12.3 Nutrients, pH, Alkalinity, and Buffering

Nutrient and Buffer Chemistry

Water-quality chemistry on the PE Civil WRE exam is practical rather than theoretical. You need enough chemistry to identify the pollutant form, predict the environmental effect, and complete common mass-balance or process-stability calculations. Nutrients, pH, and alkalinity appear in surface-water impairment, wastewater treatment, groundwater quality, drinking-water treatment, and total maximum daily load contexts.

Nitrogen Forms

Nitrogen changes form as water moves through natural systems and treatment processes. Organic nitrogen is bound in organic matter. Ammonia nitrogen is reduced nitrogen, written as NH3-N or NH4-N depending on pH. Nitrite (NO2-N) is an unstable intermediate oxidation form. Nitrate (NO3-N) is highly oxidized, soluble, and mobile in groundwater; the federal drinking-water maximum contaminant level is 10 mg/L as N (1 mg/L as N for nitrite).

Two relationships drive most exam problems:

TKN = organic nitrogen + ammonia nitrogen

Total nitrogen = TKN + nitrite-nitrogen + nitrate-nitrogen

Do not add ammonia again if it is already inside TKN. This double-counting error is the single most common nitrogen mistake when a problem lists several lab results separately.

Phosphorus and Eutrophication

Phosphorus may be reported as orthophosphate, dissolved phosphorus, particulate phosphorus, or total phosphorus. In many freshwater systems phosphorus is the limiting nutrient, so a small additional load can trigger algal growth. Algae produce oxygen during photosynthesis, but their later decay consumes oxygen and worsens low-DO conditions. Nitrogen is often limiting in estuarine and marine settings and is central to nitrate groundwater concerns.

pH and Speciation

pH is a logarithmic measure of hydrogen ion activity, pH = -log[H+]. A pH of 6 is ten times more acidic than pH 7, and pH 5 is one hundred times more acidic than pH 7. Many treatment and toxicity questions turn on speciation. At higher pH a larger fraction of ammonia exists as un-ionized NH3, which is far more toxic to aquatic life than the ammonium ion NH4+. Metal solubility, corrosion, and disinfection (free chlorine vs. hypochlorite) also depend strongly on pH.

Alkalinity and Buffering

Alkalinity is the water's ability to neutralize acid, commonly mg/L as CaCO3. It is not the same as pH: pH is the current condition, while alkalinity is resistance to pH change. The carbonate-bicarbonate system supplies most of this buffering in natural water and wastewater, with bicarbonate (HCO3-) dominant near neutral pH.

Nitrification produces acidity and consumes alkalinity. A standard design approximation is:

Alkalinity consumed = 7.14 mg/L as CaCO3 per mg/L NH4-N oxidized

Denitrification recovers about half, often approximated as 3.57 mg/L as CaCO3 per mg/L nitrate-nitrogen reduced. These factors decide whether a biological nutrient-removal system retains enough buffer to hold pH in the viable 6.5 to 8.0 range; designers commonly target a residual alkalinity of at least 50 to 100 mg/L as CaCO3 after nitrification.

Calculation Workflow

1. Identify whether each lab value is reported as the element (as N, as P) or as a compound.
2. Assemble nitrogen species without double-counting TKN.
3. Convert nutrient concentration and flow to load when comparing sources or permits.
4. Check whether nitrification or chemical addition changes alkalinity.
5. Interpret pH as logarithmic and alkalinity as buffering capacity.

Exam Strategy

If a nutrient problem also reports low DO, look for organic decay, ammonia oxidation, algal growth, or algal decay. If a biological reactor becomes unstable, check alkalinity before assuming the biology failed. If the question asks for receiving-water impact, connect the nutrient form to its pathway: ammonia drives oxygen demand and toxicity, nitrate moves readily with groundwater toward wells, and phosphorus usually controls freshwater eutrophication.

Worked Alkalinity-Balance Example

A nitrifying activated-sludge plant treats 4.0 MGD with influent ammonia-nitrogen of 30 mg/L and influent alkalinity of 220 mg/L as CaCO3. Determine whether buffering is adequate. Alkalinity consumed by full nitrification = 30 x 7.14 = 214 mg/L as CaCO3. Residual alkalinity = 220 - 214 = 6 mg/L as CaCO3, far below the 50 to 100 mg/L target, so pH will fall out of the viable range and nitrification will stall. To restore a 70 mg/L residual, the deficit is 70 - 6 = 64 mg/L as CaCO3. As a load, that is 4.0 x 64 x 8.34 = 2,135 lb/day of supplemental alkalinity as CaCO3.

If partial denitrification recovers 3.57 mg/L per mg/L nitrate reduced and the design reduces 15 mg/L of nitrate, recovery is 15 x 3.57 = 54 mg/L, which would substantially close the gap and reduce chemical demand. This style of stepwise check, demand versus supply with a target residual, is exactly how the exam frames buffering stability.

Reporting-Basis Conversions

Many nutrient distractors come from mixing the as-element basis with the as-compound basis. Convert with molar mass ratios.

Always confirm whether a criterion such as the 10 mg/L nitrate maximum contaminant level is stated as N (it is) before comparing it to a lab value reported as the nitrate ion, or you will be off by a factor of more than four.