Environmental Microbiology of Aquatic and Waste Systems

10.3 Systems for the Treatment of Sewage 265

duced and this may sometimes be collected and used
as a source of energy. Figure 10.17 shows the various
processes to which sludge may be subjected and some
anaerobic sludge digester designs.

10.3.2.2 The Septic Tank

Septic tanks are small scale sewage systems not con-
nected to main sewage systems. About 26% use this
system in North America. Both in North America and
in Europe, they are limited to the rural areas. In many
parts of the developing world, however, they are the
main method of disposing of domestic sewage
(Fig. 10.18).
A septic tank generally consists of a tank of between
1,000 and 1,500 gal (4,000–5,500 l) which is con-

nected to an inlet wastewater pipe at one end and to a

septic drain field or soak away or soakage pit at the

other. These pipe connections are generally made via T

pipes which allow liquid entry and outflow without

disturbing any crust on the surface; i.e., direct current

between the inlet and outlet is prevented using baffles

and pipe tees.

In the US, the size of the septic tank to be con-

structed depends on the local authority approving the

construction of the residence. Ultimately, it is based on

the number of persons expected to use the building. In

practice, some authorities base it on the expected flow

of wastewater per day, while others base it on the num-

ber of rooms Table 10.1 shows tank sizes and the

number of persons they should serve.

AEROBIC

Sun

V

V

V

V

V V

V

V

New cells

CO 2 , NH 3 , PO 4 , H 2 O, H 2 S

Bacteria

Algae

O 2

New cells

FACULTATIVE

SETTLEABLE SOLIDS

Bacteria

CH 4 , NH 3 , CO 2 , H 2 S ANAEROBIC

O 2

l

l

ll

l

l

ll

(Winds) (Turbines)

Soluble

components

Raw or partially

treated sewage

Sunlight

Natural

Aeration

Mechanical

Aeration

Fig. 10.16 Scheme depicting the microbiological activity in an oxidation pond (From Okafor 2007. With permission)