Sentence:  This method uses a <WSD>slug</WSD> of 100 mg/L chlorinated water as a <WSD>slug</WSD> that moves along the length of the pipeline. The <WSD>slug</WSD> is a percentage of the total length of the pipeline. 
Meaning of the word marked with <WSD> tags:  A discrete mass of a material that moves as a unit, usually through another material.