8.4 Nitrification, Denitrification & Advanced Biological Treatment

Beyond BOD: why nutrients show up on the exam

Many Texas TPDES permits limit ammonia-nitrogen, and an increasing number address total nitrogen (TN) and total phosphorus (TP) to protect lakes and streams from eutrophication. Activated sludge plants meet these limits with nitrification, denitrification, and sometimes enhanced biological phosphorus removal or chemical precipitation. Exam questions mix microbiology names, alkalinity math, zone types (aerobic vs anoxic), and temperature effects.

Quick Answer: Nitrification (aerobic): NH₃ → NO₂⁻ → NO₃⁻ via Nitrosomonas then Nitrobacter/Nitrospira, consuming ~7.1 mg alkalinity as CaCO₃ per mg NH₃-N. Denitrification (anoxic): NO₃⁻ → N₂ gas using organic carbon, recovering some alkalinity. Cold water and low DO stall nitrifiers; recycle sends nitrate to anoxic zones for TN removal.

Nitrification: two aerobic steps

Nitrification is performed by autotrophic bacteria that grow much more slowly than typical BOD-removing heterotrophs:

1. Ammonia-oxidizing bacteria (classically taught as Nitrosomonas):
   (\mathrm{NH_3/NH_4^+} \rightarrow \mathrm{NO_2^-}) (nitrite)
2. Nitrite-oxidizing bacteria (classically Nitrobacter; many plants also emphasize Nitrospira):
   (\mathrm{NO_2^-} \rightarrow \mathrm{NO_3^-}) (nitrate)

Teaching point for exams: if nitrite accumulates, the second step is impaired (toxicity, low DO, shock, or startup). Complete nitrification means ammonia drops and nitrate rises (unless denitrification follows).

Conditions nitrifiers need:

• Adequate DO in aerobic zones (teaching target often ~2 mg/L residual)
• Sufficient SRT/MCRT (nitrifiers wash out if sludge age is too short)
• Adequate alkalinity and stable pH
• Absence of severe toxicity (some heavy metals, extreme pH, certain organics)
• Warm enough temperature—rates fall sharply in cold water

Alkalinity consumption (memorize the teaching factor)

A standard operator teaching value is that nitrification consumes about 7.14 mg/L alkalinity as CaCO₃ for each 1 mg/L NH₃-N oxidized (often rounded to 7.1).

Worked example — alkalinity. Influent ammonia-N is 28 mg/L and the plant fully nitrifies. Approximate alkalinity demand = 28 × 7.1 ≈ 199 mg/L as CaCO₃. If influent alkalinity is only 160 mg/L as CaCO₃ and little is recovered, pH can crash and nitrification stalls even when DO and SRT look fine. Operators may need supplemental alkalinity (e.g., lime, caustic, magnesium hydroxide) where permitted and designed.

Denitrification returns roughly 3.6 mg/L alkalinity as CaCO₃ per mg/L nitrate-N reduced (teaching value)—one reason anoxic zones help stabilize pH in BNR plants.

Denitrification and anoxic zones

Denitrification converts nitrate to nitrogen gas:

(\mathrm{NO_3^-} \rightarrow \mathrm{N_2}\uparrow)

It is performed mainly by facultative heterotrophs when:

• Nitrate is available (from upstream nitrification)
• Free DO is absent or very low (anoxic conditions—oxygen bound in nitrate is used)
• Organic carbon is available as an electron donor (influent BOD, endogenous carbon, or supplemental carbon such as methanol where used)

An anoxic zone is not the same as an anaerobic zone:

Oxidation ditches and plug-flow trains can create anoxic pockets by controlling aerator placement and DO. Many modern plants use dedicated anoxic selectors plus internal nitrate recycle from the aerobic end back to the anoxic zone.

Recycle streams that matter for nitrogen

• RAS recycle: Returns biomass (and some nitrate) to the front of the plant.
• Internal mixed-liquor recycle (nitrified recycle): Intentionally pumps nitrate-rich mixed liquor to anoxic zones for denitrification—key for high TN removal.
• Sidestreams from digesters/dewatering can dump ammonia back to the headworks—sudden ammonia spikes are often sidestream-related, not mysterious "bug death."

Total nitrogen (TN) and total phosphorus (TP) concepts

TN typically includes ammonia, nitrite, nitrate, and organic nitrogen. A nitrifying-only plant converts ammonia to nitrate but may not greatly reduce TN; nitrification + denitrification is required for substantial TN removal.

TP removal options:

• Chemical: metal salt precipitation (alum, ferric) — reliable, increases sludge
• Biological (EBPR): alternating anaerobic/aerobic zones so phosphorus-accumulating organisms (PAOs) take up luxury phosphorus aerobically after anaerobic release
• Combined chemical + biological for tight limits

Exam questions usually expect concepts and zone purposes rather than full EBPR kinetics.

Temperature effects (Texas winters still count)

Nitrifier growth rate drops as temperature falls. A plant that easily hits ammonia limits in August may struggle after a North Texas cold front if SRT is short or DO/alkalinity is marginal. Operator responses taught in training:

• Increase aerobic SRT (waste less) before cold weather if inventory allows
• Protect DO and alkalinity
• Avoid sudden toxic or hydraulic shocks when the population is already slow
• Watch for nitrite blips during transitions

Heat has the opposite problem for oxygen: biology is fast but oxygen dissolves poorly—hold aerobic DO carefully when nitrifying in summer.

Worked example — thinking through a TN train

A plant must meet ammonia < 3 mg/L and a seasonal TN limit. Aerobic SRT is 10 days, DO is 2.2 mg/L, and ammonia is 0.5 mg/L—but TN is still high because nitrate is 12 mg/L. Diagnosis: nitrification is working; denitrification is not. Checks: Is there a true anoxic zone (ORP/DO near zero)? Is internal nitrate recycle operating? Is enough carbon reaching the anoxic zone, or is all BOD removed before denitrification can occur? Adding more air will not lower TN and may destroy anoxic conditions.

Operator exam triage for nutrient questions

1. Ammonia high + nitrate low → nitrification failure (DO, SRT, alkalinity, toxicity, cold).
2. Ammonia low + nitrate high + TN high → need denitrification/anoxic/recycle/carbon.
3. pH falling while nitrifying → alkalinity deficit.
4. TP high with good BOD → chemical feed or EBPR zone problem—not "more MLSS" by default.

Master the organism names, the ~7.1 alkalinity factor, zone definitions, and the difference between ammonia removal and TN removal. That package covers most advanced biological treatment items on the TCEQ wastewater path.