Page 327 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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8.7 Dual Water Supply Systems 287

Table 8.3 Specific Speed versus Leakage Losses for Double
Suction Pumps in Example 8.11
Specific Speed Leakage Losses in
S % of Power Input
500 9.5%
1,000 4.3%

1,500 2.6%
2,000 1.8%

2,500 1.4%
3,000 1.2%
3,500 1.0%

For Pump B:
0.5
S 3,600 (200) >260 0.75 786
Pump B would have an inherent loss of about 6%.

Solution 2 (SI System):
For Pump A:
0.5 0.75 0.5 0.75
S 51.7 NQ /H sv 51.7 1,900 (0.2145) >54.9 2,255
Pump A’s expected inherent loss would be about 1.6%
For Pump B:
0.5
S 51.7 3,600 (0.0126) >79.2 0.75 786
Pump B’s expected inherent loss would be about 6%.

8.7 DUAL WATER SUPPLY SYSTEMS
The term dual water supply systems refers to two water supply distribution systems in a
city. One is a freshwater system for potable use; the other system is for lower quality water
(seawater or untreated raw freshwater, or treated/reclaimed wastewater) for firefighting
and toilet flushing purposes. In Hong Kong and Saudi Arabia, dual water supply systems
have been in use for more than 40 years.
Water supply networks are sized so that they can provide the large flows needed for
firefighting, but this creates potable water quality problems due to the long time water can
spend in the network. What is needed, argues Okun (2007), is a switch to dual systems,
with potable supplies provided through a smaller bore network using a material such as
stainless steel, allowing reclaimed water to be used in existing networks.
Note: This section is based on a timely article written by Dr. Daniel A. Okun a few
months before he passed away in 2007. The article appeared in the February 2007 issue of
Water 21, a magazine of the International Water Association, pages 47–49; reproduced
courtesy of IWA Publishing.