Rule-of-Thumb: When in doubt, you can safely assume the MLVSS/MLSS ratio is 0.80. Having said that, I do not hesitate to use a slightly lower value, say 0.75 or 0.78, when I need to calculate the quantity of mixed liquor volatile solids in the bioreactor as part of determining the food-to-microorganism ratio. In this case I want to be conservative in my estimate (underestimating the quantity of biomass) to be certain I have enough bugs on hand for a given organic load. If I’m calculating the oxygen demand in the bioreactor I want to be conservative again but in the opposite direction. So I might use a ratio value of 0.82 or 0.85 to overestimate the oxygen demand to be certain I have enough aeration equipment to satisfy that demand.

The total suspended solids concentration in the bioreactor (activated sludge process) represents the mixed liquor suspended solids concentration or MLSS. The MLSS value includes inorganic or non-biological solids in addition to organic bacterial cells or biomass. As such, the MLSS “overestimates” the concentration of microorganisms in the reactor.

In order to generate a more representative number for the concentration of microorganisms in the bioreactor, the mixed liquor volatile suspended solids (MLVSS) concentration is determined. Burning off the organic fraction of the MLSS at 550C leaves behind the inorganic fraction of the mixed liquor, the weight of which is then subtracted from the MLSS to obtain a better estimate of the organic, biomass quantity. The lab testing for producing MLSS and MLVSS is done easily enough but I often find discrepancies in the data I get. For example, the MLVSS will be reported as being 97% of the MLSS concentration. And though many wastewater plants generate MLSS values daily there can be gaps in the MLVSS data because this testing is done only occasionally.

When you do not have the MLVSS concentration it can be estimated simply by multiplying the MLSS concentration by the typical value of 0.80 (MLVSS = MLSS x 0.80). In this case you are estimating that the organic fraction of the MLSS, the fraction that contains, in theory, microorganisms, is approximately 80% of the total suspended solids in the bioreactor. It should be recognized that there will be dead cells and burst, or lysed, cells that, though organic, are nonetheless not part of the active biomass. I provide you with several sources of information in support of what I am saying regarding the MLVSS/MLSS ratio, shown below. The range of likely MLVSS/MLSS ratios is also shown graphically, below, after the references, in Figures 1, 2, and 3.

1. Metcalf & Eddy. Wastewater Engineering: Treatment and Reuse. 4th ed. Boston: McGraw-Hill, 2003. Pgs. 682-683.

    …the VSS fraction of the total biomass is about 0.85, based on the cell composition given in Table 7-4 (Table 7-4 has been reproduced, below).

2. von Sperling, Marcus. Activated Sludge and Aerobic Biofilm Reactors. Volume 5. London: IWA Publishing, 2007. Pgs. 20-21.

    MLVSS/MLSS ratio is typically 80 percent.

    Conventional activated sludge ratio MLVSS/MLSS = 0.70 to 0.85.

    Extended aeration activated sludge ratio MLVSS/MLSS = 0.60 to 0.75.

3. Water Environment Federation. Activated Sludge. 2nd ed. Manual of Practice OM-9. Alexandria, VA, 2002. Pg. 4.

    MLVSS/MLSS ratio ranges from 70 to 80 percent.

Figure 1: Range of Mixed Liquor Suspended Solids Values

MLSS Range

It is my experience that MLSS values above 3,600 mg/L most often occur because of insufficient wasting, not by design or intent to achieve optimal operation. It should also be realized that as the MLSS concentration increases, the rate at which the MLSS will settle in the secondary clarifier actually decreases. The slower rate of settling, due to a higher MLSS concentration, can then lead to a higher total suspended solids concentration leaving the clarifier. But it does also happen where I find the MLSS is purposely increased to lower the F:M ratio. Unfortunately, if a plant is suffering from chronically high F:M ratios the real solution is more aeration volume rather than an increase in the MLSS concentration.

Figure 2 shows the expected ratio of mixed liquor volatile solids (MLVSS) to mixed liquor suspended solids (MLSS), a range that spans 60 to 85%. I have seen MLVSS/MLSS ratio values as high as 90% but I consider such high values to be suspect.

Figure 2: Ratio of MLVSS to MLSS


With a typical MLSS concentration that ranges from 1,800 to 4,000 mg/L in conventional activated sludge systems and an expected MLVSS/MLSS ratio that ranges from 60 to 85%, we can expect the MLVSS concentration to range from 1,080 to 3,400 mg/L as shown in Figure 3.

Figure 3: Range of Mixed Liquor Volatile Suspended Solids Values


For those of you that have an interest in statistics, or, more specifically, in uncertainty, risk, or probability analysis, I provide some additional details on how to better estimate the MLVSS concentration using a great software program called @Risk from the Palisade Corporation ( This software is a big help in developing any model, allowing you to refine and clarify your modeling effort using a probability distribution to define the risk profile associated with a given variable, and to improve confidence in the analysis you generate and the recommendations you often need to make based on model results.

A very basic model for estimating the MLVSS concentration from MLSS data starts simply from the few Excel inputs shown Figure 4, below. Here, I’ve arbitrarily assumed the MLSS concentration is 2,500 mg/L. This MLSS value itself could be an educated guess; it could be based on an actual lab analysis; it could be an average value obtained from many recent values; or it could have been easily generated in the field using a spectrophotometer.

Figure 4: Basic Excel Model for Estimating MLVSS from MLSS Using a Ratio Value

Basic Excel model

The true value of the MLVSS/MLSS ratio is often much harder to determine than the MLSS. It can be hard to tell if some ratio values are due to sampling error, lab error, or data entry error. In Figure 5 you can see a histogram (produced using Minitab statistical analysis software) with a normal curve superimposed on the data showing several years worth of ratio data for MLVSS/MLSS. I frequently use a histogram with any data set as a means to look for outliers and as you can see, we have outliers at both ends of the spectrum. The mean MLVSS/MLSS ratio is shown to be 85.9%. But we can’t use this value because we know we have some number of values in the data that are greater than 100% which is impossible. So we either need to clean up the data, deleting excessively low values and certainly all values equal to, or greater than, 100%. Or we can use @Risk to help us determine a likely range of MLVSS concentration values.

Figure 5: Histogram with Normal Curve Produced by Minitab

Minitab histogram

There is such simplicity and beauty in modeling uncertainty with @Risk and I never tire of doing it. In Figure 6 you can see how I have set up my first assumption, where I have a starting MLSS concentration of 2,500 mg/L and I believe this concentration to be normally distributed. I could have picked another distribution and many are available within @Risk as shown in Figure 7. With enough data I could also have used @Risk to automatically fit a distribution to the data.

Figure 6: Setting Up an @Risk Model in Excel

Normal distribution applied to MLSS starting value

Figure 7: Many Probability Distributions Are Available Within @Risk

Many distributions are available for fitting to the data

In Figure 8, having originally chosen a normal distribution as being representative of the most likely range of values for the MLVSS/MLSS ratio, the results of a simulation are shown. From Figure 8 you can see that there is a 90% probability that the MLVSS/MLSS ratio is likely to range from 6.2 to 86.7%.

Figure 8: Results of a Simulation Run Using @Risk

Distribution used to estimate the true MLVSS/MLSS ratio

From the model results, starting with a normally distributed MLSS concentration of 2,500 mg/L and a normally distributed MLVSS/MLSS ratio centered on a value of 75%, we can be 90% confident that the actual MLVSS concentration will range from 1,454 to 2,324 mg/L as shown in Figure 9.

Figure 9: Model Results of a Simulation Run Using @Risk Showing Range of MLVSS Concentration

MLVSS range of expected values