What is a good SVI range?

Most conventional activated sludge plants run best between 80 and 150 mL/g. Below 50 suggests dense, over-aged sludge that can leave pin floc in the effluent. Above 150 settling is degrading, and above 200 usually means filamentous bulking.

What units is SVI expressed in?

Milliliters per gram (mL/g). It represents the volume occupied by one gram of suspended solids after settling for 30 minutes.

What does a low SVI number indicate?

A low SVI means dense, fast-settling sludge. Below roughly 50 mL/g it usually indicates old sludge with a low F/M ratio, where excessive endogenous decay has broken down floc structure. The result is pin floc — small dense particles that do not settle with the blanket and raise effluent turbidity.

Can SVI be used to find the maximum RAS concentration?

Yes. The settleometer result gives SSC30, the maximum concentration the sludge can compact to in 30 minutes: divide 1,000 mL by the settled volume in mL/L and multiply by MLSS. Pulling RAS slower than this will not thicken it further, because the sludge has reached its compaction limit.

SVI Calculator — Sludge Volume Index with Instant Interpretation

Calculate Sludge Volume Index

30-Minute Settled Volume (SV₃₀) Settleometer reading after 30 minutes

mL/L

MLSS Concentration From your lab solids test

120

mL/g

Normal

<5050–8080–150150–200200+

SVI = SV₃₀ (mL/L) ÷ MLSS (mg/L) × 1,000 · SSC₃₀ = (1,000 ÷ SV₃₀) × MLSS

The Formula

SVI (mL/g) = SV₃₀ (mL/L) ÷ MLSS (mg/L) × 1,000

SV₃₀ is the volume your mixed liquor occupies after settling undisturbed for 30 minutes in a 1-liter settleometer. MLSS is the mixed liquor suspended solids concentration from your lab. The 1,000 converts milligrams to grams — which is why the answer comes out in milliliters per gram: the volume one gram of solids occupies after settling.

Worked example

A settleometer reads 300 mL after 30 minutes. The lab reports MLSS at 2,500 mg/L.

SVI = 300 ÷ 2,500 × 1,000 = 120 mL/g

120 mL/g sits in the normal band. The sludge is settling and compacting the way a healthy conventional plant should.

What Your Number Means

SVI (mL/g)	Condition	What You'll See
Below 50	Over-dense / old sludge	Settles fast but leaves pin floc; turbid effluent despite a low blanket
50 – 80	Dense	Excellent compaction; watch for fines carrying over at long sludge age
80 – 150	Normal	Target range for conventional activated sludge
150 – 200	Marginal	Settling degrading; blanket climbing; find the cause now
Above 200	Bulking	Filamentous bulking likely; solids risk going over the weirs

One reading is a data point, not a diagnosis. A single SVI of 165 means far less than an SVI that has climbed from 95 to 165 over ten days — the trend is the operating signal, which is why the settleometer is a daily test.

Why the Calculator Also Gives You SSC₃₀

The same settleometer reading tells you the maximum concentration your sludge can compact to in 30 minutes:

SSC₃₀ = (1,000 mL ÷ SV₃₀) × MLSS

That's the ceiling on your RAS concentration. If your calculated RAS is already at SSC₃₀, slowing the RAS pumps further won't thicken it — the sludge has hit its compaction limit, and all you'll accomplish is a deeper blanket and a longer residence time for denitrification to start lifting solids. If your RAS is well below SSC₃₀, there's room to reduce flow and thicken.

If Your SVI Is High

Above 150 and climbing, the cause is usually one of these:

Low dissolved oxygen — chronically under 1.0 mg/L favors S. natans and H. hydrossis. The single most common cause.
Long sludge age with low F/M — favors M. parvicella and Type 0041; often paired with thick brown foam.
Septic influent — hydrogen sulfide in the collection system feeds Thiothrix, Beggiatoa, and Type 021N.
Nutrient deficiency — insufficient nitrogen or phosphorus relative to BOD, common with industrial loading.
Clarifier overload — if SVI is normal but the blanket still rises, the problem is solids loading rate, not sludge quality.

Confirm with a microscope before you act. The filament type points at the condition, and the condition points at the fix — chlorinating the RAS line when the real problem is a fouled diffuser treats the symptom and damages your floc formers.

If Your SVI Is Low

Below 50, the sludge is over-aged. Excessive endogenous decay has broken down the exopolymer that holds floc together, so what's left is dense fragments that compact hard but don't flocculate — pin floc drifting over the weirs while your blanket looks perfect. The fix is usually increasing the WAS rate to bring sludge age back down, gradually, over several days.

Practice Questions on SVI and Settleability

Activated Sludge: Bugs, Basins & Beyond covers the settleometer test, SVI, SSC₃₀, solids loading, and every common settling problem — 150 questions with a detailed explanation behind every answer.

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