Hello Lam,
Geodome is quite right. The lignin is removed by addition of a flocculant (this is one of my many areas of specialty in industrial chemistry). The flocculated material can be floated to the top by the addition of small bubbles of air to the water, or it can be removed by mechanically scraping the lignin from the bottom of the settling vessel (either technique works well).
The method of removal is usually coagulation by chemical charge neutralization.
I am not familiar with the product called pure clean, but I have used over 14 variations of chemicals that work very well.
There are other methods to remove lignin, but geodome mentioned one of the more efficient methods.
I hope this helps,
Eugene