DISCUSSION
CHAPTER 7 DISCUSSION
7.1 WATER USAGE
7.1.6 REDUCING WATER USAGE
Optimal values for rinse criteria for cleaner, pre- and post plating rinses l62 (see Table 7.8) as well as concentration levels163'l64'l65 (see Table 7.9) which should not be exceeded in the final (cleanest) rinse solution have been published in the literature. However the rinse criteria are given as a broad range of values while published concentration limits are scarce. Further the concentration of metals in the exiting (flowing rinse) wastewater must be within the local authority's discharge limits.163 The metal concentration in the cleanest rinse should also be below the suggested quality criteria for the contaminant limit.164 The contaminant limit has been defined as the concentration of that metal that is acceptable in the rinse water because it
has no negative effect on the plate. Table 7.9 gives the effluent discharge limits for the metals for which published contaminant limits were also found.
Table 7.8 Optimal values for rinse criteria
Rinsing process
Post degreasing and pickling Pre electroplating
Post electroplating
Post decorative chromium plating
Dilution factor 100-1000 500-2000 1000-5000 5000-10000
The dilution factor found for the nickel in the plating solution (in terms of its optimal concentration) and the final rinse tank of the nickel rinse system (Tank 13) was 1.2xl04 and
1.4xl04 for the two days' analyses results. These values exceed (by a factor of about 2.6) the top of the range literature value for the dilution factor required in post plating rinses (see Table 7.8). Similarly in the final rinse tank of the chromium plating rinse system (Tank 18) the dilution factor for chromium range from 2.3x104 to 1.7xl05 over the monitoring period. The dilution factors measured for the chromium plating operations were found to be greater (by a factor of between 2 and 17) than the literature value for the dilution factor for post decorative chromium plating as given in Table 7.8.
Table 7.9 Effluent discharge and contaminant concentration limits values
Substance Cr03
Calculated as Cr Ni
cr so*-
Total metals Zn
Cu Fe Pb
Total dissolved solids Conductivity
PH
Effluent discharge limit163 Not specified Not specified
5 mg/L 500 mg/L 250 mg/L 20 mg/L
5 mg/L 5 mg/L 5 mg/L 5 mg/L 500 mg/L 4000 uS/cm
6.5 to 10
Contaminant limitm-165 40 mg/L
19.2 mg/L 8 mg/L 50 mg/L 200 mg/L
5 mg/L Not specified Not specified Not specified Not specified
250 mg/L 200 uS/cm
6.0 to 8.0
The Sewage Disposal Bylaws issued by Durban Metro's Department of Waste Management only specifies a total chromium discharge limit {Cr(VI), Cr(III), etc} of 5 mg/L when the
effluent is going into a small sewage works. A small sewage work is defined as a plant which deals with less than 25 ML of effluent per day. The Sewage Disposal Bylaws require that the sum of the concentrations of the individual metal ions does not exceed 20 mg/L for a small sewage works.163 All these concentration limits are greater for a large sewage works.
Tables 7.10a and 7.10b presents the data used in estimating the potential financial savings from reducing the volume of water used by the flowing rinses following the nickel and chromium plating solutions. Such calculations could not be performed on the other two rinse systems because these are also used by the other plating processes in the Chrome Shop.
Table 7.10a shows the water volume that can be saved by halving the flow rate in both flowing rinse systems. This (and not the static or reactive rinses) is considered to be the part of the rinsing process where water usage can be reduced. This is based on the finding that the dilution factors calculated using the optimum process solution concentration and the measured concentration in the final rinse solution are about double the top of the range literature value for the dilution factor given in Table 7.8 (dilution factor of 5000 for nickel and 10000 for chromium) for good rinsing. The projected annual water used in the flowing rinse is obtained by multiplying the new flow rate by the number of working hours per year (5616 hours) and the annual volume of water saved is estimated to be (4156m3). The annual water savings (R19949) is equal to the difference between the annual water used in the flowing rinse and the projected annual water used in the flowing rinse.
Table 7.10a An estimate of annual water savings for the plating processes
Rinse system
Ni Cr
Annual water used in the flowing rinse (m3)
3369 1798
New flow rate (m3/hr)
0.1 0.08
Projected annual water used in the flowing rinse
562 449
Annual water savings m
2807 1349
R 13474
6475
Table 7.10b considers the rinsing and dilution in two steps namely that taking place in the drag-out and that obtained by the flowing rinse. The first dilution factor describes the situation between the process solution and the drag-out (or reactive rinse in the case of chromium) and the second dilution factor describes the situation between the drag-out and the final solution of
the flowing rinse. The dilution factor between the nickel plating solution and the nickel drag- out was calculated using the average drag-out concentration over the two monitoring days and the specified concentration of 82.5 g/L of nickel in the plating solution. Equation 2.6 given on page 66 in Chapter 2 was used to approximate the value for the average drag-out volume (D in Table 7.10b). The measured flow rate (Q in Table 7.10b), the average concentration in the final rinse (CFR in Table 7.10) and average drag-out concentration (Co in Table 7.10b) were used in Equation 7.4 to calculate the drag-out volume (D in Table 7.10). The values for Q are given in Table 6.5 in Chapter 6 and the average values CFR and CD are calculated from the analytical results for nickel and chromium in Tables 6.9 and 6.10. The new flow rate (QN in Table 7.10b) was then estimated using the calculated drag-out volume (D) and a dilution factor value calculated for the flowing rinse by dividing 5000 by the dilution value achieved by the static rinse. The volume of water (1291 m3) used in the flowing rinse has been estimated by multiplying the new flow rate by the number of working hours per year (5616 hours). It is assumed in this calculation that the drag-out volume will stay the same despite changes in solution concentration. The symbol "n" in Table 7.10 and in Equation 7.4 represents the number of stages in the flowing rinse system. The nickel rinse system has a three stage flowing rinse and the chromium rinse system has a two stage flowing rinse.
i
Equation 7.4
^(N) _ J CD
D FR J
Table 7.10b An estimate of annual water savings for the plating processes Rinse
system Ni Cr
Q m3/hr
0.2 0.16
Average Cm (mg/m3)
0.0064 0.0036
Average Co mg/m3
3.6 1.6
D m3/hr 8.1xl03 3.8 x ! 0J
Dilution factor
217 105
m3/hr 0.15 0.08
Annual water savings
m3 842 449
A comparison of the flow rate results for obtained for the nickel and for the chromium rinses from Table 7.10a and 7.10b show a good correlation for chromium and a higher (by 33%) value for nickel using the method based on Equation 7.4. However as the rates were measured on site using a crude (bucket and stop watch) method this is probably not such a bad result after all.