Nitrogen Recovery from Hydrolysed Urine Using Power-Free Microbial Fuel Cell Electrodialysis
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Typen talteenotto hydrolysoituneesta virtsasta energiaomavaraisen elektrokonsentraation avulla
Nutrients, especially the most widely used macronutrients nitrogen, phosphorus and potassium, are essential for sufficient food production to feed the constantly growing human population. However, fertiliser production deals with scarce, non-renewable raw-materials and high energy consumption, which is why more efficient nutrient recovery and re-use from human waste streams should be encouraged. Human urine is an especially interesting source for nutrient recovery since it contains most of the nutrients consumed and excreted by humans. This study focused on utilising bioelectrochemistry for nitrogen recovery from source-separated urine. The reactor design was a combination of microbial fuel cell and electrodialysis technology. The aim was to optimise the performance of both the anode and the cathode of the reactor towards a power-free nutrient recovery method. On the anodic side, the target was to enrich an acidophilic electroactive consortium, which would be able to enhance the nitrogen recovery as well as make more complete use of the high buffering capacity of urine. Of the three studied pH – 5.5, 6.5 and 7.5 – the highest one, 7.5, proved to be the most suitable one for the enriched culture, resulting in a maximum current density of 16 A m‑2 at Ewe = 0 V vs. SHE. On the cathodic side, an air-cathode using carbon cloth as the electrode material, carbon nanoparticles as the catalyst layer and commercial PTFE spray as diffusion layers was developed. Of the different layer materials tested, the best performance was obtained using acid-pre-treated nitrogen-doped carbon nanotubes as the catalyst and four layers of WD-40 PTFE spray as the diffusion layers. With this combination, an onset potential of +0.1 V vs. SHE for the reduction reaction was obtained and the maximum current reached in a cyclic voltammetry test was 25 A m‑2 at -0.6 V vs. SHE. When the best performing anode and cathode were combined in a reactor, current densities in the range of 1–2 A m‑2 were obtained at short circuit. When the cell voltage was increased to 0.5 V, the current production approximately doubled, but further increase in cell voltage did not have a notable effect on the current density obtained. Based on the preliminary studies, higher current densities should have been obtained, which indicates that further optimisation in the reactor configuration and operation is needed.