Alfonso Jose Lag-Brotons

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 Abstract

Editorial: Resource Recovery From Waste

The application of digestate from anaerobic digestion to grassland soils is of growing interest as an agricultural practice. However, significant uncertainties surrounding the potential impacts of digestate application on processes associated with the soil microbial community remain, particularly for processes governing Carbon Use Efficiency (CUE) and the broader soil C cycle. In this research, we examined how the C:N stoichiometry of digestate and the nutrient status of soil influenced the impact of digestate application on the soil C cycle.   Three fractions of digestate (whole [WD], solid [SD] and liquid [LD]), spanning a range of C:N, were each applied to two soils of contrasting starting nutrient status (high and low) and compared to unamended controls (Ctr). Two short-term incubations, each lasting seven days, were undertaken. In the first, applications of WD, SD and LD each achieved the same total N input to soils. In the second, digestate applications were adjusted to provide consistent total C input to soils. In each incubation, CO2-C efflux, microbial biomass C (Cmicro) and pH were determined.   In each of the two incubations, the application of digestate significantly increased cumulative CO2-C efflux compared to control soils. However, the precise effect of digestate application varied between the two incubations and with both soil nutrient status and digestate fraction. Microbial biomass C was largely unchanged by the treatments in both incubations. During the first incubation, soil pH decreased substantially following each digestate treatment in both soil types. A similar pattern was observed within the second incubation in the high nutrient soil. However, in contrast, soil pH increased substantially following LD and WD application to the low nutrient soil in the second incubation. Varying CUE responses are likely to be observed following the application of digestate to agricultural soils, dependent on digestate fraction, C:N ratio of the digestate, and the initial soil nutrient status. Therefore, digestate application rates and soil management must be carefully planned in order to avoid adverse impacts of digestate application to land.   

Applying digestate to soil is of growing interest in agriculture. However, the impacts of digestate on soil biogeochemical cycles often remain unclear, especially after solid-liquid separation of whole digestate (WD). We used a 21 d incubation to examine the effects of WD and solid digestate (SD) on CO2-C efflux, dissolved organic carbon (DOC), microbial biomass C (Cmicro), phospholipid fatty acid (PLFA) and carbon use efficiency (CUE) within two grassland soils of contrasting nutrient status. Application rates for SD and WD were based on recommended N inputs to grassland soils for these organic materials. Compared to un-amended controls, cumulative CO2-C efflux, Cmicro and the fungal:bacterial in soils increased significantly following SD application, regardless of the soil nutrient content (+20% CO2-C, +29% Cmicro, +58% fungal:bacteria for high nutrient soil; +563% CO2-C, +36% Cmicro, +18% fungal:bacteria for low nutrient soil). In contrast, WD produced a significant effect on CO2-C efflux and fungal:bacterial only in the low nutrient soil. Our results also indicated that both digestate fractions and the initial soil nutrient status affected CUE. Applying both SD and WD to a low nutrient soil potential leads to decreases in soil C stocks, whilst the application of SD to a high nutrient soil can potentially enhance soil C stocks. Digestate application must be carefully planned, accounting for both the nature of the digestate and of the soil, in order to avoid adverse impacts on soil C stocks.  

While bioenergy generation represents a step towards sustainability, residues and by-products are generated as a result of this process. In order to take the next step and ‘close the loop’ on the generated residues, resource recovery from these materials is required. In this chapter we will focus on the use of blends of biomass ash and anaerobic digestate, derived from industrial scale bioenergy production processes, as potential alternatives to conventional inorganic fertilisers. Topics will not only cover waste streams, their nature and relevance to agriculture, but also experimental assessments at different scales of the effects of these waste stream combinations on the soil–plant system and the wider environment.

A number of policy mechanisms have driven the development of a circular economy in organic waste management. This chapter explores the journey of organic wastes from a waste management problem to a renewable energy solution, and then looks to their future as a viable market alternative to conventional inorganic fertilisers. This transition reflects changes in the wider waste policy landscape with a shift from waste management to resource recovery—a viewpoint that is to become increasingly important as the UK and EU look to pursue circular economy strategies. Producing alternative fertilisers from bioenergy by-products provides a neat principle for returning nutrients to soils and closing the loop. However, myriad barriers make this challenging from regulatory viewpoints. Regulations are necessarily risk-conservative, yet arguably innovation-prohibitive. This chapter will explore the challenges that may arise from possible conflicts in regulations, and seeks to establish a way forward for the entry of bioenergy by-products into the circular economy.