Nutrient concentration of blackwater digestate using an air gap membrane distillation process

Abstract

Blackwater from vacuum toilets contains significant amounts of nutrients that can be repurposed as fertilizer. This study aimed to evaluate an air gap membrane distillation process to concentrate nutrients in blackwater digestate. In the first experiments, various temperatures (40 °C, 55 °C and 70 °C) and pH levels (2.5, 2.8, 3.0, 4.0 and 6.0) were tested on the feed, resulting in operating conditions of 55 °C and pH 4 for the study's second experiment. Under these conditions, a 15-fold volume reduction was achieved. In this second experiment, the average permeate flux was 1.95 Lm⁻² h⁻¹ during the first 18 h, decreasing to 0.81 Lm⁻² h⁻¹ after 150 h of operation. The membrane was cleaned when the flux dropped to 0.39 Lm⁻² h⁻¹. The final concentrate had an NPK elemental weight ratio of 1:3:0.3. A chemical model indicated that most ammonium and phosphorus were dissolved in the concentrate, with some phosphate compounds precipitating. The product contained essential micronutrients and low levels of harmful substances like As, Hg, Cd, and Pb, with NaCl at 0.3 % of the weight. The main challenge was membrane wetting, leading to 33 % of nitrogen loss and 13 % of phosphorus loss. Despite the challenges the process successfully produced a nutrient-rich concentrate beneficial for agriculture.