MCRT, SRT, DSRT: What’s It All About?

Did you ever wonder why there are so many names for what is basically the same thing? I did. Our industry is full of acronyms and abbreviated names of things. Take for example the acronyms MCRT, SRT and DSRT.

As a trainee years ago, I asked my licensed co-workers what these terms meant, and I received many different and sometimes colorful answers. By far the most common responses were “I don’t know,” “Go look it up,” and “Why do you ask so many questions?” Let’s look at these acronyms, what they mean, and why they’re important in the quest to control the activated sludge treatment process.

Variations on a theme

In MCRT, the M is for “mean,” which is a substitute for “average.” The remaining letters, CRT, represent cell residence time — how long a particular bacterial cell, or a pound of bacteria, remains in the activated sludge secondary treatment train. MCRT is normally expressed in days.

SRT — solids retention time — is sometimes used as a synonym for MCRT. They typically mean the same thing, but they may be calculated using different pieces of data. SRT can be viewed as the total mass of the solids in the treatment system, whereas MCRT is the mass of the bacteria in the system. SRT is also expressed in days.

In essence, MCRT would be calculated with the volatile suspended solids (VSS) values (for example, mixed liquor VSS, effluent VSS, waste sludge VSS), whereas SRT would be calculated using the total suspended solids (TSS) values (for example, mixed liquor TSS, effluent TSS, waste sludge TSS).

DSRT is the acronym for dynamic solids retention time — about which, more later. For now, let’s focus on MCRT and SRT.

Taking the measure

Various reference manuals explain methods of calculating MCRT and SRT. One alternative for calculation is to include the solids in the secondary clarifiers. If a treatment facility has multiple clarifiers and carries a high sludge blanket (greater than 3 feet) in each, then accounting for the solids in the clarifiers can become important for accurate mass balance process control approaches.

Looking at the calculation for MCRT and SRT reveals how closely related these two are:

MCRT/SRT (Days) = Pounds of Solids Under Aeration

(Pounds per Day of Solids in effluent) +

(Pounds per Day of Solids Wasted from System)

In practice, how do you measure MCRT or SRT? First, to obtain each clarifier’s total suspended solids amounts, use a core sampler (Sludge Judge is one brand-name device) to take a routine sludge blanket depth reading. Do not empty the core sampler contents back into the clarifier — instead place it into a clean bucket (both sludge and water) and mix well. Then collect a portion in a sample container. After performing a TSS analysis on the sample, you will know the milligram per liter (mg/L) value of solids in the clarifiers. You can then calculate the total pounds of solids in the facility.

What to include

Bacteria in the treatment system do the work of capturing suspended and dissolved solids and organic material and converting them into new cell mass that will settle in the clarifiers. They exit the treatment system by one of three methods:

By way of waste activated sludge flowing out of the secondary process to digesters or other treatment units.

By “jumping over the weirs” of the final clarifiers and exiting with the plant effluent flow.

Through endogenous respiration — the decomposition of the bacteria (death) into more simple compounds (CO2, H2O, energy). Endogenous respiration is common in extended aeration activated sludge treatment modes and aerobic sludge digesters.

MCRT and SRT are mathematical calculations used to describe how long bacteria are in the system before they leave the plant via one of the first two methods listed above.

If a wastewater treatment plant is running well, experiencing very few times where suspended solids actually leave the plant, the operator may choose to exclude the minimal pounds of effluent TSS portion of the MCRT/SRT calculation and just use the wasted solids pounds. This makes the calculation simpler.

One disadvantage of using MCRT/SRT is the daily flow variations experienced at some treatment plants. When calculating MCRT/SRT, the daily flow rates of plant effluent and waste sludge are important values. But what if a facility does not waste sludge every day? Some plants waste sludge just a few times per week.

To help make the calculations a little easier to use in process control decisions and operation of a facility, you might use moving averages to smooth out the data, especially if using trend charts or computer-based trending software. A moving average is simply the daily data averaged over a period of time, commonly 5, 7 or 14 days. This helps alleviate upward or downward swings in data that is trended over time, especially waste sludge flow rates.

The accompanying graph shows very high spikes in the SRT value. These represent days where no wasting of sludge occurred and the SRT value increased by more than 20 days in just one day. This of course doesn’t make sense, and the moving average helps eliminate such error when trending the data.

A look at DSRT

DSRT, an alternate method of sludge age calculation that has been discussed in the wastewater process engineering community, is a more accurate method of accounting for the total biomass produced and removed from the system each day. The DSRT sludge age calculation includes the amount of solids wasted from the system, but also includes the solids entering the system and possibly the amount of new solids produced in the facility on a daily basis. DSRT could include information such as influent BOD, TSS and VSS in the calculation.

DSRT has been used to help establish process control methods where sludge age is critical in determining the minimum amount of MLSS needed to provide nitrification of ammonia and meet regulatory nitrogen discharge limits

One disadvantage of DSRT is that it incorporates more data than MCRT or SRT, and this data may be difficult or expensive to obtain from the lab. Of course, this depends on your individual situation and laboratory budgets.

In summary, MCRT, SRT and DSRT can be very useful process control tools for operators of activated sludge treatment plants. Knowing the best MCRT or SRT for your plant when it is running its best can be a real asset when you find it running poorly and need some answers for making corrective action decisions.

Whichever method you choose, remember that the information it provides can only be as accurate as the information it receives from you. Representative sampling, accurate flow measurement, proper laboratory procedures and precise calculations are essential with these and any process control methods you use at your plant. For more information about these or other process control methods, please feel free to contact me via email anytime.

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.