Biotransformation of Organic Waste into High Quality Fertilizer: Potentials and Limitations for Compost Process Optimization and Nutrient Recycling

Research output: Book/ReportPh.D. thesisResearch

  • Sofie Bryndum
Agriculture faces several challenges of future provision of nutrients such as limited P reserves and increasing prices of synthetic fertilizers and recycling of nutrients from organic waste can be an important strategy for the long-term sustainability of the agricultural systems. Organically derived nutrient sources, however, generally suffer from three major shortcomings compared to synthetic nutrient sources: (1) the relatively low concentration of nutrients makes them bulky when applied in the required amounts; (2) the immediate plant availability of organically-bound nutrients, e.g. N, P and S, is often low; and (3) the unbalanced composition of nutrients rarely matches crop demands.
Therefore the objective of this project was to investigate the potential for (1) recycling nutrients from agro-industrial wastes and (2) compost biotransformation into high-quality organic fertilizers, with specific focus on reducing nutrient losses during processing and improving bioavailability of nutrients in the final fertilizer product. The project was formulated at two levels of analysis; a contextual-system level and a compost-system level. Costa Rica was used as a contextual case to conduct a nutrient inventory study which quantified the total mass and nutrient content of agroindustrial
derived wastes and by-products and identified flows into different uses (Paper III). The compost-system level included two studies; a pilot scale study conducted in Costa Rica (Paper I) and a laboratory scale study, including three succeeding compost experiments, conducted in Denmark (Paper II). The composting studies investigated three management interventions to enhance the compost fertilizer quality and accelerate compost turnover: (1) the addition of nutrient rich material, (2) the compost turning frequency and (3) the inoculation with functional microorganisms.
The nutrient inventory study showed that the current distribution of uses had been optimized according to the most cost-effective use, considering organic waste quality, facilitated handling and transportation constraints, and redirection of more organic waste from other uses into fertilizer use would be unlikely. An estimated ~50 % of the total organic waste pool, primarily consisting of animal manure and waste from the processing of sugar cane, coffee, oil palm and oranges, is currently being re-used as “fertilizers”, meaning it is eventually returned to the soil with varying
degrees of pre-treatment (Paper III).
The compost studies showed that enrichment with various N and P rich materials can increase the fertilizer quality of the final compost product and accelerated the compost process (Paper I and II). Delayed addition of poultry manure until after the peak temperature phase reduced N losses and resulted in twice the mineral N concentration in the final compost (Paper I). Enrichment with pig manure or rock phosphate combined with ammonium sulfate increased the final concentrations of available N and P (Paper II). Compost turning frequency of three days compared to nine days did
not accelerate the rate of decomposition, contrary there was a tendency that frequent turning reduced the rate of decomposition due to excessive moisture loss (Paper I). The inoculation with functional microorganism did not influence the availability of N and P in any of the inoculated composts neither did inoculation accelerate compost process (Paper II).
The main conclusions of the project are (1) increasing the nutrient recycling from agro-industrial waste in Costa Rica by redirection of more waste from other uses into the “fertilizer use” pool is not likely however there is a potential to increased nutrient recycling by reducing nutrient losses through improved management of the organic waste already designated as fertilizers and (2) composting has the potential to transform plant-derived organic waste into high-quality fertilizers if nutrient rich materials are added. Delaying the addition of nutrient rich material until after the peak
temperature phase can be a strategy to reduce N losses and improve the final mineral N concentration. (3) composting studies are very challenging because of the dynamic and complex nature of the compost process and it is difficult to manipulate selected parameters while maintaining others to investigate the effect of specific management strategies.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Publication statusPublished - 2014

ID: 124446217