13.2 pH, Temperature, DO & Settleable-Solids Analysis
Key Takeaways
- Measure rapidly changing physical parameters at representative locations.
- Verify probes with appropriate standards or independent checks.
- Use the correct apparatus and reporting basis for settleable solids.
- Interpret profiles and related process data rather than trusting one sensor.
13.2 pH, Temperature, DO & Settleable-Solids Analysis
2025 WPI alignment: This section teaches conducting and interpreting physical analyses such as pH, temperature, dissolved oxygen, and settleable solids in Laboratory Analysis, the 15-question area containing 3 recall, 12 application, and 4 calculation items.
Why this laboratory task matters
Physical measurements often change quickly and guide immediate operation. Representative location, probe condition, temperature effects, calibration, mixing, timing, and correct apparatus are essential.
Analytical foundation
| Element | What makes the result defensible |
|---|---|
| pH measurement | A glass electrode and reference system require appropriate buffers, clean condition, temperature awareness, and representative sample. |
| Temperature | Temperature affects biological rates, oxygen saturation, chemical reaction, and sensor response and must be recorded at the relevant point. |
| Dissolved oxygen | Membrane or optical probes require verified calibration, clean sensing surfaces, flow/response, and location suited to the process question. |
| Settleable solids | An Imhoff cone or specified apparatus measures settled volume per liquid volume after the method-defined settling period. |
| Settleometer versus Imhoff | Activated-sludge settleability observations and wastewater settleable-solids tests have related appearance but distinct purposes and reporting. |
| Field immediacy | Transport and storage can change pH, temperature, and DO, so method-required field or prompt measurement protects meaning. |
Laboratory workflow
- Choose the representative point and inspect for stratification, bubbles, deposits, sunlight, mixing, and safe access.
- Verify pH buffers, DO calibration/check, temperature sensor, cone cleanliness, timer, and sample identity.
- Collect or immerse without aerating, warming, cooling, or contaminating the sample.
- Allow probes to stabilize under the method and record temperature with pH/DO when required.
- For settleable solids, mix the sample as directed, fill to mark, settle undisturbed, and read the correct interface.
- Compare duplicate/field checks and process profiles, clean/store instruments, and record calibration and maintenance.
Quality and diagnostic evidence
| Finding | Meaning | Correct response |
|---|---|---|
| DO differs across basin | Real zoning or mixing differences may exist | Map locations and verify probe before averaging them away. |
| pH drifts in one sample | Temperature, junction, coating, low ionic strength, or poor stabilization may affect reading | Clean/check probe and follow the method. |
| Bubbles cling to DO sensor | Measurement can be biased | Reposition or remove bubbles without altering the sample improperly. |
| Imhoff interface unclear | Floc, floatables, or wall adhesion complicates reading | Use the SOP-defined reading and qualifier, not an invented precision. |
Calculation and interpretation
WPI provides Celsius/Fahrenheit conversion and may require interpreting measured values rather than complex formulas. pH is logarithmic, so arithmetic averaging of pH values is not always chemically meaningful for blending. Settleable solids is reported on the method basis, commonly a volume per liter. DO saturation comparisons must account for temperature and other stated conditions; use provided tables or instrument-approved functions.
Worked laboratory scenario
A fixed DO probe reads 2.0 mg/L, but a calibrated portable probe reads 0.4 mg/L beside it and ammonia is rising. The operator inspects the fixed sensor and finds a fouled membrane/surface. The portable verification and process response outweigh the unverified control signal; cleaning and recalibration are required before retuning aeration.
Common exam traps
- Do not transport a sample for hours and treat its DO or temperature as the original field condition.
- Settleable solids in an Imhoff cone is not TSS mass in mg/L.
- A two-point pH calibration should bracket the expected measurement range under the SOP.
- One DO value does not describe an entire staged basin.
Field-to-exam checklist
- Measure rapidly changing physical parameters at representative locations.
- Verify probes with appropriate standards or independent checks.
- Use the correct apparatus and reporting basis for settleable solids.
- Interpret profiles and related process data rather than trusting one sensor.
Profiles instead of isolated points
Physical parameters are especially useful as profiles: DO along an aeration train, pH before and after chemical addition, temperature across a heat exchanger, or settleable solids through treatment. Use consistent depth, location, time, and instrument checks so differences represent the process. A profile can locate oxygen depletion, poor mixing, short-circuiting, or chemical contact that a single final value conceals. Mark offline equipment and recycle state on the profile.
Calibration practice and the saturation reference
pH meters are calibrated with buffers that bracket the sample. Two points are the normal minimum — typically 7 and 4 for acidic samples or 7 and 10 for alkaline ones — and the meter reports an electrode slope that should fall inside the manufacturer's acceptance window. A slope drifting steadily outside that window indicates an aging or coated electrode, not a changing wastewater, and no amount of recalibration restores an electrode past its useful life.
Dissolved-oxygen instruments are verified against a known condition. Water-saturated air is the standard field check; a Winkler titration is the laboratory reference. Membrane probes consume oxygen at the membrane and therefore need flow across the sensing surface — a stagnant reading drifts downward — while optical probes do not consume oxygen but still require cap replacement on the manufacturer's schedule.
Saturation is a temperature-dependent reference, not a constant. Clean water at sea level holds about 9.1 mg/L at 20 °C and about 7.6 mg/L at 30 °C; saturation also falls with elevation and with dissolved salts. A reading of 6 mg/L therefore represents about 66 percent of saturation in a 20 °C basin but about 79 percent in a 30 °C basin — the same number describing two quite different oxygen conditions.
Settleable solids has a prescribed sequence. Fill the Imhoff cone to the 1 L mark, settle for 45 minutes, gently spin the cone to dislodge solids adhering to the walls, settle a further 15 minutes, and read at the 60-minute mark, reporting mL/L. Skipping the wall-dislodging step commonly understates the result.
Worked conversion. The WPI table supplies temperature conversion: 25 °C = (25 x 9/5) + 32 = 77 °F, and conversely 50 °F = (50 − 32) x 5/9 = 10 °C.
What does an Imhoff-cone settleable-solids test report?
A fixed DO probe disagrees with a verified portable probe and ammonia is rising. What is the best first action?