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10.7 Treatment and Disposal or Reuse of Wastewater 351
Treated wastewater is weaker in proportion to the amount of putrescible matter
removed or destroyed. Where emphasis is on water supply, recreational enjoyment of
water, and conservation of fish and other useful aquatic life, dilution or treatment be-
comes more urgent.

EXAMPLE 10.4 WASTEWATER TREATMENT AND DILUTION IN STREAM WATER
2
3
2
The low-water flow of a normally rapid stream draining 2,000 mile (5,180 km ) is 0.1 ft /s/mi 2
2
3
(0.0656 m /min/km ). Estimate the extent to which domestic wastewater from a city of 80,000 peo-
ple must be treated before discharge into the stream if nuisance is to be avoided; also determine the
resulting dilution ratio of wastewater to stream water. Assume a per capita flow of wastewater of
100 gpd 378 L/d.
Solution 1 (U.S. Customary System):
3
1. Low-water flow 0.1 2,000 200 ft /s.
2. Required flow for disposal of domestic wastewater if it is left untreated 6
3
(80,000>1,000)>1.4 340 ft /s.
3. Percent removal of pollutional load needed 80 (340 200)>340 33%.
4. Dilution ratio (80,000 100): (200 7.48 60 60 24 ) 1:16.

Solution 2 (SI System):
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3
2
3
2
1. Low-water flow (0.0656 m /min/km )(5,180 km ) 340 m /min 5.67 m /s.
2. Required flow for disposal of domestic wastewater if it is left untreated 0.17
3
(80,000>1,000)>1.4 9.71 m /s.
3. Percent removal of pollutional load needed 80 (9.71 5.67)>9.71 33%.
4. Dilution ratio (80,000 378>1,000): (5.67 60 60 24) 1:16.

10.7.5 Disposal onto Land
The objectives of terminal discharge of wastewater onto land or into the soil are safe dis-
posal and, possibly, crop production. In municipal practice, disposal by irrigation is viable
when the water resources of the region are poor and large tracts of suitable land are avail-
able. The extent to which wastewater should be treated before irrigation depends on hy-
gienic considerations and available standards. If wastewater is not sufficiently treated,
there is the obvious hazard of contaminating food raised on irrigated soil and infecting an-
imals pastured on irrigated land. By contrast, the discharge of settled wastewater into the
ground through agricultural tile pipes in the disposal of wastewater from isolated
dwellings, known as subsurface irrigation, can be quite safe.
In one sense, shallow earth basins holding wastewater for a number of days and called
wastewater lagoons or stabilization ponds are purposely inundated or waterlogged irriga-
tion areas producing suspended (algal) rather than rooted crops. There is much evaporation
from the ponds and some seepage. In another sense, the displacement of pond waters
is often fast enough to approach the natural purification of sluggish receiving streams.
Under favorable climatic conditions, pond loadings may be as high as 500 persons per acre
(1,235 persons per ha) for raw domestic wastewater.