Recovery of terephthalic acid from alkali-decrement wastewater by zinc salt-based coagulation

The effective recovery of terephthalic acid (TPA) from alkali-decrement wastewater remains a challenge for textile printing and dyeing enterprises. In this research, various salt-based coagulation were tried and zinc salt (zinc chloride) was found the relatively optimized coagulant for the TPA recovery without pH adjustment. After that, the operating conditions of zinc chloride coagulation for recovery of TPA was further investigated, and the optimal conditions were 120 mg/L of zinc chloride, 70 min of coagulation time and 1.0 mg/L of polyacrylamide (PAM) with the recovery efficiency of 99.29% for TPA and the removal efficiency of 89.31% for chemical oxygen demand (COD), respectively. Finally, economy costs analysis showed the investment and operational costs of the zinc salt-based coagulation were separately 1–2 yuan and 2–3 yuan per m³ alkali-decrement wastewater treated. Taken together, the zinc salt-based coagulation offers some significant advantages, including low operation cost (no pH adjustment), easy maintenance and best recovery efficiency, which makes this process a viable alternative for the treatment of alkali-decrement wastewater.