Activated Sludge Bulking and Effluent TSS Excursion
A 2.5 MGD activated sludge plant with a conventional aeration basin and secondary clarifiers is running an SVI of 280 mL/g. The mixed liquor suspended solids are 2,200 mg/L, and the return activated sludge rate is 0.5 times influent flow. The secondary clarifier blanket is rising, and effluent TSS has climbed from 12 mg/L to 38 mg/L against a 30 mg/L permit limit. Dissolved oxygen in the aeration basin is 1.8 mg/L at the tail end. The influent has a strong septage load and a low F/M because the plant has been wasting lightly to build solids for a winter margin. Filamentous organisms are visible on the microscope. The plant has no selector and no ferric feed available. The permit also has an ammonia limit that is currently being met.
1. Diagnose the bulking and rank the corrective actions you would take in the next 24 hours.
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SVI of 280 with rising blanket and filamentous organisms is classic filamentous bulking. The low F/M (light wasting to build solids) is the likely driver: low substrate gradient favors filamentous growth, and the septage load gives a shock of readily degradable substrate that filaments exploit. DO at 1.8 mg/L at the tail is adequate, so low DO is not the primary cause.
Next-24-hour actions, ranked:
- Protect the permit first: increase RAS rate to pull the blanket down and reduce solids carryover, and check the clarifier for a rising blanket or sludge washout. If TSS is still over 30 mg/L, consider temporary flow reduction or hauling, and notify the state if an exceedance is unavoidable.
- Start wasting to raise F/M toward the target range (roughly 0.2–0.4 lb BOD/lb MLSS/day for conventional). The current low F/M is feeding the filaments.
- Manage the septage: meter and equalize it, or reduce the septage acceptance rate until the biomass stabilizes, so you stop feeding the shock load.
- Confirm DO is maintained at 2.0 mg/L or above throughout the basin, especially at the tail.
Without a selector or ferric, you cannot fix the filaments structurally today, so control F/M and solids inventory and protect the clarifier.
2. What longer-term changes would you recommend, and how do you avoid trading the bulking problem for an ammonia violation?
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Long-term fixes target the selection pressure that favors filaments:
- Add an anoxic or anaerobic selector ahead of the aeration basin to create a substrate gradient that favors floc-formers over filaments.
- Consider a temporary or permanent chemical aid (ferric or polymer) at the clarifier if the state allows, to control solids until the biology improves.
- Tighten septage receiving: flow-paced, equalized, with a load limit and a hauler tracking log.
- Set a wasting schedule tied to SRT and F/M targets rather than building solids for a winter margin; if you want a winter cushion, build it with a controlled SRT, not by starving the biomass.
Avoiding an ammonia violation: nitrifiers are slow growers and are easily washed out when you increase wasting. When you raise the waste rate, do it in steps and watch effluent ammonia daily. Do not drop SRT below the nitrifier washout threshold for the current temperature. If ammonia starts to climb, back off wasting, increase aeration to hold DO, and consider reducing the septage load, which carries high ammonia. Keep the selector design from creating anaerobic conditions that harm nitrification.