16.2 Chemical Analyses & Laboratory Quality Control: COD, Nutrients, Metals, WET Testing & SOPs

16.2 Chemical Analyses & Laboratory Quality Control: COD, Nutrients, Metals, WET Testing & SOPs

Exam Focus: The Laboratory Analysis content area names chemical analyses (COD, nutrients, metals), biological analyses including WET, and both "Follow laboratory Standard Operating Procedures (SOPs)" and "Operate and maintain laboratory instrumentation." Section 7.2 covered BOD and TSS; this section covers everything else the criteria list.

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1. Chemical Oxygen Demand (COD)

COD measures the oxygen equivalent of matter that can be chemically oxidized. A sample is digested in a sealed vial with potassium dichromate in concentrated sulfuric acid at 150 degrees C for two hours, with a silver catalyst and mercuric sulfate to suppress chloride interference. The amount of dichromate consumed is measured colorimetrically and expressed as mg/L of oxygen.

COD vs. BOD

Because COD captures non-biodegradable material as well, COD always exceeds BOD5 on the same sample. Each plant develops its own reasonably stable COD-to-BOD ratio for a given stream, and once that ratio is established a same-day COD result can be used to estimate the loading long before the five-day BOD is available. A sudden change in the COD-to-BOD ratio is itself a valuable signal — it usually means a non-biodegradable industrial discharge has entered the plant.

Waste handling. Spent COD vials contain dichromate, mercury, silver, and strong acid. They are a hazardous waste stream and must be accumulated and disposed of properly. Pouring them down the drain sends heavy metals straight into the plant's own influent and is a serious violation.

2. Nutrient Analyses

The distinction between total phosphorus and orthophosphate is a frequent exam point: total phosphorus requires a digestion step to convert organic and condensed phosphates into orthophosphate before color development, while orthophosphate is measured directly on a filtered sample with no digestion.

3. Metals

Metals analysis normally goes to a contract laboratory, but the operator collects the sample and the sample determines the result:

• Preserve with nitric acid to pH less than 2 unless the method directs otherwise. Without acid preservation, dissolved metals adsorb onto the container walls and the reported concentration is low.
• Use the container type the method specifies — plastic for most metals, and note that mercury has its own requirements.
• Total versus dissolved metals is a real distinction: dissolved metals require field filtration before preservation. Filtering after acidification gives a meaningless result.
• Metals matter to a treatment plant in two directions: they inhibit the biological process (nitrifiers fail first), and they limit biosolids land application under 40 CFR Part 503 (Section 6.4). A metals excursion usually means an industrial user needs attention through the pretreatment program.

4. Whole Effluent Toxicity (WET) Testing

WET testing is listed under biological analyses in the Need-to-Know Criteria. It exposes live test organisms to a series of effluent dilutions and measures their survival, growth, or reproduction.

• Acute tests measure survival over a short exposure, typically 48 hours.
• Chronic tests measure sublethal effects — growth and reproduction — over a longer exposure.
• Common freshwater test organisms are the water flea Ceriodaphnia dubia and the fathead minnow Pimephales promelas.

Why WET exists. Chemical-specific tests only find what you decide to look for. WET testing measures the aggregate toxicity of the effluent as the receiving stream actually experiences it, including combined effects of multiple substances and toxicity from compounds nobody analyzed. A plant that passes every chemical limit can still fail a WET test — and when it does, the response is a toxicity identification and reduction evaluation, working systematically to find and eliminate the source, very often an industrial contributor.

5. Quality Control — What Makes Data Defensible

Approved methods are not optional. For NPDES compliance, results must be produced by a method approved under 40 CFR Part 136 for that parameter. An unapproved method — however accurate, however convenient — produces a number that cannot be reported for compliance. Many states additionally require the laboratory or the individual analyst to be certified for the parameters reported.

6. Laboratory Instrumentation and Records

The Need-to-Know Criteria includes "Operate and maintain laboratory instrumentation." That means:

• Analytical balance — level it, keep it away from drafts and vibration, and verify with class weights on a documented schedule.
• Incubators, ovens, water baths, and refrigerators — record temperatures daily against a NIST-traceable thermometer. The fecal coliform incubator's 44.5 plus or minus 0.2 degrees C is unforgiving, and an out-of-range incubator invalidates every result run in it.
• Pipettes — verify volume gravimetrically on schedule.
• Reagents — label with contents, concentration, preparation date, expiration, and preparer. Discard expired reagents; an expired buffer or standard silently biases every result calibrated against it.
• Glassware — dedicated cleaning procedures, and never use detergent residue-contaminated glassware for BOD work.

Laboratory Safety

• Work with acids, digestion reagents, and volatile solvents in a functioning fume hood.
• Always add acid to water, never water to acid.
• Eyewash and safety shower must be unobstructed, accessible within seconds, and flushed on schedule.
• Store incompatible reagents separately — acids apart from bases, oxidizers apart from organics.
• Keep the safety data sheet for every chemical accessible, and wear the eye, hand, and body protection the SDS specifies.
• Never pipette by mouth, and never eat, drink, or store food in the laboratory.