Activated Sludge Plant Losing Biomass in the Monsoon
A 30 MLD conventional activated sludge plant serves a mixed residential and textile-dyeing town. The aeration tank volume is 6,000 m³ with MLSS of 3,000 mg/L in the dry season, and there are two secondary clarifiers of 24 m diameter each. Three blowers (two duty, one standby) supply fine-bubble diffusers at 4.5 m submergence, and sludge age is controlled by wasting 200 m³/d at 7,000 mg/L. Dry-season influent is 30 MLD with BOD 250 mg/L, SS 250 mg/L and TKN 35 mg/L, and the plant meets the discharge norm. In the monsoon the flow rises to 45 MLD with BOD 110 mg/L and TKN 22 mg/L; MLSS falls to 1,400 mg/L, effluent BOD rises to 40 mg/L and SS to 60 mg/L, DO in the tank rises to 4.5 mg/L, SVI is 240 mL/g and a sludge blanket appears on the clarifier surface. Filaments are visible under the microscope and the inlet sewer smells strongly of sulphide.
1. Compute F/M, HRT and SRT for both seasons and identify which parameter has actually failed.
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Dry season
- BOD load = 30,000 × 0.250 = 7,500 kg/d; VX = 6,000 × 3.0 = 18,000 kg → F/M = 0.42 d⁻¹
- HRT = 6,000/30,000 = 0.20 d = 4.8 h
- Sludge out = wasting 200 × 7.0 = 1,400 kg/d + effluent SS ~20 mg/L × 30,000 = 600 kg/d = 2,000 kg/d → SRT = 18,000/2,000 = 9 d
Monsoon
- BOD load = 45,000 × 0.110 = 4,950 kg/d; VX = 6,000 × 1.4 = 8,400 kg → F/M = 0.59 d⁻¹
- HRT = 6,000/45,000 = 0.133 d = 3.2 h
- Sludge out = 1,400 (wasting) + 45,000 × 0.060 = 2,700 kg/d = 4,100 kg/d → SRT = 8,400/4,100 = 2.0 d
Diagnosis: the F/M barely changes (0.42 → 0.59) and is not in the low-F/M bulking range at all, so low-F/M filament bulking is not the mechanism. What collapses is SRT, from 9 d to 2 d, and that loss is entirely through the effluent (2,700 of the 4,100 kg/d leaving in the overflow) - a hydraulic washout, not a wasting decision. HRT also falls to 3.2 h, below the ~4–6 h needed to damp a shock organic load in a fully mixed tank. The biomass is being flushed out faster than it can grow.
2. What causes the bulking and the clarifier blanket, and what does the 4.5 mg/L DO reading tell you?
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The high DO is diagnostic. At 4.5 mg/L the aeration system is not limiting; low-DO filamentous bulking (Type 1701, Sphaerotilus) is ruled out, and the low organic load in the monsoon means less oxygen demand. So the filaments are substrate/electron-acceptor driven, not oxygen driven.
Bulking mechanisms that fit the data:
- Short HRT with high soluble substrate entering in a fully mixed tank gives dispersed growth and favours fast-growing filaments over floc formers.
- Sulphide in the inlet is the decisive clue. Long detention in the sewers, plus sulphate-rich dyeing effluent, produces sulphide and a low redox potential; sulphur-oxidising filaments (Thiothrix, Type 021N, Beggiatoa) thrive on reduced sulphur compounds. This explains both the 240 mL/g SVI and the septic odour.
Blanket: at SVI 240 mL/g with MLSS 1.4 g/L, the 30-minute settled volume is 240 × 1.4 = 336 mL/L, i.e. about a third of the clarifier depth, while the hydraulic flow is 50% higher and the RAS pumps (sized for 30 MLD) cannot return the required 1.5–2.0 Q. The sludge rises rather than being under-returned because of the area. Solids loading = (45,000 + 18,000) × 1.4 / 904 m² ≈ 98 kg/m²·d, which is within the acceptable band - so the blanket is a settleability and return-capacity failure, not a clarifier-area failure.
3. Give immediate operational actions and the permanent works needed, with the size of the permanent works quantified.
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Immediately (this monsoon):
- Divert or equalise the excess 15 MLD. A storm-holding tank of 15,000 m³/d × 0.5 d = 7,500 m³, or throttled diversion to a storm overflow, protects the biomass.
- Stop wasting until MLSS recovers above 2,500 mg/L; run all RAS pumps at maximum and check the RAS capacity - it must handle 1.5–2.0 Q at 45 MLD, i.e. 67,000–90,000 m³/d, not the current 15,000 m³/d.
- Dose PAC or polyaluminium chloride to the clarifier as a short-term settling aid, and pre-aerate the incoming sewage to strip sulphide before it reaches the aeration tank.
- If filaments persist, dose the return sludge with 2–3 kg Cl per tonne MLSS for 3–5 days, watching for nitrifier kill.
Permanent works:
- Aeration volume. For a 6 h HRT at 45 MLD the tank must be 45,000 × 0.25 = 11,250 m³, an increase of 5,250 m³; alternatively add an anoxic/selector compartment ahead of the tank, which suppresses filaments far more cheaply than extra volume.
- Third clarifier or larger RAS capacity to keep the solids loading below about 120 kg/m²·d at the peak flow.
- Source control: sulphate/sulphide limits and pre-treatment for the dyeing cluster, plus sewer rehabilitation to cut infiltration - the flow rise from 30 to 45 MLD is a collection-system problem, not a treatment problem.
- Install DO and MLSS control loops, since the monsoon DO of 4.5 mg/L shows the blowers are running flat out for no benefit.