NET SLUDGE YIELDS OBTAINED DURING SINGLE-STAGE
NITRIFICATION STUDIES AT CHICAGO'S
WEST-SOUTHWEST TREATMENT PLANT
Alan W. Obayashi, Research Chemist III
Booker Washington, Sanitary Chemist II
Cecil Lue-Hing, Director
Department of Research and Development
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois 60611
INTRODUCTION
The Metropolitan Sanitary District of Greater Chicago (MSDGC) has for the past
several years conducted full-scale single-stage nitrification studies of each of its three
major plants, namely the Calumet, the North Side and the West-Southwest treatment
plants.   These studies were conducted in order to establish design criteria for the expansion of the major plants, which will have to achieve ammonia removal as well as meet
stricter effluent water quality criteria in terms of 5-day biochemical oxygen demand
(BOD) and suspended solids (SS).   In particular, studies were conducted of the District's
West-Southwest (W-SW) treatment plant, the world's largest secondary treatment plant.
The results of the study, which will be reported on in a subsequent paper [ 1 ], proved
to be quite interesting.
In general, provided that there was sufficient dissolved oxygen in the aeration tank, the
key to successful nitrification during   the winter months at W-SW was found to be the
maintenance of a solids retention time (SRT) which was long enough to promote the
growth during the colder temperatures of the relatively slow-growing autotrophic bacteria
which utilize the ammonia as an energy source.   With the assumption that the total mass
of suspended solids in the activated sludge system was equal to the suspended solids under
aeration, (i.e., no solids in the clarifers),  unstable nitrification (30% to 70% removal of
NH4-N) was found to occur at SRTs of 3 to 4 days at wastewater temperatures of
11-13 C.   In contrast, operation at SRTs in the range of 6 to 10 days at the same temperatures provided NH4-N removals in excess of 90%, with effluent NH4-N values being
generally less than 1 mg/1.
The above results were not entirely unexpected, since theoretically, in order to maintain a culture of nitrifying bacteria in the activated sludge process, the process must be
operated such that the net growth rate (1/SRT), i.e., the discharge of suspended solids
per unit time divided by the total SS in the system, must not exceed the maximum
growth rate of the nitrifying bacteria, otherwise the bacteria will "wash out" of the
system.   Since the growth rate is equal to the reciprocal of the SRT of the system, in
order to maintain successful nitrification the SRT must be significantly greater than (or
growth rate significantly lower than) the SRT at which the growth rate is maximum at a
given temperature.   This is assuming that dissolved oxygen and pH are not limiting.   In
order to evaluate how much greater the SRT must be, Lawrence and McCarty [2] have
introduced the following concept of the safety factor (SF):
6g
SF = —
0m
759