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Efficient removal and recycling of nitrogen from organic waste as fertiliser

Final Report Summary - NIREC (Efficient removal and recycling of nitrogen from organic waste as fertiliser)

The NIREC project aimed to develop a new sustainable technology for nutrient recovery and recycling from anaerobic digesters, which concurrently enhanced the digestion process. In addition, the project evaluated and converted residuals into marketable products, with standard quality specifications. The finalised proposal offered an integrated solution with composted solid residuals, liquid residuals that could be recycled and zero emissions.

Alternative technologies were investigated and developed in a laboratory scale during the first project period. Several pilot plants were then operated for the technologies' upscale as part of nine different, yet well connected, work packages (WPs) which were related to the following activities:
1. project management and coordination.
2. examination of alternative pretreatment technologies of possible input materials in an anaerobic digestion plant. Laboratory research was performed to select approaches and operating conditions and to define the effects of this selection on ammonia, solubilisation of solid waste and anaerobic biodegradability. The finalised alternatives were an ultrasonic disintegration, a thermal hydrolysis and a thermochemical treatment. Relevant pilot scale experiments were elaborated with a high pressure reactor.
3. development of a technology for nitrogen direct removal from the anaerobic digester through transformation into soluble ammonia. Laboratory trials on ammonia release with different enzymes and specialised microorganisms were carried out. Three concepts were tested to define the most suitable solution and steam stripping of ammonia was chosen as the most promising method. Moreover, the performance of an existing plant was compared to laboratory results and, subsequently, the maximum nitrogen concentration in the reactor was determined in order to obtain satisfactory results. Finally, a feasibility study and a complete process scheme were developed based on the WP findings.
4. refinement of the separation technology for dry solid and liquid residues with low organic matter. The solid residues would subsequently be composted while the liquid residues would be nitrogen stripped. A centrifugal extractor and a thickener were utilised and evaluated under varying operational conditions. Given that the thickener required polyelectrolyte in order to provide a clarified liquid, alternative electrolytes were assessed in the laboratory so as to select the most adequate for the project purposes. Both examined devices had operational advantages and disadvantages; thus the final selection in case of application would depend on each plant manager priorities.
5. stripping and liquid residuals management in different experimental scales. Considering a zero emission plant, the goal was to obtain either direct discharge quality or recycling of the generated wastewater within the plant. Trials were carried out with varying pH, temperature and alkalinity, which were of major influence to stripping efficiency. In addition, a membrane pilot plant, operating with microfiltration and reverse osmosis, was optimised in terms of suspended solids management and performance improvement.
6. solid anaerobic residues treatment, in order to develop a fertiliser product which would add value to the proposal. A set of organic mineral fertilisers, according to the current regulations, was produced and applied, so as to assess the products' performance, focusing on their leaching potential in common soils. Experimental reactors were built to simulate soil plots, on which selected rainfall events were reproduced and leaching tests were performed. None of the produced fertilisers presented noticeable positive or negative characteristics in comparison to conventional products.
7. definition of best operational parameters of the prototype plant so as to minimise ammonia and odour emissions. Acidic absorption, scrubbing liquids and bio-filtration were employed for treating the emitted gases. The laboratory experiments were verified within a composting trail. Application of H2SO4 was promising in terms of ammonia removal; nevertheless no significant odour reduction was achieved. An odour monitoring device was constructed and applied, but further improvement remained an essential post-project activity.
8. analysis of market acceptance, economic viability and legal framework for the potential use of the treated residues. The analysis mainly focused on different effluents' management options from an environmental point of view, outlining the environmental benefits which had monetary impact and the various externalities. Useful information for optimisation of the process and integrated production management was derived and a reference model was created. Cash flows were also examined, so as to easily compare alternative investments.
9. knowledge dissemination and exploitation strategies. Marketing actions for the commercial exploitation of the project outcomes were defined and the required technical training of enterprises' personnel was organised. Finally, information on NIREC innovations was disseminated to numerous firms operating in the waste treatment sector at an international level.

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