Creating a Circular Nitrogen Bioeconomy in Agricultural Systems through Nutrient Recovery and Upcycling by Microalgae and Duckweed: Past Efforts and Future Trends
Authors: Femeena Pandara Valappil , House Gregory R. , Brennan Rachel A. 
Published: March 2022

HighlightsAquatic vegetation-based nutrient recovery offers an alternate approach for treating agricultural wastewater.Microalgae and duckweed can upcycle waste nutrients into valuable bio-based products.Producing feed, fertilizer, and fuel from manure-grown aquatic vegetation promotes a circular N-bioeconomy.Abstract. The massive amounts of nutrients that are currently released into the environment as waste have the potential to be recovered and transformed from a liability into an asset through photosynthesis, industry insight, and ecologically informed engineering design aimed at circularity. Fast-growing aquatic plant-like vegetation such as microalgae and duckweed have the capacity to enable local communities to simultaneously treat their own polluted water and retain nutrients that underlie the productivity of modern agriculture. Not only are they highly effective at upcycling waste nutrients into protein-rich biomass, microalgae and duckweed also offer excellent opportunities to substitute or complement conventional synthetic fertilizers, feedstocks in biorefineries, and livestock feed while simultaneously reducing the energy consumption and greenhouse gas emissions that would otherwise be required for their production and transport to farms. Integrated systems growing microalgae or duckweed on manure or agricultural runoff, and subsequent reuse of the harvested biomass to produce animal feed, soil amendments, and biofuels, present a sustainable approach to advancing circularity in agricultural systems. This article provides a review of past efforts toward advancing the circular nitrogen bioeconomy using microalgae- and duckweed-based technologies to treat, recover, and upcycle nutrients from agricultural waste. The majority of the work with microalgae- and duckweed-based wastewater treatment has been concentrated on municipal and industrial effluents, with &lt;50% of studies focusing on agricultural wastewater. In terms of scale, more than 91% of the microalgae-based studies and 58% of the duckweed-based studies were conducted at laboratory-scale. While the range of nutrient removals achieved using these technologies depends on various factors such as species, light, and media concentrations, 65% to 100% of total N, 82% to 100% of total P, 98% to 100% of NO3-, and 96% to 100% of NH3/NH4+ can be removed by treating wastewater with microalgae. For duckweed, removals of 75% to 98% total N, 81% to 93% total P, 72% to 98% NH3/NH4+, and 57% to 92% NO3- have been reported. Operating conditions such as hydraulic retention time, pH, temperature, and the presence of toxic nutrient levels and competing species in the media should be given due consideration when designing these systems to yield optimum benefits. In addition to in-depth studies and scientific advancements, policies encouraging supply chain development, market penetration, and consumer acceptance of these technologies are vitally needed to overcome challenges and to yield substantial socio-economic and environmental benefits from microalgae- and duckweed-based agricultural wastewater treatment. Keywords: Circular bioeconomy, Duckweed, Manure treatment, Microalgae, Nitrogen, Nutrient recycling, Wastewater treatment.