Recovery of ammonia as struvite from anaerobic digester effluents

The effects of environmental conditions on ammonia removal as struvite (Magnesium ammonium phosphate, MAP) were studied in a laboratory scale batch reactor. MAP precipitation was carried out by adding phosphoric acid and magnesium source either as MgCl2 or MgO. The effect of temperature, pH, Mg:N:P ratios were studied. Temperature did not significantly affect ammonia removal between 25-40 degreesC and over 90% removal was obtained. The effect of pH, however, was significant and highest removal was reached at pH 8.5-9.0. The various stoichiometric ratios of ammonium to Mg and P have been tested and slight excess of Mg and P found to be beneficial for higher recovery of ammonia as struvite. However further increase in Mg and P ratios did not result in further ammonia removal which is also costly for the practical application of the process. When MgO was used as M source, the ammonia recovery was 60-70% whereas the use of MgCl2 has increased this figure up to 95%. In addition a two step purification process was developed to recover MAP crystals from impurities of the anaerobic digester. Firstly, precipitates were dissolved in acid and impurities were removed by centrifugation. The clarified supernatant was re-precipitated by adjusting its pH with caustic. It was shown that in the two steps process white MAP crystals could be obtained with over 85% recovery to be used for another applications. The economical analysis of the process has shown that ammonia in the digester effluents can be recovered at the cost of $7.5-8.0 kg(-1) NH4+-N. The rate of reaction is very fast and is completed almost in minutes. This simplifies the process design resulting in a smaller reaction vessel.