Treatment of dairy manure effluent using freshwater algae: Algal productivity and recovery of manure nutrients using pilot-scale algal turf scrubbers

Cultivating algae on nitrogen (N) and phosphorus (P) in animal manure effluents presents an alternative to the current practice of land application. The objective of this study was to determine values for productivity, nutrient content, and nutrient recovery using filamentous green algae grown in outdoor raceways at different loading rates of raw and anaerobically digested dairy manure effluent. Algal turf scrubber raceways (30 m2 each) were operated in central Maryland for approximately 270 days each year (roughly April 1–December 31) from 2003 to 2006. Algal biomass was harvested every 4–12 days from the raceways after daily additions of manure effluent corresponding to loading rates of 0.3 to 2.5 g total N (TN) and 0.08 to 0.42 g total P (TP) m−2 d−1. Mean algal productivity values increased from approximately 2.5 g DW m−2 d−1 at the lowest loading rate (0.3 g TN m−2 d−1) to 25 g DW m−2 d−1 at the highest loading rate (2.5 g TN m−2 d−1). Mean N and P contents in the dried biomass increased 1.5–2.0-fold with increasing loading rate up to maximums of 7% N and 1% P (dry weight basis). Although variable, algal N and P accounted for roughly 70–90% of input N and P at loading rates below 1 g TN, 0.15 g TP m−2 d−1. N and P recovery rates decreased to 50–80% at higher loading rates. There were no significant differences in algal productivity, algal N and P content, or N and P recovery values from raceways with carbon dioxide supplementation compared to values from raceways without added carbon dioxide. Projected annual operational costs are very high on a per animal basis ($780 per cow). However, within the context of reducing nutrient inputs in sensitive watersheds such as the Chesapeake Bay, projected operational costs of $11 per kg N are well below the costs cited for upgrading existing water treatment plants.