Phosphorus is naturally present in very small quantities in the soil and in water. A significant concentration of phosphates can lead to seaweed expansion since it is a nutrient for plants. However, seaweed is responsible for the eutrophication of stagnant waters. This eutrophication phenomenon is then more or less important according to the phosphate content in the wastewater. However, phosphorus is a major constraint to reduce eutrophication.
Based on a study, 1g of phosphate-phosphorus (PO4-P) can cause an seaweed expansion of 100g. When these seaweeds die, they require about 150 g of oxygen to decompose. This phenomenon starts at very low P-PO4 concentrations:
- 0.1-0.2 mg/l in tap water
- of 0.005-0.01 mg/l in stagnant water.
The chemical forms of phosphorus in wastewater are very diverse. They can be soluble or particulate, mineral and organic.
Total Phosphorus = Particulate Phosphorus + Dissolved Phosphorus = Mineral Phosphorus + Organic Phosphorus
In order to meet the recommended limits, a targeted phosphorus removal process is then required in wastewater treatment plants. This process can be chemical or biological, and often both processes are used.
Orthophosphates and total phosphorus
Orthophosphates (PO4 ions) are the simplest and most common form of phosphates in water. Phosphates (also called orthophosphates) are an oxidized mineral form of phosphorus. Phosphates are present in dissolved, colloidal or solid form.
These different forms of phosphates, from the orthophosphoric acid salt (H3 PO4), are present in water because they ionize:
- in H2 PO4-,
- HPO4- -,
- PO4- – –
Orthophosphate is the most common component of the total phosphate load. In general, orthophosphates have limited toxicity to fish. They are also used in fish farming to increase plankton biomass. However, they can promote eutrophication when present in excessive quantities.
The amount of phosphates in water is measured in mg/l. of P-PO4. But the most commonly used parameter is total phosphorus, which is the sum of organic and inorganic phosphorus. It is expressed in mg/l Pt. The difference between total phosphorus and orthophosphates is a measure of the organic fraction of phosphorus in the water.
The treatment process must allow the removal of phosphorus components until they reach the legal limits allowed at the outlet of the treatment plant. There are two ways to do this: biological and chemical removal (or chemical precipitation of phosphates).
Biological removal of phosphorus compounds
Biological dephosphatization consists in the overaccumulation of phosphorus in a biomass. While phosphorus levels of 2-3% in the sludge are obtained under normal conditions of degradation of an organic substrate, the mechanism of overaccumulation requires stressing the biomass, alternatively in anaerobic (without oxygen) and aerobic phase. However, the denitrification zone, characterized by the presence of oxygen via nitrates, is therefore assimilated to an aerobic zone, and does not allow biological dephosphatation.
Under stress conditions (phase alternation), the micro-organisms accumulate phosphorus, up to 10% of their dry weight, in the form of polyphosphate granules.
The reactions occurring in each area can be summarized as follows:
- anaerobic area: the micro-organisms use their reserves and release intracellular phosphorus;
- aerobic area: overaccumulation of phosphorus in the form of polyphosphate granules.
These phenomena, which are essential to the biological dephosphatisation process, will condition the design of treatment plants. The basic layout should include at least one anaerobic area where the release of phosphorus will occur and at least one aerated area where the over-assimilation reactions will take place.
To achieve good biological phosphorus removal, the essential element is the presence of a sufficient quantity of easily assimilable organic matter in the water to be treated. From a conventional urban wastewater, an average ratio of 3.5% of phosphorus removed per BOD5 consumed is obtained. This leads to a biological elimination of only 50 to 65% of the phosphorus. This performance is often not sufficient to meet discharge standards. It is necessary to add a chemical precipitation step where the remaining phosphorus will be precipitated by the addition of reagent.
Biological disposal is only effective when sufficient quantities of readily biodegradable organic matter (BOD5) are present.
Chemical precipitation of phosphates
This process takes into account the assimilation of soluble phosphorus precipitation with a metal salt component. It can be iron or aluminum salt. Iron or aluminum salts are also capable of combining with phosphate ions to form an iron or aluminum phosphate precipitate (Fe PO4 or AIPO4). We talk about coagulation.
Due to the competitiveness of the hydroxide or phosphate precipitate formation reactions, the molar ratio to be used between Fe/P or Al/P varies from 1 to 3.
Chemical precipitation is generally carried out in structures dedicated to settling.
The metal phosphate precipitates flow and are extracted in the wastewater sludge.
The addition of coagulants can be done during primary settling, or once in the biological tank, or in an additional treatment structure located after the biological tank.
However, it must be remembered that the level of sludge increases considerably due to the precipitation of phosphate salts. To limit the costs of sludge treatment and coagulants, most wastewater treatment plants combine biological and chemical processes for phosphorus removal.
COD/Pt ratio: Prediction of phosphorus removal efficiency
For wastewater, variations in phosphorus removal efficiency by assimilation alone are mainly correlated
- COD/P (or BOD5/P) ratio
- in the sludge age
The removal efficiency increases in a quasi-linear way when COD/P increases: as the sludge production is proportional to the applied COD load, an increase in the COD flow for a constant P flow causes an increase in the assimilated P flow and thus a better overall removal.
In addition, low sludge ages improve phosphorus efficiency, while very long sludge ages decrease phosphorus efficiency.
For a median COD/P ratio of 70 g COD/g P, the removal efficiency can be :
- 38% with a sludge age of 20 days
- 38% with a sludge age of 20 days
TSS concentration has only a small effect on phosphorus removal efficiency. The lower the concentration, the better the efficiency.
In wastewater, the normal COD/Pt ratio is between 25 and 100.