Abstract:
Nutrient pollution—mainly nitrogen and phosphorus—caused by organic waste continues
to impact the environment. The implementation of a circular economy is integral to alleviating these
effects. Liquid digestate, which is a byproduct of anaerobic digestion (a waste-valorising process), is a
nutrient-dense organic fertiliser with vast applications in agriculture. Using an aerobic polishing unit,
this study developed a viable method for the preparation of a hydroponic fertiliser by investigating
the effect of pH on the nutrient recycling capabilities of said system. The heterotrophic bacteria
present in the biofilm, identified by 16S gene sequencing, are responsible for 90% of organic carbon
(as TOC) removal with minimal ammonium loss. This is ideal for promoting optimal nitrification in
hydroponic systems in the absence of organic carbon to ensure plant growth is not affected. Although
pH 8 was found to be ideal for batch operation, this pH condition resulted in decreased microbial
longevity and, therefore, increased ammonification due to microbial decay. Therefore, continuous
operation at pH 7 proved to be a better option owing to the ammonium-rich effluent (>220 mg/L)
which was produced, which is on par with the nitrogen concentration of a Hoagland solution. The
continuous carbon polishing of liquid digestate provides an efficient way of utilising organic fertilisers
in hydroponic systems.
