How I Increase MLSS in the Aeration Tank (Without Crashing the Plant)

How I Increase MLSS in the Aeration Tank (Without Crashing the Plant)

I’ve seen a lot of operators panic when MLSS starts dropping.

Effluent gets cloudy. Bugs look thin. Someone says, “We need more solids in the system.”

And then someone makes a huge adjustment… and things get worse.

I’ve done that too.

Now I handle it differently—and it works without crashing the plant.

Here’s exactly how I increase MLSS step by step.

🔍 First—What MLSS Actually Tells Me

MLSS (Mixed Liquor Suspended Solids) is basically:

👉 How much biology I have in the system

• Low MLSS → not enough bugs → poor treatment
• High MLSS → too many solids → settling issues

So when MLSS drops, I don’t panic—I diagnose.

⚠️ Step 1: Confirm It’s Actually Low (Don’t Guess)

Before I adjust anything, I check:

• Recent MLSS trend (not just one test)
• Effluent clarity
• Sludge blanket level

If MLSS is trending down over a few days, then I act.

🔧 Step 2: Reduce Wasting (This is the main move)

If I want MLSS to go up, I keep more solids in the system.

So I:

👉 Reduce wasting rate

• Close the WAS valve slightly
• Cut wasting time back
• Monitor daily

🔁 Why This Works

• Less sludge removed → more solids stay in system
• More solids → MLSS increases
• MLSS increases → sludge age increases

⚙️ Step 3: Make Small Adjustments Only

This is where most operators mess up.

I don’t shut wasting off completely.

Instead:

• I reduce wasting 10–20% at a time
• Watch the trend over 1–2 days
• Adjust again if needed

👉 Slow changes = stable plant

🌬️ Step 4: Support It with Proper DO

As MLSS increases, oxygen demand goes up.

If you ignore this, you’ll create a new problem.

So I check:

• DO levels (keep around 1.5–3.0 mg/L typically)
• Air distribution

👉 More bugs = more oxygen needed

🌀 Step 5: Watch Settling Closely

Higher MLSS isn’t always better.

As I increase MLSS, I watch:

• Settling in clarifier
• Sludge blanket depth
• Effluent clarity

If settling starts getting worse:
👉 I know I’m going too far

⚡ Step 6: Know When to Stop

I don’t chase a number—I watch performance.

I stop increasing MLSS when:

• Effluent is clear
• Settling is stable
• MLSS is in a reasonable range

👉 There’s no “perfect” MLSS—only what works for your plant

📊 Real Example (How I Handle It)

Let’s say:

• MLSS dropped from 3,000 → 2,000
• Effluent starting to get cloudy
• Ammonia creeping up

Here’s what I do:

1. Reduce wasting slightly
2. Monitor MLSS daily
3. Increase aeration if needed
4. Watch settling

Within a few days:
👉 MLSS climbs back up and stabilizes

🚫 Mistakes I Avoid Now

These will cause bigger problems fast:

• ❌ Shutting off wasting completely
• ❌ Ignoring DO while increasing MLSS
• ❌ Chasing a specific MLSS number
• ❌ Increasing MLSS when settling is already poor

🔗 How This Connects to Everything

Once you understand MLSS control:

• Low MLSS → reduce wasting
• High MLSS → increase wasting
• Bulking → often too high sludge age
• Instability → usually poor control of solids

👉 It all ties back to wasting + sludge age

💡 What Took Me Too Long to Learn

MLSS isn’t something you “fix” once.

It’s something you manage daily.

Small adjustments, watching trends, and staying patient—that’s what keeps the plant stable.

🔚 Final Thought

If you can control MLSS, you can control your entire biological process.

Everything else becomes easier:

• Settling
• Effluent quality
• Stability

Read more…

How to calculate sludge wasting rate

How I fix bulking sludge