Application of cerium and lanthanum coagulants in wastewater treatment—A comparative assessment to magnesium, aluminum, and iron coagulants

Biological wastewater treatment plays a central role in removing nutrients from wastewater. However, during the dewatering of biological sludge before it is transported to a landfill, a concentrated stream (centrate), loaded with phosphate, nitrogen, organic matter, and suspended solids, among others, is formed. Recycling these elevated concentrations, especially soluble phosphorus, back to the head of the plant defeats the sole purpose of enhanced biological treatment. In this study, a centrate stream was treated with lanthanides through coagulation. The study also established comparative performance between lanthanides and other commonly used coagulants, like FeCl3, Al2(SO4)3, Na2Al2O4, and Mg(OH)2. Coagulant performance was assessed based on their capacity to remove dissolved phosphorus and other impurities while maintaining alkalinity, sludge production, and operation cost. While adjusting the pH of the centrate (4.0–9.0), Al2(SO4)3, FeCl3, Na2Al2O4, LaCl3, and CeCl3 demonstrated strong removal of phosphate (>50%) at pH ≥ 6.0. However, as the pH dropped (≤5.0), the performance of both alum and FeCl3 plummeted to less than 15%. Na2Al2O4, CeCl3, and LaCl3 coagulants are not affected by pH; they maintained at least 85% phosphate precipitation at all tested pH values. The trend for moles of metal ion to phosphorus ratio required to treat centrate is as follows CeCl3 [1.2] = LaCl3 [1.2] < FeCl3 [1.5] < Na2Al2O4 [1.8] < alum [2.5] < Mg(OH)2 [3.1]. Evaluating each coagulant's performance based on purchase price, sludge disposal cost, and alkaline/acid reagent cost, Mg(OH)2 coagulant is the least expensive to treat wastewater in the given case study. However, based on effluent water quality, lanthanides outperform all the tested coagulants. Assessment of the lanthanide precipitates reveals that there is direct bonding between lanthanide and phosphate.