Phosphorus depletion is an emerging and serious global environmental issue. At this moment, research and policy discussion on phosphorus scarcity is still limited. This research investigates the possibilities of phosphorus recovery by controlled precipitation of struvite from digested sludge.
At Waternet’s wastewater treatment plant Amsterdam West, plans for a struvite reactor are scheduled. The advantage of phosphorous recovery through struvite precipitation from digested sludge is three-fold. First, struvite can be directly used as fertilizer. Second, undesirable struvite precipitation in the wastewater treatment plant is prevented by reducing the phosphorus concentration in the dewatering reject stream which is fed back to the inlet of the treatment plant. Third, sludge dewaterability improves due to the addition of MgCl2.
This thesis investigates the influence of mixing speed, aeration rate, magnesium dosing method and crystal recycle method on struvite growth and phosphorus removal, as well as separation of struvite from sludge. For that purpose, experiments have been performed in a crystallization reactor and a counter-current washing column at lab scale at wastewater treatment plant Amsterdam West. MgCl2 was added under varying reactor conditions, struvite constituent concentrations were measured and struvite growth was assessed.
First, it is demonstrated that struvite recovery is well possible in a stirred sludge environment at neutral pH commonly applied in sludge digesters (7.0 - 7.1). Phosphorous removal under these circumstances is at least 85%. More complete mixing by stirring at a higher speed further improves struvite recovery by keeping supersaturation low.
Secondly, a significant difference in struvite recovery was observed between experiments in which MgCl2 is dosed instantly versus experiments in which MgCl2 is dosed gradually. Gradual MgCl2 dosage, and therefore rapid mixing, improves recovery compared to instant dosage. Mixing at a higher stirring speed further improves recovery.
Thirdly, it is found that struvite recovery under given circumstances is poor in a combined aerated and stirred sludge environment. In such environment higher aeration rates deteriorate struvite recovery further, while struvite recovery improves with decreasing aeration rates at a higher stirring speed.
Fourthly, struvite separation experiments have verified that separation is well possible in a counter-current washing set-up, separating 86% of detectable struvite within 15 minutes at an upflow velocity of 1.3 mm/s.