Struvite Crystallization

Overall Summary

Primary Application

• The primary application is for dairies interested in phosphorous partitioning to a value-added, market-ready solid.
• Raw and/or digested manure can be used, although considerable pre-treatment or manure characteristics are required before treatment—namely, very dilute manure with low suspended solids concentration after significant primary solids removal.
• The technology is applicable to any scale however economics might preclude its use to specific dairies, with certain needs and existing conditions.
• No restrictions on climate conditions is required.
• A value-added co-product is produced from the system—struvite crystals, that are of low moisture and fine granular structure suitable for immediate use as a fertilizer.

Economic/Return on Investment Considerations

• The capital cost of struvite crystallization systems is relatively high.
• The operating costs of these systems are also quite high as the process requires. additives to alter pH/crystal availability and increase the recovery if enough magnesium is not available in the feedstock.
• The economic viability of struvite crystallization of dairy manure is more difficult than other animal manures because of the higher calcium content. Calcium ties up the orthophosphate preventing struvite crystal formation and growth. This can be overcome by raising the pH of the solution, but this increases the cost.

Industry Uptake

• Only one farm in the U.S. has an installed struvite crystallization reactor for managing manure.

Technology Maturity

• Struvite recovery technologies are well established for waste water treatment (preferably dilute waste water) across the world however only one has been installed on a U.S. dairy due to chemistry/economic concerns unique to dairy manure.
• There are several commercial struvite recovery technologies available that could be scaled down for the adoption by dairy farms to treat their waste streams if the economic feasibility was proven.

Primary Benefits

• Compared to other phosphorus-recovery products from dairy manure, the solid struvite crystal product is a highly concentrated dry product that can easily be applied as fertilizer using existing equipment—positively impacting application costs and value.
• Performance at the sole U.S. commercial size test facility utilizing dairy manure from a flush system with coarse solids separation and chemical pretreatment to convert all phosphorous to an available form, showed a recovery of total nitrogen (TN) by 10% and total phosphorous (TP) by 70-80% in the effluent.
• The volume reduction is negligible due to the small concentration and dry matter content of the struvite crystals as compared to other phosphorous separation systems.
• With limited impact to ammonia and organic matter separation, the process also has limited impact on odor and GHG.
• Chemical dosing and pH adjustment can have a positive impact on pathogen reduction, but data on specific impact is lacking.

Secondary Benefits

• Struvite crystallization has no sludge-handling problems and lesser heavy metal contaminant concentration compared to commercial fertilizers which are manufactured from phosphate rocks.
• The lower solubility of struvite crystals enables them to be re-used as slow releasing fertilizer which provides plant essential nutrients including magnesium, nitrogen and phosphorus.
• Struvite fertilizers can be a sustainable phosphorus source with lesser carbon footprint for fertilizing crop production.
• Use of struvite can potentially decrease the use of limited non-renewable phosphate deposits to produce fertilizers, which also avoids detrimental environmental impacts of phosphate mining.

How it works?

• Dairy manure with suitable low suspended solids content is adjusted for pH to release the bound phosphorous from the calcium/magnesium ions and upon later pH/magnesium adjustment flows past a seed crystal of struvite. Like an elementary school demonstration where a solution of salt causes a seed crystal to grow larger and larger by moving from the solution to the crystal, this cone-shaped reactor does the same, producing more and more granules of struvite that can be removed by auger to a container. The treated manure liquid, now significantly reduced in phosphorous, is then sent to a lagoon for storage and subsequent field application.
• Struvite, magnesium ammonium phosphate, (MAP) is a phosphate crystal that contains magnesium, ammonium, and phosphate.
• Struvite crystallization is high P removal technology which typically produces a product that consists 13% of phosphorus, 6% nitrogen, and 10% magnesium and that can be used as a slow-release, granular fertilizer.

Pretreatment and/or Post-treatment Required

• The complex composition of raw dairy manure, particularly its suspended solids and calcium concentrations, interferes with the struvite formation process.
• Struvite recovery from dairy manure must use prior solid-liquid separation technologies as well as chemical and pH adjustment to achieve suitable separation performance.

Limitations

• Struvite recovery technologies work well for dilute waste water streams, but not for scrape manures or manure without solids separation.
• The cost of chemical and pH adjustment negatively impacts the widespread adoption of this technology by dairy farms.

Other Considerations

• The success of struvite output formation depends on characteristics of waste stream and thermodynamics of solid and liquid phases (e.g. solution pH, chemical composition of the waste water, agitation parameters and processing temperature).
• There is need for developing recommended agronomic practices to utilize struvite as fertilizers in crop production.
• Use of pretreatment chemicals preclude organic certification of the produced struvite.