33 Chemistry

1 mgl of platinum in combination with ~ mgl of metallic cobalt is taken as 1 standard color unit The yellow-brownish hue produced by these metals in solution is similar to that found in natural waters Comparison tubes containing standard platinumshycobalt solutions ranging from 0 to 70 color units are used for visual measurements however laboratoshyries often employ a colorimeter for readings

Turbidity Insoluble particles of soil organics microorganshyisms and other materials impede the passage of light through water by scattering and absorbing the rays This interference of light passage through water is referred to as turbidity In excess of 5 units it is noticeable by visual observation Turshybidity in a typical clear lake water is about 25 units and muddy water exceeds 100 units

The earliest method for determination of turshybidity used a Jackson candle turbidimeter in which a candle flame was viewed through a colshyumn of water contained in a calibrated glass tube Units of tmbidity using this apparatus are expressed as Jackson TUrbidity Units (JTU) Since the lowest value that could be measmed directly by this technique was 25 units the Jackson canshydle turbidimeter was limited in application to tmbid waters

Measurement of turbidity in treated drinking water commonly less than one unit is measured using a precalibrated commercial turbidimeter (nephelometer) Units of turbidity using a nephshyelometer are expressed as Nephelometric TUrbidshyity Units (NTUJ The tmbidimeter shown in Figure 2-13 can measure turbidities in the nonrashytio mode in excess of 40 NTU up to 10000 NTU and in the ratio mode from less than 100 NTU down to O Light from a tungsten-filament lamp is focused and passed through the water sample Transmitted and forward scatter detectors receive light passing through the sample The back scatshyter detector measures light scattered back toward the light somce The 90deg scatter detector receives light scattered by particles in the water at a right angle to the light beam In the nonratio mode for moderate to high turbidity the measurement is the light received by the 90deg scatter detector The back scatter detector is incorporated to permit

(a)

Forward Back 90

0

detector ~ scatter scatter _ detector

detector ~ -t bullbullJf r-I~~- _J_tn----lL

~

bull ~ middotbullbullbull~Ibull

Lamp Lens Sample

cell Transmitted

light detector

(b)

Fi~ure 2-13 Turbidimeter (nephelometer) and light path diagram (Courtesy of Hach Company Loveland CO)

measurement of very high turbidity In the ratio mode for low turbidity forward scatter is negligishyble compared to transmitted light and the meashysurement is a ratio of 90deg scattered light to transmi~ted light This ratio mode provides calishybration stability linearity over a wide range and negates the affect of color in the water sample

Jar Tests

The effectiveness of chemical coagulation of water or wastewater can be experimentally evalshyuated in the laboratory by using a stirring device as illustrated in Figure 214 The stirrer consists of six paddles capable of variable-speed operation between 5 and 300 rpm In making tests 1 liter or more of water is placed in each of the square conshytainers and is dosed with different amounts of

Figure 2-14 tirring device lIsed in jar tests for chemical

coagulation with outlets for sample withdrawal from the chambers ICou rl e ~ 01 PhiP lJs amp Bird Inci

coagulant After rapid mixing to disperse the chemicals the samples are stirred slowly for floc formation and then are allowed to settle under quiescent conditions The containers are mixed at a speed of 60 to 80 rpm for 1 min after adding the coagulant solution and then are stirred at a peed of 30 rpm for 15 min The stirrer can run

sequentially through a programmed sequence of paddle speeds and time duration Programme in iIIIlation is retained even with the power oft permitting day-to-day test reproducibility After the stirrer is stopped the nature and settling characteristics of the floc are observed and recorded in qualitative terms as poor fair good or excellent A hazy sample indicates poor coagushylation while properly coagulated water contains iloc that is well formed with the liquid clear benTeen particles

The lowest dosage that provides good turbidity removal during a jar test is considered the first trial dosage in plant operation Ordinarily a full-scale treatment plant gives better results than a jar test at the same dosage For example results from a jar test for coagulation of a turbid river water using alum are given in Table 2-10 The lowest recommended dosage in treating this water is 40 mgl of coagulant Since such other factors as temperature alkalinity and pH influence coagulation jar tests can also be run to evaluate these parameters and to determine optimum dosage under differing conditions

Chapter 2

Tahlt- l-I 4)

Results of Jar Test for Coagulation

JAR NUMBER

ALUMINUM SULFATE DOSAGE

(MUJAR) (MGIL) FLOC

FoRMATION

I 10 None 2 2 20 Smoky 3 3 30 Fair 4 4 40 Good 5 5 50 Good 6 6 60 Heavy

Fluoride

Excessive fluoride ions in drinking water cause dental fluorosis or mottling of teeth On the other hand communities whose drinking water conshytains no fluoride have a high prevalance of dental caries Optimum concentrations of fluoride proshyvided in public water supplies generally in the range of 08 to 12 mgl reduce dental caries to a minimum without causing noticeable dental fluoshyrosis Several fluoride compounds (Table 2-3) are used in treating municipal water all of these disshysociate readily yielding the fluoride ion (F- )

Electrode and colorimetric methods are curshyrently most satisfactory for determining fluoride ion concentration Both methods are susceptible to interfering substances for example chlorine in the colorimetric method and polyvalent cations such as Al+++ through which complex fluoride ions hinder electrode response Fluoride can be separated- from other constituents in water by distillation of hydrofluoric acid (HFj after acidifyshying the sample with suliuric acid Pretreatment separation is performed using a distillation apparashytus Samples not containing significant amounts of interfering ions can be tested directly

The fluoride-ion-activity electrode is a specific ion sensor designed for use with a calomel reference electrode and a pH meter having a millivolt scale The key element in the fluoride electrode is the laser-type doped single lanthanum fluoride crystal across which a potential is established by the presshyence of fluoride ions The crystal contacts the samshyple solution at one face and an internal reference