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, often written as NH3-N or NH4-N depending on pH. Nitrite is an intermediate oxidation form. Nitrate is highly oxidized, soluble, and mobile in groundwater.

A common exam relationship is:

TKN = organic nitrogen + ammonia nitrogen

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

Do not add ammonia again if it is already included in TKN. This double-counting error is common when a problem lists several lab results.

Phosphorus and Eutrophication

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

pH and Speciation

pH is a logarithmic measure of hydrogen ion activity. 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 is present as un-ionized NH3, which is more toxic to aquatic life than ammonium. Metal solubility and corrosion behavior also depend strongly on pH.

Alkalinity and Buffering

Alkalinity is the water's ability to neutralize acid, commonly reported as mg/L as CaCO3. It is not the same as pH. pH is the current condition; alkalinity is resistance to pH change. The carbonate system provides much of this buffering in natural water and wastewater.

Nitrification consumes alkalinity because ammonia oxidation produces acidity. A standard design approximation is:

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

Denitrification restores about half of that, often approximated as 3.57 mg/L as CaCO3 per mg/L nitrate-nitrogen reduced. These factors are especially useful when a biological nutrient removal question asks whether a system has enough buffering capacity to keep pH in a viable range.

Calculation Workflow

1. Identify whether each lab value is reported as the element, such as N or 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 mentions 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 the pathway: ammonia affects oxygen and toxicity, nitrate moves readily with groundwater, and phosphorus often controls freshwater eutrophication.