This document discusses sludge bulking, which occurs when sludge has poor settling and compaction characteristics in secondary treatment. Sludge bulking can be caused by mechanical issues, solids characteristics like filaments, or nutrient deficiencies. Filamentous bulking is the most common type and is caused by high levels of filamentous bacteria that interfere with sludge settling. Remedies include adding chemicals to improve settling or identifying and addressing the root causes, such as low dissolved oxygen, nutrients, or organic loading. Selectors may help by providing initial high organic loading to control filament growth.

1. Sludge Bulking
PAPR 3531

2. Bulking Sludge
General definition:
A secondary (biological) sludge with poor
settling and compaction characteristics
Bulking sludges are more of a concern with activated
sludge treatment but also can occur in aerated
stabilization basins

3. Significance
Why care?
 High TSS and associated BOD
(~0.1-0.8 lb BOD/lb TSS) in
effluent, possibly resulting in
a permit violation
 Difficulty in achieving desired
solids in RAS and therefore
difficulty in controlling
activated sludge system

4. Origins of High TSS and/or
Sludge Settling Problems
Equipment design and condition (mechanical
problems):
 Uneven flow splitting (parallel clarifiers)
 Inlet and outlet structures (flow distribution, velocity
dissipation)
 Flocculation chamber
 Weir placement and condition
 Sludge removal mechanism
 Hydraulic overloading or surges
 Solids overloading or surges
 Wind-created or thermal-induced currents

5. Origins of High TSS and/or
Sludge Settling Problems
Problems related to solids
characteristics:
 Pin floc (from high shear/over-aeration,
low F/M, toxicity)
 Dispersed growth (from high F/M, low
nutrients)
 Rising sludge (from anaerobic gases)
 Severe foam or scum (from surfactants,
high F/M or low F/M filaments)
 Viscous bulking
 Filamentous bulking

6. Viscous Bulking
 a.k.a. slime, polysaccharide, hydrous, non-filamentous or
zoogloeal bulking
 Extracellular polymeric substances (EPS) surround
bacterial cells and consist of carbohydrates (i.e.,
polysaccharides), proteins, and other substances
 EPS is critical to ability of bacteria to flocculate, settle,
and dewater
 With viscous bulking, polysaccharide content is higher
than normal, which causes the density of bacteria to be
reduced (poor settling)
 Generally caused by nutrient deficiency (not enough
nitrogen or phosphorus)

7. Filamentous Bulking
The most common of settling and
compaction problems

8. Filamentous Bulking
The culprits: Filamentous
organisms (bacteria, possibly
fungi)
 Their shape is long and narrow
(length >> width)
 They remove BOD (good)
 In small concentrations they provide
physical support to flocs (good)
 At high concentrations they interfere
with settling in secondary clarifiers
(bad)

9. Filamentous Bulking Characteristics
 Poor compaction of sludge blanket (high
blanket level)
 If there is a supernatant, it is usually very clear
 High Sludge Volume Index (SVI)
 Large, strong flocs, but they settle poorly
 Filamentous bacteria predominate, as can be
seen with MLSS sample and a 100x microscope

10. Activated Sludge Under the Microscope
Good floc structure Poor floc structure

11. Sludge Volume Index (SVI)
 It is the volume in mL of one gram of settled sludge
 It is measured with a settling test using a 1-liter
graduated cylinder* and a 30-min. settling time
SVI = (settled volume in mL/L)(1000 mg/g)
initial MLSS in mg/L
 As SVI increases, the likelihood of higher effluent
TSS increases
 SVI > 150 mL/g often defined as bulking sludge, but
a “too high” SVI is treatment-plant specific
* Or larger-diameter “settleometer”

12. MLSS = 3,000 mg/L = 3 g/L
Bulking? Yes (> 150)
Is it a problem?
Settled sludge volume
(SSV30) = 930 mL/L
SVI = 310 mL/g
Is this a bulking sludge?
SVI = (930 mL/L)/(3 g/L)

13. SVI as a Function of Filaments
Palm, J.C.; Jenkins, D.; and Parker, D.S. 1980. Relationship between organic loading, dissolved oxygen
concentration and sludge settleability in the completely-mixed activated sludge process. Journal of the
Water Pollution Control Federation. 52(10):2484-2506.

14. Remedial Measures
Treat the symptom (may be a short-term or
long-term solution)
 Add settling or weighting aids at secondary
clarifier
□ Polymers
□ Lime
□ Ferric chloride
□ Alum

15. Remedial Measures
Treat the symptom (continued)
 Add toxic agents
□ Oxidants (chlorine/hypochlorite,
peroxide)
2 - 10 lb Cl2/day/1000 lb MLSS
□ Proprietary biocides
□ pH shock (usually add acid)

16. Remedial Measures
Eliminate the cause (longer-term solution)
 Specific conditions have been associated with the
proliferation of certain filament types
 Identify probable cause(s)/condition(s) by identifying
the predominant filamentous organisms (identification
requires expertise)
 If necessary, examine or collect other information to
confirm the probable cause(s)
 Make operational and/or equipment changes to correct
the problem

17. Causes of Filamentous Outbreaks
(Conditions Favoring Filaments)
Common causes for activated sludge treatment
plants in the paper industry:
 Septicity (high sulfide and/or volatile organic acid levels)
 Low F/M ratio (less than ~0.2 - 0.3)
 Low dissolved oxygen (less than ~2 mg/L, but a function
of F/M & temperature)
 Nutrient deficiency (nitrogen or phosphorus, or possibly
trace metals)
Richard, M.G. 1997. Recent changes in the prevalence and causes of bulking filamentous bacteria in
pulp and papermill activated sludge systems. TAPPI Proceedings - Environmental Conference &
Exhibition, 553-556.

18. Causes of Filamentous Outbreaks
(Conditions Favoring Filaments)
 Approximate minimum nutrient residuals
(measured before secondary clarifier):
Ammonia (NH4
+): ≥1 mg/L as N
Orthophosphate (PO4
3-): ≥ 0.5 mg/L as P
 Some mill treatment plants with strict permit
limits for phosphorus are able to operate
successfully at lower levels of orthophosphate
(as low as ~0.1 mg/L)

19. Causes of Filamentous Outbreaks
(Conditions Favoring Filaments)
Less common causes for paper industry treatment
plants:
 Low pH (<6.5) or pH swings
 Nature of the BOD (high amount of easily degraded
organic matter)

20. Filament Types in Pulp and
Paper Activated Sludge Systems
FILAMENT TYPE ASSOCIATED CONDITION
More prevalent:
Thiothrix II septicity; low nutrients (N)
Thiothrix I septicity; low nutrients (N)
Nostocoida limicola II septicity
Type 0914 septicity
H. hydrossis low dissolved oxygen
Nostocoida limicola III septicity; low nutrients (P)
Type 1851 low organic loading (low F/M)
Type 1701 low dissolved oxygen
Type 021N septicity; low nutrients (N)
(N) = nitrogen deficiency (P) = phosphorus deficiency

21. Filament Types in Pulp and
Paper Activated Sludge Systems
FILAMENT TYPE ASSOCIATED CONDITION
Less prevalent:
Type 0092 septicity
Type 0411 septicity
Type 0675 low organic loading (low F/M)
Sphaerotilus natans low dissolved oxygen
Type 0041 low organic loading (low F/M)
Type 0581 septicity
Type 0803 low organic loading (low F/M)
Type 0211 septicity
Richard, M.G. 1997. Recent changes in the prevalence and causes of bulking filamentous bacteria in pulp and
papermill activated sludge systems. TAPPI Proceedings - Environmental Conference & Exhibition, 553-556.

22. Eliminating the Cause
Operational and/or equipment changes
might involve:
 Add (more) nutrient
 Increase aeration
 Raise F/M
 Install a selector

23. Selectors
 A selector is an initial contact zone (separate tank or
sectionalized compartment) of short duration (15-60
min. HRT) where primary effluent and RAS are combined
so that the F/M ratio is high and the DO level is
controlled.
 A selector may be aerated (aerobic) or not (anaerobic or
anoxic).
 Selectors are little used in the paper industry (~dozen
mills or so in US). Most are aerobic. Generally seem to
work. NOT suitable solution for low nutrients.

24. This really is a clarifier
Thanks to Paul Klopping of Callahan and Brooks for permission to
use most of the photographs that appear in these lecture notes.