Sludge Bulking vs Settling: Biotech Companies in India
“Our MLSS is quite high, but we are not getting enough settling. “ or “Our biomass development is very good as our MLSS is high, but we have very little BOD/COD reduction”. these statements are often given by EHS managers. However, the concept of MLSS is completely misunderstood; it’s never the quantity of MLSS, it’s always the quality of MLSS. The settling of sludge and BOD reduction always correspond with how good the MLSS is, and not how much it is.
This blog intricately explains the difference between sludge bulking and sludge settling, and which factors are necessary to look out for.
Sludge Settling vs Sludge Bulking:
With the growing awareness of operational efficiency, several biotech companies in India are now addressing sludge bulking challenges through microbial innovation and advanced diagnostics.
Healthy Sludge Settling:
In a well-operating secondary clarifier, biomass flocs are compact, dense, and settle rapidly. The supernatant above appears clear, and the sludge blanket remains stable.
Sludge Bulking:
Here, the sludge appears fluffy, loose, and struggles to compact at the bottom. The supernatant turns turbid, and sludge blankets may rise or disperse.
| Parameter | Healthy Settling | Sludge Bulking |
| SVI (Sludge Volume Index) | 80–120 mL/g | >150 mL/g |
| Sludge appearance | Dense, compact flocs | Loose, filamentous flocs |
| Supernatant | Clear | Turbid |
| Settling time | 20–30 mins | >45 mins |
| Cause | Balanced system | Filamentous overgrowth, F/M imbalance |
Why Good MLSS ≠ Good Settling
Operators often celebrate high MLSS as a sign of strong microbial population. But MLSS is a mass reading-It doesn’t distinguish between healthy floc-formers and problem-causing filamentous organisms.
“ Think of it like body weight: Two individuals weigh the same, but one may be with lean muscle, the other with excessive fat.
In bulking scenarios, the bulk of MLSS is held together by filamentous bacteria-these long, thread-like organisms stretch out of flocs, creating open, web-like structures that trap water and resist compaction.
Reliable biocultures companies have been instrumental in developing floc-forming microbial strains specifically tailored for bulking control.
What Causes Sludge Bulking?
- Filamentous Bacteria Overgrowth
Common species: Type 021N, Sphaerotilus, Microthrix parvicella, Thiothrix
These bacteria thrive under specific conditions such as:
Low DO (<1.0 mg/l) – especially at floc centers.
High F/M ratios – excess food leads to dominance of fast-growing filaments
Nutrient Imbalance– N and P deficiency affect floc formation
Surfactants and FOG – common in food, dairy, and textile industries
Hydraulic surges – shock loading from upstream process
Leading microbial companies in India are providing industry-specific solutions for complex ETP issues, helping clients achieve consistent results in variable conditions.
- F/M Ratio Imbalance
Too much organic load relative to MLSS results in excessive microbial growth, and filamentous bacteria often outcompete floc-formers.
Ideal F/M ratio: 0.2-0.5 kg BOD/kg MLSS/day
Bulking is more likely when F/M > 0.6 or < 0.1, especially during inconsistent feed conditions.
- pH and Toxic Shocks
Sudden changes in pH (below 6.5 or above 8.5) , or toxic loads (solvents, phenols, metals) can kill floc-formers and allow filaments to dominate during regrowth. However, Solutions like those from Team One Biotech, a known player among bioculture for ETP STP plant manufacturers, are reshaping how industries manage MLSS health and sludge behavior.
Decoding SVI and other key Indicators
Sludge Volume Index (SVI) is the gold standard for assessing settleability.
- SVI = ( Settled sludge volume in 30 mins, mL/L) / MLSS (g/L)
- SVI < 100 = Good settling
- SVI 120–150 → Early warning of bulking
- SVI > 200 → Severe bulking
Other red flags:
- Rising sludge in the clarifier
- Scum layer formation
- Poor TSS in final discharge
- Varying DO and pH patterns in aeration tanks
Countermeasures- How to fix Bulking?
In addition to microbial solutions, industrial odor control systems also play a pivotal role in overall ETP performance and workplace hygiene.
Short-Term Fixes:
- Chlorination or Peracetic Acid Dosing: Targets filamentous bacteria selectively. Start with 0.5–1 ppm, monitor response.
- Increase DO Levels: Maintain >2.0 mg/L throughout the aeration tank, especially in large tanks or tanks with dead zones.
- Sludge Wasting: Reduce SRT (sludge retention time) to control filament growth. Remove excess MLSS.
- Polymers in Clarifier: For emergency clarity issues, short-term use of cationic polymers can compact sludge.
Long-Term Solutions:
- Nutrient Balancing: Maintain COD:N:P at approx. 100:5:1. Add urea or DAP if needed.
- Equalization Tank: Smooth out hydraulic/organic loading rates to the aeration tank.
- Bioculture Regeneration: Consider seeding with robust floc-forming consortia after bulking episodes.
- Upgrade Aeration: Switch to fine-bubble diffused aeration systems to improve oxygen transfer.
- Micronutrient Support: Trace metals like iron, cobalt, and molybdenum support healthy floc formers.
If you’re exploring biocultures for ETP plant manufacturers in India or need effective bacteria solutions for wastewater treatment, Team One Biotech offers proven blends tested across sectors.
Conclusion:
Remember one quote: What settles well, treats well. MLSS and BOD tell only one part of the story – settleability, floc health, and microbial balance complete the picture.
As experts and EHS leaders, we must look beyond the dashboard. A 3500 mg/L MLSS might impress, but if your sludge floats and supernatant clouds, your ETP is already sending you a warning.
Looking for a trusted waste water treatment company to resolve sludge settling problems? Contact Team One Biotech today for tailored solutions and microbial consultation.
📧 Email: sales@teamonebiotech.com
🌐 Visit: www.teamonebiotech.com
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