State-of-the-art wastewater treatment plants emphasize biological nutrient removal, which results in a digester stream that is exceptionally rich in phosphate and ammonia. These streams have great value if nutrients can be recovered economically. Struvite is a relatively insoluble mineral that forms in waters rich in nitrogen and phosphorous, especially in municipal wastewater treatment plants. Struvite precipitation in processing equipment and pipes may cause significant processing problems. Struvite formation can be controlled by the addition of an acid, since its solubility increases as pH decreases from circumneutral values. Subsequently, the pH can be recovered by the addition of a base, allowing for struvite recovery. The proposed solution to struvite control and recovery in the plant is to use carbon dioxide in residual biogas, a byproduct of anaerobic digestion, in place of chemical addition for struvite control. Since carbon dioxide readily forms carbonic acid when dissolved in water, it is possible to reuse residual biogas to lower the pH of wastewater to prevent the formation of struvite. Subsequently, carbon dioxide may be stripped from solution in order to increase the pH and recover struvite for sale as slow-release fertilizer. The purpose of this study is twofold. First, we aim to increase the understanding of the chemistry of carbon dioxide dissolution in complex matrices, such as treated wastewater. Second, we will investigate the feasibility of using biogas for struvite control instead of acidification with other chemicals. Preliminary results show that injection of biogas into wastewater streams can be used to lower the pH to achieve a more than 100-fold increase in struvite solubility and that struvite recovery using this process is more economical than traditional methods.

Sustainable struvite control and recovery in wastewater treatment plants using residual biogas

253rd National Meeting (2017)

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