Nutrients: Nitrogen and Phosphorus in Water and Wastewater

Why Nutrients Matter in Water and Wastewater Treatment

Nitrogen and phosphorus are essential plant nutrients, but when they enter lakes, rivers, and estuaries in excess, they trigger algal blooms, oxygen depletion, and fish kills — a process called eutrophication. Discharge permits issued under the Clean Water Act set limits on these nutrients, making nutrient control a core operator responsibility. Understanding the chemical forms of each nutrient and how they move through the environment is the foundation of effective treatment.

The Nitrogen Cycle

Nitrogen exists in several forms in water and wastewater, and operators must track all of them.

Organic nitrogen is nitrogen bound in proteins and urea — the dominant form in raw sewage and agricultural runoff.

Ammonia nitrogen (NH₃-N / NH₄⁺-N) forms when bacteria break down organic nitrogen in a step called ammonification. Ammonia is toxic to aquatic life at elevated concentrations and exerts an oxygen demand in receiving waters.

Nitrite (NO₂⁻-N) is an intermediate oxidation state produced during biological nitrification. It is unstable and rapidly converted to nitrate under normal aerobic conditions, but can accumulate during process upsets.

Nitrate (NO₃⁻-N) is the fully oxidized, stable form. It is the most common form of nitrogen in treated effluent and in groundwater affected by agricultural fertilizers. Infants exposed to high nitrate in drinking water are at risk for methemoglobinemia (blue-baby syndrome), which is why the Safe Drinking Water Act establishes a maximum contaminant level for nitrate.

Total Kjeldahl Nitrogen (TKN) is the lab measurement that captures both organic nitrogen and ammonia nitrogen together. Operators use TKN along with nitrite and nitrate results to account for all nitrogen in a sample.

Nitrification is the two-step aerobic biological process in which specialized bacteria (Nitrosomonas converts ammonia to nitrite; Nitrobacter converts nitrite to nitrate). Nitrification requires adequate dissolved oxygen (typically above 2 mg/L), appropriate temperature, and sufficient solids retention time in the treatment system.

Denitrification occurs under anoxic conditions: bacteria use nitrate as an electron acceptor in place of oxygen, converting nitrate to nitrogen gas (N₂), which escapes to the atmosphere. This is the main biological route for removing total nitrogen from wastewater. A carbon source — often the influent BOD or an added chemical like methanol — is required to drive the reaction.

The Phosphorus Cycle

Phosphorus enters water systems through human sewage, detergents, fertilizers, and animal waste. Unlike nitrogen, phosphorus has no significant atmospheric pool; it moves between water, sediment, and organisms.

Orthophosphate (PO₄³⁻), also called reactive phosphorus, is the dissolved, biologically available form. It is taken up directly by algae and plants and is the form most commonly targeted in effluent limits.

Polyphosphate refers to chains of phosphate molecules found in some detergents and industrial products. Polyphosphates hydrolyze over time to orthophosphate in the environment.

Organic phosphorus is phosphorus incorporated into cell tissue and organic matter. It is released as orthophosphate during decomposition.

Total phosphorus (TP) is the sum of all forms and is the standard permit parameter.

Phosphorus is removed in treatment by chemical precipitation (adding alum, ferric chloride, or lime) or by enhanced biological phosphorus removal (EBPR), in which bacteria accumulate luxury amounts of phosphorus under alternating anaerobic and aerobic conditions and are then wasted from the system with the sludge.

Eutrophication: The Consequence of Excess Nutrients

When nutrient-rich effluent enters a slow-moving or enclosed water body, algae and aquatic plants grow explosively. When the bloom dies and decomposes, aerobic bacteria consume the dissolved oxygen, creating hypoxic (low-oxygen) or anoxic conditions that suffocate fish and macroinvertebrates. Decaying algae also produce taste and odor compounds that complicate drinking water treatment. Some cyanobacteria (blue-green algae) release toxins dangerous to humans and animals.

Reducing nutrient loads through advanced treatment — biological nutrient removal, chemical precipitation, or tertiary filtration — is the primary strategy. Source control, such as limiting fertilizer applications in a watershed, is a complementary approach.

Safety Notes for Nutrient Monitoring Work

Chemicals used in nutrient removal present real hazards. Alum (aluminum sulfate) and ferric chloride are corrosive; wear chemical-splash goggles and acid-resistant gloves when handling. Methanol, used as a carbon source for denitrification, is flammable and toxic — store away from ignition sources and ensure ventilation. When collecting samples from aeration basins or clarifiers, follow confined-space and fall-protection procedures. Always wear a life jacket or use a buddy system near open water structures. Lab work with nitrogen and phosphorus reagents requires eye protection and awareness of specific chemical hazards listed on the Safety Data Sheet (SDS).