Integrating chemical coagulation with fixed-bed column adsorption using rice husk-derived biochar for shipboard bilgewater treatment: Scale-up design and cost estimation

Abstract

Discharging shipboard bilgewater (SBW) into seas and oceans without proper treatment could poison marine organisms and negatively impact transportation-related activities. This study focuses on the treatment of synthetic SBW using a simple and cost-efficient physico-chemical process. This treatment system included coagulation-flocculation (CF) using alum coagulant and fixed-bed column adsorption using rice husk-derived biochar (RHB). The optimization and effect of CF process parameters, viz., pH, fast stirring speed, and alum dosage, on surfactant and COD removal efficiencies were studied using a central composite design-response surface methodology (CCD-RSM). At optimum conditions (pH of 6.4, rapid mixing speed of 160 rpm, and coagulant dosage of 140 mg/L), the surfactants and COD removal efficiencies were 46.31±1.48% and 84.67±2.61%, respectively. The adsorption column optimum conditions were flow rate= 5 mL/min and bed depth= 16 cm, giving a surfactant removal efficiency of 96.6%. The primary adsorption mechanism was physisorption, including electrostatic attraction, hydrogen bonding, pore-filling, and hydrophobic interaction, as revealed by SEM, EDX, and FTIR characterizations. The adsorption data fitted well with Thomas model predictions, giving the best kinetic constant (KTh)= 0.305 mL/min/mg and equilibrium surfactant uptake (qo)= 11.05 mg/g (R2= 0.996). The Thomas model's coefficients were successfully used to predict the breakthrough curves and determine the column dimensions for pilot-scale (3 L/h) and large-scale (300 L/h) fixed-bed adsorption units. The estimated cost for the on-board treatment of SBW by the proposed system was 2.51 US$/m3. Future studies are required to implement the proposed combined coagulation/flocculation/adsorption system to treat real SBW from medium-sized ships.

Keywords

• Biochar-based adsorbent
• Breakthrough curve modeling
• Physico-chemical treatment
• Process optimization
• Ship wastewater
• Techno-economic