Turbid Effluent and a Creeping Blanket at a 0.75 MGD Extended-Aeration Plant
A 0.75 MGD extended-aeration plant (aeration basin 0.30 MG) serves a town with a poultry-processing discharger. For ten days the effluent has been turbid and gray, with TSS 38 mg/L and BOD 22 mg/L, and the clarifier blanket has crept up to 1.5 ft with a 30-minute settled sludge volume of 430 mL/L. Today's data: influent BOD 240 mg/L, MLSS 1,800 mg/L, SVI 240, aeration basin DO 2.5 mg/L, RAS 4,200 mg/L, WAS 0.030 MGD at 4,200 mg/L. The previous operator raised the WAS rate two weeks ago after a blanket carryover and has not touched it since. Effluent ammonia and chlorine residual are normal. The superintendent wants the WAS rate raised again to stop the carryover.
1. Using the data given, calculate F/M and MCRT. What condition do the two numbers together point to?
Read the worked answer
Loading and inventory
- BOD load = 0.75 MGD × 240 mg/L × 8.34 = 1,501 lb/day
- MLSS inventory = 0.30 MG × 1,800 mg/L × 8.34 = 4,504 lb
- F/M = 1,501 ÷ 4,504 = 0.33 lb BOD per lb MLSS per day
Solids leaving the system
- WAS = 0.030 MG × 4,200 mg/L × 8.34 = 1,051 lb/day
- Effluent solids = 0.75 × 38 × 8.34 = 238 lb/day
- Total = 1,289 lb/day
- MCRT = 4,504 ÷ 1,289 = 3.5 days
Interpretation: a high F/M (0.33) paired with a short MCRT (3.5 days) and SVI 240 is a classic young sludge / bulking signature. The biomass is being wasted out faster than slow-growing floc formers can establish, so the mixed liquor is dominated by dispersed, fast-growing organisms that settle poorly. The metric that condemns the plant is the MCRT, not the SVI by itself; the SVI only tells you the floc is the wrong age and type. Note that the wasting rate was set two weeks ago and never revisited - the mass balance has moved underneath a fixed number.
2. The superintendent says to increase wasting again. What wasting change actually addresses the problem, and what must be monitored as you make it?
Read the worked answer
The instinct is backwards. Wasting harder drives MCRT even lower (younger sludge, worse SVI, more turbidity). The fix is to hold more solids, not fewer.
Setting a target of 8 days MCRT
- Total solids out allowed = 4,504 ÷ 8 = 563 lb/day
- Effluent already removes 238 lb/day, so WAS = 563 − 238 = 325 lb/day
- WAS flow = 325 ÷ (4,200 × 8.34) = 0.0093 MG ≈ 9,300 gpd, down from 30,000 gpd
Caveats to state explicitly
- The calculation assumes MLSS stays at 1,800 mg/L. It will not - cutting wasting raises the inventory, which raises the target WAS mass. Recompute MCRT daily from actual MLSS and adjust, rather than setting the flow once and walking away.
- Reduce wasting in steps (e.g., 30,000 → 20,000 → 14,000 → 9,000 gpd over several days) so the clarifier is not hit with a sudden blanket rise.
Monitor
- Clarifier blanket depth (at least once per shift) and effluent TSS - the immediate risk of holding solids is a blanket washout.
- SVI and 30-minute settle with supernatant clarity - the leading indicator that the floc is maturing.
- Aeration basin DO - a bigger inventory consumes more oxygen; if DO falls below ~2.0 mg/L the fix becomes another problem.
- RAS rate and RAS suspended solids, plus microscopic exam weekly to confirm floc formers and protozoa returning.
3. Before you commit to "young sludge," what other causes of a turbid effluent and rising blanket must be ruled out, and how would you rule them out?
Read the worked answer
Run these in parallel rather than serially, because several look identical on the bench:
- Denitrification rising sludge. Blanket rises in chunks, often with visible gas bubbles, and the supernatant is clear, not turbid. Check nitrate in the clarifier and the DO in the RAS line (low DO + nitrate = rising). Here the turbid supernatant argues against it, but confirm with a nitrate reading.
- Septic/anaerobic influent. Gray, sulfide-odored influent, low ORP, low DO recovery. Check influent sulfide, ORP, and whether the collection system has long force mains or lift station wet wells that go septic overnight.
- Industrial slug from the poultry plant. High BOD/ammonia/FOG loading, rapid DO depression, pH swings. Check the discharger's flow and the plant's daily BOD/ammonia trend against production schedules; review any pretreatment reports.
- Nutrient deficiency or low pH. Confirm effluent ammonia and phosphorus are not exhausted and that basin pH is near neutral; a sudden nitrification failure can strip the biomass of nitrogen.
- Physical/hydraulic issues. Short-circuiting in the aeration basin, a stalled mixer, a plugged diffuser grid, or a clarifier hydraulic overload from high RAS - any of these mimic poor biology.
- Sampling or analysis error. Confirm the MLSS and SVI were run on a genuinely mixed grab, that the 30-minute settle used a well-mixed sample, and that the effluent TSS filter was not overloaded. Reproduce the numbers before changing the process.
Only after these are excluded is "young sludge from over-wasting" the defensible diagnosis - which is exactly what the MCRT of 3.5 days predicts.