EVALUATION OF STRUVITE AS A REPLACEMENT FOR TRADITIONAL SOURCES OF NITROGEN AND PHOSPHORUS IN THE PRODUCTION OF MICROALGAE IN OUTDOOR RACEWAYS .

Fossil fuels are a finite resource with no true consensus as to remaining reserves. The production of microalgae to generate biofuel is a way to help alleviate the ever growing demand on fossil fuels. A persistent challenge with growing microalgae is the continued availability of nutrients required for its production in mass quantities. Phosphorus is an essential nutrient required by all plants, however, it is a finite resource whose global depletion is expected to occur at earliest estimates by 2050. If methods to conserve and cost effectively recycle phosphorus are not developed, depletion could eventually lead to a food vs. fuel crisis. A possible means of conserving phosphorus lies in struvite, a crystal compound with a 1:1:1 molar ratio of Mg2+:NH4+:PO43- formed by the combination and reaction of magnesium, phosphate, and ammonia. Struvite is typically derived from waste streams and has good potential for use as a microalgae fertilizer. Eight 15-day outdoor trials and two 90-day semi-continuous microalgae culture trials using Nannochloropsis salina (CCMP 1776), Phaeodactylum tricornutum (local isolate), or a mixture of both evaluated struvite’s ability to replace or supplement nitrogen and phosphorus. All trials were conducted outdoors in 557 L raceways. Control raceways (n=3) were supplemented with ammonium sulfate and phosphoric acid at a 1:1 ratio for N replacement trials and a 16:1 ratio for P replacement trials. Experimental raceways (n=3) were supplemented with struvite to replace 100, 67, or 33% of the N and/or P in the control. Struvite supplemented raceways performed statistically similar or better than control raceways. Maximum productivity values in 15-day trials ranged from 5.02 to 19.42 g AFDW/m2/day. Mean productivities of the 90-day trials ranged from ≈11.5 (winter) to ≈16.5 g AFDW/m2/day (summer). Results indicate that struvite is able to completely replace N and P without negative effects on growth.