Sentence:  The average volatile suspended solids in the effluent was 75 mg / l while MLSS averaged 170 mg / <WSD>l</WSD> volatile suspended solids . 
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)