Page 767 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

722 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

1. Diffused-air aeration or activated-sludge process
TABLE 23.1 proper. In this method of treatment, air is blown
Sampling of Biological Reactors into sewage as it flows through tanks, activated
sludge being added to the incoming sewage and
Suspended Growth Attached Growth
Reactors Reactors Reaction Type settled out in sedimentation basins from the tank
effluent. A part of the sludge produced is fed into
Activated sludge
the influent as ‘‘return sludge,’’ and a part is disposed
Conventional Trickling filters Aerobic
of as ‘‘excess sludge.’’
Plug-flow Bio-filters Anoxic
2. Mechanical aeration or bio-aeration. Mechanical
Complete-mix Rotating biological Nitrification
apparatus is employed in this method, to aerate the
contactors
sewage and keep the tank contents in circulation.
Extended aeration Soil, gravel Anaerobic
Absorption of air takes place from the atmosphere
Tapered aeration Granular activated Sludge
at the surface of the sewage.
carbon digesters
Hatfield process Anaerobic
reactors Metcalf and Eddy go on to describe three distinct functions of
Pasveer oxidation diffused air: (1) to provide oxygen necessary to attain aerobic
ditch conditions and promote the growth of oxidizing organisms,
chiefly bacteria; (2) to cause the activated sludge to move
Hydraulic
through the sewage and provide contact between the gelatin-
Ponds Hybrid Reactors Classification
ous surfaces of the sludge and the organic matter contained
Aerobic Imhoff tanks Continuous-flow
in the sewage; and (3) to prevent deposition of the sludge.
Anaerobic Septic tanks Plug-flow
Figure 23.1 shows two variations of tank sections.
Facultative Leach fields Complete-mix
Table 23.2 illustrates the extent of activated-sludge prac-
Wetlands Land application Batch
tice by 1930. The Houston North Side Plant was built in 1917,
Aquaculture Sequencing batch
only 3 years after the Ardern and Lockett paper. Depths,
Constructed wetlands
widths, aeration times, etc., are essentially the same as current
diffused aeration practice. Air consumption is on an ‘‘air
volume per gallon of sewage’’ basis. Table 23.3 reports the
only. This sludge accumulating in this manner and inducting performance of activated-sludge plants of the day. Parameters
such active nitrification was called ‘‘activated sludge.’’ Hatton measured included suspended solids, BOD, ammonia, and
at Milwaukee, Rank at Baltimore, and others, finally showed nitrates. Plant performances were similar to contemporary
that the process could be operated on a practical, continuous plants of similar designs; actually, the designs have not
basis by running sewage through aeration tanks, the activated changed much in concept. Reactors remain ‘‘plug-flow’’ and
sludge being mingled with the entering sewage, and later are long and narrow with a rectangular cross section, and the
separated from it after its passage through the tanks. spiral flow was used until about 1980. Detention times were
Metcalf and Eddy further reviewed the ‘‘state of the art’’ less than plants since the 1950s, however, and the surface
for the design of activated-sludge reactors. They report on the loading has not been used, probably since the book was
variations (p. 637) as follows: published. A volumetric loading was used by 1950.

Air header
Air header

Diffuser

Diffusers
(a) (b)

FIGURE 23.1 Two variations of conventional activated sludge using diffused air. (a) Ridge and furrow tank and (b) spiral flow tank.
(Adapted from Metcalf, L. and Eddy, H.P., Sewerage and Sewage Disposal, McGraw-Hill, New York, 1930.)