Overcoming challenges to recovering phosphorus as struvite from anaerobically digested dairy waste

Many dairy farmers are facing increasing pressure to upgrade their manure management systems due to new regulations and inadequate land area to absorb manure nutrients. Cost-effective solutions to manure management have been a challenge since off-site disposal is highly cost-intensive and on-site treatment technologies are limited. Anaerobically digested dairy manure liquid was collected for this study and characterized to have high magnesium (160-423 mg L^-1 Mg²⁺), ammonium (370-1800 mg L^-1 NH ₄⁺-N), and phosphate (93-328 mg L^-1 PO ₄^3--P) concentrations, making struvite (MgNH ₄PO₄·6H₂O) precipitation a viable option for nutrient removal and recovery. Struvite is crystallized, separated from wastewater, and directly applied as a slow-release solid fertilizer for various land-use applications. Anaerobic digestion appears to enhance struvite formation ability in dairy manure by increasing phosphate and ammonium concentrations, however it turns out extraction of phosphate from stable solid particles is required prior to it being recovered as struvite. This study explored water and acid extraction techniques to increase orthophosphate content for struvite formation. Water and acid extraction of nutrients increased Ca²⁺ concentration in manure liquid by 112% and 28% respectively, presenting additional challenges to struvite formation. Little Mg²⁺ increase (<7%) during both extraction methods indicated few Mg-phosphate solid particles to be present in the dairy manure. By investigating dilution and acidification techniques for nutrient extraction, the feasibility of using struvite precipitation for phosphorus recovery can be further assessed for anaerobically digested dairy manure.