This steady-state model for biological P removal comes from Biological Phosphorus Removal: Manual for design and operation, published by IWA Publishing (2002).
The steady-state bio-P model is represented in an Excel spreadsheet, which can be accessed by clicking here. The spreadsheet is 'protected'. Wastewater and process data can be added to the spreadsheet. For some input data, instructions are given in an attached note. Biological Phosphorus Removal gives information on the use of the steady-state model.
A calculation example using the bio-P model is given in Appendix IV of the manual.