What an MLSS wastewater reading counts
The term is worth unpacking before the number is used. MLSS stands for mixed liquor suspended solids in wastewater, and it has three parts. Mixed liquor is the contents of the aeration tank: the incoming flow, the sludge returned from the clarifier, and the culture of micro-organisms grown in it. Suspended solids is the mass of dry solids carried in those litres, reported in milligrams per litre. An MLSS wastewater figure is therefore a concentration and not a quantity: it says how densely the tank is populated, not how large the population is. Two tanks at the same concentration and different volumes hold different amounts of biology, which is why the concentration is never read on its own.
Three kinds of material stand behind the number, and only one of them does the work. The first is the living biomass, which oxidises the organic matter and, where the sludge age allows it, the ammonia. The second is inert particulate matter that arrives with the influent and leaves only with the waste sludge, together with the dead fraction of the culture. The third is mineral: grit and silt that got past the inlet works, and the hydroxide precipitates that form when a metal salt is dosed.
The measurement is a weighing rather than an instrument reading. A known volume of mixed liquor is drawn through a filter of standard pore size, dried at about 103 to 105 degrees Celsius, and the residue is weighed. The pore size and the drying temperature belong to the conventional gravimetric method rather than to your plant, which is one reason two laboratories can return different values for the same sample.
MLSS is the figure a conventional plant is steered by, because it closes the mass balance. The mass of solids held in the system divided by the mass taken out each day is the sludge age, so at a steady concentration the wasting rate sets the age. That is why the concentration appears in the aeration tank volume, in the return sludge ratio and in the food to microorganism ratio.
MLSS concentration in an aeration tank
Commonly quoted figures put conventional activated sludge at roughly 2,000 to 4,000 mg/L and extended aeration at roughly 3,000 to 6,000 mg/L. Read them as a starting method rather than as a specification: the concentration a given plant can hold is set by its clarifier, not by a table, and the ceiling is the point at which the sludge blanket stops settling low enough to stay out of the effluent.
What fixes the figure is a chain. The sludge volume index describes how well the sludge settles, the return sludge concentration follows from it, and the concentration the tank runs at follows from the return rate together with the mass of solids the clarifier can carry. A tank asked to hold more solids than its clarifier can return will lose them over the weir, whatever the worksheet says.
Sampling decides whether the reading means anything at all:
- Draw the sample from the same point every time. Mixed liquor is not uniform along a tank: a sample near the inlet carries a different concentration from one at the outlet, and a reading from a moving point cannot be compared with last week's record.
- Take it from the aeration tank, not from the return sludge line. Return sludge is the concentrate the clarifier has already produced, and its concentration is a different quantity with a different use.
- Sample with the tank in its normal state of mixing and aeration, and note the dissolved oxygen at the same time, so a change in concentration can be read against the conditions it was measured in.
- Report the figure with its date, its point and its method, because a concentration without those three cannot be compared with anything, and treat a single grab sample as a point in time rather than as the plant.
What the volatile fraction adds
MLVSS is the volatile part of the same measurement: mixed liquor volatile suspended solids. It is produced from the same filter, which is then ignited, conventionally at about 550 degrees Celsius, and weighed again. What burns off is the volatile fraction; what remains is the fixed or ash fraction. Expressed as a concentration, the loss on ignition is the MLVSS figure.
In an activated sludge plant, volatile suspended solids in wastewater treatment is used as an estimate of the organic fraction, and the organic fraction is largely the culture. That is the useful part of the measurement, and it is also the part most often over-read. MLVSS is a proxy for biomass, not a measurement of it. Dead cells are still volatile; so is organic matter that arrived with the influent and has not yet been oxidised; so are the stored products inside the cells; and so are any organic chemicals the plant doses. A rising volatile fraction can mean the culture is growing, or it can mean the influent is carrying more organic particulate than the plant can break down in the time available.
The measurement earns its place as a trend rather than as an absolute. A volatile fraction falling while MLSS holds steady says the tank is accumulating mineral matter. A volatile fraction rising after a change in the influent says the load has changed. In both cases the direction of movement carries more information than the decimal place.
The mlvss to mlss ratio in wastewater
Divide MLVSS by MLSS and the result describes the composition of the solids rather than their quantity. For municipal activated sludge the mlvss to mlss ratio in wastewater is commonly quoted in the range of about 0.6 to 0.8, and as with every band in this article it belongs to common practice rather than to your plant.
Several things move it, and most of them are worth knowing before the number is interpreted.
- Sludge age. A long sludge age holds solids in the system long enough for the inert fraction to accumulate, because inert matter is not consumed and leaves only with the waste sludge. The volatile fraction of the same tank therefore drifts down as the plant gets older, with nothing wrong with the biology.
- Chemical dosing. A metal salt dosed for phosphorus removal or for coagulation leaves a hydroxide precipitate in the sludge. It is mineral, it is retained with the sludge, and it raises MLSS without raising MLVSS. Lime and any other inorganic chemical reaching the tank does the same.
- Grit and silt. Material that should have been caught at the inlet works arrives in the aeration tank as fixed solids, and the ratio of the tank falls with it.
- Influent composition. An industrial discharge high in inorganic suspended solids pulls the ratio down; a discharge of largely soluble organic matter adds nothing to the fixed fraction.
- Sludge age in the other direction. A young sludge carries a smaller inert fraction per unit of biomass, so the ratio tends to sit higher, until the plant begins losing solids it cannot settle.
Why the two numbers are read together
MLSS is the number the process is controlled with; MLVSS is the number that checks whether MLSS is telling the truth. Read together, they separate a plant that is genuinely carrying more biology from one that is carrying more mineral matter. The distinction matters where the figures become design inputs: a food to microorganism ratio computed on MLSS alone is a ratio against total solids, of which only part consumes the food, so where the inert fraction is large the ratio understates the load on the biology.
The practical use is modest and reliable. Record both numbers at the same point and read the ratio as a slow-moving line rather than either figure on its own. A ratio that steps down and stays down is a fact about the solids inventory, and what changed in the influent or in the wasting is answered from the operating record rather than from the measurement itself.
Common mistakes
- Reading the ratio as a measure of biological health. It measures the composition of the solids, and a plant with a falling ratio can be working well.
- Comparing MLSS or MLVSS values from two laboratories with different filter pore sizes and drying temperatures, and treating the difference as a change in the plant.
- Sampling the return sludge line or the effluent and recording the result as the aeration tank concentration.
- Leaving the clarifier out of the inventory. The sludge blanket holds solids the sludge age is computed from, and the aeration tank figure alone does not describe the system.
- Reporting MLVSS as biomass. Volatile solids include unoxidised organic matter from the same water, so the figure is a fraction of the mass and not a count of organisms.
- Copying a target MLSS figure from another plant on different water and a different clarifier.
- Changing the sample point or the sampling time and then reading the step change as a process change.
Self-check before the two numbers are used
- Is the sample drawn at a defined point in the aeration tank, at a defined time in the cycle, and recorded as such?
- Are the filter pore size and the drying temperature stated with the result, so that it can be compared with earlier readings?
- Is the ignition temperature for the volatile fraction recorded as well, since it belongs to the method rather than to the sample?
- Has the MLSS wastewater concentration been checked against the sludge the clarifier can return, rather than against a table?
- Is the volatile fraction being read as a trend across the record, rather than as a value with a pass mark?
- Has the dosing of any metal salt or lime been noted on the same date, since it moves the ratio without moving the biology?
- Is the wasting rate recorded alongside the two numbers, so that the sludge age the plant is actually running can be calculated?
- Where the numbers feed a ratio or an age calculation, is the same basis being used as last time?
From a reading to a plant duty
The two numbers describe a tank that already exists, and they are read backwards when one has to be sized. The mass of solids the biology needs comes from the load and the sludge age, the concentration the clarifier can hold sets the volume, and the two together fix the aeration tank and the air demand. The tank volume and HRT chart turns a residence time into a basin volume, and the integrated MBR equipment page covers the membrane version of the same process. What a single unit will hold on your water is confirmed with the factory for your influent and your discharge standard rather than assumed from a table.