Sludge Bulking: Polysaccharide (slime)
versus filamentous bulking
WHAT IS A BULKING SLUDGE?
By definition, a bulking sludge is a sludge that has a sludge volume index (SVI) of >150 mL/gram. In
practical applications, the SVI for each plant where problems of solids loss over the clarifier weirs is site
specific due to hydraulics and solids loading rates. In simple terms, if the sludge doesn’t settle well in the
clarifier, it may be a bulking sludge.
WHAT ARE THE CAUSES FOR SLUDGE BULKING?
 Fats, Oil, and Grease (FOG)
 Low Dissolved Oxygen
 Low F/M Ratio
 Nutrient Deficiency
 Septicity (Organic Acids and Sulfide)
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Reference Richard et al
POLYSACCHARIDE (SLIME) BULKING
Don’t assume that poor sludge settling is caused by filamentous
bacteria. Non-filamentous bulking (slime bulking) can due to
proliferation of zooglea bacteria, which have a thick slime capsule,
or the buildup of polysaccharide under nutrient limited conditions.
In these conditions, chlorination of the RAS line is not
recommended as this can produce serious foaming problems (can
make problems worse, not better). Note that zooglea and nutrient
deficiency can both cause problems due to excessive polysaccharide
but they are caused by separate things and there is no relation in
the material that we reference and our practical experience.
Zooglea bacteria do not grow due to nutrient deficiency. Nutrient
deficiency is caused by an improper balance of BOD (food) to
macronutrients (nitrogen and phosphorus) and very rarely due to
lack of micronutrients (iron, and other trace elements). In municipal
wastewater systems, nutrient deficiency is almost never observed
unless there is a severe degree of industrial contribution or unless
too much phosphate is precipitated through primary treatment with
addition of iron salts. Zooglea bacteria favor higher F/M ratios and grow due to certain alcohols and
organic acids in the wastewater.
FILAMENTOUS BULKING
Filamentous bulking is caused by the proliferation of
bacteria (filaments) that outcompete floc forming bacteria.
The causes for filamentous bulking can be viewed on the
previous page. Filamentous bacteria can be identified and
with careful evaluation using ranking systems such as Dr.
Michael Richard’s scoring system, the cause of the root
cause of the bulking sludge problem can be identified.
Generally, filamentous bulking occurs once filamentous
abundance reaches very common or greater (>15 filaments
per average floc). Note that floc strength is very important;
strong flocs may be able to support higher amounts of
filaments and not cause bulking, while weak flocs do not
have the weight and strength, and thus the SVI can be
elevated at lower filament abundance in these situations.
Filamentous bulking can occur within the floc, where weak
and diffuse flocs are created, and also can occur as
filamentous bacteria bridge the flocs. In situations where
inter-floc bulking is occurring, it is often not possible to view
these filaments without use of a phase contrast microscope.
SUMMARY
Filamentous bulking and polysaccharide (slime) bulking are common causes of poor settling sludge.
Other possibilities for poor settling may include hindered settling due to denitrification (there is an
upward force working against settling in which nitrogen gas wants to float the sludge), or hindered
settling due to the mixed liquor concentration being too high. If your sludge is not settling as it should, it
is recommended to send out a sample for microscopic evaluation to a trained professional to determine
the cause of bulking, and also the root cause of the problem. Once this information is obtained, short
term and long term control strategies can be evaluated.
REFERENCE

3rd Edition Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other
Solids Separation Problems, D. Jenkins. M.G. Richard, G. T. Daigger, 2003