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.

Most nutrients ingested by humans are excreted in urine and faeces, forming the primary components of blackwater together with toilet paper and water. Due to its high concentrations of carbon and nutrients, blackwater presents a significant opportunity for nutrient and energy recovery. Blackwater can be treated with an anaerobic digester, which transforms a large share of the carbon into methane and carbon dioxide. The remaining digestate requires further treatment to recover the nutrients in a concentrated form.

The aim of this thesis was to investigate processes to concentrate blackwater digestate at different pH levels and temperatures, using two different technologies that can be driven using low-grade heat as the main energy source. The concentration processes were evaluated with respect to the volume reduction factor and nutrient losses, including by use of mineral and gas formation simulations. Furthermore, the quality of the concentrate and its potential for use as a fertiliser were assessed, as were energy use and the decrease in flux over time.

A lab-scale experiment with an air gap membrane distillation set-up, a mathematical model for the air gap membrane distillation, and a pilot-scale low-temperature evaporator all formed part of this thesis work. Chemical simulations for sorption, mineral, and gas formation were developed for the pH-adjusted blackwater digestate and the concentrates of both experiments. The air gap membrane distillation experiment included three parts: (i) evaluation of permeate flux with tap water at different temperatures, (ii) concentration of pH-adjusted blackwater digestate (using phosphoric acid) at pH levels of 2.5, 2.8, 3.0, 4.0, and 6.0, and temperatures of 40°C, 55°C, and 70°C. Based on these results, a final experiment (iii) was conducted where pH 4.0 and 55°C were selected for further concentration, up to 15 times. The pilot-scale low-temperature evaporator operated at 60°C with the pH adjusted to 2.8 and 6.0 using nitric acid. The volume was reduced in four separate batches, each starting with about 1000 litres. By the end of the process, the total volume was reduced to less than 50 litres. For the membrane distillation, the permeate flux dropped to 15% of its initial value during the first 170 hours. The membrane was cleaned when the flux dropped to 0.39 L/m²/h. The final concentrate had an NPK ratio of 1:3:0.3. Membrane wetting caused losses of nitrogen (33%) and phosphorus (13%). The condensate flux in the evaporator decreased slightly over time, which resulted in higher energy use at higher volume reduction factors. The concentrate at pH 6 had an elemental NPK ratio of 6.0:0.3:1.2, while at pH 2.8, it was 6.7:1.5:1.2.pH significantly influenced the retention of ammonium, phosphorus, calcium, and magnesium. The concentration of harmful substances (arsenic, aluminium, lead, cadmium, mercury) was higher at pH 2.8, which was most probably due to leakages from the equipment.

Based on membrane distillation experimental results, the model predicted permeate flux at various temperature differences. A model parameter related to the concentrations was estimated, which confirmed the wetting observed in the permeate quality.

Both setups tested can be improved by changing the materials or design. The extent to which the blackwater digestate should be concentrated has to be carefully evaluated because the losses are accumulative. Overall, these technologies produced a nutrient-rich liquid with low levels of problematic substances, making it promising for agricultural use.