Water treatment

1. A process for treating waste water, the process comprising:

a. providing plant-derived anionic nanofibrillar cellulose, said plant-derived anionic nanofibrillar cellulose having a zero sheer viscosity of 2,000 to 100,000 Pa*s, and a yield stress of 2 to 50 Pa, when dispersed to a concentration of 0.5 wt.-% in waste water;

b. adding said plant-derived anionic nanofibrillar cellulose in an amount of 0.005 to 25 kg/m 3 waste water, based on the dry weight of the nanofibrillar cellulose, to carry out a purification treatment comprising sorption of ions included in the wastewater, to a hydrogel of said plant-derived anionic nanofibrillar cellulose;

c. separating the added plant-derived anionic nanofibrillar cellulose hydrogel with sorbed ions from the waste water, wherein said ions comprise positively charged metal ions and oxyanions selected from the group consisting of sulfate, sulfite, nitrate, phosphate, selenate, selenite, antimonite, dichromate and arsenate ions; and

d. recovering treated waste water, the waste water having a pH of below 6.

2. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose has a zero sheer viscosity of 5,000 to 50,000 Pa·s when dispersed to a concentration of 0.5 wt.-% in waste water.

3. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose has a yield stress of 3 to 20 Pa, when dispersed to a concentration of 0.5 wt.-% in waste water.

4. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose comprises nanofibrillar cellulose manufactured from oxidized cellulosic raw material having a carboxylate content above 0.5 mmol/g based on the weight of the cellulosic raw material.

5. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose comprises nanofibrillar cellulose manufactured from carboxymethylated cellulosic raw material having a degree of substitution above 0.1.

6. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose comprises nanofibrillar cellulose manufactured from anionized cellulosic raw material having a degree of substitution of at least 0.08.

7. The process according to claim 1 , wherein the process comprises adding plant-derived anionic nanofibrillar cellulose to waste water in an amount of 0.01 to 20 kg/m 3 waste water based on the dry weight of the nanofibrillar cellulose.

8. The process according to claim 1 , wherein the pH value of the waste water is below 4.

9. The process according to claim 1 , wherein the metal ions comprise at least one of aluminum, sodium, iron, manganese, nickel and zinc ions.

10. The process according to claim 1 , wherein the purification treatment further comprises mixing of said plant-derived anionic nanofibrillar cellulose and said waste water.

11. The process according to claim 1 , wherein the the step of adding the plant-derived anionic nanofibrillar cellulose comprises forming a layer of the plant-derived anionic nanofibrillar cellulose hydrogel on a top of a filtration fabric or membrane.

12. The process according to claim 11 , wherein the purification treatment further comprises passing said waste water through said layer of plant-derived anionic nanofibrillar cellulose hydrogel.

13. The process according to claim 1 , wherein the the step of separating the added plant-derived anionic nanofibrillar cellulose hydrogel with ions sorbed thereto comprises filtration and/or centrifugation enabling trapping of the positively charged metal ions and the oxyanions included in the waste water into a nanofibril network of the plant-derived anionic nanofibrillar cellulose hydrogel.

14. The process according to claim 1 , further comprising separating the added plant-derived anionic nanofibrillar cellulose hydrogel with ions sorbed thereto from treated waste water by utilizing filtration or centrifugation.

15. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose consists essentially of cellulose I.

16. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose comprises TEMPO oxidized nanofibrillar cellulose.

17. The process according to claim 1 , wherein said plant-derived anionic nanofibrillar cellulose comprises carboxymethylated or sulphonated nanofibrillar cellulose.

18. The process according to claim 1 , wherein the pH of the waste water is below 4.