Evaluating the Environmental Impacts of Fertilizer Production Using Photosynthetic Recovery of Nutrients in Livestock Waste

Current waste management methods in the dairy industry yield significant environmental repercussions. Failure to implement proper management practices will result in the degradation of farming soil, air, and water resources. These impacts manifest as emissions of greenhouse gases and the leaching of phosphorus and nitrogen into the environment [1]. Two of the main practices that are employed are: i) solid-liquid separators (SLS) to make the manure easier to transport to long-term storage containers and ii) anaerobic digesters (AD) that can capture the methane emissions and clean the captured methane, which can be used as a renewable biofuel. Neither of these practices mitigates nutrient leaching. A promising avenue to reduce nutrient leaching and improve greenhouse emissions involves utilizing cyanobacteria (CB) [2]. By using CB, it is possible to create the Renewable Nutrients from Algae (ReNuAl) process.

The ReNuAl process presents three distinct versions: ReNuAl (0), which integrates CB into an AD system, harnessing the captured methane in an on-site generator; ReNuAl (1), which similarly employs an AD but utilizes all the captured methane as an additional revenue stream; and ReNuAl (2), which eliminates the AD and methane collection. CB functions as an optimized fertilizer, able to deliver the required nutrients to crops in the ideal ratio. Additionally, CB exhibits the potential to curb emissions by sequestering carbon dioxide.

Multiple integration pathways are feasible for incorporating CB into the waste management system for dairy farms. We performed a life cycle assessment to assess the environmental impacts of these diverse systems and compare them with prevailing practices employed by permitted farms (with more than 1000 Animal Units). We used the software openLCA with the Environmental footprint method [3]. The current practices for this analysis are assumed to be using i) an SLS and ii) an SLS and an AD. Our analysis shows that despite the larger energy requirements of ReNuAl, the emissions are substantially reduced for all proposed ReNuAl systems. However, other factors must be considered, such as the environmental effects of the current grid composition and the use of water, plastic, and synthetic nitrogen.

[1] Hu, Y., et al. Managing Conflicting Economic and Environmental Metrics in Livestock Manure Management. ACS ES&T Eng., 2022

[2] Ma, J., et al. Infrastructures for phosphorus recovery from livestock waste using cyanobacteria: Transportation, techno-economic, and policy implications. ACS Sustain. Chem. Eng., 2021

[3] Ciroth, A., et al. openLCA 1.10: Comprehensive user manual, GreenDelta, Berlin, Germany, 2020.