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344 Chapter 10 Introduction to Wastewater Systems
10.7 TREATMENT AND DISPOSAL OR REUSE OF WASTEWATER

The sewerage system is a simple and economical means of removing unsightly, putresci-
ble, and dangerous wastes from households and industry. However, it concentrates potential
nuisances and dangers at the terminus of the collecting system. If rivers and canals, ponds
and lakes, and tidal estuaries and coastal waters are not to become heavily polluted, the
load imposed on the transporting water must be unloaded prior to its disposal into the re-
ceiving bodies of water. The unloading is assigned to wastewater treatment plants to pre-
vent (a) contamination of water supplies, bathing places, shellfish beds, and ice supplies;
(b) pollution of receiving waters that will make them unsightly or malodorous and eutroph-
ication of ponds and lakes; (c) destruction of food fish and other valuable aquatic life; and
(d) other impairment of the usefulness of natural waters for recreation, commerce, and in-
dustry. The required degree of treatment before disposal depends on the nature and extent
of the receiving water and on the regional water economy.
In the treatment of wastewater before reuse in irrigation, full recovery of the water
value is intended together with as much recovery of fertilizing value as is consistent with (a)
avoiding the spread of disease by crops grown on wastewater-irrigated lands or animals pas-
tured on them; (b) preventing nuisances such as unsightliness and bad odors around disposal
areas; and (c) optimizing, in an economic sense, wastewater disposal costs and agricultural
returns. The design of the irrigation areas themselves is based on the nature and size of
available lands and the purposes they can serve in the regional agricultural economy.
As countries grow and their waters are used more widely and intensively, the dis-
charge of raw or inadequately treated wastewater into their streams, lakes, and tidal waters
becomes intolerable. The daily load of solids imposed on domestic wastewater amounts to
about half a pound per person. The resulting mixture of water and waste substances is very
dilute—less than 0.1% solid matter by weight when wastewater flows are 100 gpcd
(378 Lpcd). Some industrial wastewaters are far more concentrated. Floating and other
suspended solids render wastewater and its receiving waters unsightly; settling solids build
up sludge banks; organic solids cause wastewater and its receiving waters to putrefy; and
pathogenic bacteria and other organisms make them dangerous.

10.7.1 Wastewater Treatment Processes
Waste matter is removed from transporting water in a number of different ways. In munic-
ipal wastewater treatment works of fair size, the following processes and devices are com-
mon (see Fig. 10.8):
1. Bulky floating and suspended matter is strained out using racks and screens that
produce rakings and screenings. Cutting racks and screens comminute the rakings
and screenings in place and return them to the wastewater.
2. Oil and grease are skimmed off after rising during quiescence in flotation tanks,
producing skimmings.
3. Heavy and coarse suspended matters are allowed to settle to the bottom of stilling
chambers, such as grit chambers, flotation tanks, settling tanks, or sedimentation
basins, producing grit, detritus, or sludge.
4. Suspended matter and dissolved solids that do not settle are converted into set-
tleable solids and become amenable to sedimentation by flocculation and precipi-
tation with chemicals. Chemical flocculation or precipitation tanks are used to pro-
duce sludge precipitates.
5. Colloidal and dissolved organic matter is metabolized and converted into a set-
tleable cell substance by biological growths or slimes. The hosts of living cells that