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 of relative sample clarity. * Color is interference but correctable by substracting with 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 between Suspended solids and Light intensity due to Light scatter. * Light detected by photocell not caused by suspended solids is called “stray light”. * Causes of stray light are reflections, scratches, fingerprints on sample cell, Or glass imperfections.

Single Beam Method * Single light source & A single photodetector Located at 90 O angle To transmitted light. * Frequent calibration necessary due to changes in light intensity over time.

Ratio Method Expands upon the single beam concept. Detectors at other angles are 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 problem With light source decay and frequent calibration still exists.

Dual Beam Method * Minimizes effect of light Source 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 at high turbidity

USEPA Specifications 1.Light source – Tungsten-Filament lamp operated at a color temperature between 2200 and 3000 O K. 2.Distance traversed by incident light and scattered light within the sample tube not to exceed 10 cm 3.Angle of light acceptance by detector – Centered at 90 O to the incident light path and not to exceed ±30 O from 90 O. The detector and filter, if used, 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 of hydrazine sulfate = 4000 NTU (nephelometric turbidity units) 3.Secondary Standards – manufacturer supplied standards equivalent to prepared primary formazin standard, microspheres of styrene- divinylbenzene copolymer, sealed latex suspension or with metal oxides in 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 temperature changes and particle flocculation and sedimentation. Wipe sample cell with Kimwipes to remove fingerprints. Use cells with no scratch marks. Condensation may interfere with results. Adjust sample temp. to room temp.