Bulking Sludge When Wasting Rates Look Normal
A 2.2 MGD activated-sludge plant has two 0.45 MG aeration basins (0.90 MG total), MLSS 2,600 mg/L, and WAS of 0.022 MG/day at 5,800 mg/L. Influent BOD is 240 mg/L. Sludge settles to 620 mL/L in a 30-minute test. Effluent TSS is 34 mg/L against a 30 mg/L permit limit; CBOD and ammonia are in compliance. DO at the end of the basin is 0.8 mg/L. The headworks sulfide reading is 2.0 mg/L; the industrial area is served by a 4-mile force main with a 6-hour detention time. The two secondary clarifiers are 70 ft in diameter, and the RAS rate is 1.0 MGD. The superintendent wants to know whether to change the wasting rate, because the sludge is bulking.
1. Compute MCRT, F/M, and SVI, and say whether changing the wasting rate is the right first move.
Read the worked answer
Solids inventory: 0.90 MG x 2,600 mg/L x 8.34 = 19,500 lb (aeration basin only; the clarifier adds more, so MCRT is slightly higher than this).
Wasting: 0.022 MG x 5,800 mg/L x 8.34 = about 1,064 lb/day.
MCRT: 19,500 / 1,064 = about 18 days.
F/M: BOD load = 2.2 MGD x 240 x 8.34 = about 4,400 lb/day. MLVSS at 0.75 x 2,600 = 1,950 mg/L, so 0.90 x 1,950 x 8.34 = about 14,600 lb. F/M = 4,400 / 14,600 = about 0.30 lb BOD per lb MLVSS-day.
SVI: 620 / 2.6 = 240 mL/g - severe bulking.
Judgment: 18 days MCRT and 0.30 F/M are both normal for a nitrifying activated sludge. So the SVI of 240 is not explained by wasting too little or too much. Changing the wasting rate first would be guessing: wasting down would worsen low-DO filaments, wasting up would cut MCRT and could hurt nitrification. Look at dissolved oxygen, septicity, nutrients, and pH first, then decide wasting.
2. Given the DO and sulfide data, what is the most likely cause, and what sequence of actions do you take? What result do you expect and when?
Read the worked answer
Most likely cause: low DO plus septic influent.
- DO of 0.8 mg/L at the end of the basin selects for filamentous organisms that compete well at low oxygen (Sphaerotilus and similar). Target 1.5-2.0 mg/L at the last pass.
- 2.0 mg/L sulfide at the headworks means septic, low-ORP wastewater after 6 hours in the force main. Sulfide consumes DO the moment it reaches the basin, drives odors and corrosion, and favors filaments.
Actions in order:
- Raise aeration to hold 1.5-2.0 mg/L at the last pass. Check diffusers for fouling and air distribution - not just blower output.
- Attack septicity: clean the force main, shorten its detention by changing pump cycles, or dose nitrate/ferric at the wet well to suppress sulfide. Find any industrial contributor.
- Check BOD:N:P and basin pH; nutrient deficiency also bulks.
- Manage the clarifier now: keep blankets low, control RAS/WAS, consider a small polymer dose to protect the 30 mg/L TSS limit while the biology recovers.
Expected result: SVI improves over roughly 2-3 sludge ages - 5 to 8 weeks at 18 days MCRT - once filaments are starved out. Do not expect a change in a week. Do not increase wasting as the first move; lowering MCRT does not fix the selected filaments and can make them worse.
3. Is the clarifier overloaded? Support the answer with hydraulic and solids loading rates, and state what that means for the fix.
Read the worked answer
Area: two 70-ft units = 2 x pi x 35^2 = 2 x 3,850 = 7,700 ft2.
Hydraulic loading: (2.2 MGD + 1.0 MGD RAS) = 3.2 MGD. 3,200,000 / 7,700 = about 416 gpd/ft2, or about 0.29 gpm/ft2. Normal secondary clarifier design is roughly 400-800 gpd/ft2, so this is inside the range.
Solids loading: MLSS x (Q + RAS) x 8.34 / area = 2,600 x 3.2 x 8.34 / 7,700 = about 9.0 lb/day/ft2. Typical allowable is 25-30 lb/day/ft2, so it is well inside the range too.
Conclusion: the clarifier is not hydraulically or solids overloaded. The failure is settling velocity. At SVI 240 the limiting flux is far below the design flux, the blanket rises, and solids carry over - hence 34 mg/L against a 30 mg/L limit. Adding a clarifier or cutting flow will not fix it.
What that means: fix the filamentous population (DO and septicity), and use interim measures to hold the permit - polymer at low dose, lower blankets, controlled RAS, and temporary RAS chlorination only if the state allows it. Track SV30/SVI daily, effluent TSS, blanket depth, and a microscopic exam to confirm the filaments are declining.