Aerobic polishing of liquid digestate for the preparation of hydroponic fertiliser

Abstract

Nitrogen and phosphorus form the backbone of most lifeforms on earth, however, improper management of these nutrients has devastating effects on the environment. These effects can be lessened by the use of the anaerobic digestion process, which provides methane-rich biogas and nitrogen-rich liquid biofertiliser. The use of liquid digestate (LD) in hydroponics can be problematic given that these systems are prone to microbial contamination. Thus, minimising the organic carbon present in the digestate before nitrification would make it ideal for hydroponic use.

By investigating the effect of pH on the use of a biological aerobic polishing unit for the removal of organic carbon with the retention of nitrogen and, to some extent phosphorus, the repeat-batch experiments displayed high levels of organic carbon removal at all investigated pH values (6, 7, and 8). Although ammonium losses were observed at pH 6 and 7, at pH 8, approximately 27% of the ammonium was recovered. Comparatively, the continuous process, wherein different three hydraulic retention times were investigated, performed better than the repeat-batch experiments as the TOC in the effluent was <100 mg L−1 which accounts for 90% of TOC removal from the LD. In this process, approximately 53% of the ammonium was recovered with the ammonium concentration in the polished product being >220 mg L−1 with the continuous addition of digestate. Excluding the nitrate, the nitrogen content in the polished product surpasses that of a standard Hoagland medium (≈ 210 mg L−1).

Findings from the metagenomic analysis of the biofilm indicate that a large fraction of the bacteria present in the biofilm are heterotrophic, hence the rapid decrease in TOC. The presence of other microorganisms responsible for the rapid uptake of phosphorus (Gemmatimonas spp.), nitrate assimilation (Aquamicrobium spp.) and nitrogen acquisition (Luteimonas spp.) are also documented in the findings. The combined effect of these microorganisms is the driving force behind the rapid carbon removal and nitrogen recovery seen in the findings.

This study provides a novel approach to the preparation of liquid digestate for hydroponic systems, which is not only more efficient but also minimises the risk of contamination in downstream processes, which will ultimately improve crop yield and quality. Although phosphate losses were observed throughout all experiments, the phosphate levels can be supplemented prior to hydroponic use. The continuous process provides a greater deal of nutrient recovery, while simultaneously converting a large fraction of the organic carbon present in the liquid digestate and is the suitable method for the implementation of a circular economy.