A COMPARISON OF STRUVITE AND TRADITIONALLY USED NUTRIENTS IN 90-DAY, SEMI-CONTINUOUS, MIXED ALGAL CULTURE PRODUCTION TRIALS IN OUTDOOR RACEWAYS

The demand for fuel and food continues to rise as the global population increases. Microalgae biomass can be effectively utilized to produce renewable energy and protein/lipid for inclusion into terrestrial and aquatic feeds without competing with food crops for land and water. Of major concern to the development of microalgae as a feedstock for biofuels and bio-products is the identification of nutrient sources. Phosphate is a required nutrient for the production of microalgae biomass, however, unlike ammonia, it is a non-renewable resource. Thus, without significant technological progress to recycle nutrients, significant expansion of algal biofuels production can be expected to start a food versus fuel crisis. Struvite, a nutrient which can be produced from various waste streams, is currently being considered as a means to effectively recycle phosphorus and nitrogen for use as a microalgae fertilizer.

The current studies were designed to compare the productivity of a mixed culture of Nannochloropsis salina (CCMP 1776) and Phaeodactylum tricornutum (local isolate) in 12 outdoor raceways (557 L) with and without struvite addition during two different growing seasons. The control raceways (n=6) were supplemented with our standard nutrient blend of ammonium sulfate, phosphoric acid and iron sulfate. Experimental treatments (n=6 each) were supplemented with commercial struvite to replace 100% of the phosphorus and 12.5% of the nitrogen in the control treatment. All experimental raceways were then supplemented with ammonium sulfate and iron sulfate to balance nutrient levels with those in the control. At nutrient "depletion" raceways received a partial harvest (75%) and nutrient supplementation. At the conclusion of the trials samples from each raceway were subjected to microbiome analysis by second-generation sequencing of small subunit rRNA genes to identify raceway composition. Algal samples were also analyzed for proximate composition for suitability as nutrient sources for aquaculture feeds. Results indicate that struvite is able to completely replace the phosphorus without negative effects on raceway production during extended production trials. Productivity data from the trial is presented in Table 1.