Cold-Weather Nitrification Loss at an Activated Sludge Plant
A 2.6 MGD extended-aeration activated sludge plant has a permitted effluent ammonia limit of 1.5 mg/L monthly average from April through October and 2.5 mg/L November through March. Influent ammonia is 26 mg/L, flow 2.4 MGD, and water temperature has fallen from 18 to 10 degrees C in three weeks. MCRT is held at 6 days by the wasting schedule the previous operator used, MLSS is 2,600 mg/L, DO at the end of the aeration tank is 1.8 mg/L, and RAS is 60 percent of influent. Secondary clarifier sludge blanket is 3.5 feet in a 12-foot sidewater depth unit, SVI is 210, and the blowers are at 95 percent of capacity. Effluent ammonia has gone from 0.4 to 3.2 mg/L, effluent pH has dropped to 6.6 from an influent 7.3, effluent alkalinity is 45 mg/L, and effluent CBOD is unchanged. An industrial user discharging 0.15 MGD of food-processing waste increased production 30 percent without notifying you.
1. Explain what is failing and identify the single most important operational change.
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The nitrifiers are being washed out faster than they can grow. Nitrifier growth rate falls sharply as temperature drops, so the minimum solids retention time needed to keep them rises; at 10 degrees C you generally need well more than the 6 days of MCRT this plant is holding, and at 6 days you are wasting nitrifiers out of the system every day. The chemistry confirms it: ammonia climbing, alkalinity consumed (about 7.1 mg as CaCO3 per mg of ammonia nitrogen oxidized), and pH falling to 6.6, which itself slows nitrification further. Intact CBOD removal means the heterotrophs are fine, so this is not a general biomass collapse.
The most important change is to cut wasting and raise MCRT, letting MLSS climb so the nitrifier population can rebuild and the sludge age exceeds the minimum for 10 degrees C. Do that gradually and watch the clarifier, because you are asking it to carry more solids. The industrial user's unreported 30 percent increase is a separate load question: call them, get the flow and strength data, and check whether the extra load is ammonia, BOD, or something inhibitory before you assume temperature is the whole story.
2. The clarifier is at SVI 210 and the blowers are nearly maxed. How much can you actually raise MLSS, and what else do you fix?
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Do the arithmetic before you raise solids. Solids loading to the clarifier is MLSS times flow times 8.34, and you must include the RAS return. If you push MLSS from 2,600 toward 3,500 mg/L at 2.4 MGD plus 60 percent RAS, the clarifier sees roughly 35 percent more solids with an SVI of 210, meaning a blanket that is already at 3.5 feet will rise further and you risk a solids carryover that trades an ammonia violation for a TSS violation. Raise MLSS only as far as the clarifier's blanket and effluent TSS allow, and hold there.
Attack the SVI. At 6 days MCRT the sludge is young and tends to be dispersed; look at a wet mount for filament type, check for septicity and sulfide coming in from the collection system, and check the dissolved oxygen in the clarifier and RAS. If filamentous bulking is confirmed, temporary RAS chlorination is the usual short-term control. On aeration, 1.8 mg/L at the end of the tank is marginal for nitrification; move DO toward 2.5-3.0 at the end of the basin by shifting blower output, and check diffuser fouling, because 95 percent blower output with low DO usually means the diffusers, not the blowers, are the limit.
3. Effluent ammonia is 3.2 mg/L and the month ends in nine days. What are your compliance and communication obligations?
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Check the permit language first: is the limit a monthly average, a weekly average, or both, and what does the reporting calendar say? At 3.2 mg/L and nine days left, you are very likely to exceed a 2.5 mg/L monthly average for November, so treat it as a reportable noncompliance rather than hoping for recovery. Notify Ohio EPA within the time the permit specifies for anticipated noncompliance, and put the cause, the corrective actions, and the expected return to compliance in writing. Keep the operating log that shows when wasting was reduced and how the MCRT responded.
Assess the actual instream risk: ammonia toxicity is driven by un-ionized ammonia, which rises with pH and temperature, and at pH 6.6 and 10 degrees C almost all of it is in the ionized form, so the acute risk is lower than the number alone suggests, but the receiving stream still deserves a look. Sample upstream and downstream and note any odor or fish-kill reports. Finally, document the industrial user contact: an unreported 30 percent production increase is a pretreatment notification issue, and you want the correspondence in the file when you explain the excursion.