Method and process arrangement of removing cations from water
A method of removing cations from water and a process arrangement for performing the method, wherein the method includes containing cations selected from a group consisting of Se, As, Sb, Cr, Mn, Mo, W, V, Te, P, Si including preparing an iron precipitate-containing sludge by electrochemical water treatment, wherein the water is led through an electrochemical water treatment unit, wherein electricity dissolves iron from an anode to the water thereby forming an iron precipitate-containing sludge to which cations to be removed are adsorbed/co-precipitated, adsorption and co-precipitation enhancing to provide a very high density sludge having an iron precipitate concentration of 50-400 g/l thereby further adsorbing cations contained in the water, and separating solids from the obtained sludge thereby producing treated water having a reduced cation content and solids comprising iron precipitate compounds and adsorbed/co-precipitated cation compounds.
1. A method of removing selenium (VI) cation(s) from water containing the selenium (VI) cation(s), the method comprising:
preparing an iron precipitate-containing sludge by electrochemical water treatment in an electrochemical water treatment unit, wherein the water is led through the electrochemical water treatment unit, wherein electricity dissolves iron from an anode to the water thereby forming the iron precipitate-containing sludge to which cations to be removed are adsorbed/co-precipitated;
densifying the iron precipitate-containing sludge by further adsorbing selenium (VI) cations contained in the water by enhancing adsorption and co-precipitation in an adsorption and co-precipitation unit thereby providing a very high density sludge having an iron precipitate concentration of 85-400 g/l; and
separating, in a solid-liquid separation unit, solids from the obtained very high density sludge, thereby producing treated water having a reduced cation content, and solids comprising iron precipitate compounds and adsorbed/co-precipitated cation compounds,
wherein the adsorption and co-precipitation unit is located after the electrochemical water treatment unit and before the solid-liquid separation unit,
wherein the iron precipitate concentration of the very high density sludge is adjusted by recycling at least part of the separated solids comprising iron precipitate and adsorbed/co-precipitated cations back from the solid-liquid separation unit to the adsorption and co-precipitation unit.
2. The method according to claim 1 , wherein the electrochemical water treatment unit comprises an iron anode, from which iron is dissolved into the water as ferrous or ferric iron.
3. The method according to claim 1 , wherein the current density in the electrochemical water treatment unit is adjusted to a level of 10-500 A/m2 and/or the charge loading is adjusted to a level of 0.1-10 MC/m3.
4. The method according to claim 1 , wherein the iron precipitate-containing sludge formed by the electrochemical water treatment unit comprises iron precipitate 0.1-2 g/l.
5. The method according to claim 4 , wherein the iron precipitate-containing sludge formed by the electrochemical water treatment unit comprises iron precipitate 0.5-1.5 g/l.
6. The method according to claim 1 , wherein the iron precipitate concentration of the very high density sludge is adjusted to a level of 100-350 g/l in before the solid-liquid separation unit.
7. The method according to claim 6 , wherein the iron precipitate concentration of the very high density sludge is adjusted to a level of 200-300 g/l before the solid-liquid separation unit.
8. The method according to claim 1 , wherein the iron precipitate concentration of the very high density sludge is adjusted to a desired level by re-circulating at least a part or all of the solids comprising iron precipitate and adsorbed/co-precipitated cations obtained before the solid-liquid separation unit.
9. The method according to claim 1 , wherein the iron precipitate-containing sludge is obtained by mixing fresh iron precipitate obtained by electrochemical water treatment and re-circulated part of the solids comprising iron precipitate.
10. The method according to claim 1 , wherein the separating of solids from the very high density sludge is performed by clarifier, thickener, hydrocyclone, dissolved air flotation or ultrafiltration.
11. The method according to claim 1 , wherein in the solid-liquid separation unit the water is allowed to pass through a bed of solids formed of iron precipitate in the solid-liquid separation unit thereby enabling adsorption/co-precipitation of the cations to the bed of solids.