Alkalinity-Limited Nitrification With Alum Phosphorus Removal
Your 1.8 MGD plant must meet effluent TP 0.5 mg/L and NH3-N 1.0 mg/L monthly, with pH between 6.0 and 9.0. Influent alkalinity averages 185 mg/L as CaCO3, influent NH3-N is 32 mg/L, and influent TP is 6.5 mg/L. For years you have fed 60 mg/L of alum into the aeration tank and met TP. This month effluent TP is 0.7 mg/L, aeration-tank pH is 6.2 and dropping, and effluent NH3-N has crept from 0.4 to 4.5 mg/L. DO is 2.0 mg/L in the last pass and the blowers are at maximum output. The lab is confident in its numbers. Your predecessor's note in the binder says only: "Add more alum if TP rises."
1. Explain with arithmetic why adding more alum will make both problems worse.
Read the worked answer
Nitrification is consuming more alkalinity than the influent supplies.
Nitrification destroys about 7.14 mg of alkalinity as CaCO3 per mg of ammonia nitrogen oxidized.
- Ammonia oxidized: 32 − 1.0 = 31 mg/L as N
- Alkalinity consumed: 31 × 7.14 ≈ 221 mg/L as CaCO3
- Alkalinity available: 185 mg/L as CaCO3
The plant is already short before any chemical is added. Alum consumes additional alkalinity through hydrolysis, on the order of 0.45–0.5 mg as CaCO3 per mg/L of alum, so 60 mg/L of alum removes roughly another 30 mg/L. Total demand is about 250 mg/L against 185 mg/L supplied: a deficit of about 65 mg/L.
Consequences.
- pH falls into the low 6s, where nitrifier growth rate drops sharply; below about 6.0 nitrification is largely inhibited. That is why ammonia is climbing while DO is fine - this is not an oxygen problem.
- Alum hydrolysis and floc formation are less effective at low pH, so each additional pound removes less phosphorus. More alum means more acid, a bigger deficit, and a worse TP result.
- The rising sulfate and dissolved solids load adds cost with no benefit.
The problem is alkalinity, not coagulant dose. Adding alum treats the symptom the binder described and accelerates the failure.
2. How much alkalinity do you need, and what are two ways to supply it? Show the arithmetic and the trade-offs.
Read the worked answer
Target: maintain aeration-tank pH around 6.8–7.0 and about 30–50 mg/L of residual alkalinity as CaCO3 in the final effluent.
Required supplement:
- Demand: 221 (nitrification) + 30 (alum) + 40 (residual target) = 291 mg/L as CaCO3
- Supply: 185 mg/L
- Supplement: ≈ 105 mg/L as CaCO3
Option A - sodium bicarbonate (NaHCO3). Conversion factor 84/50 = 1.68 mg NaHCO3 per mg as CaCO3.
- 105 × 1.68 ≈ 176 mg/L NaHCO3
- Mass: 1.8 MGD × 8.34 lb/gal × 176 × 10⁻⁶ ≈ 2,640 lb/day
Safe, easy to control, adds sodium and dissolved solids, and is the most expensive per pound of alkalinity.
Option B - caustic soda (NaOH, 100%). Conversion factor 40/50 = 0.80.
- 105 × 0.80 ≈ 84 mg/L NaOH
- Mass: 15.0 × 10⁶ lb water/day × 84 × 10⁻⁶ ≈ 1,260 lb/day
Cheaper per unit of alkalinity and far more hazardous. It is easy to overshoot and drive pH above 8.5, which scales diffusers, drives free ammonia toxicity, and can swing the alum chemistry.
Also reduce demand, not just supply it. Evaluate whether the alum dose is being applied where it does the most work (it should be dosed on soluble reactive phosphorus after biological uptake, and a lower dose with a polymer aid may achieve 0.5 mg/L). Every pound of alum removed also removes its alkalinity demand. Whatever you choose, add the chemical ahead of the aeration tank with good mixing, not into the last pass.
3. Write the monitoring and control plan you would put in place, including what you would watch to avoid creating a new problem.
Read the worked answer
Instrumentation and alarms
- Continuous pH in the last aeration pass and final effluent, low alarm at 6.5, high alarm at 8.5, with the alarm to the on-call operator's phone.
- DO in each pass, ammonia analyzer if available.
Routine laboratory (daily until stable, then 3×/week)
- Filtered alkalinity: influent, last pass, final effluent. Report as CaCO3 and flag when the last-pass residual falls below 50 mg/L.
- NH3-N and NO3-N in influent, last pass, effluent; compute the alkalinity demand from the actual ammonia oxidized.
- Effluent TP and soluble reactive P; jar tests at the actual plant pH, not at a lab-adjusted pH.
Control logic
- Feed alkalinity to hold last-pass pH at 6.8–7.0.
- Trim the daily chemical feed from the previous day's alkalinity balance, and verify the two methods agree within about 10%.
What to watch so you don't create a new problem
- Caustic overshoot: pH above 8.5 causes carbonate scaling on diffusers and blowers and shifts free ammonia to the toxic form. Feed with a pH controller and a hard high-high shutdown.
- Sodium loading: both chemicals raise TDS and can push effluent conductivity limits.
- Cost tracking: dollars per pound of phosphorus removed, so the coagulant dose is optimized rather than assumed.
- Never use alum addition as the first response to a TP rise again; write the alkalinity check into the SOP.