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Struvite crystallization from digester supernatant-reducing caustic chemical addition by CO₂ stripping Zhang, Ying


The recovery of phosphorus from wastewater, mainly digester supernatant/ centrate, into struvite, a slow release fertilizer, is an important research topic. The recovery of phosphorus helps to solve severe problems that have occurred in the secondary wastewater treatment plants (especially those with enhanced biological nutrient removal process). Also, it suggests an alternative, but more sustainable, source of phosphorus for domestic use. The crystallization of struvite in the liquid solution requires a relatively high pH environment, thus, caustic chemicals, e.g., NaOH, Ca(OH)₂, are usually added to the solution to adjust its pH value to above 7.5. This raises the costs of recovering struvite. Considering that the digester supernatant/centrate are usually saturated with dissolved CO₂, gas, which is relatively easy to remove, the idea of stripping CO₂, from the digester supernatant to raise pH, and thus reducing the caustic chemical usage, was adopted in this research. In this study, a cascade CO₂ stripper was designed and first tested with three different synthetic solutions: 1) tap water with CO₂, saturated, 2) NaHC0₃ solution with CO₂ saturated, and 3) NaHCO₃ + NH₄Cl solution with C0₂ saturated. It turned out that the stripper’s C0₂, removal efficiency was dependant on several parameters, such as the characteristics of the influent, including total alkalinity, temperature, and initial concentration of dissolved C0₂ gas, influent flow rate, effluent recycle rate, aeration rate, and baffle numbers in the stripper. A C0₂ stripping model was developed using these parameters, based on the stripper’s performance on the three synthetic solutions. The C0₂ stripper was also tested in a real struvite crystallization process. In this test, two struvite crystallizers, A side reactor and B side reactor, were operated in parallel. The difference between these two reactors was that, the downpipe that links the reactor’s seed hopper and the external clarifier in A side process was partially substituted by the CO₂ stripper in the B side process. The results revealed that, under different operating conditions, the addition of caustic chemical can be reduced by 46% to 65%, by employing a C0₂ stripper in the process flow scheme. Once optimized, this new approach has the potential to significantly reduce the costs of struvite production and recovery, under full-scale conditions.

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