13.1 COD, Nutrient & Metals Analysis

13.1 COD, Nutrient & Metals Analysis

2025 WPI alignment: This section teaches conducting and interpreting chemical analyses such as COD, nutrients, and metals in Laboratory Analysis, the 15-question area containing 3 recall, 12 application, and 4 calculation items.

Why this laboratory task matters

Chemical analyses use analyte-specific preparation and measurement. COD estimates chemically oxidizable material; nutrient methods distinguish forms and digestion; metals work demands contamination control, preservation, digestion or filtration, calibration, and matrix-aware quality control.

Analytical foundation

Laboratory workflow

1. Confirm analyte, fraction, method, container, preservation, filtration timing, digestion, range, and reporting limit.
2. Prepare standards and reagents with traceable lots and verify blanks, calibration, glassware, and instrument condition.
3. Homogenize, filter, digest, dilute, or otherwise prepare samples exactly as required.
4. Analyze calibration and QC at prescribed frequency and prevent carryover from high-concentration samples.
5. Apply dilution, blank, digestion-volume, or calibration factors and review all QC.
6. Compare chemically related forms and process data, qualify limitations, and report the correct analyte name and units.

Quality and diagnostic evidence

Calculation and interpretation

Chemical results often use a calibration response multiplied by dilution or preparation factors. WPI’s loading = flow × concentration × 8.34 converts a valid concentration to lb/day in US units. A nutrient mass balance must compare the same chemical form at each point. A “total” result cannot be subtracted from a “dissolved” result without understanding how both samples were prepared.

Worked laboratory scenario

Effluent total phosphorus increases while filtered orthophosphate stays low and TSS rises. The combined evidence points to particulate phosphorus carryover from solids separation, not loss of soluble precipitation alone. The operator validates digestion and filtration records, then investigates clarifier/filter performance rather than simply doubling coagulant.

Common exam traps

• COD is faster than BOD but does not measure the identical property.
• Orthophosphate and total phosphorus are not interchangeable.
• Filter timing changes the dissolved-metals basis and must follow the method.
• A matrix spike problem does not mean the sample concentration should be adjusted by guesswork.

Field-to-exam checklist

• Name the exact analyte and fraction before interpreting a result.
• Use digestion, filtration, preservation, and QC specified by the method.
• Combine nutrient fractions with TSS/process data to locate the failure.
• Convert concentration to mass only after result validity is established.

Fraction and preservation logic

Decide the reported fraction before touching the sample. Filtering in the field versus later in the laboratory can change a dissolved result as particles release or adsorb material during transport. Acid preservation that is correct for a total-recoverable metals sample may be wrong before a dissolved fraction is filtered. For nutrients, digestion deliberately converts several forms for a total result. Document these preparation choices because they determine what the number means, not merely how stable it remains.

For metals, verify whether the required result is dissolved, total, or total recoverable before filtration or digestion. Once a sample is prepared on the wrong basis, arithmetic cannot restore the missing fraction.

Method families and interference control

COD digestion has a built-in interference control. The dichromate method digests the sample at 150 °C for two hours with a silver catalyst and mercuric sulfate. The mercuric sulfate is there specifically to complex chloride, which the dichromate would otherwise oxidize and report as oxygen demand. A high-chloride industrial or coastal sample analyzed without adequate chloride suppression reads high, and no amount of careful colorimetry corrects it.

Nitrogen forms are a hierarchy, not synonyms. Ammonia can be measured by ion-selective electrode, by salicylate or phenate colorimetry, or by distillation and titration. TKN adds a digestion step that converts organic nitrogen to ammonia, so TKN equals organic nitrogen plus ammonia. Total nitrogen adds the oxidized forms, either by persulfate digestion of the whole sample or by summing TKN with nitrate-plus-nitrite. Reporting a TKN result against a total-nitrogen limit understates the true value by the entire nitrate fraction.

Phosphorus forms follow the same logic. Orthophosphate is measured directly by the ascorbic-acid colorimetric step. Total phosphorus requires a persulfate or acid digestion first, to convert condensed and organic forms. Filtering before digestion produces total dissolved phosphorus — a third, distinct result that is neither of the other two.

Metals depend on when filtration happens. Total recoverable metals are digested from the unfiltered sample; dissolved metals are filtered through a 0.45 µm membrane before preservation. Once the sample is prepared on the wrong basis, the missing fraction cannot be recovered arithmetically.

Worked mass consequence. A 5.0 MGD discharge at 0.6 mg/L total phosphorus releases 0.6 x 5.0 x 8.34 = 25 lb/day. Tightening the limit to 0.2 mg/L reduces the allowable mass to 8.3 lb/day — a two-thirds cut that in practice usually requires chemical precipitation plus tertiary filtration rather than biological optimization alone.