Denoise EEG signal by using Daubechies function
You might simply use soft/hard thresholding. This operation is quite standard and called Wavelet Denoising.
Here are some resources:

http://www.stthomas.edu/mathematics/pdfs/MSAD/Denoising%20via%20Wavele.pdf
http://scholar.lib.vt.edu/theses/available/etd-12062002-152858/unrestricted/Chapter4.pdf
http://www.ee.iitb.ac.in/~icvgip/PAPERS/202.pdf

An overview of noise removal for EEG:
http://www.ijarcce.com/upload/february/3_A%20survey%20on%20different.pdf

Some source code on image denoising via VisuShrink:
http://www.mathworks.com/matlabcentral/fileexchange/43996-image-denoising-using-visushrink