Environmental Engineering FOURTH EDITION

200 ENVIRONMENTAL ENGINEERZNG

wastewater treatment systems. Such land treatment systems have been used in Europe
for many years and have recently been introduced in Noah America. They appear
to represent a reasonable alternative to complex and expensive systems, especially
for smaller communities. Irrigation is probably the most promising land treatment.
Depending on the crop and the soil, about 1000 to 2000 ha of land are required for
every cubic meter per second of wastewater flow. Nutrients like N and P that remain
in secondarily treated wastewater are usually beneficial to crops.

CONCLUSION

A typical wastewater treatment plant, shown schematically in Fig. 9-21, includes pri-
mary, secondary, and tertiary treatment. The treatment and disposal of solids removed
from the wastewater stream deserve special attention and is discussed in the next
chapter.
Figure 9-22 is an aerial view of a typical wastewater treatment plant. Well-operated
plants produce effluents that are often much less polluted than the receiving waters into
which they are discharged. However, not all plants perform that well. Many waste-
water treatment plants are only marginally effective in controlling water pollution, and
plant operation is often to blame. Operation of a modem wastewater treatment plant

Solids

treatment

& disposal

From town

Pdmary

(physical)

Bar Screen

Commlnutor

Grit chamber

A

Settling

Aemtlon

Secondary

(Aeroblc

biological)

Final

Disposal Solids Settllng J Tostream

I

L---t

To stream

Figure 9-21. Block diagram of a complete wastewater treatment plant.