Sludge Volume Index Formula: How to Calculate and Interpret SVI

What Is the Sludge Volume Index?

The Sludge Volume Index (SVI) tells you how well your activated sludge settles. It's the volume in milliliters that one gram of sludge occupies after 30 minutes of settling. The sludge volume index formula is one of the most tested calculations on wastewater certification exams.

You run a settleability test, grab your MLSS result from the lab, plug both numbers into the formula, and you've got a single number that tells you whether your bugs are settling like champions or floating around like a filamentous nightmare. SVI shows up in daily process control at every activated sludge plant.

The "× 1,000" converts milligrams to grams so your final answer comes out in mL/g. That's where most exam mistakes happen, so let's break this down step by step.

How Do You Run the Settleability Test?

Before you can calculate SVI, you need two numbers: the 30-minute settled sludge volume and the MLSS concentration.

Here's how you get the settled volume:

1. Grab a sample of mixed liquor from your aeration basin.
2. Pour it into a 2,000 mL (2-liter) settleometer or a 1,000 mL graduated cylinder.
3. Start your timer.
4. After 30 minutes, read the volume of settled sludge at the top of the sludge blanket.
5. If you used a 2,000 mL settleometer, divide your reading by 2 to convert to mL/L.

For the MLSS, you'll run a total suspended solids test on a separate mixed liquor sample. Many plants run MLSS daily or per shift, depending on plant size, staffing, and process needs.

How Do You Calculate SVI Step by Step?

The SVI calculation takes four steps: write the formula, plug in your settled volume and MLSS, divide, then multiply by 1,000. Let's walk through a typical exam problem.

That's it. Four steps. The formula itself is simple, but the exam will try to trip you up with the setup. Let's look at another example with a twist.

Notice that second example? The 2,000 mL settleometer is the trap. If you forget to divide by 2, you'd calculate an SVI of 219 mL/g and incorrectly conclude your sludge is bulking. That's exactly the kind of wrong answer choice you'll see on the exam.

What Do the SVI Numbers Actually Mean?

Here's a quick reference table for interpreting your SVI wastewater results:

In the plant, SVI trending is more useful than any single reading. If your SVI has been sitting at 95 mL/g for weeks and suddenly jumps to 140, that's your early warning to start investigating. Check your DO levels in the aeration basin, look at the food-to-microorganism balance, and get a sample under the microscope.

SVI connects directly to several other process control calculations. SVI informs both your sludge age and MCRT decisions (managing clarifier blanket depth and biology) and your wasting decisions. A climbing SVI might also point to an F:M ratio that's out of whack - if you haven't reviewed how to calculate F:M ratio, that's a good next step.

When Does SVI Fall Short?

SVI isn't perfect. It has a known weakness at high MLSS concentrations. When your mixed liquor is above about 4,000 mg/L, sludge gets compressed by its own weight during the 30-minute settle, and SVI can look artificially good. The sludge might actually be settling poorly, but the sheer volume of solids forces compaction.

That's why some plants use the Diluted SVI (DSVI) for high-MLSS situations. You dilute the mixed liquor sample with final effluent to a known ratio (such as 1:1 or 1:2) until the 30-minute settled volume falls between 150-250 mL/L. Then you calculate SVI using the original MLSS multiplied by the dilution fraction - this corrects for the dilution so you get a meaningful result. In many states and ABC-aligned programs, the exam probably won't ask you about DSVI at the Grade 1 level, but it might show up on Grade 3 or 4 exams. Check your specific exam blueprint to be sure.

For everyday process control at typical MLSS levels (1,500 - 4,000 mg/L), standard SVI works just fine.

What Are the Most Common SVI Exam Mistakes?

Here are the traps that catch operators on test day:

Forgetting the × 1,000. Without it, you get an answer in mL/mg instead of mL/g. Your answer will be a tiny decimal like 0.09 instead of 90. If your answer doesn't look reasonable, check whether you multiplied by 1,000.

Not converting settleometer volumes. As we covered above, a 2,000 mL settleometer reading must be divided by 2. Read the problem carefully for the container size.

Mixing up the fraction. It's settled volume divided by MLSS, not the other way around. If you flip it, you'll get a nonsensical number. Remember: the settled volume goes on top.

Calculator carry-over. Clear your calculator before every problem. If you've got a leftover number in memory from a detention time calculation three questions ago, you'll get garbage out.

Not checking reasonableness. Your SVI answer should almost always fall between 40 and 400 mL/g. If you get 5 or 5,000, something went wrong. Go back and check your setup.

How Does SVI Connect to Process Control?

In real plant life, SVI is one leg of the process control stool. You don't make decisions on SVI alone - you look at it alongside MLSS, sludge age, DO, F:M ratio, and what you see under the microscope.

Here's a practical scenario. Your SVI trends from 100 to 170 over two weeks. What do you do?

• Check DO. Low dissolved oxygen in the aeration basin favors filamentous organisms. If DO is below 1.0 mg/L, that could be your culprit.
• Check F:M. A low F:M ratio (underloaded system) can also encourage filaments. You might need to increase wasting to bring MLSS down.
• Look at the bugs. A microscopic exam tells you whether filaments are actually present and what type they are, which points you toward the root cause.
• Review your return sludge rate. Adjusting RAS rate primarily manages clarifier sludge blanket depth. Increasing RAS returns more solids to the aeration basin, which typically raises MLSS rather than diluting it. Wasting (WAS) is the separate control for managing sludge age and F:M ratio.

The EPA's NPDES program sets the effluent limits your plant must meet, and poor SVI often leads to solids carryover in your final effluent. That's a permit violation waiting to happen. Catching an upward SVI trend early gives you time to fix it before your clarifier starts losing solids.

The Sacramento State OWP manuals cover SVI in detail if you want the deeper textbook treatment, but for exam prep, the formula and interpretation table above are what you need to nail down.