How this instrument works
In an activated-sludge wastewater treatment process, MLSS (mixed liquor suspended solids) measures every suspended solid in the aeration tank — living microorganisms, inert grit, and other non-biological material all counted together. That total doesn't distinguish active treatment biomass from inert solids, so operators also track MLVSS (mixed liquor volatile suspended solids), the portion of MLSS that burns off when a dried sample is combusted at high temperature in the lab, which corresponds closely to the organic, biologically active fraction actually doing the treatment work.
The relationship is a simple percentage: MLVSS equals MLSS multiplied by the volatile solids fraction, the share of total suspended solids that is organic rather than inert. Determining that fraction requires a lab test — drying a sample to find total suspended solids, then igniting it (commonly at 550°C) and measuring the weight lost to combustion, since only the organic material burns off while inert mineral solids remain as ash.
A typical well-operated activated-sludge plant runs a volatile solids fraction (also expressed as the MLVSS/MLSS ratio) somewhere around 70-80%, meaning roughly a quarter or more of total suspended solids in the mixed liquor is inert rather than active biomass. Operators track MLVSS because it feeds directly into process-control calculations like food-to-microorganism ratio and sludge age, both of which are meant to reflect active biomass, not the inert solids that happen to be suspended alongside it.
- Enter your lab's measured suspended solids concentration into MLSS — mixed liquor suspended solids (mg/L).
- Enter the volatile solids fraction from your lab's ignition test into Volatile solids fraction (%) — typically 70-80% for a well-operated activated-sludge system.
- Read the result from MLVSS — mixed liquor volatile suspended solids (mg/L).
- Recheck the volatile solids fraction periodically — it can shift with influent character or process upsets, and process-control math depends on a current reading.
Worked example — 3,000 mg/L MLSS at 75% volatile
Enter 3000 into MLSS — mixed liquor suspended solids (mg/L) and 75 into Volatile solids fraction (%) — a standard activated-sludge reading with a typical VS/TS ratio for biological sludge. MLVSS — mixed liquor volatile suspended solids (mg/L) reads 2250.00: 3,000 x 0.75 = 2,250 mg/L.
That 2,250 mg/L MLVSS reading means roughly three-quarters of the 3,000 mg/L total suspended solids in the aeration basin is organic, biologically active material, with the remaining 750 mg/L made up of inert solids — grit, mineral matter and other non-volatile material that doesn't contribute to the treatment process.
Questions
What's the actual difference between MLSS and MLVSS?
MLSS counts every suspended solid in the aeration tank, biological and inert alike, while MLVSS isolates just the organic (volatile) portion, determined by igniting a dried sample and measuring the weight lost to combustion. MLVSS more closely tracks the active microbial biomass actually doing the treatment work, which is why process-control calculations lean on MLVSS rather than raw MLSS wherever biomass, not total solids, is the relevant quantity.
What's a typical MLVSS/MLSS ratio for a healthy activated-sludge plant?
Commonly around 70-80%, meaning roughly a fifth to a third of total suspended solids in the mixed liquor is inert rather than biologically active. A ratio noticeably below that range can signal a buildup of inert or inorganic solids in the system, while operators generally treat 70-80% as a normal, healthy operating range rather than a strict target.
How is the volatile solids percentage actually measured?
In the lab: a mixed liquor sample is filtered and dried to determine total suspended solids, then the dried residue is ignited in a furnace (commonly at 550°C) and reweighed. The organic material burns off during ignition while inert mineral solids remain as ash, so the weight lost to combustion, as a percentage of the dried weight, is the volatile solids fraction.
Why does MLVSS matter for day-to-day plant operation?
Because several key process-control calculations — including food-to-microorganism (F/M) ratio and estimates of active biomass in the system — are meant to reflect living microorganism mass, not total suspended solids including inert grit and mineral matter. Using MLVSS instead of raw MLSS in those calculations keeps process-control decisions anchored to the biomass that's actually treating the wastewater.
Can MLVSS ever equal MLSS exactly?
Only in the theoretical case where 100% of suspended solids are volatile (organic) — entering a volatile solids fraction of 100% makes MLVSS equal MLSS in this formula. In practice, real mixed liquor always contains some inert, non-volatile material, so MLVSS is consistently a bit lower than MLSS, typically landing in that 70-80% range rather than reaching it.