Abstract
"Diminishing global phosphorus (P) reserves, and the consequences of its excessive release into the environment, mean that a sustainable P management system is urgently required. This thesis explores the potential of untapped biomass wastes, which are a well-known sink of valuable plant nutrients. Hydrothermal carbonisation (HTC) is a promising thermochemical upgrading technology for wet biomass waste. Under aqueous conditions, large quantities of inorganic nutrients are leached from the biomass providing a significant opportunity for nutrient recovery, whilst rendering much of the calorific content into biochar, which is a potential future fuel for combustors currently powered by coal and/or woodchip.
In this study, a process combination of HTC, nanofiltration and struvite precipitation was developed for the recovery of P from a model biomass waste, namely spent coffee grounds (SCG). HTC liberated more than 82% of the total-P from SCG grounds as dissolved ortho-P. This is the highest reported level of ortho-P extraction using unmodified HTC available in the literature. Efficient concentration of the inorganic nutrients within the HTC process water, using a nanofiltration NF270 membrane, was made possible by the selective removal of hydrophobic organic compounds with a Macronet™ MN200 resin. Crystallisation of struvite from the nanofiltration concentrate resulted in the recovery of 92.8% of the dissolved P. X-ray analysis of the recovered struvite produced the first known crystal structure of a struvite compound containing both NH₄-N and K. This compound is best described as a solid solution of ammonium struvite and potassium struvite. The overall process resulted in a P recovery of 73.6% from the native SCG far exceeding that of current state-of-the-art industrial solutions.
A state-of-the-art thermodynamic PHREEQC model was adapted to improve the predicted product composition of struvite precipitation reactions from complex wastewaters. This was achieved through the addition of user-defined-compounds, NK-struvite and amorphous hydroxyapatite, to the existing database and the optimisation of their solubility constants using the experimental data obtained in this study. The resulting amorphous model provided, for the first time, an accurate representation of the experimental results for precipitation of phosphates from complex wastewaters found in the literature."