Ammonia limits are some of the tightest in many discharge permits, and nitrification is one of the most sensitive parts of the activated sludge process. When effluent ammonia starts to climb, it usually means one of a handful of conditions has gone out of range for the nitrifying bacteria. The good news is that those conditions are well understood and you can check most of them in an afternoon.
A quick review of nitrification
Nitrification is a two step biological process carried out by autotrophic bacteria:
- Ammonia-oxidizing bacteria convert ammonia (NH4+) to nitrite (NO2-).
- Nitrite-oxidizing bacteria convert nitrite to nitrate (NO3-).
These organisms get their energy from ammonia and nitrite, not from BOD, and they get carbon from carbon dioxide and alkalinity. That makes them very different from the heterotrophs that remove BOD:
- They grow slowly. Much more slowly than heterotrophs, and their growth rate drops sharply in cold water.
- They need oxygen. About 4.57 lb of oxygen per lb of ammonia-N nitrified.
- They consume alkalinity. About 7.14 mg/L as CaCO3 per mg/L ammonia-N nitrified.
- They are sensitive. Many chemicals inhibit them at concentrations that barely affect BOD removal.
So when ammonia rises, ask six questions in order: Is there enough oxygen? Enough alkalinity and the right pH? Enough SRT? What is the temperature? Has the load changed? Is something inhibiting them?
1. Dissolved oxygen
Nitrifiers lose rate quickly below about 1.5 to 2.0 mg/L DO. In plug flow basins, the first part of the basin often has the lowest DO and the highest load, so nitrification may start late and not finish before the mixed liquor leaves.
Check DO at several points along the basin and at peak load, not just at one probe. A DO trend at 15 minute resolution will reveal afternoon dips that a daily average hides.
At the fictional White River WRRF, a blower failure from September 7 to 12 cut average DO to about 1.0 mg/L. Ammonia began to bleed through at peak hours, and after the September 13 storm washed out solids, effluent ammonia reached 3.4 mg/L on September 14, above the 3.0 mg/L daily maximum limit.
Fix: restore air, move air to the starved zone, raise the DO setpoint at the end of the basin, and check for fouled diffusers.
2. Alkalinity and pH
Nitrification consumes alkalinity. If the influent alkalinity is modest and you nitrify fully without denitrifying, you can run out. Below about 50 mg/L residual alkalinity, or below pH about 6.5 to 6.8, nitrification slows.
Worked example: influent alkalinity 180 mg/L, influent ammonia-N available for nitrification 30 mg/L, no denitrification, target residual 70 mg/L.
Alkalinity consumed = 30 x 7.14 = 214 mg/L
Remaining = 180 minus 214 = negative 34 mg/L. You need 70 + 34 = 104 mg/L more.
At 2.0 MGD: 104 x 2.0 x 8.34 = 1,735 lb/d as CaCO3. With magnesium hydroxide (about 0.58 lb per lb CaCO3), that is roughly 1,010 lb/d of Mg(OH)2. The alkalinity calculator runs the numbers with credit for denitrification and a choice of chemicals.
White River's influent alkalinity is about 260 mg/L, and its anoxic zones recover some alkalinity, so its effluent pH stays near 7.2. Plants with soft water sources are much more likely to see trouble.
Fix: add an anoxic zone or cycle aeration to recover alkalinity (about 3.57 mg/L per mg/L nitrate-N denitrified), or feed alkalinity.
3. SRT
Nitrifiers must stay in the system longer than it takes them to reproduce. If you waste faster than they grow, they wash out. At 20 C, an aerobic SRT of 3 to 5 days may be enough; at 10 C, 8 to 12 days or more may be needed, plus a safety factor.
What lowers SRT without anyone deciding to?
- A storm that washes solids out over the weirs.
- Increased wasting to fix a settling problem.
- A WAS meter reading low, so more is wasted than recorded.
- Taking a basin out of service.
Calculate actual SRT from actual data for the past two to three weeks. See How Do I Calculate Sludge Age?.
Fix: reduce wasting. Recovery takes time: nitrifier populations double in a day or two in warm water but may take a week in cold water.
4. Temperature
Temperature affects nitrifier growth more than any other factor you cannot control. As a rough rule, nitrifier growth rate drops by about half for every 8 to 10 C drop. A plant that nitrified easily in August at 24 C may struggle in December at 12 C with the same SRT.
Fix: plan ahead. Raise the SRT target in early fall, before the temperature drops, so the inventory is in place. Check that the clarifiers can handle the higher MLSS using the state point tool.
5. Ammonia load
Look for new or changed ammonia sources:
- Dewatering returns. Centrate and filtrate from anaerobically digested sludge often contain 500 to 1,000 mg/L ammonia-N or more. At White River, centrate runs 700 to 900 mg/L ammonia-N. A centrifuge running 99 gpm feed returns roughly 90 to 95 gpm centrate, which at 800 mg/L is about 0.137 MGD x 800 x 8.34 = 910 lb/d of ammonia-N at that rate, or about 38 lb for every hour the centrifuge runs. That rate is a large fraction of the plant influent ammonia load of about 1,300 lb/d (6.4 MGD at 24 to 26 mg/L), and it arrives only while the centrifuge runs.
- Septage and hauled waste.
- Industrial dischargers such as food processing or fertilizer.
- Population growth or new connections.
Fix: schedule dewatering for low load periods, equalize return streams, meter hauled waste, and use the ammonia loading calculator to see the real daily load.
6. Inhibition and toxicity
Nitrifiers are inhibited by many substances: heavy metals (copper, nickel, zinc, chromium), cyanide, some solvents and organics, high free ammonia or free nitrous acid, sulfide, and some cleaning chemicals and biocides. Signs of inhibition:
- Ammonia rises suddenly with DO, alkalinity, SRT and temperature unchanged.
- DO rises at constant air (less oxygen is being used).
- Protozoa are sluggish or missing.
- Nitrite accumulates (nitrite oxidizers are often more sensitive).
Fix: find and stop the source with your pretreatment staff, protect the remaining biomass by reducing wasting, and consider reseeding. See the toxic upset guide.
A profile tells the story
The single most useful test when ammonia is high is a profile along the basin: sample at the inlet, every quarter of the basin, and the outlet. Measure ammonia, nitrate, DO and pH. Patterns:
- Ammonia drops steadily but not to zero at the outlet: not enough capacity (SRT, DO, or temperature). Nitrifiers are present but outnumbered.
- Ammonia barely drops anywhere: severe inhibition or washout.
- Ammonia drops then stalls where DO is low: oxygen limit.
- Nitrite builds up: partial inhibition or a recent disturbance.
Recovery expectations
After you correct the cause:
- DO or alkalinity problems usually recover within a day or two.
- SRT and temperature problems take one to three weeks.
- Toxic events can take weeks, depending on how much biomass was lost.
Keep records of what you changed and when. Nitrification problems often repeat seasonally, and next year's operator will thank you for the notes.
Checklist
- DO by zone and time of day.
- Effluent alkalinity and pH.
- Actual aerobic SRT (last 2 to 3 weeks).
- Mixed liquor temperature trend.
- Ammonia load including return streams.
- Signs of inhibition (sudden change, DO rise, nitrite, microscope).
- Basin profile.