Nitrification Falters in a Cold-Weather MLE Basin
A 3.6 MGD MLE plant treats municipal wastewater with a small dairy contribution. Design influent: TKN 40 mg/L, alkalinity 240 mg/L as CaCO3, BOD 210 mg/L, winter temperature 14 C. The aeration basin holds 1.5 MG at MLSS 2,600 mg/L; internal mixed-liquor recycle is set at 2.0 times influent flow and RAS at 0.6Q. Effluent ammonia-N has crept from 0.4 to 6.8 mg/L over two weeks. The last aeration zone pH has fallen from 7.1 to 6.5 while DO is held at 2.0 mg/L. Effluent nitrate is 18 mg/L. The lab's SRT calculation shows 6.0 days. BOD removal and effluent TSS are unchanged.
1. Do an alkalinity and pH balance for the basin and decide whether alkalinity is limiting nitrification.
Read the worked answer
Alkalinity demand. Nitrification destroys 7.14 mg/L of alkalinity as CaCO3 per mg/L of ammonia-N oxidized. About 40 - 6.8 = 33 mg/L N is being oxidized, so the demand is roughly 33 x 7.14 = 236 mg/L as CaCO3 against 240 mg/L entering.
Denitrification credit. Each mg/L of nitrate-N reduced in the anoxic zone returns about 3.57 mg/L as CaCO3. With the internal recycle at 2.0Q and effluent nitrate at 18 mg/L, you are denitrifying on the order of 15-20 mg/L of nitrate, returning roughly 54-71 mg/L as CaCO3.
Balance. 240 - 236 + about 60 = roughly 60-65 mg/L as CaCO3 leaving the basin, which is consistent with the measured pH of 6.5. Alkalinity is not exhausted, but the buffer is gone, and pH 6.5 in the last zone is below the comfortable window for Nitrosomonas and Nitrobacter - growth is near optimum at pH 7.5-8.5 and falls off sharply below about 7.0. That by itself can explain part of the ammonia rise.
Verify before acting. Confirm the pH probe calibration and read pH immediately on a fresh grab sample, because CO2 stripping in an open sample biases pH upward - the basin may be lower than the lab reports.
2. Is a 6-day SRT adequate at 14 C? Show the growth-rate reasoning and list what else you would rule out before recommending capital spending.
Read the worked answer
Temperature-corrected growth rate. Use mu_max = 0.47 exp[0.098(T - 15)]. At 14 C: 0.47 x exp(-0.098) = 0.47 x 0.906 = 0.43 per day. Subtract nitrifier decay of about 0.06 per day for a net 0.37 per day, giving a minimum SRT of 1/0.37 = about 2.7 days.
The safety factor is the point. Minimum SRT is not design SRT. Real plants need a factor of roughly 2.5 to 3 to absorb load swings, cold snaps, pH excursions and toxicity - that puts the target at about 7 to 10 days at 14 C. Six days leaves essentially no margin, and this basin is already spending part of that margin on pH stress.
Rule out before spending capital.
- Toxicity: the dairy contributor, cleaning chemicals, or a slug of high-sulfide or high-ammonia waste. Check influent ammonia, collection-system pH swings, and run a nitrification-rate or respirometry test.
- DO in the last zone specifically, not the basin average, and whether those diffusers are fouled.
- Internal recycle actually running at 2.0Q and not short-circuiting; RAS chloride or inert solids buildup.
- Nutrient and trace-metal limitation for nitrifiers (phosphorus, copper, molybdenum) in an industrial-influenced flow.
3. Recommend corrective actions in priority order, with the chemical dose calculations for restoring alkalinity.
Read the worked answer
1. Raise the SRT. Reduce waste sludge flow to take MCRT from 6 to about 10 days - roughly 40% less wasting at constant inventory - in steps, watching the clarifier, since a higher SRT raises MLSS and solids loading. Confirm with the mass balance rather than habit.
2. Restore alkalinity and pH. To add 60 mg/L as CaCO3 at 3.6 MGD: 3.6 x 8.34 x 60 = 1,802 lb/day as CaCO3.
- As sodium hydroxide (equivalent weight 40 vs 50 for CaCO3): 1,802 x 40/50 = about 1,440 lb/day of 100% NaOH.
- As soda ash (equivalent weight 53): 1,802 x 53/50 = about 1,910 lb/day. Feed downstream of the anoxic zone so the chemical is not consumed by denitrification, and control on basin pH - hold the last zone above about 6.8-7.0.
3. Buy alkalinity for free with denitrification. Raise the internal recycle from 2.0Q toward 3-4Q and confirm the anoxic zone is truly anoxic with readily biodegradable carbon available. Every mg/L of nitrate-N denitrified instead of discharged returns 3.57 mg/L of alkalinity.
4. Do not chase the symptom. Raising DO will not fix an SRT or alkalinity problem, it just spends energy. With effluent ammonia at 6.8 mg/L and rising, notify the regulator early and plan for a possible permit exceedance while the fixes take effect.