Abstract
Handling of food waste and agro-industrial waste digestates, and fly and bottom wood ashes represents an economic and environmental problem for society and industries where they are produced due to the requirement of large capacity storage facilities with special conditions. Since both materials contain important nutrients for crops, two different blending combinations were examined as potential slow-release fertilizers to enhance the circular economy. High share of biomass ashes were selected aiming the chemical alkaline stabilization of the digestates. The concentrations of nitrogen, carbon, and phosphorus in the water-soluble extract of the blends, which was obtained by adding ten parts of milli-Q® per each part of sample and subsequent solid–liquid separation, were measured during a 10-h incubation at 22 °C and 100 rpm. The volatilization of ammonia was the main route of losing nitrogen, with the greatest rate of volatilization at the time of blending and approximately 50 mg NH3-N/kg blend/hour by the end of the incubation. The hydrolysis taking place at alkaline pH was found responsible of the conversion of organic nitrogen to ammoniacal nitrogen and of the increase of the amount of water-soluble carbon, which rose progressively during the incubation. The release of CO2 was prevented by the high pH (> 10) of the blends. The drop in the concentration of water-soluble orthophosphate at the time of preparing the mixtures was driven by adsorption onto the ashes. The desorption of some of the orthophosphate was found to be inversely related to the amount of ashes added to the digestates.Graphic
Highlights
This work contributed to the development of the technology of alkaline stabilization of organic wastes
There was no significant difference between the pH values of the digestates, it was expected that the food waste digestate (FWD) would have greater buffer capacity to prevent the increase of the pH when adding the ashes because of the greater content of NH4+-N (Eq (1))
It should be noted that less than 1% of the phosphorus contained in the ashes was soluble (Table 3) because it was in the form of insoluble calcium compounds, such as hydroxyapatite (Ca5(PO4)3OH) [9]
Summary
This work contributed to the development of the technology of alkaline stabilization of organic wastes. Considering the interrelations between the three levels of stabilization, a holistic approach was proposed for the intensification of the process of manufacturing of advanced fertilizers. In this way, the mixing ratios of lowpollutant fly and bottom wood ashes with food waste and agro-industrial waste digestates, which were primarily selected to assess the chemical stability of the blends (i.e. the availability of carbon and nutrients), seek positive impacts on biological and physical stabilities. The management of digestates and ashes represents a problem for the industries where they are produced Whilst both issues are operational expenditure challenges, for ashes it is predominantly related to the gate-fee for disposal in landfills. Organic slow-release fertilizers are known for having low nutrient availability, which is typically enough for the utilization by the soil biota while preventing important losses due to runoff waters and gas exchange [3]
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