8.4 Activated Sludge Troubleshooting
Key Takeaways
- A sludge volume index above roughly 150 indicates filamentous bulking, while an SVI below about 80 indicates pin floc from an over-oxidized sludge.
- Low dissolved oxygen, low food-to-microorganism ratio, nutrient deficiency, and septic influent are the classic causes of filamentous bulking.
- Nocardia and Microthrix foaming produces thick brown greasy foam and is addressed by removing foam from the system rather than spraying it down.
- Rising sludge in the secondary clarifier is denitrification lifting settled solids on nitrogen gas bubbles, corrected by increasing return sludge rate.
- Billowing sludge indicates hydraulic or solids overload, while ashing indicates excessive sludge age or grease.
8.4 Activated Sludge Troubleshooting
Activated sludge is the process most likely to fail on your shift, and the exam tests diagnosis rather than definitions. The pattern is always the same: an observation is given, and you must name the cause and the correction.
1. Read the Settleometer
A one-liter settleometer run for 30 minutes is the fastest process diagnostic available.
| Observation at 30 minutes | Interpretation |
|---|---|
| Settles to 150–250 mL with a clear supernatant and a crisp interface | Healthy, well-flocculated sludge |
| Settles very little, stays diffuse, 300–800 mL | Bulking — investigate filaments |
| Settles rapidly to a compact, small volume with turbid, cloudy supernatant | Pin floc / old sludge — over-oxidized |
| Settles well but with fine particles suspended above the blanket | Straggler floc — young sludge |
| Settles, then the blanket rises after 20–60 minutes | Denitrification in the settleometer — rising sludge |
Sludge Volume Index
| SVI | Meaning |
|---|---|
| Below ~70 | Sludge too old; pin floc, dense, turbid effluent |
| ~80–120 | Ideal — good settling and compaction |
| ~120–150 | Marginal; monitor closely |
| Above ~150 | Bulking — poor compaction, blanket rising, washout risk |
Worked example. Settled volume 420 mL/L, MLSS 2,600 mg/L.
2. Filamentous Bulking
Filamentous organisms extend beyond the floc, bridging particles apart so the sludge occupies far more volume and will not compact. Some filaments are essential — they form the backbone of good floc — but excess growth is the most common activated sludge failure.
Causes and the filament each favors
| Cause | Typical filament | Correction |
|---|---|---|
| Low dissolved oxygen | Sphaerotilus natans, type 1701 | Raise DO to 1.5–2.0 mg/L in the aeration basin |
| Low F/M ratio (long sludge age) | Types 0041, 0675, 021N | Increase wasting to lower MCRT and raise F/M |
| Nutrient deficiency | Thiothrix, type 021N | Restore BOD:N:P of 100:5:1; supplement N or P |
| Low pH | Fungi | Add alkalinity |
| Septic influent / sulfides | Thiothrix, Beggiatoa | Aerate the collection system; pre-aerate; control the septic source |
| Grease and FOG | Nocardia (Gordonia), Microthrix parvicella | Source control through the FOG program; remove foam from the system |
Interim control
Chlorination of the return activated sludge at roughly 2–3 lb Cl₂ per 1,000 lb MLVSS per day selectively kills filaments, which protrude from the floc and are exposed first. This is a symptom treatment, not a cure — find and fix the underlying cause. Overdosing destroys the floc-forming organisms too and turns a bulking problem into a total process failure.
3. Foam — Read the Color
| Foam | Meaning | Action |
|---|---|---|
| White, billowy, light | Young sludge / high F/M — often a startup condition or after washout | Reduce wasting to build MLSS and sludge age |
| Crisp, light tan or brown, moderate | Healthy, well-balanced sludge | None — this is what you want |
| Dark brown, thick, scummy | Old sludge / low F/M — excessive sludge age | Increase wasting |
| Thick, greasy, chocolate-brown, persistent, viscous | Nocardia or Microthrix filamentous foaming | Physically remove the foam from the system. Do not spray it down with water — that returns the organisms to the process and simply recycles the problem. Selective foam wasting, surface skimming, and FOG source control are the answers |
| Black | Septic conditions, inadequate aeration | Increase DO; check for anaerobic zones and dead spots |
4. Clarifier Problems
| Problem | Appearance | Cause | Correction |
|---|---|---|---|
| Rising sludge | Clumps of well-settled sludge float to the surface, often with gas bubbles visible; sludge appears in patches and is otherwise dense | Denitrification — nitrate is reduced to nitrogen gas in the anoxic sludge blanket and the bubbles lift the solids | Increase RAS rate to reduce blanket detention; reduce sludge age; increase clarifier sludge withdrawal |
| Bulking sludge | Blanket high throughout, uniform, poor compaction, SVI over 150 | Filamentous growth | See Section 2 above |
| Billowing / washout | Large volumes of solids carried over the weirs | Hydraulic or solids overload — storm flow, excessive MLSS, RAS pump failure | Reduce MLSS, restore RAS, use equalization, put another unit in service |
| Pin floc | Small dispersed particles that will not settle; effluent turbid though blanket is compact | Sludge age too long; floc sheared into fragments | Increase wasting to lower MCRT |
| Straggler floc | Small light particles floating over an otherwise good blanket | Sludge too young | Reduce wasting |
| Ashing | Fine dark ash-like particles on the surface | Excessive sludge age, denitrification, or grease | Increase wasting; skim |
| Short-circuiting | Uneven flow, solids on one side | Density currents, wind, poor baffling, uneven weirs | Level the weirs, check baffles, verify flow split |
5. Nutrient Deficiency
The target ratio is BOD : N : P = 100 : 5 : 1 by weight. Domestic wastewater is naturally near this ratio; industrial wastewater is frequently deficient, which is why nutrient bulking is more common at plants receiving significant industrial load.
Worked check. A plant receives 4,000 lb/day of BOD. Nitrogen requirement is 5% of BOD:
If the influent supplies only 120 lb/day of nitrogen, the plant is 80 lb/day short and will grow filaments until it is supplemented.
6. Toxic Upset
Signature: dissolved oxygen rises sharply for no apparent reason — the biology has stopped respiring — accompanied by a loss of floc structure, a dispersed and turbid effluent, and dead protozoa under the microscope.
Response:
- Confirm with the DO trend and a microscopic examination.
- Identify and stop the source — the industrial pretreatment program exists for this.
- Reduce wasting to zero to preserve remaining biomass.
- Reseed from another plant's return sludge if the loss is severe.
- Bring load back gradually as the population recovers, which typically takes days to weeks.
Exam anchor: rising DO with no change in aeration is the classic toxicity signature. A falling DO points to increased load, not toxicity.
Settled sludge volume is 380 mL/L and MLSS is 2,200 mg/L. What is the SVI and what does it indicate?
Clumps of dense, well-settled sludge float to the surface of the secondary clarifier with visible gas bubbles, while the sludge itself settles well in a settleometer. What is happening?
A plant has thick, greasy, chocolate-brown foam persisting on the aeration basin surface, associated with Nocardia. What is the correct response?
Dissolved oxygen in the aeration basin rises sharply with no change in blower output, and the effluent becomes turbid with dispersed floc. What has most likely occurred?
Which nutrient ratio should be maintained for healthy activated sludge, and which type of plant most often falls short of it?