Page 61 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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24 Chapter 1 Introduction to Water Systems
1.10.2 Individual Small Supplies

The term individual describes those situations in which the needs and amenities of water
supply and wastewater disposal are normally satisfied by relatively small and compact sys-
tems individually owned, developed, and operated, and kept within the property lines of
the owner. Normally, this implies construction of wanted or required systems through indi-
vidual rather than community effort. But there have been developments for villages and
communities with scattered buildings in which local government has taken the initiative
and assumed responsibility for construction and care of individualized systems. Property
owners, as well as the community, then enjoy the benefits of adequate planning, design,
construction, management, and supervision. Otherwise, unfortunately, necessary works
are rarely designed by qualified engineers and often end up not satisfying their purposes,
both in a sanitary and an economic sense.
Reasonably good results can be obtained (a) if engineering departments of central
health authorities publish manuals of design, construction, and operation that fit local
conditions, and (b) if they give needed advice and supervision as well as provide for regu-
lation. Nevertheless, villages and fringe areas are best served, in the long run, by the exten-
sion of central water lines and sewers, or by incorporation of water and sewer districts
comprising more than a single unit of local government.

1.11 INDIVIDUAL WATER SYSTEMS
Because of the natural purifying capacity and protection of the soil, individual and rural
water supplies are generally drawn from springs, infiltration galleries, and wells. Where
groundwater is highly mineralized or unavailable, rainwater is next best in general safety
and quality. Only in uninhabited and well-protected upland areas should ponds and
streams be tapped without purifying the waters drawn.
Some of the safeguards for groundwater works are illustrated in Figs. 1.11 to 1.13.
They share the following features in common:
1. Diversion of surface water from intake structures
2. Drainage of overflow or spillage waters away from intake structures
3. Water tightness of intake works for at least l0 ft (3 m) below the ground surface
and, if necessary, until the aquifer is reached
4. Prevention of backflow into intakes; where there is no electric power, water is
pumped by hand, wind, water, or gasoline engines

Individual and rural water supplies are not without their purification problems.
Gravity and pressure filters are employed to improve waters of doubtful purity, and zeolite
softeners and other ion-exchange units are used for the removal of unwanted hardness.
Iron-bearing groundwaters that issue from their source sparklingly clear but become rusty
on exposure to air (by oxidation and precipitation of iron) are best treated in manganese-
cation exchange units. Hexametaphosphates may keep iron from precipitating, but this re-
quires skillful management. It may be advisable to seek an iron-free source instead.
Some soft groundwaters containing much carbon dioxide are highly corrosive.
Passage through marble or limestone chips takes calcium into solution and reduces the car-
bon dioxide proportionately. Hardness is increased, but corrosiveness is decreased. For the
chlorination of polluted rural supplies, there are solution-feed dosing devices that propor-
tion the amount of added chlorine to flow. Instead, the householder may prefer to boil his
drinking and culinary water. Investment in an inherently safe and satisfactory supply, how-
ever, is usually wisest in the long run.