Thermophilic and hyper-thermophilic anaerobic digestion as novel treatment technologies for safe nutrient recovery from concentrated black water

To decrease the dependency on industrial processes or non-renewable resources for fertiliser production, the nutrient loop with human excreta has to be closed. Macronutrients, being nitrogen, phosphorus and potassium, are the most applied nutrients in agriculture to meet increasing food demands as consequence of the global human population growth. Phosphorus and potassium are obtained from mines, and the amount of accessible nutrients in these mines is decreasing. Furthermore, these mines are concentrated in a few regions around the world causing geopolitical issues. Another macronutrient, nitrogen, is obtained by fixation from ambient air through the industrial Haber-Bosch process. This process is very energy intensive and thus has a high impact on climate change through fossil energy consumption. Nutrients consumed by humans are excreted through urine and faeces and end-up in centralised wastewater treatment plants where nitrogen is removed and the other two macronutrients, potassium and phosphorus, are recovered to some extent.

The research in this thesis is performed within the framework of the EU Horizon2020 project Run4Life (Recovery and utilisation of nutrients for low impact fertilisers) and revolves around so-called new sanitation. In the new sanitation approach domestic waste streams are separated at the source. Bathroom and laundry water (grey water), rainwater and toilet water (black water) are separately collected. The benefit of source separated collection of domestic waste stream is the isolation of pollutants (like personal care products in grey water) and concentrating nutrients and organics in black water. Often vacuum toilets, and in this thesis even ultra-low flush volume vacuum toilets are applied to 1) increase nutrient concentrations 2) decrease the energy consumption during black water treatment and 3) decrease (drinking) water consumption for flushing. The main issue, however, with the reuse of nutrients from human excreta is the high concentration of pathogens and pharmaceuticals in black water. Therefore, this thesis focusses on thermophilic (55 °C) and hyper-thermophilic (70 °C) anaerobic digestion of concentrated black water aiming at removal of pathogens and pharmaceuticals and simultaneous recovery of hygienised and nutrient-rich effluent streams. The temperatures of 55 and 70 °C are commonly applied conditions for pathogen elimination. Anaerobic digestion is a well-developed method for treatment of (concentrated) waste streams. Energy production, nutrient mineralisation and higher applicable loading rates are amongst the advantages of anaerobic treatment over aerobic treatment.

Thermophilic anaerobic digestion of concentrated black water (collected through ultra-low flush volume vacuum toilets) outperforms the hyper-thermophilic anaerobic digestion. Thermophilic anaerobic digestion reaches higher organics removal (70%) and methane production (60%) than hyper-thermophilic anaerobic digestion. The majority of the nitrogen is mineralised to dissolved ammonium in the liquid effluent during both thermophilic and hyper-thermophilic anaerobic digestion. The phosphorus is mainly (>70%) retained in the sludge at thermophilic conditions. In the hyper-thermophilic reactor <70% of the phosphorus is retained. Thermophilic anaerobic digestion of black concentrated is therefore the best performing technology with the highest nutrient recovery potential. Even though there is an increased risk of ammonia toxicity due to the temperature-induced pKa shift and elevated ammonia concentrations of 1.0-1.5 gN/L as a consequence of vacuum collection, thermophilic AD reaches the same levels of methanisation and COD removal as mesophilic AD. Chemical modelling showed that high organics and ionic strength from the BW matrix result in lower toxic ammonia concentrations. With respect to pathogen removal during thermophilic and hyper-thermophilic anaerobic digestion both conditions achieve the same performance. Two pathogen indicator organisms, Escherichia coli and antibiotic resistant E. coli, are completely removed at thermophilic and hyper-thermophilic conditions, with significantly higher removal rates than more conventional mesophilic (35 °C) anaerobic digestion of vacuum collected black water.  Also with respect to pharmaceuticals there is no increased benefit for hyper-thermophilic anaerobic digestion over thermophilic anaerobic digestion.

Thermophilic treatment of concentrated black water has high potential for nutrient recovery (e.g. through ammonia stripping or phosphorus precipitation), and would thus be an interesting technology for community areas with source separated collection of domestic wastewater as for instance the “Lemmerweg” in Sneek, The Netherlands.