Civil Engineering Reference
In-Depth Information
flow, increases or decreases in the filtration rate are accomplished slowly, minimizing
filter surging and the chance for turbidity breakthrough.
The modular unit filter is designed to backwash automatically once a preset filter
headloss is reached, or the operator may override the automatic controls and backwash
the filter manually. During backwash, the material accumulated in the tube settlers is
automatically drained from the unit. Combining backwashing with draining of the tube
settlers for sludge removal eliminates the need for an operator to judge how often or
how much sludge should be wasted from the clarifier. This particular feature simplifies
operation and reduces the required skill level of the operator.
Prior to the end of filter backwash, the drain valve on the tube settler basin is
closed, allowing the remaining backwash water to refill the settling tube compartment.
Upon completion of the backwash cycle, the treatment plant is returned automatically
to service. Operational requirements are only to replenish chemical feeds, establish
proper dosages, conduct routine water-quality tests, and carry out routine daily main-
tenance activities.
Roberts Water Technologies, Inc.—RELIANT System
Steel and stainless steel Reliant treatment units are available in both modular and
packaged (preassembled) designs. The package Reliant units are available with rates
of up to 100 gpm; modular systems are available with rates of up to 700 gpm. Units
utilize mechanical flocculation to mix chemicals and to form a large, settleable floc.
These units include a high-rate, tube settler—equipped clarifier, with an overflow rate
of about 2.5 gpm / ft
2
. The unit also includes an open gravity filter with a design flow
rate of 5 gpm / ft
2
. The filter is supplied with either dual media (12 inches 0.5-mm
sand and 18 inches 1.0-mm anthracite) or tri-media for improved performance. Effluent
from the filter can be by gravity flow or pumped. The unit includes air backwash (2.5
scfm / ft
2
) and water backwash (15-18 gpm / ft
2
). Sludge is withdrawn automatically
from the clarifier through a sludge collection system. An influent-throttling valve that
operates off the water level in the treatment plant controls plant flow. The plant is
designed to operate unattended, requiring only replacement of treatment chemicals and
adjustment of chemical dosage as raw water conditions change. Figure 25-4isa
drawing of a typical Reliant unit and Figure 25-5 is a photograph of a package Reliant
unit.
CONTACT CLARIFICATION-FILTRATION PACKAGE SYSTEMS
A package system developed around 1980 and now available from several manufac-
turers incorporates a high-rate flocculation-clarifier unit with fixed or floating media
followed by mixed or dual media filters. The unit is preceded by coagulant addition
and high-energy mixing. The State of California and others have adopted the term
contact clarification-filtration
to identify this technology.
Studies in California have demonstrated effective treatment in conformance with
the Surface Water Treatment Rule on waters where the median total coliform MPN is
less than 500 per 100 mL and the turbidity is less than 15 NTU.
1
A substantial number
of particle count and organism challenge studies have been completed in California,
with treatment systems designed in conformance with this technology. These studies
in California concluded that ''additional pilot plant studies should not be necessary
