Struvite formation, analytical methods and effects of pH and Ca2 +

Struvite formation is mainly controlled by concentrations of Mg2+, NH4+ and PO4 3+, pH, temperature, and other ions like Ca2+. Experiments evaluating the effects of pH and Ca2+ on struvite formation indicated that XRD is only a qualitative method to analyze the struvite content in precipitating compounds, which was also reflected in microscopic images. The element analyses preceded by a dissolution method were introduced to quantitatively determine the struvite content and were shown to be an efficient enough method. Based on element analyses, the struvite content could be calculated according to the N content in the precipitations, based on the molar ratios (1:1:1) of Mg, N and P in pure struvite (MgNH4PO4 x 6H2O). It was found that the optimal pH ranges for the struvite content >90% were respectively at 7.5 approximately 9.0 with ultra pure water as solute and at 7.0 approximately 7.5 with tap water (mainly consisting of ground water) as solute. Applying a pH > 8.0 in real wastewater containing Ca2+ might result in impure struvite contents in the precipitate due to the effect of Ca2+.