Table 7. Water Analysis Waranga Channel at Rochester Following Chemical Conditioning (Unless otherwise noted, all analyses in mg/ L) As Ion
Calcium Magnesium Sodium Potassium
As CaC0 3
pH 7.5 The TDS of the conditioned water will be 163 mg/L
The same procedure as above is again followed. Addition of lime wi ll increase the alkalinity and calcium of the water. The new water analysis will be: • cations: Ca=36.2+2.5X6.0X40.1/74 .1 = 44.3 mg/L CaC03
• anions: alkalinit y: 9.0+6.0X1 .35-1.5X1.41= 15.0 mg/L chloride: there will be an increase in the chloride level equivalent to the decrease in alkal in ity due to chlorine addition , i.e. 2.1 mg/L.
The change co the CO 2 level is aga in calc ulated. •
Initial CO 2 : 6.3 mg/L as is
• CO 2 added from destruction of al kalin ity by chlorine add ition = 1.5X1.24 = 1.9 mg/L • CO 2 destroyed by lime addition = -6.0X1.19 = -7.1 mg/L •
Final CO 2 concentration=1.1 mg/L
The final pH of the co ndition ed water wi ll be: pH = log(2.24X106X15.0/1.1) = 7.5.
Th e condition ed water now has a p H of 7.5 and a LSI of - 1.3, which is satisfactory from a corrosivity standpo int. The CCPP value can also be checked using the graphical method ou tlined earlier. In this case: • (ALK-Ca) = 15.0-44.3= · 29.3 mg/L CaC03. • AC IDITY = 17 .0 mg/L CaC0 3 from Equation (6).
From Figure 1 (Point B), the [Ca,, ,] value may be read-off as 48.0 mg/L, and hence:
new TDS of the conditioned water found. Th e new water analysis is shown in T able 7. Note that Mg, Na and K levels in the raw water all remain unchanged as a consequence of water conditioning with lime. Th e difference between the sum of cations and the su m of anions is + 2.2 mg/ L, which is the sam e as the original water analysis and hence the calculations are correct. This procedure can also be repeated using caustic soda and soda ash as alternative alkalis for conditioning. Table 8 co mpares the three different alkalis using WaterQual, a water treatment and quali ty assessm ent model developed by Fish er Stewart. Th e differences between the results from WaterQual and the above calculations are du e to the use of more accurate data in the model and co rounding errors. Note the operating cost differential in using lime compared to caustic soda and soda ash . The additional advantage in using lime is also apparent: this conditioned water has the lowest CC PP. T he TDS increase associated with using soda ash is also evident: l15 to 195 mg/ L, a 70% rise.
References Australian Drinki ng Water Guidelines (1996), NHMRC and AR.MCANZ, Canberra . Arnerican Water Works Association (1996), The Rothberg, Tamburini and Winsor Model for Corrosion Control and Process C hernistry, D enver, C olorado, U SA . Caldwell, D.H. and Lawrence, W .B. (1953), Ill//. Ell,~ - Chem; 45 , 3, 535-548. Rossu rn , J. R.. and M e rrill, O.T. (1983), JA WWA, Feb; 95-100. Singley,J. Edward (1981),JAWWA, N ov; 580.
The Author Peter Gebbie is a Senior Engineer in the Water Industry Group at Fisher Stewart, M elbourne responsible for water and wastewater treatment process and detailed design. Tel: 03 8517 9268. Email: peterg@ fisherstewart.com .a u
Alkali Dosed Parameter
CCPP = (44.3-48.0) = - 3.7 mg/L CaC0 3.
Th e precise va lu e o f CC PP 1s -3.4 mg/ L, suggesting the water is " passive" and has been conditioned to a satisfa ctory level. The assumed lime dose of 6.0 mg/L used in the calcul ations is therefore su fficient. The fin al sum of the cations and an ions can also be determin ed and from this, the
Alkali Dose: pre-treatment, mg/L
WATER MARCH 2001
Water stability indices have been reviewed w ith a focus o n the Langelier Saturation Index and Calciu m Carbonate Precipitation Potential. Methods of calculating these parameters ha ve been outlin ed. An approach to determining the impact that different treatm ent regimes can have on treated water quality and how a treated water can be condition ed to make it less corrosive has also been presented . The te chniqu es outlined are straightforward and can be readily adapted to a PC-spreadsh eet, providing a useful tool to allow water treatment plant optimisation. Water stabil ity indices will no doubt be used more freq u ently in the future as the general trend to improve treated water quality and redu ce plant operating costs continues. Th e methods prese nted in this paper wi ll hop efull y contri bute towards these goals.
Table 8. Comparison of Different Alkalis for Treatment of Rochester Water. Raw Water Quality: true co lour 60 Pt/ Co Units, turbidity 40 NTU, alum dose 50 mg/ L. Disinfection: 1.5 mg/L gaseous chlorine
pH of Conditioned Water
Alkal i dose: post-disinfection, mg/ L
LSI of Conditioned Water
CCPP Cond itioned Water, mg/L CaC0 3
TDS of Conditioned Water, mg/L "as is"
Total Alkali Dose, mg/ L
Operating Cost of Alkal i Dosing, $/ML