Abstract
AbstractThe liquid fraction after liquid/solid separation of biogas digestate has a high potential as a fertilizer due to its high nutrient concentration. However, the direct application of digestate in agricultural fields results in practical problems due to its voluminous nature. One solution to this could be to concentrate nutrients onto sorbents such as biochar or zeolites, which can subsequently be used as a fertilizer. This study investigated the ability of biochar and zeolite ‘clinoptilolite' enriched with digestate nutrients to supply nitrogen (N) when used as a fertilizer. A pot experiment with ryegrass was conducted to test the effect of a nutrient‐enriched biochar and clinoptilolite by determining plant biomass growth and N uptake. This included untreated biochar and clinoptilolite as controls and two levels of N application (15 and 45 mg N per pot) each at two initial loading ratios (low and high). Nutrient‐enriched biochar and clinoptilolite increased plant biomass yield (up to 1.02 and 2.39 g per pot) and N uptake (up to 11.23 and 39.94 mg N per pot) compared to the untreated sorbents treatments. Initial loading ratio had a significant effect on plant biomass response and apparent N recovery (ANR) for enriched clinoptilolite, and lower initial loading ratio improved plant growth. In contrast to clinoptilolite, higher initial loading ratio resulted in higher ANR. In conclusion, our results reveal that N released from enriched clinoptilolite and biochar could be taken up by the plants, clinoptilolite performed more effectively than biochar, and initial loading ratio affected the performance of the sorbents when used as a fertilizer.
Highlights
Digestate from anaerobic digestion in biogas plants shows comparable properties to animal slurry, both physically and as a fertilizer
Biomass yield and N uptake were significantly lower for the control with 0.62 (– 0.05) g per pot biomass yield and 5.62 (– 0.34) mg N per pot N uptake, compared to the N-enriched materials in all ANOVA comparisons (P < 5% after Holm correction), which indicates that application of enriched biochar and clinoptilolite increased both biomass and N uptake (Fig. 2)
Slopes for biomass are interpreted as the biomass response (BR) and slopes for N uptake are interpreted as apparent nitrogen recovery (ANR)
Summary
Digestate from anaerobic digestion in biogas plants shows comparable properties to animal slurry, both physically and as a fertilizer. This is the case when manure is the only feedstock, and when additional organic waste feedstock is used for co-fermentation. In areas with a high animal density and, a nutrient surplus compared to crop requirements, mechanical slurry separation can be used as a method to facilitate redistribution of nutrient surpluses to other areas (Møller et al, 2000; Paavola and Rintala, 2008; Jørgensen and Jensen, 2009). The solid fraction resulting from the separation contains most of the dry matter and phosphorus (Hjorth et al, 2009, 2010), but constitutes only 10–25% of the total mass, so it can be transported more and economically to regions with lower animal densities and no nutrient surplus (Møller et al, 2000, 2007).
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