Page 333 - Industrial Wastewater Treatment, Recycling and Reuse

Urban Wastewater Treatment 305

only level of treatment, the clarified wastewater is disinfected and then dis-
charged into the receiving water body.
In enhanced primary treatment (also called advanced primary treatment) che-
micals are added to the sedimentation tanks to help waste particles bond
together and settle out more readily.

7.3.1.2 Secondary Treatment
Secondary treatment typically involves biological treatment of wastewater
usually following the primary treatment stage. Some secondary treatment
plants do not include the primary treatment process. Naturally occurring
bacteria break down the organic components of wastewater, and additional
settling occurs. These solids are either reused in the biological process or
removed for further treatment and disposal. If secondary treatment is the
final level of treatment, the clarified wastewater is disinfected and then dis-
charged into the receiving water body.

7.3.2 Urban Wastewater: Tertiary Treatment Options

The technical factor is often considered as least important for the success of
water reuse projects as today’s available technologies make it possible to reach
any water quality required by users and for regulatory compliance. The range
of technologies that can be applied is broad, from the well-proved standard
ones to the newest advanced types (IAEA, 2007). However, the final choice
will be strongly dependent upon local conditions, plant size, and water quality
standards. An adequate choice, which means the selection of the most
appropriate or “the best available” technology in a given situation, plays a
key role for its future reliable operation and provides the guarantee of having
a suitable water quality at lower operation and maintenance costs.
To achieve an adequate disinfection performance, in which fecal indica-
tors are to be kept below the detection limits, it is recommended to imple-
ment a pretreatment to decrease the concentration of suspended solids. This
is of special importance if UV light is used as disinfection agent. The com-
bination of UV with other disinfection agents provide a greater reliability
and higher efficiency for inactivation of different types of microorganisms.
During the last decades, membrane treatment has been approved as one
of the best available technologies for the production of high-quality recycled
water for indirect potable reuse and industrial applications. Among the
membrane processes used for wastewater treatment, MBR technology
is advancing rapidly worldwide. Microfiltration and ultra filtration are
becoming the preferred pretreatment options over reverse osmosis (RO).