Recovery of dairy manure nutrients by benthic freshwater algae

Harnessing solar energy to grow algal biomass on wastewater nutrients could provide a holistic solution to nutrient management problems on dairy farms. The production of algae from a portion of manure nutrients to replace high-protein feed supplements which are often imported (along with considerable nutrients) onto the farm could potentially link consumption and supply of on-farm nutrients. The objective of this research was to assess the ability of benthic freshwater algae to recover nutrients from dairy manure and to evaluate nutrient uptake rates and dry matter/crude protein yields in comparison to a conventional cropping system. Benthic algae growth chambers were operated in semi-batch mode by continuously recycling wastewater and adding manure inputs daily. Using total nitrogen (TN) loading rates of 0.64–1.03 gm−2 d−1, the dried algal yields were 5.3–5.5 gm−2 d−1. The dried algae contained 1.5–2.1% P and 4.9–7.1% N. At a TN loading rate of 1.03 gm−2 d−1, algal biomass contained 7.1% N compared to only 4.9% N at a TN loading rate of 0.64 gm−2 d−1. In the best case, algal biomass had a crude protein content of 44%, compared to a typical corn silage protein content of 7%. At a dry matter yield of 5.5 gm−2 d−1, this is equivalent to an annual N uptake rate of 1430 kgha−1 yr−1. Compared to a conventional corn/rye rotation, such benthic algae production rates would require 26% of the land area requirements for equivalent N uptake rates and 23% of the land area requirements on a P uptake basis. Combining conventional cropping systems with an algal treatment system could facilitate more efficient crop production and farm nutrient management, allowing dairy operations to be environmentally sustainable on fewer acres.