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The wood ash-based treatment of anaerobic digestates is a novel technology to achieve the stabilization and improve the properties of organic amendments as slow-release fertilizers. The present work evaluated 3 strategies to optimize the blending of these materials under minimum acidification. The first strategy consisted in the titration of 2 samples of digestate and 2 samples of wood ash with 4 acids (H2SO4, HCl, HNO3, and CH3CHOHCOOH). The stability of the samples was inferred from the pH measurements and the simulations with Visual MINTEQ allowed to confirm some of the on-going phenomena. The bottom ash and the agro-waste digestate required less acid to reach the targeted pH of 5.5, which prevents ammonia volatilization, due to low reactivity of the coarse particles and the low ammoniacal nitrogen content, respectively. The calculation of the pH of zero point of charge further minimized the use of acid reagents, since the optimum pH for the treatment of the digestates could be 11.90 ± 0.50 and 12.43 ± 0.06, when employing the finer and coarser fractions of the ashes, respectively. In the second strategy tested, commercial doses of acids were applied to the digestates before the addition of the ashes. The sorption of ammonia onto the fly ash at pH 10.49 ± 0.76 was found to be key to preserve the nitrogen of the food waste digestate, while saving HCl or HNO3. The third strategy involved washing the ashes repeatedly with ultrapure milli-Q® water to remove the impurities and reach a pH closer to the zero point of charge without using acids. The removal of impurities and most reactive alkali elements (e.g. sodium and potassium) from the ashes led to approximately 5 % lower pH and higher carbon, calcium, and magnesium contents, which could enhance sorption processes upon combination of these materials with the digestates. The integration of washing the wood ashes and commercial acidification of anaerobic digestates would provide the best conditions for the preparation

The depletion of world´s mineral resources increases the cost of production of chemical fertilisers for agriculture, due to the need for extraction of raw materials in poorly accessible and inconvenient sites. An enhancement of the circular economy by means of reprocessing clean nutrient-rich waste materials will also address environmental challenges associated with the low use efficiency of the nutrients, which leads to pollution of water, soil and air due to the excessive fertilisation of fields to maintain crop yields and provide enough foodstuff. This thesis investigates the treatment of anaerobic digestate with wood ash and commercial acids (i.e. sulphuric, hydrochloric, nitric and lactic acids) for the preparation of a stable slow-released fertiliser and mineral products, such as ammonium sulphate. The extraction of the biogas from the anaerobic digestate implies the mineralisation of some organic nutrients (e.g. organic nitrogen; Norg), thus becoming more available for plants (e.g. formation of ammoniacal nitrogen; NH4+-N). The application of this material to land represents a problem due to the volatilisation of ammonia and leaching of nitrate (after nitrification of ammonium in the soil) and phosphate. The role of the wood ash as sorbent could be used to control the availability of the nutrients already present in the anaerobic digestate. Moreover, the wood ash provides additional phosphorus to generate a more balanced organic fertiliser, according to the type of crop. The investigation of the wood ash treatment of the anaerobic digestate under different pHs was necessary to find the best blending conditions of these two waste streams. A food waste digestate (FWD) and a post-harvest vegetable waste digestate (PVWD) were the two types of organic material considered. A wood fly ash (WFA) and a wood bottom ash (WBA) were the two types of sorbents employed and they were produced in the same combined heat and power plant. Among these samples, the best controlled

The present study combined two nutrient management strategies to improve the marketability of a waste-derived fertilizer: (a) isolation of ammoniacal nitrogen and (b) preparation of a bulk soil amendment. The wood fly ash with low content of pollutants was added to an agrowaste anaerobic digestate as alkaline stabilizer, which promoted the volatilization of ammonia and adsorption processes, and as nutrient supplement. The 39.71 ± 1.44 g blend was incubated for 60 hours at 20°C and 100 rpm in a closed chamber (250-mL Schott Duran® bottle) with a 5.21 ± 0.10 mL sulfuric acid trap of 10 different concentrations (0.11, 0.21, 0.32, 0.43, 0.54, 0.64, 0.75, 0.86, 0.96, and 1.07 mol/L). For analytical purposes, the sulfuric acid, water-soluble, and water-insoluble fractions of the blend were isolated after the incubation. The 1.07 mol/L sulfuric acid solution contained 23.69 ± 5.72 % more of ammonical nitrogen than the 0.11 mol/L solutions. However, in all cases the amount of nitrogen in the H2SO4 compartment was lower than the one in the water-soluble and water-insoluble fractions. Only the 15.52 ± 2.13 % of the nitrogen accounted after the incubation was found in the H2SO4 trap. The bottleneck of the NH3 stripping process was the rate of mass transfer at the interface between the blended fertilizer and the headspace of the closed chamber. The organic phosphorus was more susceptible to be adsorbed during the alkaline treatment with non-intrusive acidification than the nitrogen and carbon. Activation of the ash as adsorbent before mixing with the digestate should improve the properties of the blend as slow release fertilizer, since more nutrients would end in the water-insoluble fraction.

The addition of wood bottom ash to an agrowaste anaerobic digestate has been proposed for improving the carbon to nutrient ratio of the soil organic amendment (C/N/P < 100/10/1). The ash-based treatment also aimed to improve properties of the anaerobic digestate as controlled-release fertilizer by decreasing the availability of nitrogen, carbon, and phosphorus. Treated (39.86 ± 0.94 g) and untreated (33.51 ± 0.84 g) digestates were incubated for 7 h at 20 °C and 100 rpm, in a 250-mL chamber with a 0.11 M sulfuric acid trap of 4.38 ± 0.02 mL. By the end of the incubation, the blend of 13.12 ± 2.87% ash and 86.88 ± 2.87% digestate released 8.95 ± 2.19 times more ammonia than the digestate alone. This was in agreement with the lower content of water-soluble ammoniacal nitrogen in the treated digestate (93.35 ± 21.00 mg/kg) than in the digestate incubated without the addition of the ash (357.64 ± 54.10 mg/kg). As per the very low levels of water-soluble nitric nitrogen found in the treated (0.16 ± 0.05 mg/kg) and the untreated (0.06 ± 0.02 mg/kg) digestate, the emissions of nitrous oxide were discarded. The availability of carbon increased 1.43 ± 0.82 times and the availability of phosphorus decreased approximately 100 times due to the addition of the ash to the digestate. The ash-based treatment was successful in improving the C/N/P of the digestate from 121.51 ± 57.18/9.94 ± 4.21/1 to 42.05 ± 9.88/2.48 ± 0.58/1.

Pollution reduction from waste management and energy generation is necessary to mitigate climate change and is one of the major challenges of the 21th century. This can be achieved through the development of innovative energy recovery technologies from biomass and wastes, such as microwave plasma gasification. An envelope of stable CO2 plasma operation is described, by varying working gas flow rate at applied microwave powers between 1 and 6 kW, whereas H2O plasma operation is possible with flow rate ranging from 20 to 50 g/min and microwave powers between 2.5 and 6 kW. The temperature generated in a large chamber connected to the plasma torch is recorded, reaching up to 850°C, showing a heterogeneous temperature distribution. In addition, optical emission spectroscopy measurements provide an insight into plasma chemistry and demonstrate the dissociation of CO2 and H2O molecules at extremely high temperatures of up to 6,300°C assuming local thermodynamic equilibrium. The experimental results demonstrate that the microwave plasma torch provides an ideal environment for gasification with high temperature and very chemically reactive species. This study provides valuable information for the design of microwave plasma gasification reactors with great potential for effective solid feedstock conversion into high quality syngas for energy production.

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.

Microwave-induced plasma (MIP) is an innovative technology with potential application for waste treatment and clean energy generation. The study presents the investigation of an MIP torch operated with carbon dioxide and steam. Optical spectroscopy measurements demonstrate the dissociation of CO2 and H2O molecules. This work shows that the MIP torch can generate a high temperature environment with very active chemical species, making it a promising technology for waste and biomass gasification.