Nutrient removal: nitrogen and phosphorus

# Nutrient removal: nitrogen and phosphorus

> Nitrogen and phosphorus ride along in your effluent unless the biology in your tank is set up to pull them out on purpose. Here is how that biology works, and the exam questions built around it.

Why the exam tests this

Nutrient removal carries less weight at Class I than the rest of Treatment Process Evaluation and Adjustment. Most Class I exams barely touch it, because a small conventional plant with no nutrient limit in its permit doesn't run one. By Class II you need to know the two processes exist and what each one does. By Class III you're expected to run and troubleshoot them, and by Class IV the exam wants you reasoning about zone configuration and carbon supply the way a design engineer would.

That climb matters because permits are getting stricter. States are folding nutrient criteria into their water-quality standards, and a permit renewal increasingly hands a small plant a nitrogen or phosphorus limit it didn't carry before. An operator who only knows BOD and TSS removal is caught flat when that letter arrives.

What you need to know

Nitrogen leaves your plant as ammonia unless you push it through two biological steps in sequence. Nitrification comes first: an aerobic process where ammonia-oxidizing bacteria turn ammonium into nitrite, then nitrite-oxidizing bacteria turn that nitrite into nitrate. Both groups are autotrophic, slow-growing, and picky about their conditions.

Denitrification comes second: an anoxic process, no free oxygen, where ordinary heterotrophic bacteria, the same kind doing your BOD removal, strip the oxygen off nitrate and release the nitrogen as a gas. That gas leaves the water and the nitrogen is gone for good. Denitrification needs carbon as its energy source, the same BOD you're already removing elsewhere in the plant.

The two steps trade alkalinity back and forth. Nitrification consumes about 7.14 mg/L of alkalinity for every mg/L of ammonia-nitrogen it oxidizes. Denitrification gives roughly half of that back, about 3.57 lb of alkalinity per lb of nitrate-nitrogen it reduces. Run an anoxic zone after your nitrification zone and you recover some of that buffer, which can cut or erase the caustic soda a straight nitrifying plant would otherwise have to feed.

Nitrification is the expensive half of the pair. It can eat up to half the plant's aeration power, because oxidizing ammonia takes far more oxygen than removing carbon does. It is also temperature-sensitive: the bacteria work across roughly 4 to 45 degrees C, run best near 35 degrees C, and their growth rate roughly doubles for every 10-degree rise. Drop below about 8 degrees C and nitrification slows sharply. A hard toxic load, a swing in DO, pH, or alkalinity, or plain cold water can wash the nitrifiers out of the system faster than they can regrow.

Denitrification runs on carbon, and if you're short on it, the process stalls. The rule of thumb is a minimum BOD-to-TKN ratio of about 4 to 5, or COD-to-TKN around 9 to 10. A plant that doesn't clear that ratio needs supplemental carbon, roughly 3.5 to 8 lb of COD per lb of nitrate-nitrogen reduced, about 3.5 with methanol specifically. Ammonia stripping, raising pH to 10.5 to 11.5 and driving ammonia off as a gas over a tower, is the physical alternative to the biological path, though it's rarely the first choice at a small plant.

Phosphorus removal splits along the same chemical-versus-biological line as nitrogen, but the mechanics differ. Chemical removal precipitates phosphorus out with alum, an iron salt, or lime. It works reliably and it's simple to dose, but it makes chemical sludge and consumes alkalinity.

Biological removal, enhanced biological phosphorus removal (EBPR), works a specific organism instead of a chemical. Polyphosphate-accumulating organisms (PAOs) cycle between an anaerobic zone, where they release stored phosphate and take up volatile fatty acids, and an aerobic zone, where they take up phosphorus beyond what growth alone requires. That aerobic zone needs dissolved oxygen at 1.0 mg/L or higher to work. EBPR traditionally needs a COD-to-TP ratio of at least 37 to 1, or a BOD-to-TP ratio near 18 to 1, with some short-chain volatile fatty acids on hand.

EBPR has a natural competitor: glycogen-accumulating organisms (GAOs). They eat the same volatile fatty acids the PAOs need but remove no phosphorus, and they outcompete PAOs once water temperature climbs above about 25 degrees C. That is one reason EBPR reliability drops off at tight limits. Across five plants studied, effluent orthophosphate under 0.5 mg/L was met only 24 to 95 percent of the time, averaging 68 percent, which is why many plants running EBPR still keep a chemical backup ready to dose.

Looking past the main process train is where the higher grades push you next. Dewatering centrate, the liquid that comes off sludge dewatering, is small by volume but carries 15 to 20 percent of the plant's total nitrogen load, sometimes 40 to 50 percent, and it lands back at the head of the plant in a spike. Treating that stream separately, sidestream deammonification with anammox bacteria, removes nitrogen for roughly a third of the cost per pound of mainstream treatment, mostly by skipping the aeration and carbon a mainstream process would need. Anammox bacteria are slow to establish and easily hurt: a biomass loss can take three months or more to recover from, and nitrite above about 4.8 mg/L damages them.

One documented case shows how far a zone-reconfiguration fix can go. At the Hillsborough, NC wastewater plant, a 3.0 MGD facility, operators facing a much tighter nitrogen limit reallocated part of an existing aerobic zone to anoxic volume and raised the internal nitrogen recycle rate to 900 percent, inside the plant's existing five-stage BNR train, because actual flow was running well under design flow. That reconfiguration alone, no added carbon and no coagulant, dropped effluent total nitrogen from about 3 mg/L to roughly 1.5 mg/L and let the utility skip a planned reverse-osmosis or ion-exchange addition. At Class IV, expect the exam to test that kind of reasoning, whether an existing multi-zone process can be reconfigured to meet a tighter limit, rather than asking you to size new tanks from scratch.

Worked examples

Example 1: alkalinity balance across a nitrification zone and an anoxic zone. Your primary effluent carries 29 mg/L of ammonia-nitrogen, and full nitrification drops it to 1 mg/L, so the process oxidizes 28 mg/L of ammonia-N. A downstream anoxic zone then reduces 19 mg/L of that resulting nitrate-N.

Alkalinity consumed by nitrification = 28 mg/L x 7.14 = 200 mg/L as CaCO3. Alkalinity returned by denitrification = 19 mg/L x 3.57 = 68 mg/L as CaCO3. Net alkalinity consumed = 200 minus 68 = 132 mg/L as CaCO3.

If your raw wastewater carries 180 mg/L of alkalinity, a net draw of 132 mg/L leaves only about 48 mg/L in reserve, tight enough that a cold snap or an ammonia spike could push pH down fast. Check this arithmetic at /tools/practice.

Example 2: denitrification carbon check. Primary effluent carries BOD of 165 mg/L and TKN of 34 mg/L.

BOD-to-TKN ratio = 165 divided by 34 = 4.9.

That sits inside the minimum 4-to-5 window, so the plant has enough carbon to denitrify without a supplemental feed, as long as the ratio holds through wet-weather dilution. Drop the BOD or raise the TKN and the ratio falls below 4, and the plant needs supplemental COD to keep denitrifying. Rework this ratio at /tools/practice until it's automatic.

Example 3: EBPR carbon-to-phosphorus check. A plant sees BOD of 140 mg/L and total phosphorus of 9 mg/L in its influent.

BOD-to-TP ratio = 140 divided by 9 = 15.6.

That falls short of the roughly 18-to-1 minimum EBPR needs to run reliably. This plant is a candidate for a chemical backup dose, or for adding a fermentation step to generate more volatile fatty acids, rather than counting on biological removal alone to hit a tight phosphorus limit.

Common traps

• Confusing nitrification and denitrification. Nitrification is aerobic and consumes alkalinity; denitrification is anoxic and returns it. Mixing up which one needs oxygen is the single most common miss.
• Treating the alkalinity math as one-directional. A plant that only counts what nitrification consumes, without crediting what a working anoxic zone returns, overestimates how much caustic it needs to feed.
• Assuming any BOD is enough carbon for denitrification. The ratio matters, not the raw number. A plant with plenty of BOD but a high TKN can still fall short of the minimum ratio.
• Defaulting to chemical phosphorus removal without checking whether EBPR's carbon-to-phosphorus ratio and VFA supply would support it, and forgetting that warm water above about 25 degrees C hands the advantage to GAOs instead of PAOs.
• Forgetting the sidestream. A plant can run its mainstream process well and still miss its nitrogen limit if dewatering centrate lands back at the headworks unmanaged.

Practice

Nitrogen and phosphorus removal questions at your grade level make up part of the wwt-1 test, found at /tools/practice. Drill the alkalinity balance and the BOD-to-TKN and BOD-to-TP ratio checks until you can run them without reaching for a reference. At Class III and IV, also work the sidestream and zone-configuration scenarios, since those tend to show up as reasoning questions rather than straight math.

Quick reference

• Nitrification: aerobic, autotrophic; ammonia to nitrite (AOB), then nitrite to nitrate (NOB); consumes oxygen and alkalinity.
• Denitrification: anoxic, heterotrophic; nitrate to nitrogen gas; needs carbon, returns alkalinity.
• Alkalinity: nitrification consumes about 7.14 mg/L per mg/L of ammonia-N oxidized; denitrification returns about 3.57 lb per lb of nitrate-N reduced.
• Nitrification runs roughly 4 to 45 degrees C, optimum near 35 degrees C; growth rate roughly doubles per 10-degree C rise; slows sharply below 8 degrees C.
• Denitrification needs a minimum BOD-to-TKN of about 4 to 5 (or COD-to-TKN about 9 to 10); short plants add supplemental COD, about 3.5 to 8 lb per lb of nitrate-N (about 3.5 with methanol).
• Ammonia stripping (the physical option): raise pH to 10.5-11.5, strip ammonia gas over a tower.
• Chemical phosphorus removal: alum, iron salts, or lime; reliable, but makes chemical sludge and consumes alkalinity.
• EBPR (biological): PAOs cycle anaerobic (release phosphate, take up VFAs) and aerobic (take up phosphorus), aerobic DO at least 1.0 mg/L.
• EBPR needs COD-to-TP of at least 37 to 1, or BOD-to-TP near 18 to 1, with VFAs present.
• GAOs compete with PAOs for VFAs without removing phosphorus, and win out above about 25 degrees C.
• EBPR reliability at tight limits (under 0.5 mg/L orthophosphate) has run 24 to 95 percent, averaging 68 percent, across studied plants; many keep a chemical backup.
• Sidestream centrate carries 15 to 20 percent (sometimes 40 to 50 percent) of plant nitrogen; sidestream deammonification with anammox treats it at roughly a third of mainstream cost per pound.