Ferrous Assisted Carbonation Mud Dosing Improves Activated Sludge Settleability

Abstract

Background: Secondary clarifier limitations and poor sludge settleability often constrain the performance of conventional activated sludge (CAS) under stringent effluent limits. Valorizing carbonation mud (CM), a CaCO₃‑rich by‑product from sugar manufacture, as an alkalinity source and ballast, particularly when paired with ferrous sulfate (FeSO₄), could improve settleability without pH excursions.

Methods: A tiered program combined jar tests with a continuous bench‑scale ASP design. Jar tests quantified settling volume (mL L⁻¹) under fixed mixing (rapid 120 rpm, 60 s; slow 20 rpm, 5 min) across FeSO₄ (0–80 mg L⁻¹ for co‑dosing; 0–400 mg L⁻¹ Fe‑only) and CM (0–720 mg L⁻¹), with pH and temperature held near 7.5–7.6 and 14–15 °C. Hydrodynamic effects were resolved by varying slow‑mix speed (10–25 rpm) and contact time (2–20 min). Cross‑run synthesis reported percent reductions versus paired controls and identified operating windows.

Results: Co‑dosing reduced settling volume by 10.3–16.5% (minimum 810 mL L⁻¹ at 80/720 mg L⁻¹ FeSO₄/CM). Lowering the slow-mix speed from 20 to 10 rpm yielded a 7.1% decrease, and extending the contact time from 2 to 20 minutes delivered a monotonic 20.2% decrease. A broad co‑variation window, FeSO₄ 50–80 mg L⁻¹ with CM 350–320 mg L⁻¹, consistently achieved 730–740 mL L⁻¹ (minimum 730 mL L⁻¹). FeSO₄‑only dosing produced the most significant single‑factor reduction (−25%) with a plateau near 350–400 mg L⁻¹. Across all conditions, pH remained neutral to slightly alkaline.

Conclusions: Moderate FeSO₄ with CM ballast, applied under gentle, prolonged mixing, substantially improves settleability while maintaining neutral pH. The practical window (FeSO₄ 40–70 mg L⁻¹ with CM 320–360 mg L⁻¹, or FeSO₄ ≥350 mg L⁻¹ alone; slow mix 10 rpm for ≥15–20 min) is operationally attainable and motivates continuous‑pilot validation with COD–N–P and SVI endpoints alongside routine metals/leachate compliance.