Overloaded Activated Sludge Plant and a High-Strength Industrial Connection
A municipal activated sludge plant designed for 8,000 m3/day receives 11,500 m3/day in the wet season because of infiltration and inflow. Influent BOD is 320 mg/L and TSS 300 mg/L. The aeration basin holds 3,000 m3 at an MLSS of 1,800 mg/L, the secondary clarifier shows a sludge volume index of 240 mL/g with pin floc carry-over, and effluent averages 85 mg/L BOD and 95 mg/L TSS against a permit of 50 mg/L BOD and 70 mg/L TSS. Separately, a new food-processing plant has begun sending 200 m3/day at 2,500 mg/L BOD and pH 4.5 into the municipal sewer without pretreatment. Since that connection the plant's aeration demand has risen, septic odours are noticeable near the outfall, and the clarifier has begun showing a rising blanket in the afternoon. The LGU owns the plant and issues the sewer connection permit.
1. Quantify the loading on the plant (BOD mass load, food-to-microorganism ratio, volumetric loading) and explain how the numbers account for the effluent quality and the bulking sludge.
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BOD mass load. 11,500 m3/day x 320 mg/L = 3,680 kg BOD/day against a design of roughly 2,560 kg/day at 8,000 m3/day and the same strength, i.e. about 44% above design.
Biomass inventory. 3,000 m3 x 1.8 kg/m3 = 5,400 kg MLSS.
F/M ratio. 3,680 / 5,400 = 0.68 per day. Conventional activated sludge runs at about 0.2-0.4 per day and extended aeration at 0.05-0.15 per day. The plant is operating as a high-rate system, far outside its design envelope.
Volumetric loading. 3,680 / 3,000 = 1.23 kg BOD per m3 per day, at or beyond the upper end for this configuration.
Interpretation. The biomass is being outrun, so substrate passes through to the effluent and the effluent BOD of 85 mg/L is consistent with the F/M of 0.68. An SVI of 240 mL/g is bulking, and filamentous organisms are favoured by the combination of high F/M, septic sewage (sulfide-rich from long sewer travel times) and probably low dissolved oxygen in parts of the basin. Hydraulic overload of 44% also raises the clarifier surface overflow rate and shortens detention, so solids wash out and effluent TSS rises to 95 mg/L. This is a loading failure, not a tuning problem, and the fix has to address flow and load, not just the clarifier.
2. Assess the food-processing discharge: how large is it really, what does the pH do, and what should the LGU require before allowing the connection to continue?
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Size of the load. BOD mass = 200 m3/day x 2.5 kg/m3 = 500 kg/day. That is 500 / 3,680 = 13.6% of the plant's entire organic load delivered in 200 / 11,500 = 1.7% of the flow. The concentration of 2,500 mg/L is four to eight times a typical sewer ordinance limit and behaves as a shock load because it arrives in a concentrated slug.
The pH problem. At pH 4.5 the discharge is corrosive to concrete and to the sewer, and it can depress the aeration basin pH enough to inhibit nitrifiers and stress the floc-forming population. Combined with the high organic strength and the existing septic conditions, it accelerates odor and sulfide generation.
What the LGU should require. The connection permit was issued without conditions that protect the plant. Reissue it with: pH neutralization to 6.0-9.0 before discharge; grease and oil separation; flow equalization to flatten the slug; screening; a mass-based BOD limit with sampling points and self-monitoring reports; and a strength surcharge that reflects the treatment cost of the extra 500 kg/day. Because 500 kg/day is a serious load, an anaerobic pretreatment step (for example a UASB reactor) is worth evaluating, since it removes most of the BOD at low energy cost and reduces the load on the aeration basin.
The LGU also has to remember that its own discharge permit under the Clean Water Act is at risk; every kilogram it fails to treat is a compliance liability, and the industrial user should be made to carry its share through enforceable permit conditions.
3. The plant manager proposes doubling the polymer dose at the secondary clarifier to fix the effluent TSS. Evaluate that proposal and set out a defensible sequence of corrective actions.
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Evaluation. Polymer will not fix this. It improves capture of solids that are already there, but the solids loss here is driven by an SVI of 240 mL/g, meaning the sludge itself settles poorly. Adding polymer to bulking sludge increases chemical cost, adds inert material to the biomass and can blind the clarifier surface, while doing nothing to the effluent BOD of 85 mg/L. It treats the symptom.
Defensible sequence:
- Cut the hydraulic load. Rehabilitate the infiltration and inflow sources and consider flow equalization or storage of wet-weather peaks. The plant is 44% over its design flow; nothing else works until that is reduced.
- Fix the biology that causes bulking. Maintain dissolved oxygen at 2 mg/L or more throughout the basin, check the BOD:N:P ratio (about 100:5:1) since the food plant's load may be nutrient-deficient, and consider an anoxic or selector zone ahead of the aeration basin to favour floc formers over filaments.
- Manage the industrial load through the permit conditions described previously, so the shock and acid loads stop.
- Adjust wasting carefully. Raising MLSS helps only after settling improves, because more solids in a bulking clarifier simply means a higher blanket.
- Use polymer or a coagulant aid only as a temporary bridge once the biology is being corrected, and only at a dose set by jar or settleability testing.
Capital reinforcement, such as additional aeration volume or clarifier area, should be planned on the measured load, not on the design population equivalent.