Turbidity Measurement

By

Douglas Rittmann, Ph.D., P.E.Water/Wastewater Consultant

To

UTEP Laboratory Course

On

February 21, 2006

Turbidity Definition and Measuring Principle

* Definition: Measurement of scattered light or measure ofrelative sample clarity.

* Color is interference butcorrectable by substractingwith filtered blank.

* High suspended solids limits side scatter & results in lower than actual readings. Solution: Sample dilution* Advantage: Easy to measure & Regulatory enforceable

Disadvantages:

Particle Effects

*Particles <0.45 microns are dissolved but will Scatter light

*Amt. of scattered light differs with particle size

* Particle color: Light Sand-reflects well; Black Carbon – absorbs light

•Particle shape – spherical- predictable results; irregular shape – varying responses

Optical Design Elements

* The angle of Detection: Forward Direction or 90 degree angle

* Light beam aperture

* Incident beam wavelength affects the consistency of scatter pattern.

* Color sensitivity of the photocell affected by type of light source, photocell, and filter.

Optical Design Limits

* Relationship betweenSuspended solids andLight intensity due toLight scatter.

* Light detected byphotocell not caused by suspended solidsis called “stray light”.

* Causes of stray light arereflections, scratches,fingerprints on sample cell,Or glass imperfections.

Single Beam Method

* Single light source & A single photodetectorLocated at 90O angleTo transmitted light.

* Frequent calibration necessary due to changes in light intensityover time.

Ratio Method

•Expands upon the single beam concept. Detectors at other anglesare added. Using a ratio of multiple detector system increases stability of values. Design cancels the effects of light intensity reduction when measuring colored liquids, making it color compensated. The problemWith light source decay and frequent calibration still exists.

Dual Beam Method

* Minimizes effect of lightSource decay.

* Single light source is split by a mirror into two beams, a measuring beam & a reference beam. The single beam measures the different intensities of both beams and eliminates frequent calibration.

* It does not address problem of instability athigh turbidity

USEPA Specifications

1. Light source – Tungsten-Filament lamp operated at a color temperaturebetween 2200 and 3000O K.

2. Distance traversed by incident light and scattered light within the sampletube not to exceed 10 cm

3. Angle of light acceptance by detector – Centered at 90O to the incidentlight path and not to exceed ±30O from 90O. The detector and filter, ifused, shall have a spectral peak response between 400 and 600 nm.

Reagents

1. Dilution Water – Pass high purity water through 0.1 micron filter

2. Stock Primary Standard – Formazin Suspension of 1 gram ofhydrazine sulfate = 4000 NTU (nephelometric turbidity units)

3. Secondary Standards – manufacturer supplied standards equivalent toprepared primary formazin standard, microspheres of styrene-divinylbenzene copolymer, sealed latex suspension or with metal oxidesin a polymer gel. Should have good agreement with Formazin standard.

Procedure

1. Nephelometer calibration – Run at least one standard in each range to be used. Follow manufacturer’s operating instructions.

2. Measurement – Gently agitate sample. Wait until air bubbles disappear &pour sample into cell. Read turbidity directly from display.

General Considerations: Measure immediately to prevent temperaturechanges and particle flocculation and sedimentation. Wipe sample cellwith Kimwipes to remove fingerprints. Use cells with no scratch marks.Condensation may interfere with results. Adjust sample temp. to room temp.