Sentence:  Removal of the suspended solids by a membrane filter yielded an average effluent containing only 20 mg / <WSD>l</WSD> BOD . 
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)