Enhancing the anaerobic digestion of poultry manure through nutrient supplementation via duckweed biomass

The main aim of this research was to investigate whether small additions of duckweed biomass would enhance the anaerobic digestion of poultry manure. Experiments were undertaken in: 1 litre batch digesters; 25 litre semi-continuous digesters; and a 2000 litre pilot plant. Early experiments, carried out in the batch digesters, showed that adding a small quantity of duckweed biomass to the digestion media could produce significantly improved rates of microbial succession. Digesters receiving no duckweed took over 40 days to reach peak methane production compared to 15 days in the duckweed enriched digesters. Subsequent experiments revealed that the stimulatory effect was due to the release of iron from the duckweed in which it was present in anomalousy high concentrations. In response to this finding a small number of duckweed culturing experiments were undertaken to assess the ability of duckweed to assimilate various heavy metals. The results from these experiments are presented alongside a literature review outlining recent advances in the field of duckweed culturing. Having identified the microbial requirement for iron, direct comparisons were drawn between digesters enriched with iron via the duckweed to ones enriched with iron from more conventional sources (ferric chloride, ferrous sulphate, etc.). These conventional sources did not stimulate the digestion process to the same degree as did equivalent additions of iron via duckweed. In the pilot plant improvements in daily gas production of over 55% were achieved following the addition of iron-rich duckweed equivalent to 20,000 mgFe m-3. There is great potential for the commercial exploitation of this finding.