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WATER

WATER CONDITIONING AND STABILITY ASSESSMENT: AN INTRODUCTION P Gebbie Abstract Currently there is no requirement under the Australian Drinking W ater Guidelines to produ ce a stable water w hich is not potentially corrosive to water treatment plant equipment and reticulation systems. This paper disc usses the concept of w ater stability, d esc ribes various indices available to ga uge the corrosivity o f a water, and m etho ds o f calculation. An approach is also outlined w hereby the stability of a ,Nater can be d e termin ed fo ll ow ing a pa rtic ular treatment regime and how it can then be conditioned to make it less aggressive . Examining different treatm ent and conditioning options for a typi ca l w ater illustrate th ese concepts. Key Words: Corrosivity, Water Stability, W ater C onditioning, Langelier Ind ex, CCPP

Introduction At present there is no requirement in the Australian Drinkin g Water Guidelin es to produce a stable water - i.e. one that is neither scale-fo rming or corrosive other than having a pH in the range 6.5 to 8.5 (ADWG 1996). H owever, ma ny surfac e waters require a c hemi cal co agulant such as alum for effective treatment and disinfection using chlorine. Som e waters can then becom e aggressive, leadin g to possible deterio ratio n of equipment, pip elines and reticulation system s, so post- treatment conditio ning 1s necessa ry.

Water Chemistry As far as co rros ivity issu es are concerned, th e most impo rtant water quality param eters are: • alka linity • pH • calcium • total dissolved solids (TDS). Th e c o nce ntrat i on o f v ario u s constitu ents in a water can be expressed in one o f tw o ways: • as the ion (or "as is"), e .g. 8 mg/L calci um (C a) as calcium, or 50

WATER MARCH 2001

• as calcium carbonate, e.g. 8 m g/L Ca becomes 20 m g/ L Ca "as calcium carbonate" (CaC O 3) . (purely an historical conceit. Ed) T o con vert from one fo rm to the other, the conversio n factors listed in T able 1 are used . T he chemical for mula, formula we ights and equi valen t weights of these constituen ts are also listed. Positively charged io ns are called "catio ns" (e .g. calcium ion or c a++) and negati vely charged ions, "anio ns" (e.g. bicarbo nate or H C O 3 -) . T he sum of the concentration of cations expressed as m g/L C aCO 3 should also equal the sum of the anions, thus giving a balanced water analysis. Having such an analysis, th e fo llowing can then be investigated: • the stability of the wa ter; • the impact that chemical addition as part of th e treatment process can have o n water quality (e.g. alum and chlorine), and • the pH of the water fo llowing chemical add itions. T able 2 is a list of chemicals corrunonly used in water treatment processes, giving th e chemical fo rmula , formula w eight, equivalent weight and the factor to

con vert to equivalen t weight as C aCO 3. T his table w ill be used wh en optio ns for con ditionin g a water to make it stable and no n-corrosive are considered.

A Typical Example As an exa mple of how to apply conditioning, consider a typical w ater suppl y: the W aranga C hannel at R ochester, Victo ria, w ith the analysis listed in Table 3. From this analysis the following can be co ncluded: • the sum of the cations is not equal to the sum of the anions: 84.9 v 82.7 mg/ L CaCO 3 . H owever, in practice a water analysis is considered balanced if the (sum of cations/sum of anions is within + or 5%. In this case it is +2.7%, so it is acceptable, • the alkalini ty of the water is 15 .0 m g/L as CaCO 3, which is relatively low, suggesting this water will require supplem e ntal alkali additi o n for effe cti ve trea tment w hen using alum , • th e total hardn ess of the wate r ([Ca]+ (Mg]) is 34.1 m g/L C aCO3 and may be described as being a soft water, and • from the conductivity (E C) of the w ater, the TDS is 115 mg/ L , using the common conve rsion factor of 0.64.

Table 1. Chemical Formula, Formula Weight, Equiva lent Weight and Factors to Convert Concentration from "AS IS" to "CaC0 3 " for Common Cations and Anions Ion

Chemical Formula

Formula Weight

Equivalent Weight

As CaCO3 Multiply by

Aluminium

Al +++

27.0

9.0

5.56

Ammoni um

18.0

18.0

2.78

Calcium

NH4+ ca++

40.1

20.1

2.50

Ferric Iron

Fe+++

55.9

18.6

2.69

Ferrous Iron

Fe++

55.9

28.0

1.79

Magnesium

Mg++

24.3

12.2

4.12

Sodi um

Na+

23.0

23.0

2.18

Potassium

K+

39.1

39.1

1.28

Hydrogen

H+

1.0

1.0

50.0

Bicarbonate

HC0 3· C03··

61.0

61.0

0.82

Carbonate

60.0

30.0

1.67

Chloride

Cl·

35.5

35.5

1.41

Hydroxide

OH·

17.0

17.0

2.94

Sulphate

S04··

96 .1

48.1

1.04

Profile for australianwater

Water Journal March 2001  

Water Journal March 2001