Synergistic integration of vermicomposting and struvite crystallization for sustainable phosphorus recovery from sewage sludge: A low-carbon approach

The conventional magnesium ammonium phosphate (MAP) crystallization method for phosphorus recovery from sewage sludge faces significant challenges, including high operational costs and limited inorganic phosphorus release efficiency. To address these limitations, this study introduces an innovative integrated approach combining vermicomposting with MAP crystallization (VCMAP). The results showed that vermicomposting pretreatment significantly enhanced nutrient availability, yielding a 73.58 % increase in ammonium nitrogen and a 16.32 % increase in orthophosphate concentration compared to raw sludge. In addition, fresh vermicompost samples exhibited superior performance, releasing 96.65 % more orthophosphate than their air-dried counterparts. Response surface methodology (RSM) identified the optimal recovery conditions from vermicompost using MAP as pH = 9.3, Mg: N: P = 1.1:1.3:1, and agitation time (AT) of 100 min. Notably, calcium ions exerted a more pronounced influence on the VCMAP process, compared to iron and aluminum ions. The integrated VCMAP system demonstrated remarkable efficiency, achieving a 226 % increase in struvite recovery (8.83 kg/t sludge) and capturing 20.28 % of total phosphorus within 10 days of vermicomposting pretreatment. Moreover, the process exhibited a negative carbon footprint of -56.32 kg CO₂/t sludge, indicating its potential for carbon credit generation. These findings establish the VCMAP as a promising low-carbon technology for enhanced phosphorus recovery from sewage sludge, offering significant improvements over conventional MAP approaches.