Heavy-Metal Accumulation in Low-Sludge Wastewater Treatment Technique: Sonication-Cryptic Growth

This paper used a sonication-cryptic growth technique in sequencing batch reactor (SBR) to reduce sludge yield and then analyzed the accumulation of eight typical heavy metals in sludge, namely, Hg, Cr, Ni, Cu, Zn, As, Cd, and Pd. SBR-1 was blank, SBR-2 used sonication, SBR-3 was dosed with heavy metals, and SBR-4 had both sonication and heavy metals. Artificial wastewater dosed with 12.5 mg/L heavy metals was used for SBR-3 and SBR-4. Results showed that the addition of heavy metals in influent caused the latter to accumulate in sludge and that each metal exhibits different behavior. Heavy-metal addition dropped sludge bioactivity by 53% and decreased the effluent chemical oxygen demand (COD) (SBR-3 versus SBR-1). Application of sonication (each day, 15% sludge was treated for 15 min by 1.2 W/ml ultrasound) decreased the excess sludgy by almost half; however, it increased sludge heavy-metal concentration only by 3.5% (SBR-4 versus SBR-3). Furthermore, sonication benefited sludge bioactivity and thus improved the effluent COD quality. It also significantly changed the individual accumulation patterns of eight heavy metals.