Описание
cells, and particulate materials make up the “fine” solids in the activated sludge
process.
Colloids are relatively large and complex molecules that vary in size from 1 to
100 nm. These molecules do not dissolve in wastewater. Because of their large
surface area, colloids remain suspended in the wastewater. Examples of colloids are
proteins that are found in domestic, dairy, and meat processing wastes.
Dispersed cells consist of microscopic unicellular organisms such as algae, bacteria, and fungi that are suspended in the wastewater because of their buoyant
nature or motility. These organisms may be found as individual cells or as small
aggregates of cells. Dispersed growth is considered to be <5mm in size and consists
mostly of bacterial cells.
Particulate materials are inert or nonliving wastes and are considered to be ≥5mm
in size. Particulate materials may be found in a variety of colors and shapes, and
much particulate material remains suspended in the wastewater because of its
buoyant nature. Particulate materials that do not settle out in the primary clarifier
enter the activated sludge process. Particulate materials such as cellulose, an insoluble starch, may be degraded in the activated sludge process if sufficient hydraulic
retention time (HRT) (Appendix I) is provided in the aeration tank.
Heavy metals commonly enter most activated sludge processes in the soluble
form. Examples of soluble heavy metals include the metal ions copper (Cu2+
), nickel
(Ni2+
), and zinc (Zn2+
). The activated sludge process quickly and efficiently removes
heavy metals.
Fine solids and heavy metals are removed from a waste stream by the activated
sludge process. Their removal is achieved through their adsorption to bacterial cells
and the activity of ciliated protozoa and metazoa (animals) or higher life forms
(Figure 1.4). The most commonly occurring metazoa in the activated sludge process
are rotifers and free-living nematodes.
The ability of the activated sludge process to degrade BOD and remove fine
solids and heavy metals is achieved primarily through the growth and maintenance
of a large, diverse, and active population of bacteria. The growth in numbers and
diversity of bacteria occurs over time or increasing mean cell residence time
(MCRT) or sludge age (Appendix I) as BOD is transformed into new bacterial cells
or sludge. The bacterial population is maintained in the activated sludge process
through the development of firm and dense mature floc particles (Figure 1.5). The
development of floc particles is known as floc formation.
Two important characteristics of the floc particle determine the treatment efficiency of the activated sludge process to degrade BOD and remove fine solids and
heavy metals. These characteristics are the activity of the floc bacteria and the structure of the floc particle (Figure 1.6). Because these characteristics determine treatment efficiency, they also affect operational costs and compliance with state and
federal permit discharge limitations.
Activated sludge processes that have two or more aeration tanks may be
designed to operate in a variety of schemes or modes of operation. The feed points
of primary clarifier effluent or activated sludge influent and return activated sludge
(RAS) determine the mode of operation of an activated sludge process. The RAS
refers to the solids removed from the secondary clarifier and returned to the aeration tanks. Modes of operation include complete mix, plug-flow, step feed, and
contact stabilization (Figure 1.7).
THE ACTIVATED SLUDGE PROCESS 7
Детали
- Год издания
- 2002
- Format