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 reused in agriculture. However, blackwater is typically too dilute for efficient transport and application. Nitrogen stabilisation and volume reduction are therefore required to produce a practical fertiliser-relevant product while limiting ammonia losses during thermal treatment.

This thesis investigates nitrogen stabilisation and low-grade heat concentration as treatment steps for producing fertiliser-relevant components from blackwater digestate decantate. Biological nitrogen stabilisation was studied through nitrification in pH-controlled moving bed biofilm reactors (MBBRs), evaluating the effects of start-up strategy, temperature and pH. In separate concentration experiments, blackwater digestate was chemically acidified and concentrated using laboratory-scale air-gap membrane distillation and pilot-scale low-temperature evaporation operated at 40–70 °C. The processes were assessed in terms of nutrient retention, volume reduction, flux behaviour and energy demand.

Start-up strategy and temperature strongly influenced the establishment of nitrification, whereas nitrifier abundance alone did not explain reactor performance. Stable nitrification rates of up to 1.8 g N m-² d-¹ were achieved at pH 6 and 30 °C. During acidic disturbances at pH 2.3, at both 20 and 30 °C, approximately half of the measured inorganic nitrogen was present as ammonium and half as nitrate. The results suggest that some complete nitrification persisted, although its extent could not be quantified because part of the nitrate may have originated from the oxidation of nitrite already present in the reactors.

A low pH before volume reduction was favourable for keeping nitrogen and phosphorus in solution. Nitrification lowered the pH to 2.3, indicating that external acid addition could potentially be avoided in an integrated treatment system.

Membrane distillation achieved up to 15-fold concentration, although permeate flux declined to approximately 15% of its initial value because of fouling and membrane wetting. Pilot-scale evaporation achieved volume reduction factors of up to 85, although nutrient losses increased at high volume reduction factors, particularly for phosphorus at pH 6.0. Lower pH improved nutrient retention by reducing volatilisation and precipitation losses. A dynamic model of condensate generation was calibrated against the evaporation experiments, yielding coefficients of determination (R²) of 0.961–0.997. The model indicated that progressive solute enrichment increased the apparent enthalpy of evaporation at high volume reduction factors. Exergy analysis showed that only approximately 12% of the supplied thermal energy was converted into useful exergy in the produced vapor. 

The concentrates retained macro- and micronutrients, with estimated N–P₂O₅–K₂O contents of 2.37–0.25–1.24% (pH 6.0 evaporation), 2.83–0.68–1.14% (pH 2.8 evaporation) and 0.92–0.19–0.31% (membrane distillation). Micropollutant fate was mainly compound-dependent: nitrification did not consistently attenuate all compounds, and several were enriched to different extents during evaporation, indicating a need for polishing before concentration.