Exopolysaccharide can be purified using the procedure of Doig, P., et al., Infection & Immunity 55: 1517-1522 (1987). I have used this procedure to purify the EPS from planktonic P. aeruginosa. It should be applicable to biofilms, but I think that you would need a lot of starting material. Preparing biofilms "in bulk" is hard work. Good luck.