Source-separated blackwater contains most of the nutrients present in domestic wastewater and therefore represents an important resource for nutrient recovery. Blackwater digestate contains nitrogen, phosphorus, potassium, and micronutrients that could be recovered and reused in agriculture, contributing to more circular sanitation systems. However, the digestate is typically too dilute for efficient transport and agricultural reuse, requiring nitrogen stabilisation and volume reduction to produce a practical fertiliser product and reduce ammonia losses during thermal treatment.

This thesis investigates the pathway towards producing a fertiliser product from blackwater digestate through two sequential treatment steps: nitrogen stabilisation and low-grade heat concentration. Nitrogen stabilisation was investigated using both biological and chemical approaches. Biological stabilisation was achieved through nitrification in pH-controlled moving bed biofilm reactors (MBBRs), where the effects of startup strategy, temperature, and pH on reactor performance were evaluated. Startup strategy and temperature strongly influenced nitrification establishment, while nitrifier abundance alone did not explain reactor performance. Stable nitrification rates up to 1.8 g N m-² d-¹ were achieved at pH 6 and 30 °C. Furthermore, complete nitrification of approximately half of the ammonium was maintained even under acidic disturbance conditions at pH 2.3 and temperatures of 20 °C and 30 °C. Chemical stabilisation was achieved through acid dosing prior to thermal concentration, using phosphoric acid in membrane distillation experiments and nitric acid in low-temperature evaporation experiments to suppress ammonia volatilisation and maintain nutrients in soluble form.

Low-grade heat concentration of blackwater digestate was evaluated using laboratory-scale air-gap membrane distillation and pilot-scale low-temperature evaporation operated at moderate temperatures (40–70 °C), suitable for waste heat utilisation. The processes were assessed with respect to nutrient retention, volume reduction, flux behaviour, and energy demand. Membrane distillation achieved up to 15-fold concentration, although permeate flux declined to approximately 15% of its initial value due to fouling and membrane wetting. Pilot-scale evaporation achieved volume reduction factors up to 85 while producing nutrient-rich concentrates. Lower pH improved nutrient retention by reducing volatilisation and precipitation losses, while potassium and sodium concentrations increased under acidic conditions.

A dynamic model describing condensate generation during evaporation was calibrated against experimental data, yielding coefficients of determination (R²) between 0.961 and 0.997. The model showed that progressive solute enrichment increased the apparent enthalpy of evaporation at high volume reduction factors. Exergy analysis demonstrated that only about 12% of the supplied thermal energy was converted into useful exergy in the produced vapor, highlighting the importance of heat recovery and waste heat integration.