Recovery of phosphorus by formation of struvite with the Phospaq™ process
Remy, M.1, Driessen, W1., Hendrickx, T.1, Haarhuis, R.2 ,1Paques BV, The Netherlands, 2Waterstromen, The Netherlands
(free)
This paper describes several case studies comprising Phospaq™ technology for recovering phosphorus and nitrogen from sludge dewatering rejects waters and anaerobically treated industrial effluents by controlled precipitation of struvite.
Long term full-scale operational results of two full-scale Phospaq™ installations are discussed in detail. Combining Phospaq™ technology with Anammox® technology allows for extended removal of phosphorus and ammonium from nutrient (N & P) rich liquors.
The quality of the produced struvite is in compliance with European legislation for fertilisers. The struvite crystals have an average particle size of 0.7 mm and are used as raw material for producing bespoke agricultural fertilisers.
INTRODUCTION: Recovery and removal of nutrients, especially phosphorus is of importance in preventing eutrophication. In addition recovery of phosphorus is of interest as it is a finite resource, which might become scarce in the medium term future.
Anaerobic treatment is effective in recovering energy from waste water in the form of biogas. As anaerobic treatment does not really remove nutrients, anaerobic effluents often still contain relatively high residual concentrations of phosphorus and nitrogen. Recovery of these nutrients can be achieved in a controlled manner using the Paques proprietary Phospaq™ process. The Phospaq™ reactor is a continuously aerated and therefore agitated reactor in which magnesium (Mg2+) is dosed to stimulate the controlled formation of magnesium-ammoniumphosphate (MAP or so called struvite). The chemical reaction can be described as follows (equation 1):
Mg2+ + NH4 + + PO4 3- MgNH4PO4 · 6 H2O (1)
The struvite crystallization is facilitated by the pH increase via stripping of carbon dioxide (CO2) and addition of magnesium-oxide (MgO). The Phospaq™ reactor is generally operated at a pH between 8 and 8.5. Dosing of MgO instead of MgCl2 is preferred as it often shows to be more economical and eliminates the need for additional dosing of caustic soda (NaOH). The Phospaq™ reactor is equipped with a specially designed internal separator for retention of struvite crystals. Figure 1 shows pictures of struvite crystals collected from a Phospaq™ reactor. The average size of the struvite crystals harvested from the Phospaq™ reactors is 0.7 mm (Abma, et al 2010). Whereas the bigger crystals settle and are harvested from the bottom of the reactor, the smaller crystals are kept in suspension in the reactor by the aeration. The smaller crystals in suspension are used as nuclei for further crystallization of struvite.
Keywords: Phosphorus, Struvite, MAP, Magnesium-ammonium-phosphate, Nutrient recovery, Fertiliser, Phospaq
