5.8 Process Control Testing: Jar Tests, Settleometer & Microscopic Examination
Key Takeaways
- A jar test replicates rapid mix, flocculation, and sedimentation at bench scale so the operator can compare coagulant type, dose, and pH before committing full-scale chemical.
- The settleometer 30-minute settled volume combined with the mixed liquor suspended solids concentration yields the sludge volume index, and the shape of the settling curve is itself diagnostic.
- Specific oxygen uptake rate is the oxygen uptake rate divided by the mixed liquor volatile suspended solids concentration and measures the activity of the biomass per unit of mass present.
- Microscopic examination identifies the protozoan and metazoan population, whose composition indicates sludge age, and identifies filaments whose type points to a specific causative condition.
- Stalked ciliates dominating with rotifers present indicates a well-operating sludge of moderate to high age, while dominant flagellates and amoebae indicate a young or upset sludge.
Process Control Testing: Jar Tests, Settleometer & Microscopic Examination
These are the tests that do not appear on a discharge monitoring report but that determine what the operator does next. They are also the tests most likely to appear as application-level exam items, because they require interpretation rather than recall.
1. The Jar Test
A jar test is a bench-scale model of the coagulation, flocculation, and sedimentation train. It answers questions no online instrument can: which coagulant, what dose, what pH, what polymer.
Standard procedure
- Fill six 1- or 2-liter jars with the same raw water, collected at the same time.
- Record the raw water turbidity, pH, temperature, alkalinity, and (if available) TOC or UV254.
- Dose each jar with a graded series of coagulant - for example 15, 20, 25, 30, 35, 40 mg/L - injecting simultaneously with pipettes or syringes.
- Rapid mix at high speed (about 100 to 300 rpm, G above 300 to 700 per second) for 10 to 60 seconds.
- Flocculate at a reduced speed (about 20 to 40 rpm) for 15 to 20 minutes, often tapering the speed.
- Settle quiescently for 15 to 30 minutes.
- Withdraw supernatant from a fixed depth using a pipette and measure settled turbidity, pH, and residual aluminum or iron.
What to observe and record
- Time to first visible floc - a rough measure of the reaction rate; slow floc formation in cold water is normal and calls for a higher dose or a coagulant aid.
- Floc size and appearance - "pinpoint," "pinhead," "shot-sized." Small, weak floc suggests underdosing or insufficient mixing; large but fragile floc suggests over-mixing in the flocculation stage.
- Settling rate and the clarity of the supernatant.
- Final settled turbidity - the primary criterion.
- Final pH - because the coagulant itself moved it.
Reading the results
- The optimum dose is the lowest dose giving acceptable settled turbidity, not the dose giving the absolute minimum. Turbidity typically drops sharply, flattens, and then rises again at very high dose - that upturn is restabilization, where excess positive charge reverses the particle charge.
- A second series varying pH at the best dose (adding lime, soda ash, caustic, or acid) frequently finds a better result than dose adjustment alone, because coagulation is a pH-dependent reaction. Alum works roughly pH 5.8 to 7.5; ferric across a much wider window.
- Enhanced coagulation for TOC removal usually requires a higher dose and a lower pH than turbidity removal alone. Run the jar test with TOC or UV254 as the criterion when DBPs are the driver.
The jar test's known limitation: it does not reproduce full-scale hydraulics, so the optimum dose transfers well but the optimum mixing energy does not. Confirm a jar-test dose change at full scale and watch settled turbidity for a full detention time before deciding it worked.
2. The Settleometer
A 1,000 mL or 2,000 mL graduated cylinder (or a settleometer with a slow stirrer) filled with aeration basin mixed liquor, read at intervals for 30 to 60 minutes.
The measurements
- 30-minute settled volume (SV30) in mL/L.
- Sludge Volume Index: SVI (mL/g) = (SV30 in mL/L x 1,000) / MLSS in mg/L
- Sludge Density Index: SDI = 100 / SVI - the reciprocal, occasionally used in older references.
| SVI | Interpretation |
|---|---|
| Below about 70 | Sludge is dense and settles fast, but often old, over-oxidized pin floc with a turbid supernatant |
| 80 to 150 | Good settling and compaction |
| 150 to 200 | Deteriorating; likely early filamentous growth |
| Above 200 | Bulking; clarifier failure is imminent or occurring |
The shape of the curve is diagnostic
| Pattern | Meaning |
|---|---|
| Rapid initial settling with a sharp, clear interface and good compaction | Healthy floc |
| Slow settling, no clear interface, sludge fills the cylinder | Filamentous bulking |
| Rapid settling, but the supernatant is cloudy with fine dispersed particles | Pin floc / over-oxidation; too high a sludge age |
| Sludge settles then rises after 20 to 40 minutes with gas bubbles | Denitrification - the cylinder is reproducing what the clarifier blanket does |
| Ashy gray sludge, foamy surface, no compaction | Young sludge, or a toxic upset |
| Straw or light tan color | Very young sludge, low MCRT |
| Dark brown to black | Old sludge, or septic conditions |
Run the test in a settleometer of the same shape every time, at the same temperature, without stirring or with the same slow stir. SV30 is highly sensitive to cylinder geometry, so a result from a 1-liter graduated cylinder is not comparable to one from a 2-liter settleometer.
3. Quick Solids Checks
- Centrifuge spin. A 15 mL graduated centrifuge tube of mixed liquor or RAS spun for a fixed time and speed gives percent solids by volume in minutes. It is not an absolute measurement, but as a relative trend it is fast enough to guide a wasting decision the same shift. Establish a site-specific correlation to gravimetric TSS.
- Total solids by drying. For sludge streams, a weighed sample dried at 103 to 105 degrees C to constant weight gives percent total solids, and igniting the residue at 550 degrees C gives percent volatile solids. These are the inputs to digester loading, dewatering, and biosolids calculations.
- Sludge blanket depth by sludge judge, as covered in the clarifier section.
4. Oxygen Uptake Rate
Oxygen uptake rate (OUR) measures how fast the biomass is consuming oxygen and is the fastest available indicator of biological health or toxic inhibition.
Procedure. Fill a BOD bottle with mixed liquor, insert a DO probe with a stirrer, seal, and record DO every 15 to 30 seconds for several minutes. The slope is the uptake rate.
OUR = mg/L of DO consumed per hour SOUR (Specific OUR) = OUR / MLVSS in g/L, expressed as mg O2 per gram MLVSS per hour
| SOUR | Interpretation |
|---|---|
| Below 1.5 | Very low activity; endogenous, old sludge - and, for aerobically digested biosolids, this is also the vector attraction reduction criterion in 40 CFR 503.33 (1.5 mg O2 per g TS per hour at 20 degrees C) |
| 3 to 10 | Typical of a conventional activated sludge process |
| Above 20 | Highly active - young sludge or a large organic load |
| Sudden collapse | Toxic inhibition - the single most useful early warning available |
A comparative OUR run on a sample of healthy mixed liquor dosed with the suspect influent versus an undosed control is the standard bench test for confirming an industrial toxic slug.
5. Microscopic Examination
A phase-contrast microscope at 100x and 400x is one of the most informative instruments in a wastewater plant, and interpreting it is a classic application-level exam topic.
Indicator organisms and what they say about sludge age
| Dominant organism | Sludge condition |
|---|---|
| Amoebae and flagellates | Very young sludge, low MCRT, high F/M, or recovery from an upset |
| Free-swimming ciliates | Young to moderate sludge; transitional |
| Crawling ciliates | Moderate sludge age; good sign |
| Stalked ciliates (Vorticella, Epistylis, Carchesium) | Well-operating sludge, moderate to high MCRT, good effluent quality |
| Rotifers | Older, stable sludge; good effluent |
| Nematodes and water bears | Very old sludge, very high MCRT; often accompanies pin floc |
| Sudden absence of higher life forms | Toxicity or a severe upset |
The progression amoebae to flagellates to free-swimming ciliates to crawling ciliates to stalked ciliates to rotifers to nematodes maps directly onto increasing sludge age, which makes a five-minute look at a slide a rough MCRT estimate.
Floc characteristics
- Floc size - large and irregular is typical of a healthy, well-flocculated sludge; small and round is typical of young sludge.
- Filament abundance - rated on a 0 to 6 subjective scale, from none to excessive.
- Filaments extending from the floc cause bulking; filaments forming an inter-floc bridge network cause severe bulking; dispersed filaments in the bulk solution cause turbid effluent.
Filaments and the conditions that cause them
| Filament | Associated condition |
|---|---|
| Sphaerotilus natans, Type 1701, Type 021N | Low dissolved oxygen or high F/M |
| Microthrix parvicella | Low F/M, high sludge age, cold weather, fats and grease - the dominant Virginia winter foaming filament |
| Nocardia / Gordonia (actinomycetes) | High grease, high sludge age; produces thick brown foam |
| Thiothrix, Type 021N | Sulfide (septic influent), nutrient deficiency |
| Nostocoida limicola, Type 0092 | Low F/M, high sludge age |
| Type 0041, Type 0675 | Low F/M |
| Beggiatoa | Sulfide |
Sulfur granule staining distinguishes Thiothrix and Beggiatoa, and Gram and Neisser staining are used to narrow identification further. The practical value is that the filament identifies the cause: finding Thiothrix points you at a septic upstream force main, while finding Microthrix points you at grease and sludge age. Chlorinating the RAS treats the symptom; correcting the condition the filament is telling you about is the actual fix.
A jar test series at 15, 20, 25, 30, 35, and 40 mg/L alum produces settled turbidities of 3.8, 1.9, 0.9, 0.8, 1.4, and 2.6 NTU respectively. What dose should the operator select and what explains the rise at the high end?
Microscopic examination of mixed liquor shows abundant rotifers and nematodes, few stalked ciliates, and a turbid supernatant with fine dispersed particles in the settleometer. The sludge volume index is 62 mL/g. What does this indicate?
An oxygen uptake rate test on mixed liquor shows a drop from 12 mg/L per hour yesterday to 1.8 mg/L per hour today, with mixed liquor volatile suspended solids unchanged. What should the operator suspect?