Wastewater-Derived Algae as a Source of Fertilizer Nitrogen.

Algae generated in wastewater treatment is often recycled through treatment processes or landfilled.  This study evaluated algal biomass as a possible fertilizer source.  Annual ryegrass, Lolium multiflorum was grown in a peat-perlite mix in a greenhouse pot study with 0, 111, 222 and 445 kg nitrogen ha^-1 of wastewater-derived algae, commercial fertilizers, and organic fertilizer (bat guano), with and without micronutrients.  After 28 days of growth, grass shoots were harvested, dried, and weighed to compare biomass production.  Leaves were analyzed for total Kjeldahl nitrogen (TKN) and the planting medium was tested for mineralizable nitrogen (MN).  Shoot biomass increased with increasing applications of algae and each application rate was significantly different from other algae rates.  Shoot biomass with commercial fertilizers increased, but 222 and 445 kg ha^-1 applications were not statistically different (p=0.05). Addition of micronutrients made no difference in shoot biomass.  Lower application rates of algae (111 and 222 kg ha^-1) resulted in biomass significantly lower than comparable rates of commercial and organic fertilizers.  The 445 kg ha^-1 rate of algae produced similar biomass (3.41 g/pot) compared to the same rate of commercial fertilizer, but was exceeded significantly by bat guano (organic fertilizer) (6.25 g/pot). TKN content of leaves was significantly higher with the commercial fertilizer (all levels) and the 445 kg ha^-1 rate of bat guano compared to all algae treatments.  Algae applied at 445 kg ha^-1 had a mean TKN of 2.61% compared to 3.75% for the highest commercial fertilizer.  Mineralizable nitrogen (MN) of the potting mix showed a general trend of higher MN with higher applied rates of nitrogen.  While lower application rates of algal biomass were not competitive with other fertilizers when considering yield, the highest application rate was similar to the 445 kg N ha^-1 inorganic and commercial treatments, and the lowest application rate of organic fertilizer. 

With its slower mineralization rate, wastewater algae has the potential to be a useful fertilizer especially where nitrate leaching is common.  The algal fertilizer also contains high levels of carbon, which could help depleted soils improve organic matter levels while providing a source of fertility. Algae harvested fresh from dissolved air flotation (DAF) processes probably acts as a slow release fertilizer, while composted algae may supply a more immediately plant available form of nitrogen based on routine fertilizer analysis.