Page 394 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch10_333-356.qxd 8/2/10 9:02 PM Page 354

354 Chapter 10 Introduction to Wastewater Systems
2. For activated-sludge plants similarly USD 55.2, 21.0, 11.4, 7.2, and 5.3 for com-
4
3
6
5
munities of 10 , l0 , 10 , and 10 people, respectively
3. For conventional trickling-filter plants USD 21.0, 7.8, and 4.5 for communities of
3
5
4
10 , l0 , and 10 people, respectively
3
4. For high-rate trickling-filter plants USD 27.6, 8.4, and 4.4 for communities of 10 ,
4
5
l0 , and 10 people, respectively.
Including interest and depreciation, as well as charges against operation and manage-
ment, the removal of domestic wastewater and its safe disposal costs from USD 300 to
USD 600/MG (USD 80 to USD 160/ML). In comparison with water purification plants,
wastewater treatment works are relatively twice as expensive; in comparison with water
distribution systems, collection systems for domestic wastewater are about half as expen-
sive. Sewer use charges, like charges for water, can place the cost of sewerage on a value-
received basis. Use charges may cover part or all of the cost of the service rendered and are
generally related to the water bill as a matter of equity.

EXAMPLE 10.6 COSTS OF SANITARY SEWERAGE AND TREATMENT PLANT
Roughly, how much money is invested in the sanitary sewerage system of a city of 100,000 people?

Solution:
1. Assuming the per capita cost of sewers at USD 450, the total first cost is
USD 450 100,000 USD 45,000,000
2. Assuming that the wastewater is treated in an activated sludge plant, the expected cost is
4 1>4
282 P>(P 10 )
4 1>4
5
5
282 l0 >(10 l0 )
5
282 10 >10 1>4
USD 15,600,000
No general costs can be assigned to separate treatment of industrial wastewaters. When they
are discharged into municipal sewerage systems, treatment costs can be assessed in terms of loads
imposed on the municipal works as suspended solids, putrescible matter, or a combination of the
two.

10.10 INDIVIDUAL WASTEWATER SYSTEMS

In the absence of public sewerage, wastewaters from individual and rural dwellings and
ancillary buildings are normally discharged into the ground. The absorptive capacity of the
soil is then of controlling importance. It is greatly increased if settleable waste matter is
first removed, for example, by sedimentation combined with digestion and consolidation
of the deposited sludge and scum. Sedimentation and digestion are accomplished more
often than not either in leaching cesspools or in septic tanks (or tight cesspools).
Subsurface absorption fields or seepage pits follow. An individual wastewater system in-
cluding a septic tank and absorption field are shown in Fig. 10.12.