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Comparing the effectiveness of CO₂ strippers to reduce operating costs for struvite formation Sabrina, Nandini

Abstract

Struvite crystallization offers the potential of removing phosphorus from wastewater and recovering it in a form that can be used as a fertilizer. In 1999, the Department of Civil Engineering at the University of British Columbia (UBC) started a phosphorus recovery project. The UBC struvite group is now researching ways to reduce the operating costs of struvite production. One of the major operational costs of struvite production is the cost of caustic chemicals that is added to obtain a desired level of operative pH. The main objective of this research was to introduce two types of CO₂ strippers into the struvite crystallization process and determine their effectiveness in reducing caustic chemical use, thereby helping to reduce the operational costs of struvite production. In this study, two CO₂ strippers were used - (i) compact media stripper and (ii) cascade stripper. The strippers were connected to two identical struvite crystallizers. The reactors were placed at the Lulu Island Wastewater Treatment Plant (LIWWTP). The strippers were tested under different operating conditions, and their effectiveness in reducing caustic use was compared. Throughout the project, a high percentage of phosphorus removal was achieved under each condition, by both the reactors/strippers. Most of the time, the phosphorus removal rate was around 90%. The compact media stripper failed to save any amount of caustic, regardless of the operating conditions. Instead, more caustic was required once the stripper was introduced. One of the reasons was that the stripper blocked the passage of stripped off CO₂, since it was mounted on top of the clarifier. Another reason was the susceptibility of stripper's packing media to become frequently clogged, which also resulted in blocking the movement of CO₂ through the stripping tower. On the other hand, the cascade stripper was very effective in saving caustic. The amount of caustic saved by this stripper ranged from 35% to 86%, depending on the operating conditions. Both strippers showed very poor performance regarding ammonia stripping, with the compact media stripper being slightly better in stripping ammonia. The harvested struvite pellets from both the reactors were composed of nearly pure struvite (94% by mass), with a small amount of calcium and traces of iron and potassium. Different operating conditions did not have any affect on the quality of harvested struvite.

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