Civil Engineering Reference
In-Depth Information
rus, and places where large quantities of metal salts are added. However, these clarifiers
can be used in more typical applications based on an appropriate analysis. DensaDeg
units are in service at plants in Wilmington, Delaware, treating river water, and in
Richfield, Minnesota, softening well water.
Actiflo Process
The Actiflo high-rate clarification system, illustrated in Figure 11-25, is in service in
several drinking water plants in the United States and is widely used in Europe. There
are separate rapid mixing, flocculation, and sedimentation compartments in the Actiflo
process. The process utilizes microsand to enhance flocculation and settling. Particles
adhere to the microsand and are removed from a center hopper in the sedimentation
compartment. Settled solids are pumped to a hydrocyclone, where the microsand is
separated and reused. The lower-density sludge is discharged from the top of the
hydrocyclone.
The first full-scale Actiflo plant in the United States was put in service in May
1998 at the North Table Mountain Water Treatment Plant in Golden, Colorado. There
are two Actiflo units in parallel that operate at a nominal overflow rate of 23,000 gpd/
ft
2
(39 m / h). Alum is the primary coagulant at a dosage of 15-25 mg / L. Polymer is
also used. The Actiflo system replaced a conventional coagulation, flocculation, and
sedimentation plant. The clarified water in the conventional plant was 4-6 NTU. After
nine months of operation, the Actiflo clarified effluent was 0.4-0.7 NTU.
FLOTATION
Introduction
Flotation processes have been used for solid-liquid separations since the early 1900s.
The main types of flotation are electrolytic flotation, dispersed-air flotation, and dis-
solved-air flotation (DAF). These processes differ mainly in how the gas bubbles are
produced. Dissolved-air flotation is by far the most common of these methods in the
drinking water industry and is the focus of this section.
While DAF is still an emerging technology for drinking water treatment in North
America, the process is well accepted in other parts of the world, including the United
Kingdom, the Netherlands, Belgium, South Africa, and the Scandinavian countries.
However, dissolved air flotation is commonly used for sludge thickening in wastewater
plants in the United States. The first application of DAF for potable water treatment
was in the early 1960s in South Africa, and other systems were built shortly thereafter
in Scandinavia.
17
In Finland, there are presently over 30 DAF plants serving about 20
percent of the country's population.
18
By 1987, contracts for more than 25 DAF plants
had been awarded in the United Kingdom.
17
The first DAF plant in the United States was built in 1982 in Lenox, Massachusetts.
There now are over a dozen full-scale DAF plants in operation in the United States.
19
Pilot studies for several large U.S. and Canadian cities have recommended including
DAF in the treatment schemes.
20,21
General Description
Dissolved-air flotation is used as a clarification process in drinking water treatment,
providing an alternative to sedimentation in a conventional treatment plant and an
