Recovery of Ammonia as Struvite from Anaerobic Digester Effluents

Abstract

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 MgCl, or MgO. The effect of temperature, pH, MgN:P ratios were studied. Temperature did not significantly affect ammonia removal between 25-40 degrees C 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 useMgCl2 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.

Nutrient Recovery from Human Urine Using Pretreated Zeolite and Struvite Precipitation in Combination with Freezing‐Thawing and Plant Availability Tests on Common Wheat

“…The use of MgCl 2 as an alternative Mg source in struvite precipitation studies [18,41] showed that more nitrogen could be recovered as ammonia. The negative effect is that struvite formation is optimal at a pH between 8.9 and 9.2, and when using MgCl2 the pH must be adjusted by adding other chemicals, such as NaOH.…”

Removal of aqueous ammonium with magnesium phosphates obtained from the ammonium-elimination of magnesium ammonium phosphate

“…As one of the attractive processes for the removal of phosphate and ammonium from wastewater, the MAP process has been received attention [1][2][3][4][5]:…”

Ammonium Removal from Landfill Leachate by Means of Multiple Recycling of Struvite Residues Obtained through Acid Decomposition

“…The formation of these products removes some of the Mg 2+ and PO 4 3− available for struvite crystallization, therefore, an overdose of reagents is necessary. In further researches, the benefit of overdosing magnesium and phosphorus was verified [18,19,32]. Clearly, the optimal dosages reported in the literature are related to many factors, such as the characteristics of treated wastewaters, the exploited reactants, and the applied process conditions.…”

“…However, the development of effective and economic methods for the abatement of the high nitrogen loads of wastewaters still represents a big issue [14,15]. In this regard, the precipitation of ammonium as struvite is a promising approach, due to its high process efficiency and the ease of management [16][17][18][19][20][21][22][23][24][25][26][27]. Struvite crystals are composed of Mg +2 , NH 4 + , and PO 4 3− in equimolar amounts (MgNH 4 PO 4 ·6H 2 O) [16,17].…”