Bulking Sludge in an Oxidation Ditch
A 0.35 MGD extended-aeration oxidation ditch has a 0.30 MG aeration basin. MLSS is 4,200 mg/L, SVI is 240 mL/g, and dissolved oxygen sits at 0.7 to 1.0 mg/L even with both blowers running. Waste activated sludge is 5,000 gallons per day at 7,000 mg/L. Effluent TSS is 34 mg/L against a 30 mg/L limit; effluent ammonia-nitrogen is 1.1 mg/L against a 2.0 mg/L limit. Influent BOD is 210 mg/L. The collection system includes long force mains, residents complain of sulfide odors at manholes, and sulfide is measurable at the headworks. Microscopy shows abundant long filamentous organisms. The plant has never calculated solids retention time.
1. Calculate the SRT and F/M ratio and interpret them.
Read the worked answer
Solids inventory: 0.30 MG x 4,200 mg/L x 8.34 = 10,508 lb MLSS
Solids leaving:
- Waste sludge: 0.005 MG x 7,000 mg/L x 8.34 = 292 lb/day
- Effluent solids: (0.35 - 0.005) MG x 34 mg/L x 8.34 = 98 lb/day
- Total: 390 lb/day
SRT = 10,508 / 390 = about 27 days
F/M: BOD load = 0.35 MG x 210 mg/L x 8.34 = 613 lb/day F/M = 613 / 10,508 = 0.058 lb BOD per lb MLSS per day
Interpretation:
- An SRT of 27 days and an F/M of 0.06 means the biomass is severely underloaded. Low-F/M conditions are one of the classic selectors for filamentous bulking, and the SVI of 240 confirms it.
- Low F/M is not the only selector here. DO at 0.7 to 1.0 mg/L favors low-DO filaments, and the septic, sulfide-bearing influent from long force mains supplies the readily biodegradable substrate and sulfur that favor organisms such as Microthrix and Type 021N.
- This is a multi-cause bulking problem. Raising the DO alone will not fix a 27-day SRT.
2. Set a wasting target and show the calculation, including the risk you are managing.
Read the worked answer
Target: SRT of about 16 days and MLSS of about 3,400 mg/L, which preserves nitrification with margin while restoring a healthier F/M.
At the target:
- Inventory: 0.30 x 3,400 x 8.34 = 8,507 lb
- Solids out at SRT 16: 8,507 / 16 = 532 lb/day
- Effluent carries 98 lb/day, so WAS must carry 532 - 98 = 434 lb/day
- WAS flow = 434 / (7,000 x 8.34) = 0.0074 MG = about 7,400 gpd
So: increase wasting from 5,000 to roughly 7,400 gpd, in steps, checking MLSS and SVI twice a week.
Risks you are managing:
- Nitrification. Ammonia is only 1.1 mg/L now because of the long SRT. Drop the SRT too far or too fast and ammonia will rise, especially as water cools. Do not go below the SRT your winter temperatures require; 16 days is a buffer, 8 days is not.
- Ramp rate. During the drawdown from 4,200 to 3,400 mg/L, the actual SRT is temporarily lower than the calculated target. Step the wasting up over one to two weeks rather than opening the valve wide.
- Solids loss. Bulking sludge at SVI 240 will wash out over the weir quickly once you start wasting; watch effluent TSS daily and be ready to adjust polymer or flow.
3. What else must change for the wasting change to actually hold, and how will you verify success?
Read the worked answer
Wasting fixes the F/M selector, but not the others.
Raise DO to about 1.5 to 2.0 mg/L in the ditch. Check whether the blowers are actually delivering air: fouled diffusers or a partially closed valve will hold DO down at high MLSS regardless of blower output. Lowering MLSS will itself reduce oxygen demand.
Attack the sulfide at the source. Septic wastewater selects for the filaments you are growing. Options include cleaning and inspecting the force main and manholes, chemical dosing (nitrate or iron salts) in the force main, and reducing the detention time in the long force mains. This also protects the concrete from sulfide corrosion.
Check for other selectors: nutrient deficiency (industrial contributors), a septic return stream, or a low pH in the ditch.
Verification - measure, do not assume:
- SVI every week with a consistent 30-minute settle test; a drop below about 150 mL/g is real progress.
- Effluent TSS trending down and no sludge blanket loss over the weir.
- Effluent ammonia stable through the drawdown.
- Repeat microscopy in two to three weeks; the filament count and type tell you whether you selected against them.
- F/M back in a reasonable extended-aeration range around 0.08 to 0.12, not 0.06.