There's also work on recovering phosphorus from seawater, eg [1], which would be an effectively unlimited source. [1] https://doi.org/10.1016/j.watres.2020.115

There's also work on recovering phosphorus from seawater, eg [1], which would be an effectively unlimited source. [1] https://doi.org/10.1016/j.watres.2020.115572 https://doi.org/10.1016/j.watres.2020.115572