HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 181

tissue. The most common type of aerobic system is the suspended-growth treat-
ment system. Suspended-growth technologies are conventional activated sludge-
treatment systems that use various process modes, sequencing batch reactors, and
anaerobic treatment reactors.
In attached-growth biological systems, the microorganisms are attached to a
supporting media and form a biofilm on the surface of the media. The media
provide a large surface area for attached growth. The media also immobilize
and retain the biomass in the reactor so the solid retention times can be longer
than hydraulic retention times. The media consist of crushed stones, plastic, tile,
or slag. After a prolonged time period, microorganisms in the biofilm become
old and die. The dead microorganisms can no longer hold on to the media,
and the biofilm detaches itself from the media. This is called sloughing .The
packing of the media may be packed (down-flow) or fluidized (up-flow). The
most common types of attached-growth systems are trickling filters and rotating
biological contactors.

3.1 Activated Sludge

An activated sludge is a suspended-growth system in which a pretreated wastew-
ater is aerated to promote the growth of microorganisms (biomass) that slowly
consume the organic compounds in the wastewater. The microorganisms accli-
mate to the specific mix of compounds present in the wastewater and therefore
use up the organic portion significantly. The activated-sludge process consists
of a reactor in which the microorganisms responsible for treatment are kept in
suspension by mixing and aeration; and a final clarifier where liquid-solids sep-
aration occurs. Biomass that is captured in the clarifier is recycled back to the
reactor, depending on the configuration of the reactor. Contact time between
waste and biomass is also controlled by wasting excess biomass. Equalization is
necessary for wastes that have variable characteristics to eliminate the inhibition
of bacteria. Equalization can be achieved by using storage tanks before pumping
the waste to the biological reactor.
Floc formation in the aeration tank is a very important feature of the acti-
vated sludge process. These flocs will form flocculent settable solids that can
be removed by gravity settling in the final clarifier (Eckenfelder and Musterman
1995). The clarification allows for a clear effluent, as well as serves an additional
microorganism source for the process. The process diagram of activated sludge
is shown in Figure 6.2.
Activated sludge is commonly applied to municipal and industrial wastewater
treatment, with several variations of the two basic process configurations. The
selection of the “best” process depends on the waste characteristics. What follows
are the process design considerations.
Activated sludge exists in two general operating modes: (1) complete mix
with or without solids recirculation or (2) plug flow with or without solids