MLVSS ≈ 0.7-0.8 × MLSS (typically)
MLVSS (Mixed Liquor Volatile Suspended Solids) represents the biological portion (active microorganisms) of the activated sludge. It's determined by igniting a dried MLSS sample at 550°C — the volatile fraction that burns off is MLVSS. Typical MLVSS = 70-80% of MLSS. Normal operating ranges: conventional activated sludge 1,500-3,500 mg/L MLVSS; extended aeration 3,000-6,000. MLVSS is used to calculate Food-to-Microorganism ratio (F/M = BOD load / MLVSS inventory) and Sludge Retention Time (SRT). Low MLVSS → poor treatment; high MLVSS → settling problems. Operators adjust waste sludge rate to maintain target MLVSS.