A back of the envelope calculation can sometimes be used for prediction but it gives only general guidance in planning the test work. If you know what other phosphate plants produce that would be your source of information for an educated guess.

When we do bench tests to determine the preliminary parameters for a process plant it includes analysing the effluents. If the red flags go up we start doing bench tests on the effluent itself. If those do not work we send samples to a more specialized lab. If that does not bring positive results we shut the project down.

Later, if the bench results were positive, we do a pilot plant run to confirm expectations in general, to better define product qualities, and to fine tune the parameters for design and operation of the actual plant including effluent control.

We must also file the results of effluent testing with the department of the environment or equivalent (varies by country) in the form of asking for an operating licence which must be in place before we can begin construction.

In other words, talk to an environmental SME in your Company...effluent analytical would include ICP-MS for the entire suite of elements, Ion Chromatography for all anionic species, pH, TDS, TSS, TOC, coliform etc.

The simplest way to get the answer is to have a representative sample sent for geochemical analysis. This would also involve sequential leaching of the ore in water or mild acid to simulate possible acid rain conditions and then assess the leach liquor for any dissolved species.

If this is a beneficiation tailings then the geochemical analysis is the best way of achieving the results you seek. I use this method on all my phosphate beneficiation feasibility studies.