Sentence:  The oxygen uptake rate in the mixed liquor averaged 0.8 mg / <WSD>l</WSD> / hr during the first four months of this study . 
Meaning of the word marked with <WSD> tags:  a metric unit of capacity, formerly defined as the volume of one kilogram of pure water under standard conditions; now equal to 1,000 cubic centimeters (or approximately 1.75 pints)