Impacts of Acidification, Chelation, and Ion Exchange on Digestion, Phosphorus, and Dewatering


  • W. O. Khunjar, R. Latimer - Hazen and Sawyer
  • M. A. Latif, C. Mehta, D. Batstone - University of Queensland
  • S. Jeyanayagam - CH2M HILL

The advancement of extractive nutrient recovery from wastestreams represents a disruptive shift in the nutrient management paradigm at water resource reclamation facilities (WRRFs). By employing this approach, facilities are able to observed benefits related to reduced chemical costs (offsetting ferric or alum addition), reduced chemical sludge production, reduced struvite scaling and reduction of sidestream nutrient loads which helps to stabilize mainstream nutrient removal. An additional benefit is the ability to manipulate the N:P ratio in the biosolids to maximize end use.

There exist multiple opportunities for improving recovery efforts. For instance, during biological stabilization processes like anaerobic digestion, metal reactive nutrients that are released tend to form inorganic compounds that precipitate. Because of this unintentional precipitation, less than 10% of the nutrient that is released remains soluble. Therefore, a significant fraction of the released nutrients is not available for recovery/extraction, and is lost in the bulk organic solids stream. Indeed, conventional struvite recovery processes are limited to recovering between 20 and 50% of the total influent P entering the plant.

To minimize the occurrence of nuisance precipitate formation, WAS P release technologies that focus on leaching P prior to biological stabilization have been employed. While this process is effective, not all plants have the capacity to implement a dedicated mixing tank and separate dewatering facility. An alternative approach is to utilize options that suppress phosphorus (P) precipitation in the dewatering stream until this stream enters the struvite crystallization reactor. This latter approach can involve operating anaerobic digestion at depressed pH conditions or through the addition of addition of complexing/chelating agents.

In this work, we will provide a summary of research efforts that have sought to maximize phosphorus availability for struvite harvesting post anaerobic digestion. As part of this work, we have also examined the benefits that these strategies can have on sludge dewaterability.

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