Nutrient concentration of blackwater digestate with a pilot-scale low-temperature evaporator

Human excreta contains most of the nutrients consumed in diets, making its recovery essential for sustainable sanitation. Separately collecting blackwater (faeces and urine) enables efficient nutrient and energy recovery. After anaerobic digestion, blackwater produces a nutrient-rich liquid that requires transportation and sanitation for use as fertiliser; concentrating this liquid reduces transport costs and environmental impact. This study investigates nutrient concentration in blackwater digestate using a pilot-scale evaporator designed to operate with waste heat as its primary energy source. Experiments were conducted under two pH conditions (6 and 2.8). The evaporator operated at 60 °C with enhanced heat transfer and gravity-based separation, achieving volume reduction factors of 87 (pH 6) and 85 (pH 2.8). pH strongly influenced nutrient solubility (phosphorus, magnesium, calcium) and equipment leaching. Pharmaceutical residues persisted in concentrates, indicating a need for additional treatment. Final concentrates contained NPK up to 6.7%, 1.5%, and 1.2%, respectively. The specific energy consumption (SEC) increased with concentration, ranging from 0.56 kWh to 1.1 kWh at 60 °C, approximately 1.4–2.8 times higher than the theoretical value. Equipment modifications for low pH and improved material selection could enhance efficiency and regulatory compliance. Future designs may also integrate heat recovery systems to reduce energy demand and improve sustainability. • 60°C evaporation concentrated digestate to VRF 19–87 (pH 6.0) and 21–85 (pH 2.8). • Nutrient recovery was nutrient- and pH-dependent (~5–65%); Ca/Mg losses reached ~90% at high VRF. • Evaporation energy was ~0.5–1.3 kWh/L (max ~2.8) and only weakly related to VRF. • 160 pharmaceuticals analyzed; detected compounds enriched, up to 67× (pH 6.0) and 34× (pH 2.8)