Filamentous Micro-organisms and the
Bulking of Activated Sludge
M. S. FINSTEIN, Assistant Professor
H. HEUKELEKIAN, Head
Department of Environmental Sciences
Rutgers, the State University
New Brunswick,  New Jersey
INTRODUCTION
It is a widely held impression that the proliferation of filamentous microorganisms can cause the bulking of activated sludge. This impression is based
largely on the observation that filaments are often very conspicuous in bulky
sludges (1). Pearse et al (2) in a review of the problem, favored the explanation
that filamentous micro-organisms cause the"Sphaerotilus" type of bulking, and to
the present time most investigators appear to concur in this judgment. However,
a minority viewpoint holds that the proliferation of filaments accompanies but
does not cause bulking (3,4).
Despite sustained interest in the subject, there appears to have been no direct
investigation of the relationship between activated sludge settling and the quantity
of filaments present in sludges. The present investigation was undertaken to provide quantitative data with which to assess the influence of filamentous microorganisms on the settleability of activated sludges.
MATERIALS AND METHODS
Aeration tank mixed liquors from four activated sludge sewage treatment
plants located in New Jersey were examined during 1964. One plant (Mb) treats
the wastes from a state institution, whereas the other three plants (Mc, Bv, and
Mt)receive domestic sewage; plant Mt also receives some industrial wastes. Plant
Mt normally operates with a mixed liquor suspended solids concentration of about
1,300 mg/l and a detention time of 1.5 hrs; the others are conventional plants
with detention times of approximately six hrs.
Samples of mixed liquor were removed near the outlet end of the aeration
tanks. Total suspended solids were determined using Gooch crucibles; in seven of
the samples values ranged from 1,300 to 1,740 mg/1, and in the eighth sample to
2,320 mg/1. The standard procedure (5) was followed for the determination of
sludge volume index (SVI).
A method for estimating the hyphal mass in soil, described by Pramer and
Schmidt (6), was modified for the present investigation. Mixed liquors were d i -
luted three- to 10-fold with distilled water and 0.10 ml portions spread uniformly
over a 10 cm area of a microscope slide and allowed to dry at room temperature.
Four slides were prepared for each sample. Using a Bausch and Lomb Tri-Simplex
Micro-projector, the image of the materials on the slides was projected horizontally onto white paper (magnification 460x). Floes and filaments were clearly
visible and staining was not required.. Fields were selected at random and in any
given field the outline of all the activated sludge units (floes with their associated
filaments) were traced onto paper. Approximately 100 units from each slide and
a minimum of 400 units from each sample of activated sludge were traced, which
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