Wastewater Sampling and Lab — BOD, TSS, Composite vs. Grab, and Settleability
Sampling and lab work looks like an easy topic on paper. Then operators get into the exam and miss a string of questions — the BOD incubation conditions, when a permit demands a composite instead of a grab, what a 30-minute settling test actually tells them. These details get tested constantly because they're easy to verify and they separate operators who know the procedure from operators who skimmed it. This guide covers the lab and sampling fundamentals that show up on every wastewater operator exam.
TL;DR
- BOD is the oxygen bugs use to eat organics; the standard test runs 5 days at 20°C in the dark. It's the main measure of organic strength.
- COD measures everything chemically oxidizable — it's faster than BOD and always reads higher.
- TSS is the solids caught on a filter; composite samples are flow- or time-weighted over a period, grab samples are a single point in time.
- Permits specify which sample type for which parameter — grabs for things that change fast (DO, pH, chlorine), composites for loadings (BOD, TSS).
- The settleometer (30-minute settling) is your fastest read on sludge health. Test yourself with the free Wastewater Class I practice test.
BOD: the headline number
Biochemical oxygen demand measures the oxygen microorganisms consume while breaking down the organic matter in a sample. It's the primary indicator of a wastewater's organic strength, which is why discharge permits put a limit on it.
The standard BOD5 test holds the sample in the dark at 20°C for five days, then compares the dissolved oxygen at the start and end — the drop is the BOD. The dark matters (light would let algae produce oxygen and skew the result), and the temperature matters (it standardizes microbial activity). When a sample doesn't contain enough of its own microorganisms — for example a disinfected or industrial sample — you add seed, a known source of bugs, so there's a population to do the consuming.
COD: the fast cousin
Chemical oxygen demand uses a strong chemical oxidizer instead of microbes, so it measures everything that can be chemically oxidized, not just the biodegradable fraction. Two consequences the exam likes: COD comes back in hours instead of five days, and COD is always higher than BOD on the same sample because it captures non-biodegradable material too. Operators often use a plant-specific COD-to-BOD ratio for quick process feedback while the real BOD incubates.
TSS and the difference from dissolved solids
Total suspended solids are the particles caught on a glass-fiber filter — you filter a known volume, dry the filter, and weigh the residue. TSS is a permit parameter alongside BOD. Don't confuse it with total dissolved solids, which pass through the filter. Percent removal across the plant, ((in − out) ÷ in) × 100, is reported for both BOD and TSS.
Composite vs. grab: the question everyone misses
A grab sample is a single sample taken at one moment. A composite sample is many small samples combined over a period (often 24 hours), weighted by flow or time, to represent the average condition.
The rule of thumb the exam tests: use grabs for parameters that change quickly or don't survive storage — dissolved oxygen, pH, temperature, chlorine residual, fecal coliform — because an average would be meaningless or the parameter would change before the lab sees it. Use composites for loadings and averages — BOD and TSS — because a permit cares about the total mass over the day, not a single instant. Your permit specifies exactly which type is required for which parameter; reading the permit correctly is part of the job.
Two more details worth knowing: samples have hold times (BOD must be analyzed quickly and kept chilled; a lab will reject a sample that sat too long), and proper preservation (chilling to about 4°C, correct bottle, no headspace where it matters) is what keeps a result defensible.
The settleometer: your fastest process read
Fill a one-liter cylinder with mixed liquor, let it settle for 30 minutes, and read the settled sludge volume in mL/L. That single number — the SSV30 — does a lot of work:
- Combined with your MLSS, it gives the SVI:
(SSV30 × 1,000) ÷ MLSS. Roughly 80 to 120 is good settling. - It estimates your return sludge rate:
RAS % ≈ SSV30 ÷ (1,000 − SSV30) × 100. - The way it settles tells you a lot: a sharp, clear interface that compacts well is healthy; a slow, billowing blanket that won't compact suggests bulking; a cloudy supernatant with dispersed floc suggests old sludge or toxicity.
It costs five minutes and a cylinder, which is why operators run it constantly.
Dissolved oxygen in the field
DO is a grab parameter measured right in the aeration basin, usually held in a band around 1.5 to 2.0 mg/L. The exam point: DO needs to be controlled, not simply maximized — too little invites filaments and poor settling, too much wastes energy and can shear floc.
Practice it
Sampling questions are pure recall once you've seen them a few times. Run the Wastewater Class I practice test, and pair this with the activated sludge process control guide since the settleometer ties the two topics together.