Sentence:  ... the pulp is strained by means of a sieve or “<WSD>knotter</WSD>” as it is called, which is usually formed of brass, having fine slits cut in it to allow the comminuted pulp to pass through, while it retains all lumps and knots ... 
Meaning of the word marked with <WSD> tags:  A screen used to remove “knots” or impurities from wood pulp in the process of making paper.