The phosphate fertilizers are a non-renewable resource and global reliance on mining capacities poses significant environmental threats, including landscape destruction, radioactive waste and water pollution through eutrophication. At the same time, Wastewater Treatment Plants aim to lower phosphorus effluent levels. Anaerobic digestion technology for sludge treatment re-solubilise phosphorus that was captured by Enhanced Biological Phosphorus Removal. This study investigates possible integrated solutions to optimize existing technology and implement phosphorus recovery as fertilizers for agriculture. Moreover, bioenergy potential of wastewater sludge could be reached.
Lab-scale, thermophilic, mesophilic and room temperature acidogenic digesters were used to ferment Thickened Waste Activated Sludge (TWAS), mixed with various organic and inorganic substrates, including whey permeate concentrate (WPC), pre-fermented WPC (FWPC), acetic acid (AA), sodium acetate (SA), and real food waste. The results demonstrate that a significant role in phosphorus solubilization plays no chemical acidification, nor biological activity. Acetic acid, which lowered pH to ~4.8, provided carbon source for the process, also achieving over 73% of P release within 6 hours, whereas SA, providing the same carbon source has not shown similar results (40% in 12 hours), keeping near-neutral pH 6.6. Pre-fermented substrates as FWPC and FUSFW consistently outperformed non-fermented ones WAS and USFW, achieving >70% P release by providing immediate VFAs and low pH environment, compared to >50% with untreated. This acidogenic stage also significantly increases soluble COD, indicating a high potential base for methane production.
The recovered phosphorus in the supernatant was successfully precipitated as struvite and brushite, achieving as high as 97% and 95% recovery, respectfully. Struvite precipitation was optimal at pH 10.0 with P:Mg ratio 1:1.2, while brushite formation was most effective at lower pH as of 8.0 and P:Ca ratio 1:1.2. This research validates a sustainable, circular economy model for WWTPs, where acidogenic co-digestion of WAS with organic wastes, can recover valuable nutrient and simultaneously pre-treat sludge for enhanced biogas production.