Removal of contaminants from liquid after the hydrothermal carbonization of sewage sludge using a combination of membrane techniques and struvite precipitation

Post-processing liquid (HTCL), generated from the hydrothermal carbonization (HTC) of sewage sludge, is a highly organic by-product and one of the primary challenges of this technology. To enable its sustainable treatment, a combination of membrane techniques and nutrient recovery via struvite precipitation was applied. Accordingly, the performance of HTCL as well as the liquid resulting from the struvite precipitation (HTCLS) has been investigated in conjunction with the use of ceramic and polymeric membranes. Three types of ceramic membranes and eleven polymeric membranes were tested in consideration of permeate properties and specific membrane characteristics. In addition, the influence of struvite precipitation on the properties of the membrane and resulting permeates were investigated. Results: demonstrated that permeate properties vary significantly among membrane types. Generally, for HTCL, the NPO30P membrane provided the most effective filtration, achieving a reduction of 24.4 % in NH4+ and 22 % in nitrogen compounds. In the case of HTCLS, the PES 5 kDa membrane resulted in the highest contaminant reduction, for example decreasing NH4+ levels by 68.8 % and nitrogen compounds by 52 %. Permeate physical and chemical characteristics, such as pH, conductivity and COD, also differed significantly between HTCL and HTCSL processes. For instance, COD values for HTCL ranged from 27390 to 12100 mg/L, while for HTCSL permeates, it was 26480 to 11210 mg/L, indicating a lower level of organic contaminants following struvite precipitation. Membrane fouling occurred more rapidly during HTCL filtration compared to HTCSL, with an increase in membrane relative permeability of 9-74 % observed for HTCLS compared to HTCL.