Phosphorus Reduction – How Low Can You Go?

If you have a phosphorus limit for your wastewater effluent, you know that it can be a real challenge to remove. If your limit is below 1 mg/l, it becomes even harder. But did you know that it may be impossible to remove all your phosphorus through biological uptake or chemical precipitation?

Know the types and concentration of your phosphorus

The types of phosphorus in your wastewater will dictate not only how your phosphorus can be removed but will also determine the lowest level of phosphorus you can achieve in your effluent. In the most basic division, phosphorus is dissolved, soluble phosphorus or is in particulate, filterable form.

Of the soluble phosphorus forms orthophosphate, PO₄^3-, is the simplest with the P atom in the middle of a tetrahedron formed by oxygen atoms at the four corners. This is the form that is used in fertilizers and is the form that is most readily bioavailable to plants, including algae. While orthophosphate is also called “reactive phosphorus”, this is a method-based term that describes what is measured in the laboratory analysis, which involves a reaction with ascorbic acid and ammonium molybdate.

Soluble inorganic condensed phosphates, the polyphosphates and the metaphosphates, contain two or more phosphate groups connected with a P-O-P linkage. These linkages are in chains for the polyphosphates and are cyclical for the metaphosphates. They are also called acid hydrolyzable phosphates because laboratory analysis uses acid to break the complex phosphates down into orthophosphates that are then measured using the same ascorbic acid and ammonium molybdate method used to measure orthophosphate directly. Soluble organic phosphates are bound to plant or animal waste material and the method used to analyze for them involves digestion using heat and acid to break the phosphates down into orthophosphate, which is then analyzed again with the ascorbic acid method. Only dissolved phosphorus is bioavailable for plant uptake.

Particulate phosphorus is suspended phosphorus attached to particles that are filterable. A portion of this particulate P is also reactive orthophosphate and is typically analyzed along with the soluble orthophosphate. As with soluble phosphorus forms, there is also an organic particulate P that is digestible and an acid hydrolyzable particulate P. These are analyzed along with their soluble counterparts. Particulate P forms are readily removed in the treatment process by clarification or filtration.

Goal of chemical precipitation is to convert soluble phosphorus into particulate phosphorus

The goal of chemical precipitation of phosphorus is to convert soluble phosphorus to particulate phosphorus, and to react the reactive particulate phosphorus so that it will settle better in the clarifier or be filtered out in a membrane or filtration system. The best chemical precipitates bind strongly with the orthophosphate and retain that bond even under shifting oxygen concentrations in clarifiers or digesters. Lanthanide salt solutions, using Lanthanum and Cerium, are often the best choice for a precipitating agent when the final phosphorus limit is below 1 mg/L.

However, even lanthanide salt solutions cannot remove the soluble non-reactive phosphorus because those won’t react with the salts. The acid hydrolysis and digestion that converts these condensed phosphates and organic phosphates into orthophosphates does not typically occur in a biological system like activated sludge. Therefore, that portion of the phosphorus that is soluble non-reactive phosphorus (SNRP) cannot be easily removed with wastewater treatment processes. The concentration of the SNRP is truly as low as you can go.