Turbidity Measurement

What is measured

Particles suspended in water scatter light. A nephelometer shines a beam through the sample and measures the intensity of light scattered at 90 degrees to the beam, which is the geometry that is most sensitive to small particles and least affected by color. The reading is compared with a suspension of formazin polymer of known concentration, and reported in nephelometric turbidity units. The regulatory methods define the light source and the detector geometry: EPA Method 180.1 uses a tungsten lamp, and the ISO 7027 method uses an infrared source, which reads differently in colored water. The method the state accepts decides the instrument.

Turbidity is not a measure of particle count, mass, or size. It is a proxy for them, adopted because it can be measured continuously and correlates with pathogen breakthrough through a filter. Combined filter effluent at or below 0.3 NTU in 95 percent of monthly measurements, and never above 1 NTU, is the surface water treatment rule requirement in the United States for conventional and direct filtration plants, and individual filter monitoring at 15-minute intervals is required as well. That is why the instruments exist and what their records serve.

Instrument ranges

The sample system

Most turbidimeter problems are sample problems. The instrument measures whatever reaches its cell.

• Sample flow: slow and steady, typically 250 to 750 mL per minute depending on the instrument, set with a flow regulator and never pulsing. A sample pump introduces bubbles; gravity or line pressure through a regulator is preferred.
• The bubble trap: every process turbidimeter has a chamber that lets entrained air rise out before the sample reaches the optical cell. It works only if the flow is slow enough and the trap is clean. Filter effluent under pressure, warm water, and supersaturated water all release bubbles.
• Sample line: short, opaque, no low points that collect sediment, and taken from a point that represents the stream: a pipe tap in the side, not the bottom where sediment moves or the top where air collects.
• Lag time: the sample takes time to travel from the tap to the cell. For individual filter monitoring at 15-minute intervals it rarely matters; for a turbidity spike alarm it does. Keep the line short.
• Cell cleaning: the optical surfaces coat with biofilm and scale. Weekly to monthly cleaning, more often on raw water, with the cleaning noted in the record.
• Drain: to a drain that cannot back up into the cell, with an air gap.

Calibration and verification

Because the instruments are compliance devices, the regulations and the instrument methods specify how they are calibrated and how often they are checked. Calibration means adjusting the instrument to read correctly against a primary standard; verification means confirming, without adjusting, that it still does.

Primary standard

Formazin, prepared or purchased at a certified concentration, or an EPA-approved alternative such as a styrene divinylbenzene polymer suspension. Used for calibration, typically quarterly for process instruments and whenever verification fails.

Secondary standard

A sealed standard, often a gel or a solid, whose value was assigned by comparison against a primary standard on the same instrument. Used for verification, typically weekly, because it is convenient and stable.

Verification

Place the secondary standard, read, and compare with its assigned value within the tolerance the method or the state sets, usually a few percent or a fixed NTU at low range. Pass: record it. Fail: clean the instrument, verify again, and calibrate with the primary standard if it still fails.

Grab sample comparison

A bench turbidimeter reading on a grab sample drawn at the process instrument, on a schedule, as a check that the sample system is delivering what the process contains.

Records

Every calibration, verification, cleaning, and standard lot, with dates and initials, kept for the period the state requires. Inspectors read these before anything else.

1. 1

   Check the standard

   In date, undisturbed, at room temperature, no bubbles or settling. A formazin dilution is prepared fresh; a sealed secondary is inverted gently if the instructions say so and never shaken.

2. 2

   Clean the cell and cuvette

   Optical surfaces clean and dry on the outside, with no fingerprints. Oil the outside of a cuvette if the instrument method calls for it.

3. 3

   Calibrate at the points the instrument requires

   A zero or a low point and one or more upscale points, in the order the transmitter menu prescribes. Let each reading stabilize.

4. 4

   Verify with the secondary

   Immediately after calibration, read the secondary standard and record its value as the assigned value for future verifications on this instrument.

5. 5

   Return to sample and compare

   Confirm the reading is stable and the bubble trap is working. Compare with a bench reading of a grab.

Using the signal

Individual filter effluent turbidity is trended at the interval the rule requires and alarmed at the rule limits with pre-alarms below them. A filter that trends upward through its run is approaching breakthrough, and the trend, together with head loss and run time, is a backwash trigger. Raw water turbidity drives coagulant dosing in plants with streaming current or feedforward control and is the earliest warning of a storm event reaching the intake. Backwash waste turbidity marks the end of the wash. None of these signals is a control input the plant can trust without the sample system and the verification record behind it.