge Ordinances, and the Urban Wastewater Treatment Directive (Directive 91/271/EEC) set limits ALWAYS referring to Total Phosphorus (P-total), not the orthophosphate molecule.
- Discharges to public watercourses (urban WWTPs in sensitive areas): Regulatory limits are usually 1 mg/L or 2 mg/L of P_total (depending on the population size).
- Industrial discharges to the sewage network: Ordinances usually set looser P-total limits (e.g., 10 – 20 mg/L of P-total), because the municipal WWTP will handle the treatment, while charging the corresponding sanitation levy.
B. How work is done in the day-to-day operation of the plant
- Daily process control (Operation): Operators use rapid kits (tube photometer type) to measure dissolved P-PO4^3- at the biological stage outlet. Why? Because it takes 15 minutes to measure and gives us an idea of how P removal is being carried out and the level of bacterial assimilation of the element.
- Legal compliance (Accreditable): Official samples are taken to an accredited laboratory to measure total P in unfiltered samples using spectrophotometry after persulfate digestion in an acidic medium.
4. What analysis methods exist on the market and which is most suitable for each situation?
ANALYSIS: we must differentiate methods based on the species we are looking for and the range we are working in for industrial applications.
- Orthophosphate:
There are direct analysis methods ranging from test kits, with ultra-fast but less precise measurements, to optical methods that offer greater precision.
TEST KITS: There are countless options on the market, with detection limits suited to most industrial applications. Ideal for daily plant management controls.
Example of an Orthophosphate test kit.
ATTENTION: The result of one of these rapid tests is NOT valid for an audit or before the Administration.
Why?
For an analytical result to have legal validity or pass an ENAC control (ISO/IEC 17025), the laboratory must comply with the principle of metrological traceability. Field methods (strips, drops, color discs) fail in audits for three main reasons:
- Subjectivity of the reading: They depend on the technician’s visual acuity, ambient light (whether they measure on a sunny or cloudy day), and whether the technician is colorblind.
- Lack of instrumental records: A laboratory photometer or an automatic titrator leaves an indelible digital record of the calibration curve, wavelength, and absorbance. A plastic strip or a drop count does not leave an “auditable” backup.
- They are not Official Standardized Methods: The law requires measuring Total Phosphorus according to strict international standards (such as UNE-EN ISO 6878 or Standard Methods 4500-P), which require prior acid digestion with persulfate at high temperature (120 degrees C) and reading in a spectrophotometer.
COLORIMETRIC METHODS: These are optical methods based on the absorption of radiation by a specific sample at a specific wavelength; optical methods are based on the BEER-LAMBERT law:
Example of a colorimetric analysis method for orthophosphate
POLYPHOSPHATES:
What about polyphosphate? How is it measured?
Polyphosphate cannot be measured directly by any conventional method.
There is an official method to calculate them indirectly: ISO 6878 / Standard Methods 4500-P D.
- The free orthophosphate content is determined: A.
- The sample is hydrolyzed, breaking the P-P bonds by boiling for 30 minutes in an acidic medium. All polyphosphate is converted to free orthophosphate. We determine the hydrolyzed free orthophosphate: H.
- The polyphosphate content is the difference between H and A.
TOTAL P:
To determine Total Phosphorus, it is not enough to measure the water directly, because phosphorus comes in three different forms: orthophosphates, polyphosphates, and organic phosphorus (linked to living matter and suspension).
The objective of the analytical method is to break down all organic matter and complex chains to convert 100% of the phosphorus present in the sample into orthophosphate, which is the only form we can measure quantitatively.
This is the step-by-step standard procedure, based on international standards (UNE-EN ISO 6878 or Standard Methods 4500-P):
General Scheme of the Method
If we look closely, it is a method similar to the one proposed for the quantification of polyphosphates, with the difference that an oxidizing agent, persulfate, is added to break down the organic matter.
Organic P: It can be determined by the difference between the Total P value and the inorganic P:
Organic P = Total P – Polyphosphate P – Orthophosphate P.
