Struvite crystals from Wikimedia Commons
Wastewater treatment facilities are now faced with permits requiring phosphate removal before discharge. If discharged into receiving streams, excess phosphate often triggers algae blooms including problematic "toxic" algae that cause off-flavors in drinking water and other direct toxicity to wildlife. To meet these new permits, engineers are installing a variety biological and chemical technologies to remove soluble phosphate from the water and entrap the phosphate into sludge. Then the question is what to do with the produced sludge - especially the sludge generated from metal salts phosphate removal technology.
Several companies have introduced new technologies for removing soluble phosphate in a form suitable for use as a fertilizer. This is important as most fertilizer grade phosphate is mined and mined phosphate is getting more expensive as natural deposits are depleted for agricultural fertilizer. The most promising new technology is based on using biological processes to concentrate soluble phosphate and nitrogen in water. This is already done during sludge digestion in anaerobic digesters. As solids going to the digester contain high levels of N & P, as the solids are digested releasing carbon in the form of methane and carbon dioxide; both phosphorus and nitrogen are released into solution. The resulting supernatant is much higher in both N & P than the influent.
The new process includes raising the digester supernatant pH to between 7 - 11. Magnesium salts are then added with mixing. The equal molar ratios of P:N:Mg result in the formation of struvite crystals which drop out of the water removing soluble nitrogen and phosphorus. The crystals are collected and are suitable for use as fertilizer. The struvite crystal has a fertilizer value of N - 5%, P2O5 - 28%, and Mg - 10% and is great as a time release fertilizer.
Several companies have introduced new technologies for removing soluble phosphate in a form suitable for use as a fertilizer. This is important as most fertilizer grade phosphate is mined and mined phosphate is getting more expensive as natural deposits are depleted for agricultural fertilizer. The most promising new technology is based on using biological processes to concentrate soluble phosphate and nitrogen in water. This is already done during sludge digestion in anaerobic digesters. As solids going to the digester contain high levels of N & P, as the solids are digested releasing carbon in the form of methane and carbon dioxide; both phosphorus and nitrogen are released into solution. The resulting supernatant is much higher in both N & P than the influent.
The new process includes raising the digester supernatant pH to between 7 - 11. Magnesium salts are then added with mixing. The equal molar ratios of P:N:Mg result in the formation of struvite crystals which drop out of the water removing soluble nitrogen and phosphorus. The crystals are collected and are suitable for use as fertilizer. The struvite crystal has a fertilizer value of N - 5%, P2O5 - 28%, and Mg - 10% and is great as a time release fertilizer.
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