Filamentous Bulking in an Oxidation Ditch
An oxidation ditch plant has a permitted flow of 2.0 MGD and receives 1.6 MGD with influent BOD5 210 mg/L, TSS 230 mg/L, and grease from a restaurant district. MLSS is 4,200 mg/L, SVI is 310 mL/g, and the clarifier sludge blanket is 4 feet above the floor with a 9-foot sidewater depth. DO in the ditch swings from 0.6 mg/L at night to 1.4 mg/L in the afternoon. RAS is set at 0.55Q and WAS is 4,000 gallons per day at 0.8% solids. Effluent TSS is 38 mg/L and BOD5 is 22 mg/L. Foam with a dark, greasy appearance covers the ditch. The plant has no selector or chemical addition.
1. Diagnose the bulking type and explain the evidence. What would you check to confirm it?
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SVI 310 mL/g with a 4-foot blanket and dark greasy foam points to filamentous bulking, most likely Microthrix parvicella or Type 0092, not a simple hydraulic overload. The F/M is low: at 1.6 MGD, 210 mg/L BOD, and 4,200 mg/L MLSS, the sludge age is long, and low DO at night (0.6 mg/L) plus grease selects filaments. The low F/M and long MCRT are the key controls; the clarifier blanket is high because filaments impede settling, so RAS alone will not fix it. Confirm by microscopic exam for filament abundance and type, SVI trend, and influent grease. The effluent TSS of 38 mg/L is consistent with blanket washout, not a clarifier mechanical failure. Do not increase RAS as the only response; that returns more filaments to the ditch and can worsen the blanket.
2. What immediate operational changes would you make to stop solids washout?
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Immediate goals are to stop washout and raise DO. Increase aeration to hold at least 2.0 mg/L at the ditch outlet, especially at night; check blowers and diffusers. Increase WAS from 4,000 gpd to lower MLSS toward 3,000–3,300 mg/L, which raises F/M and shortens MCRT; do this in steps and watch the blanket. Keep RAS at a rate that holds the blanket at 1–2 feet, not 4 feet; if the blanket remains high, use a low-dose polymer to the clarifier temporarily and increase wasting. Remove grease at the headworks and check for septic return flows. Do not add chlorine indiscriminately; a short, low-dose RAS chlorination trial can knock back filaments but can also harm nitrifiers. Document DO, SVI, MLSS, and blanket depth daily.
3. What long-term capital and program changes would reduce recurrence?
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Long-term, add a selector or anoxic zone ahead of the ditch to create a substrate gradient that favors floc-formers over filaments. Add grease control in the restaurant district: interceptors, hauling records, and an ordinance. Improve aeration control so DO does not swing below 1.5 mg/L at night; install a DO probe and blower control. Recalculate the solids inventory and set a target MCRT and F/M, then waste consistently rather than only when the blanket rises. Consider a clarifier cover or foam control if Microthrix persists. Track SVI weekly and perform microscopic exams monthly. If the plant must nitrify, do not shorten MCRT too far; balance the bulking fix against nitrification. A pilot selector test before construction is cheaper than a failed full-scale retrofit.