Struvite precipitation is common in waste water systems containing high concentrations of dissolved orthophosphates, free and saline ammonia and magnesium ions, such as in anaerobic fermentation systems, often leading to severe fouling. It appears that a reduction of the partial pressure of CO2, ie. pCO2 is a trigger mechanism for struvite precipitation. An algorithm was developed, based on equilibrium chemistry, to quantitatively predict the struvite precipitation potential of the water in such a process. This was then extended, to predict the change in state together with precipitation potential due to chemical perturbations, particularly variations of pCO2. Experimental verification of the algorithms is presented.