What are the different compounds of wastewater

Phosphorus is naturally present only in very small quantities in soil and water. A high concentration of phosphates can cause seaweed blooms which is a nutrient for plants. However, algae are responsible for the eutrophication of stagnant water. This phenomenon of eutrophication is then more or less important depending on 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 running water
  • 0.005-0.01 mg/l in stagnant water.

The chemical forms of phosphorus in wastewater are very varied. 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. We implement a process that could be chemical or biological, and often both cases.

Orthophosphates and total phosphorus

Orthophosphates (PO4 ions) are the simplest and most common form of phosphates in water. Phosphates (called orthophosphates) are an oxidized mineral form of phosphorus. Phosphates are present in dissolved, colloidal or solid form.

These different forms of phosphates, derived 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. Furthermore, they are used in fish farming to increase the planktonic biomass. However, they can promote eutrophication when present in excessive quantities.

The quantity 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 used to measure the organic fraction of phosphorus in 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. Two methods are available to do this:

  • biological removal
  • Chemical precipitation of phosphates or chemical removal

Biological removal of phosphorus compounds

Biological dephosphatization consists in the overaccumulation of phosphorus in a biomass. If phosphorus contents of 2-3% are reached in the mud under normal conditions of degradation of an organic substrate, the overaccumulation mechanism 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 zone: 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 shall include at least:

  • an anaerobic zone where the release of phosphorus will take place
  • an aerated area where over-assimilation reactions will take place.

To obtain good biological phosphate removal, the essential element is the presence of a sufficient quantity of easily assimilable organic matter in the water to be treated. From 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 elimination is only effective when readily biodegradable organic elements (BOD5) are present in sufficient quantities.

Chemical precipitation of phosphates

This process includes the precipitation of soluble phosphorus with a metal salt compound. It could 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 usually carried out in structures dedicated to settling.

The metal phosphate precipitates flow and are extracted in the wastewater mud.

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.

On the other hand, it should not be forgotten that the level of the sludge increases considerably due to the precipitation of phosphate salts. To limit the costs of mud treatment and coagulants, most wastewater treatment plants combine biological and chemical processes for phosphorus removal.

COD/Pt Ratio: Prediction of Phosphorus Removal Yield

For wastewater, variations in phosphorus removal efficiency by assimilation alone are mainly correlated:

The removal efficiency increases in a quasi-linear manner when COD/P increases: since sludge production is proportional to the COD load applied, an increase in COD flux for a constant flow of P causes an increase in the assimilated P flow and therefore a better overall removal.

In addition, low mud ages improve phosphorus efficiency, while very long mud ages decrease phosphorus efficiency.

For a median COD/P ratio of 70 g COD/g W, the removal yield can be:

  • 38% with a mud age of 20 days
  • 38% with a mud age of 20 days

The concentration of suspended solids has only a small influence on the removal efficiency of phosphorus. The lower the concentration, the better the efficiency.

In wastewater, the normal COD/Pt ratio is between 25 and 100.

FAQ

What is the difference between orthophosphates, polyphosphates, and organic phosphates?

Orthophosphates are inorganic forms of phosphorus that are immediately absorbed by microorganisms and plants. Polyphosphates, used in industrial detergents and additives, slowly break down into orthophosphates. Organic phosphates are bound to organic matter (feces, food residues) and require biological degradation to be transformed. Together, they make up the total phosphorus (TP) measured in wastewater.

Phosphorus measurement can be done continuously using online analyzers, or occasionally with portable photometers and tank tests. Analyzers provide real-time monitoring, which is useful for processing adjustments, while spot methods are cost-effective for periodic checks. The choice depends on the size of the facility, the budget, and regulatory requirements.

Biological methods allow a reduction of 20 to 30% of phosphorus, while physicochemical treatments generally achieve 80 to 90% removal. A combination of the two can further improve efficiency, especially in medium to large stations. The yield also depends on the type of effluent, the equipment and the maintenance of the system.

Yes, some studies have demonstrated the effectiveness of natural materials such as laterite, sandstone or slate shale in trapping phosphates in passive systems (e.g., artificial wetlands). These low-cost solutions show promise for rural areas or developing countries, although their performance varies depending on hydrodynamic conditions and water composition.

In sensitive areas, even low concentrations of phosphorus can trigger eutrophication episodes, promoting the proliferation of toxic algae (cyanobacteria). This disrupts ecosystems, affects recreational water use, and increases downstream treatment costs. This is why some regulations, such as in Quebec, impose limits as strict as 0.1 mg P/L.

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