MLVSS
 Mixed liquor volatile suspended solids

MLVSS -  Mixed liquor volatile suspended solids

MVLSS is generally defined as the microbiological suspension in the aeration tank of an activated-sludge biological wastewater treatment plant.

The biomass solids in a biological waste water reactor are usually indicated as total suspended solids (TSS) and volatile suspended solids (VSS). The mixture of solids resulting from combining recycled  sludge with influent wastewater in the bioreactor is termed mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS). The solids are are comprised of biomass, nonbiodegradable volatile suspended solids  (nbVSS), and inert inorganic total suspended solids (iTSS).

source: Metcalf and Eddy page 551 and 581