Removal of heavy metals from laboratory waste liquid (LWL) generated from water and wastewater analytical laboratories

Abstract

The laboratory waste liquid (LWL) generated from water and wastewater testing laboratories contains high acidity and heavy metal concentrations, thus necessitates its management at the source itself. This study presents a combined approach of neutralization cum precipitation with sodium hydroxide (NaOH) and enhanced settling using coagulation-flocculation (alum + cationic polyelectrolyte) for the removal of a wide range of heavy metals from LWL. Studies were conducted across different LWL: Distilled Water dilution ratios (1:10, 1:8, 1:6, 1:4, and 1:2) to optimize the heavy metals removal. The overall results indicated that the removal efficiencies for Ag, Cr, Fe, and Hg consistently exceeded 90 %, with the removal trend following the order: Ag > Fe > Hg > Cu > Cr > Zn > Mn > Pb. The optimal pH level for achieving over 90 % removal efficiency for all heavy metals, except Mn, was between 8 and 8.5. However, Mn was more effectively precipitated at a pH of 9.5–10. Coupling neutralization with coagulation + flocculation (Alum ∼330–350 mg/L + Cationic Polyelectrolyte ∼1.5–3 mg/L) enhanced the removal of heavy metals. The scale-up from laboratory to pilot-scale studies further confirms the effectiveness of the laboratory findings and establishes a basis for future full-scale implementations in laboratories. The prescribed effluent discharge standards were met for all the heavy metals except Hg. The residual Hg removal after neutralization and precipitation can be achieved through various adsorption methods, and these studies are currently underway.