Environmental Engineering Reference
In-Depth Information
Primary
settling
Secondary
settling
Trickling
filter
Recirculating
Primary
settling
Secondary
settling
Trickling
filter
Recirculating
Primary
settling
Secondary
settling
Trickling
filter
Recirculating
figure 7.5 Common forms of recirculation.
Recirculation is used to reduce organic loading, improve sloughing,
reduce odors, and reduce or eliminate filter fly or ponding problems.
The amount of recirculation is dependent on the design of the treatment
plant and the operational requirements of the process. Recirculation
flow may be expressed as a specific flow rate (e.g., 2.0 MGD). In most
cases, it is expressed as a ratio (e.g., 3:1, 0.5:1.0). The recirculation is
always listed as the first number and the influent flow as the second
number. Because the second number in the ratio is always 1.0, the ratio
is sometimes written as a single number (dropping the “:1.0”)
Flows can be recirculated from various points following the filter to
various points before the filter. The most common form of recirculation
removes flow from the filter effluent or settling tank and returns it to the
influent of the trickling filter as shown in Figure 7.5.
Maintenance requirements include lubrication of mechanical equip-
ment, removal of debris from the surface and orifices, adjustment of flow
patterns, and maintenance associated with the settling tank.
7.3.5 general Process description
The trickling filter process involves spraying wastewater over a
solid media such as rock, plastic, or redwood slats (or laths). As the
wastewater trickles over the surface of the media, a growth of microor-
ganisms (bacteria, protozoa, fungi, algae, helminthes or worms, and lar-
vae) develops. This growth is visible as a shiny slime very similar to the
