14.3 Nutrient Removal and Effluent Quality
Key Takeaways
- Nutrient removal extends secondary treatment to target nitrogen and phosphorus that drive eutrophication, oxygen depletion, and permit violations.
- Nitrification converts ammonia to nitrate aerobically, consuming about 4.57 lb O2 and 7.14 lb alkalinity (as CaCO3) per lb ammonia-nitrogen oxidized; denitrification reduces nitrate to nitrogen gas anoxically with a carbon source.
- Phosphorus is removed biologically (anaerobic then aerobic, by phosphorus-accumulating organisms) or chemically (alum, ferric, lime); in both cases P leaves mainly with the solids, so wasting must follow.
- Effluent limits may be concentration based, load based, or both; convert to load with lb/day = MGD x mg/L x 8.34 for any total maximum daily load (TMDL) or mass-cap question.
- Advanced processes (filtration, ultraviolet, ozone, membranes, activated carbon, advanced oxidation) are chosen for a specific residual contaminant or reuse goal, not a generic wish for cleaner water.
Nutrients Link Treatment to Receiving Water
The WRE specification lists nutrient removal, disinfection, and advanced treatment inside Wastewater Collection and Treatment, while the surface-water-quality topic separately covers dissolved-oxygen dynamics, TMDLs, nutrients, and load allocation. That overlap is the clue: wastewater nutrient questions ask not only how the plant operates but whether the effluent can meet the receiving-water objective.
Nitrogen Forms and Environments
Wastewater nitrogen appears as organic nitrogen, ammonia, nitrite, nitrate, or total nitrogen (TN). Nitrification is the aerobic biological conversion of ammonia to nitrite then nitrate; it needs oxygen, slow-growing nitrifying biomass, adequate SRT (often 8-15 days, longer in cold weather), temperature above roughly 10 C, and alkalinity. Denitrification is the anoxic reduction of nitrate to nitrogen gas; it needs nitrate, a carbon source (influent BOD or supplemental methanol), and low DO so denitrifiers use nitrate as the electron acceptor.
| Process | Main condition | Main result | Exam warning |
|---|---|---|---|
| Ammonification | Biological hydrolysis | Organic N to ammonia | Can raise ammonia before nitrification |
| Nitrification | Aerobic, adequate SRT | Ammonia to nitrate | Consumes ~4.57 lb O2 and ~7.14 lb alkalinity/lb N |
| Denitrification | Anoxic, carbon present | Nitrate to N2 gas; recovers ~3.57 lb alkalinity/lb N | Suppressed by high DO or low carbon |
| Biological P removal | Anaerobic then aerobic | P stored in biomass | Needs solids removal to truly remove P |
| Chemical P removal | Metal salt or lime | Precipitated P solids | Increases sludge production |
Two nitrification constants are exam staples: 4.57 lb O2 per lb ammonia-N oxidized and 7.14 lb alkalinity as CaCO3 per lb ammonia-N oxidized. Denitrification returns about 3.57 lb alkalinity per lb nitrate-N reduced, which is why upstream anoxic zones help hold pH. A plant that loses alkalinity loses pH control and then nitrification.
Phosphorus Removal
Phosphorus is usually the limiting nutrient in freshwater eutrophication. Chemical removal uses alum, ferric or ferrous salts, or lime to form settleable precipitates; a typical metal-salt molar dose is roughly 1.5-2.0 mol metal per mol phosphorus to hit low effluent targets. Biological removal (enhanced biological phosphorus removal, EBPR) exposes phosphorus-accumulating organisms to anaerobic then aerobic zones so they over-store phosphorus. In both routes, phosphorus leaves chiefly in solids, so clarifier performance and sludge wasting govern actual removal.
Effluent Quality and Polishing
Effluent limits may be concentration based, load based, or both. A total phosphorus limit of 1.0 mg/L at 10 MGD is ten times the load of 1.0 mg/L at 1 MGD. Use lb/day = MGD x mg/L x 8.34 for any TMDL allocation, mass cap, or receiving-water load.
Disinfection follows solids removal. Chlorine depends on dose, demand, residual, and contact time (the CT concept); dechlorination with sulfur dioxide or bisulfite may be required before discharge. Ultraviolet (UV) disinfection leaves no chemical residual but needs good UV transmittance and low TSS. Ozone is powerful but capital-intensive. Effluent filtration, membranes (microfiltration through reverse osmosis), granular activated carbon, and advanced oxidation are polishing tools matched to turbidity, fine solids, trace organics, or reuse.
Calculation Workflow
- Identify the regulated endpoint: ammonia, nitrate, TN, total phosphorus, BOD, TSS, pathogen indicator, turbidity, or a specific contaminant.
- Convert flow and concentration to load if the permit or TMDL is load based.
- For ammonia removal, check oxygen demand (4.57) and alkalinity (7.14) before assuming nitrification proceeds.
- For TN removal, provide both aerobic nitrification and anoxic denitrification unless nitrate is acceptable.
- For phosphorus, remember removal means solids separation after biological uptake or chemical precipitation.
- Match advanced treatment to the contaminant.
Distractors place the right process in the wrong environment: nitrate will not denitrify in a fully aerobic basin, ammonia will not nitrify in an anoxic zone, and chemically precipitated phosphorus is not removed until its solids are separated.
Configurations for Total Nitrogen Removal
The exam expects familiarity with named biological nutrient removal (BNR) layouts. The Modified Ludzack-Ettinger (MLE) process puts an anoxic zone ahead of the aerobic zone and recycles nitrate-rich mixed liquor back to the anoxic basin, using influent BOD as the carbon source; it is the workhorse for moderate TN limits. The four-stage Bardenpho adds a second anoxic-aerobic pair to reach very low TN. The A2O layout prepends an anaerobic zone for phosphorus removal.
When a question demands both low TN and low total phosphorus, the answer is a configuration that supplies anaerobic, anoxic, and aerobic zones in the right order, not a single change to DO.
Disinfection Sizing and CT
Disinfection problems hinge on the CT concept: the product of disinfectant residual concentration and contact time must meet a target for the pathogen and water quality. Chlorine contact basins are sized for plug flow at peak flow, commonly 15-30 minutes of contact at peak hour. UV dose equals intensity times exposure time and depends on transmittance, so high TSS or turbidity shields organisms and demands a higher dose.
If a question gives a required CT and a peak flow, solve for the contact volume (volume = CT-driven contact time times flow) and verify the basin geometry avoids short-circuiting, which a baffling factor in the CT calculation accounts for. Matching the disinfectant to the residual requirement, contact volume, and effluent clarity is the recurring judgment.
A plant treats 2.0 MGD and reduces ammonia-nitrogen from 24 mg/L to 2 mg/L by nitrification. Using 4.57 lb O2 per lb ammonia-nitrogen oxidized, what is the approximate oxygen requirement for nitrification?
A wastewater plant nitrifies successfully but has high effluent nitrate and a low total nitrogen limit. Which process change most directly addresses the remaining nitrogen?
An operator nitrifying a low-alkalinity wastewater sees effluent pH falling and nitrification stalling. Using 7.14 lb alkalinity (as CaCO3) consumed per lb ammonia-N, which corrective action is most appropriate?