MUNICIPAL WASTEWATER 733
loadings are in the region of 100 pounds BOD per 1000 ft^3
filter volume.
Filter packing by plastic media is finding wider appli-
cation. Design criteria, however, call for quite deep filters
and this appears to be uneconomical in terms of power
requirements.
Modifications of the trickling filter process over the
years have dealt with improvements in media, air circula-
tion, and loadings. One of great interest is that proposed by
Ingram. Sewage is introduced at various levels in a very deep
filter in an attempt to distribute the load more uniformly over
the whole filter depth. Hydraulic loadings up to 500 mgad
have been successfully achieved.
A new development in trickling filter technology is the
Rotating Biological Contractor (RBC). A rotating drum is
partially immersed in wastewater. A zoogleal mass devel-
ops on the drum surface, functioning in the same manner as
trickling filter slime. Such installations may be completely
enclosed in plant buildings, thus avoiding any effects of
extremes in outside temperature. An excellent example of
the application of this process for upgrading the municipal
treatment works at North Bergen, New Jersey.
Activated sludge serves the same function as trickling
filter slime. The major difference lies in the filter slime being
fixed to the filter media while the activated sludge is carried
along with the flowing wastewater. Development of the acti-
vated sludge process began with attempts to purify sewage by
blowing air into it. It was observed that after prolonged aera-
tion, flocs composed of voraciously feeding organisms devel-
oped. This floc settled after aeration was stopped. Addition of
fresh sewage to tanks containing the settled sludge produced
high purification in a practical time. The name activated
sludge was assigned this means of waste treatment. At first,
this was operated as a fill and draw system. Research showed
that continuous operation could be practiced and this is the
present means or operation. The process involves:1) return of activated sludge to the aeration tank influ-
ent and discharge of excess sludge to digestion
2) aeration of the sludge-sewage mixture to maintain
purification and
3) settling of the aeration tank effluent to remove
floc before final discharge.Step (3) is necessary for the same reason as the comparable
portion of the trickling filter process-removal of waste mate-
rial transferred to the microbial cell mass. Floc is formed
in the tank through aerobic growth of unicellular and fila-
mentous bacteria. Protozoa and other organisms will also
be found in the floc. This is a strictly aerobic process and
air requirements are high. Two aeration systems are used,
(1) diffused air units and (2) mechanical aeration. Air dif-
fusers are more commonly used in North America but
mechanical aeration systems may be found in plants of less
than 1 million gallons per day (mgd) capacity. Both methods
of aeration perform three functions, (1) transfer of oxygen
to the mixture and maintenance of aerobic conditions,(2) intimate mixing of floc and sewage, and (3) keeping the
floc in suspension.
Aeration tanks are normally rectangular in cross section,
10 to 15 feet deep and 30 feet wide. Length to width should
be greater than 5 to 1 in order to avoid short circuiting.
Detention periods are from 4 to 6 hours. Air is introduced
from diffusers in such a way as to set up a spiral flow pat-
tern, thus aiding in mixing of floc and sewage and helping
to prevent dead spaces in the tank. It was found that oxygen
requirements decreased as the waste proceeded through the
tank. The number of diffusers was, therefore, increased at
the beginning of the unit and decreased at the effluent end.
This is now the common practice and is known as tapered
aeration. Mechanical aeration has the same function as air
diffusers but is accomplished by rotating paddles or brushes.
Peripheral velocity is about 2 ft/sec.
Floc returned to the aeration tank has the purpose as
trickling filter slime but floc concentration can be varied as
operation needs dictate. Returned sludge varies from about
10 to 30%. Mixed liquor suspended solids (MLSS) will
vary from 600 mg/l to 4000 mg/l, on a dry weight basis.
An important parameter in routine process control is the
ratio of the volume of MLSS to the dry weight of MLSS.
This is known as the sludge volume index (SVI) and is in
the range of 50 to 100 in well operating plants. When the
value approaches 200 operating difficulties can be expected.
Factors which promote or inhibit microbiological growth are
important and these include pH, temperature, and oxidation-
reduction potential (ORP). Hydrogen ion potential, pH, will
have a great effect on the dominant species of organisms.
Bacteria predominate above pH of 6.5 while fungi assume
greater importance below this value. There must be adequate
buffering capacity if metabolic products are acidic.
Modifications of the basic activated sludge process have
come about for solution to specific operating problems.
The municipal treatment plant at Peoria, Illinois received
a waste high in carbohydrates. The resulting nitrogen defi-
ciency caused a light and poorly settling activated sludge floc
with attendant poor waste stabilization. Kraus, for whom the
modification is named, aerated digester supernatant. This,
added to the influent, gave a nitrifying activated sludge. The
result was a readily settleable sludge with improved organic
removals.
New York City has plants scattered throughout the five
boroughs, treating more than 1 billion gallons of sewage per
day. A major modification resulting from experimentation
with plant operation has come from this city. In conventional
plants sewage was added at one end of the aeration tank and
allowed to flow through. This gave a high initial microbial
food supply and correspondingly high oxygen requirement.
The New York City modification involves introduction of
sewage at intervals along the tank. This smooths out the food
supply and lowers the oxygen requirements. The sewage is
added at discrete steps along the unit and the name applied
is step aeration.
A low mixed liquor suspended solids concentration of
200 to 500 mg/l is maintained in the high rate process. This
gives a high food to microbial mass ratio. This keeps theC013_007_r03.indd 733C013_007_r03.indd 733 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM