Sawdust is mainly fibers or splinters of wood, which does not dissolve in water, so it can be separated by decanting for the larger particles. The smaller particles may be more of a challenge.
Decanting is the chemical procedure for separating solids from mixtures with liquids, or immiscible liquids from each other. In the case of lighter substances like sawdust mixed with heavier liquids, the decanting is also referred to as flotation. Some of the sawdust could then be scooped off the top of the water.
There is a picture here:
Smaller particles of the sawdust may be a challenge to remove because of their small weight to size ratio. They may become fixed in the water due to its surface tension, and will not separate by decanting. The problem was solved by using a sieve in this experiment: http://seperationchallenge.weebly.com/part-b.html