SRT Formula: How to Calculate Mean Cell Residence Time

What Is Mean Cell Residence Time (MCRT)?

Mean Cell Residence Time (MCRT) - also called SRT or Solids Retention Time - is the average number of days a microorganism stays in the activated sludge system before it leaves via wasting or in the effluent. The SRT formula for wastewater divides the total pounds of suspended solids in the system by the total pounds of suspended solids leaving the system each day.

Here's the thing that trips everyone up: MCRT and SRT are essentially the same calculation. "Solids Retention Time" (SRT) is the textbook term. "Mean Cell Residence Time" (MCRT) is the lab-friendly version. Same math, same answer, same concept. You might also hear "sludge age" used casually on the plant floor to mean the same thing - but watch out: some exams use "Sludge Age" to mean Gould's Sludge Age, which is a different formula entirely. Gould's Sludge Age = Suspended Solids in Aeration / Suspended Solids in Influent per Day. If an exam question gives you influent TSS and flow instead of WAS data, they may be asking for Gould's Sludge Age. Always check which data the question provides before picking your formula.

Think of it this way. If you had 1,000 lbs of bugs in your system and you wasted 100 lbs per day, your average microorganism hangs around for 10 days. That's your SRT.

Why Does SRT Matter in the Plant?

SRT is one of the most important process control parameters in activated sludge. It directly controls:

• What type of organisms grow. Short SRT (3-5 days) favors fast-growing bacteria. Longer SRT (10-25+ days) allows nitrifiers to establish, which you need for ammonia removal.
• Effluent quality. Too short and you're washing out bugs faster than they can reproduce. Too long and you get old, pin-floc sludge that's hard to settle.
• Sludge production. Longer SRT means organisms consume more of each other (endogenous respiration), so you produce less waste sludge.

When your permit requires nitrification, you typically need an SRT above 10 days (sometimes higher in cold weather) to give those slow-growing nitrifiers enough time to establish. If you're just doing BOD removal, conventional activated sludge runs around 5-15 days.

Your F:M ratio and SRT are closely related. A high F:M means a short SRT, and vice versa. They're two ways of looking at the same process control picture.

How Do You Calculate SRT?

The standard SRT formula divides the total solids inventory in the aeration basin by the total solids leaving in both the waste stream and the effluent each day:

Expanded, the MCRT calculation looks like this:

Where:

• MLSS = Mixed Liquor Suspended Solids (mg/L) in the aeration basin
• Aeration Volume = Volume of the aeration basin (MG)
• WAS SS = Waste Activated Sludge suspended solids concentration (mg/L)
• WAS Flow = Wasting flow rate (MGD)
• Effluent SS = Effluent suspended solids (mg/L)
• Effluent Flow = Plant effluent flow rate (MGD)
• 8.34 = Conversion factor (lbs per mg/L per MG). In other words, 1 mg/L in 1 million gallons weighs 8.34 lbs.

Notice that 8.34 appears in every term - numerator and denominator. That's important, and we'll come back to it. If you need a deeper refresher on how the 8.34 factor works across wastewater math, check out the pounds formula.

Worked Example: Full SRT Calculation

The 8.34 Shortcut: When You Can Cancel It Out

Look at the expanded SRT formula again. The 8.34 factor shows up in every single term. When you're using MGD for all flows and mg/L for all concentrations, the 8.34 cancels out completely:

Let's verify with the same numbers:

This shortcut saves time on the exam. But here's the catch - it only works when all your units are consistent (mg/L and MGD throughout). If the problem gives you WAS flow in gallons per day or concentrations in percent solids, you can't skip the conversion step.

What If the Problem Ignores Effluent Solids?

Some exam questions (and some textbooks) simplify the sludge age formula by dropping the effluent solids term entirely:

This simplified version only accounts for solids removed through intentional wasting, not solids escaping in the effluent. In practice, when your effluent TSS is low (say, under 20 mg/L) and your WAS flow is significant, the effluent solids are a tiny fraction of total solids leaving, so it barely changes the answer.

But on the exam, use whichever version the question sets up. If they give you effluent TSS and effluent flow, include it. If they don't, use the simplified version. Don't overthink it - just use the data they give you.

This is similar to how the SVI calculation has a straightforward setup once you know which values to plug in.

Common SRT Exam Traps and Mistakes

Mixing up the numerator and denominator. The system solids go on top, the solids leaving go on the bottom. If you flip it, you get a fraction of a day instead of several days. If your answer is 0.17 days, you did it backwards.

Using the wrong solids concentration for WAS. The WAS concentration is NOT the same as MLSS. WAS is much more concentrated - most commonly drawn from the RAS line, though some plants waste directly from mixed liquor. Typical WAS is 6,000-12,000 mg/L, while MLSS runs 1,500-4,000 mg/L. If the problem says "return sludge concentration" or "underflow concentration," that's your WAS SS.

Forgetting to include secondary clarifier solids. Some versions of the SRT formula include solids in the clarifier as part of the system inventory (numerator). If the exam gives you clarifier volume and a solids concentration for it, add those pounds to the numerator. If they don't mention it, don't worry about it.

Unit conversion on WAS flow. WAS flow is often given in gallons per minute (gpm) or gallons per day (gpd) rather than MGD. A plant wasting at 50 gpm needs to be converted:

50 gpm x 1,440 min/day = 72,000 gpd = 0.072 MGD

Confusing MLSS with MLVSS. Some problems use MLVSS (volatile fraction) instead of MLSS. If the question specifies MLVSS, use it everywhere consistently. Don't mix MLSS in the numerator with MLVSS in the denominator. The EPA's NPDES program focuses on TSS for permits, but process control calculations might use either.

How Is SRT Used for Process Control?

In the real world, SRT is how you decide your wasting rate. Here's the operator logic:

1. You pick a target SRT based on your process goals (nitrification? Just BOD removal?).
2. You calculate your current SRT from lab data.
3. If SRT is too high, you increase wasting. If it's too low, you decrease wasting.

The relationship is simple: waste more, shorter SRT. Waste less, longer SRT.

Most plants calculate SRT daily or a few times per week using lab results. It's one of the core numbers, right up there with your dissolved oxygen readings and settling tests, that tell you how your bugs are doing.

The Sacramento State OWP manuals cover SRT-based process control in detail and are considered the gold standard reference for exam prep.

Quick Reference: SRT Ranges at a Glance