Effects of electrokinetic and ultrasonication pre-treatment and two-step anaerobic digestion of biowastes on the nitrogen fertiliser value by injection or surface banding to cereal crops

Summary

Biogas production from anaerobic digestion (AD) of biowastes is restricted by the recalcitrant nature of many
substrates, and this may also reduce the fertiliser value of the produced digestate. The degradability of substrates
can potentially be enhanced by physico-chemical pre-treatments before AD, and/or the degradation can be
increased by a longer digestion time. In this study, we evaluated the effects of electrokinetic (high voltage) and
ultrasonication pre-treatments of biowastes in a two-step AD process on nitrogen fertiliser replacement value
(NFRV) of digestates obtained from two biogas plants with contrasting hydraulic retention time (HRT) in the
primary AD step. The fertiliser value was tested by direct injection to spring barley and surface-banding to winter
wheat, and the ammonium N was 15N-labelled to evaluate ammonia losses. The electrokinetic pre-treatment step
significantly (p < 0.05) increased the NH4+-N/total N in the digestates before the second AD step but had an
insignificant effect on the fertiliser value in winter wheat and spring barley. Ultrasonication pre-treatment had
also no significant effect on the fertiliser value. The two-step AD significantly (p < 0.001) increased 15N recoveries
and mineral fertiliser equivalence of labelled ammonium-N in winter wheat and reduced ammonia
losses, with a significant effect (p < 0.001) observed in digestates sourced from a shorter HRT biogas reactor. The
fertiliser equivalence of labelled ammonium-N in the digestates was 80–88% after injection, indicating relatively
low N immobilisation with all the digestates. NFRV in the crops was mainly explained by the NH4+-N/total N
ratio, C/N ratio and dry matter content of the digestates. The findings suggest that electrokinetic and ultrasonication
pre-treatments combined with a second AD step have no considerable impact on the fertiliser value of
digestates, whereas a second AD step significantly reduced ammonia losses after application by surface-banding
in winter wheat.