Overloaded 5 MLD STP near Bhopal: Diagnosing Biological and Clarifier Limits
The 5 MLD sewage treatment plant for a Bhopal suburb has an aeration tank of 1,200 m³, MLSS of 3,000 mg/L, SVI of 180 mL/g and one secondary clarifier of 12 m diameter. Design influent is 250 mg/L BOD and 300 mg/L SS. Because a downstream water-supply intake and a bird sanctuary lie within 8 km, the consent condition is now BOD 10 mg/L, SS 10 mg/L and ammoniacal nitrogen below 5 mg/L. Monitoring shows effluent BOD 38 mg/L, SS 55 mg/L, occasional pin flocs, and low river dissolved oxygen for 3 km downstream. Winter water temperature falls to 15 °C. The operator proposes to raise MLSS to 4,500 mg/L and increase aeration, saying that more biomass means better treatment. You must evaluate the biological and hydraulic loading, judge the operator's proposal, and recommend an upgrade sequence for a site with only 20 m of spare land.
1. Evaluate F/M, solids retention time and clarifier loading, and identify the controlling bottleneck.
Read the worked answer
Biological load. BOD load = 5,000 × 0.25 = 1,250 kg/d. F/M = 1250/(1,200 × 3.0) = 0.35 kg BOD per kg MLSS per day, which is conventional-range and too high for a 10 mg/L plus nitrification duty.
SRT. Take Y = 0.6, kd = 0.06 per day, ΔS = 0.250 - 0.038 = 0.212 kg/m³:
- Sludge produced = YQΔS - kd V X = 0.6 × 5000 × 0.212 - 0.06 × 3,600 = 636 - 216 = 420 kg/d.
- SRT = VX/Px = 3,600/420 = 8.6 days. Nitrification at 15 °C needs roughly 12 to 15 days, so ammonia will pass through - consistent with the low downstream dissolved oxygen.
Clarifier. Area = π × 6² = 113 m². Surface overflow rate = 5000/113 = 44 m³/m²·d against a design band of about 16-24; solids loading with return sludge (R ≈ 0.75) is about 230 kg/m²·d against 100-150. The clarifier is roughly twice overloaded.
Controlling bottleneck: clarification, not aeration. The 38 mg/L BOD is largely carried-over solids, and the pin flocs and 55 mg/L SS confirm it.
2. Test the operator's proposal against the sludge-settling data.
Read the worked answer
With SVI = 180 mL/g, the maximum attainable return sludge concentration is Xr = 10⁶/SVI = 5,556 mg/L.
Solids balance for the desired MLSS with return ratio R: X = Xr·R/(1 + R), so required Xr = X(1 + R)/R.
- At MLSS 3,000 and R = 0.75: required Xr = 3000 × 1.75/0.75 = 7,000 mg/L, above the attainable 5,556. The plant cannot even hold 3,000 mg/L; R would have to rise to about 3,000/(5,556 - 3,000) ≈ 1.2, which the existing RAS pumps probably cannot do.
- At MLSS 4,500 with R = 0.75: required Xr = 4,500 × 1.75/0.75 = 10,500 mg/L - impossible with this sludge.
Raising MLSS by 50 per cent also raises the clarifier solids loading by 50 per cent and the oxygen demand, so the operator's proposal would worsen the overflow of solids, increase effluent SS and BOD, and risk a rising sludge blanket. More biomass is useful only if it can be separated and retained; the correct response is to increase the retained inventory without raising the mixed-liquor concentration, or to increase clarifier capacity.
3. Recommend an upgrade sequence within the 20 m land constraint and justify the order of works.
Read the worked answer
Sequence by bottleneck, cheapest first:
- Clarification (immediate). Add a second secondary clarifier of 12 m diameter (113 m²), which fits within 20 m of spare land with access. Overflow rate then falls to about 22 m³/m²·d and solids loading to about 115 kg/m²·d, giving immediate improvement in SS and BOD. Correct the RAS pumping to R ≈ 1.0-1.2 and install a proper sludge blanket detector; add a DAF unit instead only if the geotechnical/land constraints prevent a clarifier.
- Solids retention time (ammonia). Increase SRT to at least 15 days. Either add about 500 m³ of aeration volume or, better within the land limit, convert the existing aeration tank to IFAS/MBBR with about 40 per cent media fill; attached biomass raises the retained inventory without increasing mixed-liquor MLSS, so the clarifier is not penalised again.
- Tertiary polishing and disinfection. Dual-media or cloth filtration plus UV/chlorination to hold SS below 10 mg/L and meet the bacteriological requirement.
- Sludge handling. About 420 kg/d of dry solids already exceeds the existing handling; provide thickening and dewatering with capacity for the upgraded plant, and return centrate upstream.
Justification: step 1 delivers the fastest consent compliance at the lowest cost, step 2 addresses ammonia, and step 3 guarantees the 10 mg/L limit under upset conditions. Extension of aeration alone (the operator's approach) would be wasted money because the clarifier remains the limiting unit.