734 MUNICIPAL WASTEWATER
floc in the active growth phase, but excess food will be dis-
charged in the effluent. BOD removals are only 50 to 60%,
but in some areas this is acceptable. New York City applied
this method successfully because of a weak sewage and
low temperatures. Philadelphia and Los Angeles met with
indifferent success because of stronger sewage or higher
temperatures.
A process originated simultaneously and independently
by Smith and Eckenfelder made use of a phenomenon
observed by many researchers but dismissed as experimen-
tal error. When activated sludge and raw sewage are mixed
together in an aeration vessel there is a noted reduction in
BOD, followed by a rise and then another reduction. The
first decrease had been ignored by most research workers.
Smith and Eckenfelder found that this was due to adsorp-
tion of waste material onto the activated sludge floc. This
came from material desorbed from colloidal particles. Plants
at Austin, Texas, and Bergen County, New Jersey were con-
verted from overloaded to underloaded by changing to the
biosorption process.
A recent activated sludge process modification is the
Deep Shaft Process. As its name suggests, two concen-
tric deep shafts (120–150 m) are sunk into the ground.
Wastewater is injected into one of the concentric shafts and
the effluent is withdrawn from the other. A constant ambient
temperature is maintained due to the surrounding geological
formations. Compressed air is injected at the bottom, giving
high dissolved oxygen concentrations and provided intimate
mixing. Waste sludge is removed in clarifiers as in conven-
tional activated sludge plants.
Putrescible material collected from the primary settling
tanks and excess sludge from humus tanks must be disposed
of cheaply and efficiently. This material is highly unstable and
a potential nuisance source. Because it is putrescible it can
be stabilized by biological means, serving as food and energy
sources for microorganisms naturally found in the sludge.
Raw sludge is about 95% water, but the water is not easily
removed. As the sludge is broken down the water content is
lessened, and the volume is markedly reduced. A rough rule
is that sludge volume is reduced by half when water is low-
ered from 95 to 90%, and by two thirds when reduced from
95 to 85%. Fresh sludge has a gray color and can be easily
pumped. Its odor is most disagreeable, being due principally
to mercaptans. Digested sludge is black in color, granular
and has a slight tarry odor.
Sludge digestion is carried out in order to reduce the
volume of sludge to be handled, and reduce the number of
pathogens. Sludge is usually withdrawn at regular intervals
from primary and secondary tanks and led by gravity to a
sludge well. It is then pumped to the digester.
Mixing is very important for efficient sludge digestion.
Temperature is equally important.
Since destruction of sludge is carried on by microor-
ganisms, kinetics of their life processes will be temperature
dependent. It has been found that sludge temperature of about
95°F will give acceptably short detention times. Even shorter
detention times for the same quality of digested sludge can
be achieved with temperatures of about 125–130°F, but thistemperature range is not widely used for reasons of econom-
ics. Above 95°F an increase in detention time is noted, up to
110°F, and then again a decrease. The reason for this is the
changing character of the predominant organisms.
Heating of sludge for efficient digestion is carried out in
one of two ways. The older installations have hot water coils
in the periphery of the tank, and heat is transmitted to the
digesting sludge. Mixing was felt to be adequately effected
by turbulence due to gas generation. Mechanical mixers
have been used. It was found, however, that mixing was not
sufficient. In addition, heating of entire tank contents was
not achieved due to “baking” of sludge in the vicinity of the
heating coils. A second method of sludge heating and mixing
was developed, involving the use of external heat exchang-
ers. Sludge is pumped from the digestion tank through a
heat exchanger and returned to the tank. Two objectives are
accomplished (1) efficient mixing of sludge, thereby reduc-
ing the amount of inadequately digested sludge, (2) more
uniform temperature throughout the tank, thus reducing
digestion time. The use of external heat exchangers has
almost completely supplanted heating coils and internal
mixers in new plant design.
Sludge gas generated during digestion is approximately
72% methane and 28% carbon dioxide. Hydrogen and H 2 S
are present in trace amounts. Gas thus generated has a calo-
rific content of about 600 BTU/ft^3. About 10 ft^3 of gas are
produced per cubic foot of raw sludge digested. Generally,
the amount of sludge gas produced is sufficient to provide
heat used in maintaining digesting sludge at the required
temperature, heating plant buildings, provide hot water and
incineration of digested sludge, when practiced, and fuel and
generators.
Volatile acids, reported as acetic acid, are perhaps the
most important parameters in control of sludge digestion.
Volatile acids below 1000 mg/l occur in a healthy digestion
process. Volatile acids of 6000 mg/l indicate a malfunction-
ing process. pH values of 6.8 to 7.2 are optimum. Values less
than 6.8 usually are due to excessive volatile acid produc-
tion. In the past liming of malfunctioning tank contents was
practiced in an effort to adjust pH to about 7.0. However,
the change in volatile acids production was due to changing
dominant process microorganisms. The lowered pH and high
volatile acids concentrations were a sign of a sick process,
rather than the cause.
Digested sludge is reasonably inert but it must be fur-
ther dewatered and the question of final disposal of raw and
digested sludge is one of the most pressing with which envi-
ronmental engineers must deal today.
Sludge can be dewatered on open or covered drying beds.
Open beds are exposed to the air and drying is accomplished
by drainage and evaporation. Covered beds resemble a
greenhouse. Temperatures are rather high and this aids evap-
oration. In both cases sludge is allowed to flow over sand
beds and let stand for a suitable period. The dried sludge is
then scraped from the beds.
Sludge can be dewatered by vacuum filtration. Filter drums
rotate slowly, picking up wet sludge at the bottom. A slight
vacuum is applied and the water drawn off is returned to theC013_007_r03.indd 734C013_007_r03.indd 734 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM