Page 38 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 1
Introduction to Water Systems
EXAMPLE 1.6 ESTIMATION OF WATERWORKS COST
Roughly, what is the replacement cost of a conventional filtration plant and other waterworks for a city of 100,000 people and what
is the average plant flow?
The following conditions are assumed:
a. A per capita first cost of $1,500 in 2008.
b. Thirty percent of the first cost is to be invested in the collection works, 10% in the purification works, and 60% in the distribution
works.
c. A water consumption rate of 150 gpcd (568 Lpcd) for the city.
Solution 1 (US Customary System):
1. Assuming a per capita cost of $1,500, the total first cost is 1,500 × 100,000 = $150,000,000.
2. Assuming that 30% of this amount is invested in the collection works, 10% in the purification works, and 60% in the
distribution works, the breakdown is as follows:
Collection works 0.3 × 150,000,000 = $45,000,000.
Purification works 0.10 × 150,000,000 = $15,000,000.
Distribution works 0.60 × 150,000,000 = $90,000,000.
3. Assuming a water consumption rate of 150 gpcd, the total water consumption of the city is 150 × 100,000 gpd = 15 MGD.
Solution 2 (SI System):
1. $1,500 × 100,000 = $150,000,000.
2. Same as Solution 1.
Collection works 0.3 × 150,000,000 = $45,000,000.

Purification works 0.1 × 150,000,000 = $15,000,000.

Distribution works 0.6 × 150,000,000 = $75,000,000.

3. Assuming a water consumption of 568 Lpcd, the total water consumption of the city is 568 × 100,000 Lpd = 56.8MLD.
Of the various system components, collection and trans- disposal are normally satisfied by relatively small and com-
portation works cost about a fourth, distribution works pact systems individually owned, developed, operated, and
slightly less than a half, purification and pumping works kept within the property lines of the owner. Normally, this
about a tenth, and service lines and meters nearly a sixth implies construction of wanted or required systems through
of the total. The initial cost of conventional water filtration individual rather than community effort. But there have
plants is about $1,500,000 per MGD ($396,000 per MLD) been developments for villages and communities with scat-
capacity, varying with plant size as the two-thirds power of tered buildings in which local government has taken the
the capacity. The cost of water treatment, excluding fixed initiative and assumed responsibility for construction and
charges, lies in the vicinity of $420 per MG ($111 per ML), care of individualized systems. Property owners, as well
varying with plant output capacity inversely as the two- as the community, then enjoy the benefits of adequate
fifths power of the daily production. Including interest and planning, design, construction, management, and supervi-
depreciation as well as charges against operation and mainte- sion. Otherwise, unfortunately, necessary works are rarely
nance, water costs $300 to $3,000 per million gallons ($80 to designed by qualified engineers and often end up not sat-
$800 per million liters) and is charged for accordingly. As isfying their purposes, both in a sanitary and an economic
one of our most prized commodities, water is nevertheless sense.
remarkably cheap—as low as 12 cents a ton delivered to the Reasonably good results can be obtained if (a) engineer-
premises of large consumers and as little as 24 cents a ton to ing departments of central health authorities publish manuals
the taps of small consumers. of design, construction, and operation that fit local condi-
tions and (b) they give needed advice and supervision as well
as provide for regulation. Nevertheless, villages and fringe
areas are best served, in the long run, by the extension of
1.10.2 Individual Small Supplies
central water lines and sewers or by incorporation of water
The term individual describes those situations in which and sewer districts comprising more than a single unit of
the needs and amenities of water supply and wastewater local government.