Extractive Nutrient Recovery is a Viable Nutrient Control Alternative for Water Resource Reclamation
- Wendell Khunjar - Hazen and Sawyer
Recovery and reuse of nutrients provides us with an opportunity to reduce our reliance on nutrient cycling and fundamentally change the way that wastestreams are managed. This paper presents technical information generated as part of a global study funded by the Water Environment Research Federation (WERF) aimed at enabling the advancement of extractive nutrient recovery.
Framework for Recovery
Results from a state of a science review indicated that a three step framework may be appropriate for advancing the adoption of extractive nutrient recovery. Struvite crystallization was found to be the most commonly applied extraction technology at full-scale WRRFs. In this process, struvite (magnesium ammonium phosphate) is precipitated in dedicated reactors to form pellets/prills. Market assessment performed in this study confirm that these struvite pellets are suitable as a slow release fertilizer and retain a market value ranging from $100 to 600 per dry ton. The assessment also indicated that there is a steady demand for other products like calcium phosphate, potassium struvite, ammonium nitrate, ammonium sulfate within the agricultural section.
Stratgies Facilitating the Implementation of Recovery
Site specific economics were identified as the primary barrier that will continue to constrain implementation of recovery technologies like struvite crystallization. To circumvent this challenge, it is important to properly quantify benefits associated with performing nutrient recovery. Quantifiable benefits include but are not limited to:
• Mitigating the impact of the sidestream nutrient content on the mainstream nutrient removal process and providing a factor of safety for mainstream nutrient removal processes,
• Minimizing nuisance struvite formation and re-gaining infrastructure capacity,
• Reducing chemical and energy costs,
• Reducing chemical sludge production (if metal salt addition is practiced),
• Manipulating the nutrient (P and nitrogen) content of biosolids, and
• Improving sludge dewaterability.
By considering these benefits in concert with revenue potential from re-sale of recovered nutrient product, it has been shown that payback of investment can occur in less than 10 years.
Lessons Learned From Implementing Nutrient Recovery
Practical lessons obtained from WRRFs that have implemented full-scale recovery facilities were identified through interviews and regular communication with plant staff. Key concepts that can help guide implementation included:
• Locate recovery facility as close to digester and dewatering facilities
• Utilize equalization if possible
• Avoid traps and use long turn elbows to reduce locations where nuisance precipitate accumulation can occurMitigating the impact of the sidestream nutrient content on the mainstream nutrient removal process and providing a factor of safety for mainstream nutrient removal processes,
• Provide flush connections on all pipe runs and incorporate flushing (acid) of lines leading to and from the nutrient recovery facility as part of routine maintenance to help control and remove nuisance precipitate buildup
• Provide duplicate piping and pumps to minimize downtime during maintenance
• Provisions for odor control should be included
• Consider staged commissioning approach to allow for training of operators
Enhancing Nutrient Recovery
Even when struvite recovery is implemented at WRRFs, recovery of P is limited to between 20 and 50% of the total influent P entering the plant. This is because of formation of nuisance precipitates/complexes during biological stabilization which act as a sink for P. Research performed as part of this work indicated that P recovery potential can be increased by between 30 and 100% if the digested sludge pH is allowed to decrease from 6.5 to 5.0. This decrease can be achieved by applying a two stage digestion processes (e.g., acid-gas), in which acid phase sludge will be dewatered to generate a nutrient rich stream for P recovery that can be combined with centrate. Suppression of pH through addition of CO2 can also achieve a similar result. However, the decision to enhance P availability for struvite recovery by performing digestion at depressed pH must be balanced with operational and capital requirements at the WRRF as well as the revenue potential.
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