Removal of humic substances from surface waters with recycled fluidized bed sand

Recycled bed sand from a power plant’s fluidized bed reactor was used to remove humic substances (HSs) from surface water samples. The performance of sand samples screened into different size fractions together with unscreened sand was evaluated in removing HSs by performing shaking and column experiments, and by monitoring the quality of the treated water samples in terms of pH, conductivity, chemical oxygen demand (COD), and colour. At the beginning of the column experiments, the used sand fractions removed HSs with over 80% efficiency. However, as the experiments proceeded, the removal efficiency rapidly decreased, reaching a steady state during which a column filled with small-particle-size screened sand removed 20–25% of the COD and colour at a 2.2 kg/h flow speed, and 25–35% of the COD and 30–35% of the colour at a slow 0.5 kg/h flow speed. With unscreened sand, the corresponding COD and colour removal efficiencies were 10–20% (COD) and 10–18% (colour) for fast column experiments, and 22–27% for COD and 25–30% for colour during slow column experiments. Elemental analysis revealed that recycled fluidized bed sand contained several cationic compounds known to form complexes with HSs. Especially calcium together with aluminium and iron are potential candidates for removing colour and COD from the water samples. Highlights: Recycled fluidized bed sand could be used as a low-cost adsorbent material for removing HSs from surface water samples Especially the COD and colour of the water samples could be reduced by the sand treatments Fluidized bed sand contained several cationic compounds forming complexes with HSs No significant amounts of heavy metals were leached during the sand treatments.