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.