Turning building waste into fertilizer: Using cooling tower blowdown brine as a chemical source for critical mineral phosphate recovery from urine

Commercial and institutional buildings generate two underutilized waste streams: nutrient-rich human urine and mineral-rich cooling tower blowdown (CTB) water. This study developed a decentralized circular-economy resource recovery strategy that integrates these co-located building waste streams to recover phosphate, reduce reliance on commercial precipitation reagents, and enable simultaneous nutrient and water recovery. CTB collected from a commercial building was mixed with real hydrolyzed human urine at different volume ratios, exhibiting natural variability in phosphate concentration (186–276 mg P·L −1 ). Raw CTB achieved high phosphate removal when the CTB-to-urine ratio was adjusted according to the phosphate load, reaching 98% removal for baseline phosphate concentrations and 86–95% removal under elevated phosphate loading. Membrane distillation (MD) concentrated Ca 2+ and Mg 2+ by approximately 3.2-fold while simultaneously recovering ~70% high-quality water. The MD concentrate achieved equivalent phosphate removal (96–98%) at approximately half the CTB volume required by raw CTB, reducing hydraulic loading and precipitation reactor volume. A large-volume experiment (6 L CTB and 3 L urine) achieved 97% phosphate removal, demonstrating reproducibility at increased treatment volume. PHREEQC thermodynamic modeling supported experimental observations by predicting favorable conditions for struvite precipitation and competing mineral formation. MD performance further showed that concentrating CTB prior to precipitation maintained stable water flux (7.0–4.5 L·m −2 ·h −1 ), whereas treating the urine–CTB supernatant resulted in greater flux decline (7.0–2.5 L·m −2 ·h −1 ) and ammonia crossover. Life cycle assessment indicated that replacing commercial magnesium reagents with CTB reduced environmental impacts, while MD utilizing existing cooling-system waste heat further improved process sustainability. These findings demonstrate the potential of transforming commercial buildings into decentralized systems for simultaneous nutrient recovery and water reuse.