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10. Write a one-page summary of the attached paper? INTRODUCTION Many problems can develop in activated...

10. Write a one-page summary of the attached paper?


Many problems can develop in activated sludge operation that adversely affect effluent quality

with origins in the engineering, hydraulic and microbiological components of the process. The

real "heart" of the activated sludge system is the development and maintenance of a mixed

microbial culture (activated sludge) that treats wastewater and which can be managed. One

definition of a wastewater treatment plant operator is a "bug farmer", one who controls the

aeration basin environment to favor good microbiology.

This paper will discuss the types of microbiological problems that can occur in activated sludge

operation. These include dispersed (non-settleable) growth, pin floc problems, zoogloeal

bulking and foaming, polysaccharide ("slime") bulking and foaming, nitrification and

denitrification problems, toxicity, and filamentous bulking and foaming. The best approach to

troubleshooting the activated sludge process is based on microscopic examination and oxygen

uptake rate (OUR) testing to determine the basic cause of the problem or upset and whether it

is microbiological in nature. These methods are easy, fast and inexpensive compared to other

approaches, and are generally understandable and accepted.


Poor Floc Formation, Pin Floc and Dispersed Growth Problems

Basic floc formation, required for activated sludge operation due to the use of gravity clarifiers,

is due to a growth form of many species of natural bacteria. Floc-forming species share the

characteristic of the formation of an extracellular polysaccharide ("slime") layer, also termed a

glycocalyx. This material, which consists of polysaccharide, protein and sometimes cellulose

fibrils, "cements" the bacteria together to form a floc. Floc formation occurs at lower growth

rates and at lower nutrient levels, essentially starvation or stationary growth conditions.

Floc-forming species may grow in a dispersed and non-settleable form if the growth rate is too

fast. This latter condition, termed dispersed growth, occurs rarely in domestic waste activated

sludge operation but occurs often in industrial waste treatment, generally due to high organic

loading (high food to microorganism ratio (F/M) conditions). Here, no flocs develop and

biomass settling does not occur, resulting in a very turbid effluent. The correct remedial action

for a dispersed growth problem is a reduction in the F/M of the system, usually done by raising

the MLSS concentration. Dispersed growth problems often occur after a toxicity or hydraulic

washout event when the activated sludge biomass is low and high F/M conditions prevail.

Small, weak flocs can be formed in activated sludge that are easily sheared and subject to

hydraulic surge flotation in the final clarifier leading to a turbid effluent. These small flocs,

termed pin floc, consist only of floc-forming bacteria without a filament backbone and usually


are <50um in diameter. Pin floc occurs most commonly at starvation conditions -- a very low

F/M and long sludge age. Chronic toxicity can also cause a pin floc condition.

Free floating filaments can, at times, cause a dispersed growth problem. Here, the cause is

filament-specific and is the same as for filamentous bulking (discussed below).


Toxic shocks can be a severe problem in activated sludge operation. In a recent study, toxicity

upset was experienced by approximately 10% of 25 Colorado activated sludge plants examined

during one year. Toxicity problems were found to be a larger problem in small communities

compared to larger cities, due to the lack of dilution of toxic releases in small systems.

Examples of toxicity events were the washing of cement or lime trucks to a manhole, dumping of

congealed diesel fuel to the sewer system, and overload of small systems with septage (which

contains a high amount of organic acids and sulfides which can be toxic).

Sulfide toxicity to activated sludge is more common than currently recognized. Sulfide may

originate from outside the activated sludge system, from septic influent wastewater or from

septage disposal, or it may originate "in-house", from anaerobic digester flows or from aeration

basins or primary or final clarifiers with sludge build-up and anaerobic conditions. Hydrogen

sulfide toxicity is highly pH dependent, due to the H2S form being the toxic agent and not HS-.

The pKa for H2S is 7.0, indicating higher toxicity at a pH of 7 or less when H2S is

predominant, and less toxicity as the pH increases above pH 7 and H2S dissociates. One mg/L

of H2S reduces the activated sludge OUR by 50% at pH 7, and the H2S dose to give a 50%

OUR reduction increases to 100 mg/L at pH values above pH 8. It is advised to add lime or

other alkaline agent to the aeration basin to raise the pH to 7.5 or above if sulfide toxicity is


Toxicity can be diagnosed microscopically, often in the following sequence:

1. an initial flagellate "bloom";

2. subsequent complete die-off of protozoa and other higher life forms;

3. biomass deflocculation, often accompanied by foaming;

4. loss of BOD removal; and

5. filamentous bulking upon process recovery.

Toxic wastes generally do not favor filaments directly (except in the case of H2S); rather, upset

conditions allow filaments to proliferate. For example, bulking by Sphaerotilus natans

frequently follows a toxic upset due to a high F/M condition. Here the "true" F/M value may be

many-fold that calculated based on total biomass present, due to low viability of the biomass.

While microscopic observations can diagnose toxicity after the fact, a better method is use of

the OUR test to detect toxicity early.


The OUR of an activated sludge fed increasing amounts of a nontoxic waste will initially rise

with increasing waste additions to the test bottle, followed by no further increase in OUR with

even higher waste additions. In contrast, the OUR of an activated sludge fed a toxic waste may

increase initially with increasing waste strength, but will decrease rather dramatically at waste

additions above a toxicity threshold value. A useful definition of microbial "death" is when the

fed OUR is less than the basal endogenous OUR.

The OUR test is simple (all that is required is as a BOD bottle and a dissolved oxygen probe)

and usually takes less than two hours to perform. The normal OUR of the activated sludge must

be known before hand, so run this test periodically to know what is normal for your plant.

Nitrification and Denitrification Problems

Nitrification can create problems in activated sludge operation. Many plants experience an

upset condition with dispersed growth and filamentous bulking every spring when warmer

temperatures induce nitrification. Some plants experience a loss of chlorine disinfection during

nitrification onset, due to a transient period (weeks) of nitrite build-up. Nitrite has a significant

chlorine demand (one part nitrite consumes one part chlorine) while ammonia and nitrate do not.

A large problem in some plants is a low pH (to as low as pH = 6) caused by extensive

nitrification and low wastewater alkalinity. This often causes pin floc and high effluent turbidity.

Some plants reduce aeration to reduce nitrification or add soda ash, lime or magnesium

hydroxide as a source of alkalinity if this becomes a problem. The use of lower dissolved

oxygen concentration (1.0 mg/L or less) to control nitrification is not without the risk of inducing

filamentous bulking by low dissolved oxygen filaments.

Another problem caused by nitrification is denitrification. Here, bacteria common in the

activated sludge floc respire using nitrate in place of free oxygen when it is lacking and release

nitrogen gas as a by-product. This gas is only slightly soluble in water and small nitrogen gas

bubbles form in the activated sludge and cause sludge blanket flotation in the final clarifier. An

indication of the occurrence of denitrification can be obtained by holding the sludge in the

settling test jar for several hours. If the sludge rises ("pops") within 2 hours or less, denitrification

problems may be occurring. Denitrification problems are more prevalent during the warmer

times of the year and can be more severe if a filamentous sludge is present, due to more

extensive entrapment of the nitrogen gas bubbles by a filamentous sludge.

Control of denitrification is either by control of nitrification (reduced sludge age or reduced

aeration); or by reducing denitrification by removing the sludge faster from the final clarifier

(increased RAS rates) or by increasing the dissolved oxygen concentration in the final clarifier.

This can be done by increasing the aeration basin dissolved oxygen concentration especially at

the clarifier end of the aeration basin. One method useful in severe cases is the addition of

hydrogen peroxide as an oxygen source directly to the center well of the final clarifier.

Nitrification and denitrification problems can be particularly troublesome in industrial waste

systems where ammonia is supplemented. Here, inorganic nitrogen (ammonia or nitrate) must be


present in the aeration basin at all times to allow proper treatment and to avoid filamentous or

slime bulking but must be kept below approximately 5 mg/L to avoid nitrification-denitrification

problems (low pH and floating sludge). The common practice of batch addition of nutrients to

the aeration basin often leads to denitrification problems due to periods of high nitrate

concentration (above 5 mg/L).

A number of industries, particularly papermills, have experienced a frothy, floating sludge in the

aeration basin. This can lead to a significant amount of the sludge inventory in the foam,

compromising process control. This problem occurs in systems with a high front-end organic

loading and a long hydraulic detention time (2 days or more). Nitrification and denitrification

occur at the back end of the system due to endogenous conditions there and the release of

ammonia from the biomass. Nitrification and denitrification often occur together within the floc,

with no finding of free nitrate when examined.

Nutrient Deficiency and Polysaccharide Bulking and Foaming

Nitrogen and phosphorus can be growth limiting if not present in sufficient amounts in the influent

wastewater, a problem with industrial wastes and not domestic wastes. In general, a

BOD5:N:P weight ratio in the wastewater of 100:5:1 is needed for complete BOD removal.

Other nutrients such as iron or sulfur have been reported as limiting to activated sludge, but this

is not common.

Extracellular polysaccharide is produced by all activated sludge bacteria and is, in part,

responsible for floc formation. Overproduction of this polysaccharide can occur at nutrient

deficiency (and also oxygen deficiency or high F/M) which builds up in the sludge (it is poorly

degraded) and leads to poor sludge settling, termed "slime bulking", and to problems in sludge

dewatering. Normal activated sludge contains from 10 to 20% polysaccharide on a dry weight

basis with the higher polysaccharide content occurring at younger sludge ages. Sludges with

polysaccharide content above 20% may have settling and dewatering problems (values to 90%

have been observed with some nutrient deficient industrial waste sludges).

Signs of nutrient deficiency include: filamentous bulking; a viscous activated sludge that exhibits

significant exopolysaccharide ("slime") when "stained" with India ink; and foam on the aeration

basin that contains polysaccharide (which has surface active properties). One check for nutrient

deficiency is to be sure that some ammonia or nitrate and ortho-phosphate remain in the effluent

at all times. The recommended effluent total inorganic nitrogen (ammonia plus nitrate) and

ortho-phosphorus concentrations are 1-2 mg/L to ensure sufficient nutrients. Note that total

Kjeldahl nitrogen and total phosphorus are not used, as these may contain organically-bound

nutrients, not rapidly biologically available ("bug bodies").


Zoogloea Bulking and Foaming

A special case related to slime bulking is zoogleal bulking. Here, fingered zoogloea proliferate in

activated sludge to the extent that sludge settling is hindered. Zoogloea overgrowth also causes

reduced sludge dewatering. The responsible organism is Zoogloea ramigera, the "classical"

floc-former. Here, large masses of this dendritic floc-former may physically interfere in sludge

settling and compaction similar to filamentous bulking.

Zoogloea occur at high F/M conditions and when specific organic acids and alcohols are high in

amount due to septicity or low oxygen conditions. Note that the sludge polysaccharide values

as measured by the anthrone test are normal (10-20%) even when zoogloea are high in amount,

due to the particular types of biopolymers formed by these bacteria (amino-sugars that don't

react in the anthrone polysaccharide test). The anthrone test is a good way to separate a

zoogloea overgrowth problem from a low nutrient polysaccharide problem.

Filamentous Bulking

Filamentous bulking and foaming are common and serious problems in activated sludge

operation, affecting most activated sludge plants at one time or another. Filamentous bulking is

the number one cause of effluent noncompliance today in the U.S.

An understanding of filamentous bulking and foaming, the causative filaments and their causes

and control, has steadily increased over the past 20 years since Eikelboom and van Buijsen

published their filament identification system in 1981 (Eikelboom and van Buijsen, 1981). This

approach to filament identification has been updated and modified by Jenkins et al. (1993,

2003) and has become used worldwide. Once the causative filaments could be identified, at

least to a recognized type, their causes could be determined and control measures appropriate

to each filament found.

A bulking sludge is defined as one that settles and compacts slowly. An operational definition

often used is a sludge with a sludge volume index (SVI) of >150 ml/g. However, each plant has

a specific SVI value where sludge builds up in the final clarifier and is lost to the final effluent,

which can vary from a SVI <100 ml/g to >300 ml/g, depending on the size and performance of

the final clarifier(s) and hydraulic considerations. Thus, a bulking sludge may or may not lead to

a bulking problem, depending on the specific treatment plant's ability to contain the sludge within

the clarifier.

A certain amount of filamentous bacteria can be beneficial to the activated sludge process. A

lack of filamentous bacteria can lead to small, easily sheared flocs (pin-floc) that settle well but

leave behind a turbid effluent. Filaments serve as a "backbone" to floc structure, allowing the

formation of larger, stronger flocs. The presence of some filaments also serves to catch and hold

small particles during sludge settling, yielding a lower turbidity effluent. It is only when filaments

grow in large amounts (approximately 107 um filaments per gram of activated sludge) that

hindrance in sludge settling and compaction occurs. In concept, bulking can be envisioned as


the physical effects of the filaments on the close approach and compaction of the activated

sludge flocs. Depending on the type of filament involved, two forms of interference in sludge

settling occur: (1) interfloc-bridging - where the filaments extend from the floc surface and

physically hold the floc particles apart; and (2) open-floc structure - where the filaments grow

mostly within the floc and the floc grows around and attached to the filaments. Here, the floc

becomes large, irregularly-shaped, and contains substantial internal voids. The untrained

observer often overlooks this latter type of bulking.

A bulking sludge can result in the loss of sludge inventory to the effluent, causing environmental

damage and effluent violations. In severe cases, loss of the sludge inventory can lead to a loss

of the plant's treatment capacity and failure of the process. Additionally, disinfection of the

treated wastewater can become compromised by the excess solids present during bulking. In

less severe cases, bulking leads to excessive return sludge recycle rates and problems in waste

activated sludge disposal. Many problems in waste sludge thickening are really filamentous

bulking problems.

The true incidence of bulking in the U.S. is unknown but has been estimated to affect at least

60% of plants, either continuously or intermittently. Recent work in Colorado suggests that at

least 90% of activated sludge plants experience a bulking episode at least once during the year.

Bulking may be one of the main reasons why approximately 50% of U.S. activated sludge

plants don't consistently meet their effluent discharge standards.

Early microbiological investigations into filamentous organisms found in activated sludge were

hampered by a lack of knowledge concerning the types of filaments that may occur. Usually,

Sphaerotilus natans was diagnosed, often without adequate identification. However, it is now

known that approximately 25 different filamentous bacteria commonly occur in activated sludge

and each may lead to operational problems. D.H. Eikelboom in Holland (Water Research

9:365, 1975) provided a rational basis to "identify" the different filamentous bacteria found in

activated sludge. This identification system is based on filament characteristics as viewed under

phase contrast microscopy for live samples (in situ) and two simple staining reactions: the Gram

and Neisser stain. Each filament can be "classified" using a four-digit code, avoiding the earlier

problems of lack of specific scientific names. This is important as many of the filaments found in

activated sludge have not been isolated in pure culture and hence their identity remains

unknown. As these filaments are isolated and properly named (a current research thrust),

generic names replace the four digit number code. Hence, the current list of filaments is a hybrid

between numbers and genus names. Currently there are 24 recognized filaments (or groups of

related filaments in some cases) that cause activated sludge bulking or foaming. These are given

in Table 1.


Table 1. Recognized Filaments That Cause Activated Sludge Bulking or Foaming

Sphaerotilus natans Microthrix parvicella*

type 1701 Nocardia spp.**

Haliscomenobacter hydrossis Nostocoida limcola I, II & III

type 021N type 0961

Thiothrix I and II type 0581

Beggiatoa spp. type 0092

type 0914 type 0411

type 0041 type 1863**

type 0675 fungi

type 1851 actinomycetes

type 0803


* this filament causes both bulking and foaming.

** these filaments cause foaming only.

Causes for almost all of the different filaments are now known (there is always a need to

improve this information). Filament causes have been determined using three separate

approaches. First, a number of filaments have been isolated in pure culture and their

competitive growth abilities examined in laboratory studies. Many of these studies are

summarized by Jenkins et al. (1993; 2003). This approach has been successful for S. natans,

type 1701, Haliscomenobacter hydrossis, type 021N, Thiothrix I and II, and Microthrix


Second, the author has microscopically examined and identified the filaments in over 10,000

activated sludge samples over the past 20 years. This extensive database has been analyzed for

positive and negative statistical associations between the different filaments. This has resulted in

a number of positive associations between filaments of known and unknown causes, establishing

a probable cause for the filament of unknown cause. Alternately, a filament of unknown cause

may be negatively associated with a filament of known cause, indicating that these filaments do

not share a common cause.

Third, practical experience at trial and error successful control methods in plants with a bulking

or foaming problem has shown the cause for some filaments not found in the above approaches.


A summary of the conditions that cause filament growth and the filaments associated with each

of these conditions is given in Table 2. There are six environments or growth conditions that

cause the overgrowth of filaments in activated sludge. Four of these occur in municipal

wastewater systems while all six occur in industrial wastewater systems, with two specific only


to industrial systems (low nutrients and low pH). Many of the filaments have been associated

with other causes in the past, but recent work has indicated the causes given in Table 2 as the

primary reason for their growth. Other modifying conditions may apply to some filaments, and

these are discussed below.

Table 2. Causes of Filament Growth in Activated Sludge

Cause Filaments

1. Low Dissolved Oxygen Sphaerotilus natans

Concentration type 1701

Haliscomenobacter hydrossis

2. Low F/M type 0041

type 0675

type 1851

type 0803

3. Septicity type 021N

Thiothrix I and II

Nostocoida limicola I,II,III

type 0914

type 0411

type 0961

type 0581

type 0092

4. Grease and Oil Nocardia spp.

Microthrix parvicella

type 1863

5. Nutrient Deficiency

Nitrogen: type 021N

Thiothrix I and II

Phosphorus: Nostocoida limicola III

Haliscomenobacter hydrossis

Sphaerotilus natans

6. Low pH fungi


Note that H. hydrossis was previously listed as a low F/M filament. This filament is caused by

low DO, but grows relatively slowly and only occurs at lower F/M and a longer sludge age.

Lower F/M is not its cause, only where it occurs.


Six specific causes of filament growth and bulking are currently recognized (see Table 2). The

information in Table 2 is now used in reverse to the way that it was developed -- from the

identification of the most significant filaments present in a bulking sludge, the "cause" for such

growth can be determined. Note that some filaments have more than one cause as shown in

Table 2. The combination of conditions listed may favor bulking by a particular filament more

so than any single condition. It is important to perform filament identification early in a bulking

episode to identify the causative filament. Once bulking continues for some time, process upset

can lead to the proliferation of other filament types (secondary filaments) which can confuse

diagnosis of the real cause.

Today, many activated sludge plants regularly monitor the occurrence and abundance of

filaments in their sludge, which has become an important process control tool. This often leads

to "heading off" a bulking episode before it becomes serious. Since the microbial population in

activated sludge changes slowly in most cases, generally requiring 2-3 sludge ages to radically

change, this microscopic observation needs to be performed only at weekly intervals. However,

during a period of bulking onset or during application of remedial actions such as chlorination,

daily observation of the activated sludge is warranted.

Filamentous Foaming

A brief review of activated sludge foams and their causes is given in Table 3. Use of

microscopic examination can readily diagnose most of these, particularly when filaments are


Table 3. Description and Causes of Activated Sludge Foams

Foam Description Cause(s)

thin, white to grey foam low cell residence time or "young" sludge

(startup foam)

white, frothy, billowing foam once common due to nonbiodegradable

detergents (now uncommon)

pumice-like, grey foam (ashing) excessive fines recycle from other

processes (e.g. anaerobic digesters)

thick sludge blanket on the final clarifier(s) denitrification

thick, pasty or slimy, greyish foam

(industrial systems only)

nutrient-deficient foam; foam consists

of polysaccharide material released

from the floc

thick, brown, stable foam enriched in filaments filament-induced foaming, caused by

Nocardia, Microthrix or type 1863


Three filamentous organisms can cause activated sludge foaming: Nocardia and Microthrix

parvicella (commonly), and type 1863 (rarely). Nocardial foaming appears to be the most

common and occurs at approximately 40% of activated sludge plants in the U.S.

Nocardial foam occurs as a thick, stable, brown foam or "scum" inches to many feet thick on

aeration basin and final clarifier surfaces. Normal scum traps (too small) and water sprays (too

weak) may be useless to control this type of foam. This foam consists of activated sludge solids

(flocs) containing large amounts of Nocardia filaments growing from their surface and is quite

stable, compared to most other foams, due to the physical "interlocking" of the Nocardia

filaments. These foams are easy to diagnose microscopically - they are dominated by branched,

Gram positive filaments and a simple Gram stain of the foam is all that is needed. The analysis

should include comparison to the underlying MLSS (prepare both samples for Gram staining on

the same slide). A true Nocardial foam will contain 10-100 fold more Nocardia than the

underlying MLSS. Nocardial foams also contain substantial lipid concentrations (hexane

extractable), up to 40% of dry weight versus 5-10% for Nocardia-free activated sludge solids

(whether this lipid content of foams is due to the Nocardia themselves or to entrapped grease

and fat is not clear). In addition, these foams contain significant entrapped air, with a bulk

density of approximately 7 g/cc.

Nocardial foams occur in all types of plants, with no particular association with specific modes

of operation or aeration. These foams may be more severe in plants with fine bubble or jet

aeration and in oxygen activated sludge plants. These foams also occur equally in plants treating

domestic, industrial and mixed wastes. Industrial wastes promoting Nocardia growth (and

foaming) include dairy, meat and slaughterhouse, food processing, pharmaceutical, and any

others that contain a significant amount of grease, oil or fat. Nocardial foaming is also associated

with high-density restaurant operation in recreational areas (e.g. ski resorts and summer camps).

Nocardial foaming has been observed to be caused by treatment of locomotive and truck

washing wastes.

Severe Nocardial foams cause a number of operational problems. These include aesthetics,

odors, and safety hazards if they overflow basins to cover walkways and handrails. In cold

weather these foams can freeze, necessitating "pick and shovel" removal. Foam may escape to

the effluent, increasing effluent suspended solids and compromising disinfection. In covered

aeration basins, foam can accumulate to exceed the available hydraulic head for gravity flow of

wastewater through the basin. Process control can be compromised if a significant fraction of a

plant's solids inventory is present in the non-circulating foam (e.g. up to 40% of the total solids

inventory can be present in such foams and process control calculations may not be correct).

There should be some concern expressed for the handling of Nocardial foams. The most

common Nocardia species found in such foams, such as N. amarae, are not pathogenic to

laboratory animals; however, other less frequently isolated actinomycete strains are known

opportunistic human pathogens (e.g. N. caviae, N. brasiliensis, N. asteroides and strains of

Mycobacterium). No actual infection has been documented, however, treatment plant workers

and nearby residents may be at risk.




The start of any problem solving has to involve microscopic examination of the activated sludge.

This reveals whether the problem is, or is not, caused by filaments. If caused by filaments, and

most are, identification of the causative filament(s) yields a direction or approach to take for the

remedy, as shown in Table 2.

Although a myriad of solutions to bulking have been used (some involving witchcraft), several

methods are most practical and proven. These include both short term (treating the symptoms)

and long term (treating the cause) changes in operation.

Short Term Control Methods

Short term measures include: "sludge juggling" - changes in return activated sludge (RAS) rates

and in waste feeding points; polymer and coagulant addition to aid sludge settling; and


Sludge Juggling

Several methods useful for intermittent bulking problems, but which will not solve a chronic

problem, are manipulation of RAS flow rate and manipulation of waste feed points to the

aeration basin to minimize the adverse effects of a bulking sludge.

It should be obvious that one must remove solids from the final clarifier faster than they are

added. Therefore, the RAS flowrate must be increased in a bulking situation to prevent loss of

solids to the effluent. There is a limit to the increase in RAS flowrate as the increased return flow

to the system hydraulically pushes more sludge from the clarifier, making effluent TSS losses

worse. Some operators report success in bulking control by holding sludge in the clarifier for

lengthy time periods. This may work for some filaments (probably by creating septic and toxic

conditions), however, in other cases it may worsen the problem (for example, by encouraging

sulfide-oxidizing filaments).

A reduction in solids loading to the clarifier can be achieved by a reduction in the system's

sludge inventory (a reduction in the aeration basin MLSS concentration). However, this may be

detrimental by actually encouraging filament growth (discussed later). A change to step feeding

of wastes, where possible, can reduce the MLSS concentration in the clarifier feed without

reducing the system's sludge inventory. Here, the MLSS concentration is highest at the head

end of the aeration basin (a form of sludge storage) and is decreased in the clarifier feed, thus

reducing clarifier loading due to MLSS dilution with wastes. This redistribution of solids in the

system usually takes less than one day.


Polymer and Coagulant Addition

There exist several methods of chemical addition to enhance activated sludge settling. Most

used are synthetic, high molecular weight, anionic polymers alone or in combination with cationic

polymers that serve to overcome the physical effects of filaments on sludge settling. These are

usually added to the MLSS as it leaves the aeration basin or to the secondary clarifier center

well. Use of polymer does not significantly increase waste sludge production but can be quite

expensive, up to $450. per million gallons treated (obviously this is only used if absolutely

necessary). A polymer supply company should be consulted for advice on selection of a

polymer and its dosage (the chemical composition of most polymers is a trade secret). Jar

testing should be performed to determine the type of polymer needed and its dosage, which is

quite plant specific. Further, this jar testing needs to be repeated often, as the needed polymer

and its dosage can change, particularly if the filament type(s) change.

In some instances, inorganic coagulants/precipitants such as lime or ferric chloride can be

beneficial. These produce a voluminous precipitate that sweeps down the activated sludge,

improving settling. Sludge production may be significantly increased if these are used. The

weighting action of inert biological solids has also been used to aid sludge settling in activated

sludge modifications such as the Hatfield or Kraus processes that recirculate anaerobic digester

contents through the aeration basin. Some papermills intentionally release fiber or clay to the

wastewater system to help sludge settling during a bulking episode.


Two toxicants, chlorine and hydrogen peroxide, have been used successfully to control

filamentous organisms and stop a bulking episode. Chlorine is most widely used as it is

inexpensive and available on-site at most plants, and only this will be discussed here.

Chlorination for bulking control is widespread, used by more than 50% of plants.

The goal of chlorination is to expose the activated sludge to sufficient chlorine to damage

filaments extending from the floc surface while leaving organisms within the floc largely

untouched. Filamentous and floc-forming bacteria do not appear to significantly differ in their

chlorine susceptibility. Chlorine dosage is adjusted such that its concentration is lethal at the floc

surface but is sublethal within the floc, due to chlorine consumption as it penetrates into the floc.

This is analogous to "peeling an orange" and removing the filaments attached to its surface. It

should be pointed out that chlorination is not a cure-all for all activated sludge microbiological

problems. Chlorination will actually make problems worse if the problem is non-filamentous,

e.g. slime bulking or poor floc development.

Chlorine can be applied from a chlorinator using chlorine gas feed or as a liquid hypochlorite. A

separate chlorinator should be dedicated to bulking control and an independent rotameter and

sampling point in this chlorine line is needed. The chlorine addition point is of most importance

and should be at a point where the sludge is concentrated, raw wastes are at a minimum, and at


a point of good mixing. Poor initial mixing results in the consumption of large amounts of

chlorine without bulking control. Three common chlorine addition points are: (1) into the RAS

stream at a point of turbulence (elbows in pipes; into the volute or discharge of RAS pumps;

and into and below the liquid level in a riser tube of an airlift RAS pump); (2) directly into the

final clarifier center well or feed channel; and (3) in an installed sidestream where the MLSS is

pumped from and returned to the aeration basin.

Chlorine addition to the RAS line(s) is the method of choice and most generally successful.

Chlorine addition to the aeration basin usually does not work and often causes floc dispersion

and system damage.

The two most important parameters are chlorine dosage and frequency of exposure of the

activated sludge to chlorine. Chlorine dose is measured conveniently on the basis of sludge

inventory in the plant – termed the overall chlorine mass dose. Effective chlorine dosages

usually are in the range 1-10 pounds chlorine/1000 pounds MLVSS inventory/day (2-4 should

work). Chlorine dosage should be started low and increased until effective. Sludge settleability

usually improves within 1-3 days if the correct chlorine dosage is applied.

Most domestic waste plants can achieve a frequency of exposure of the activated sludge

inventory to chlorine of three or greater per day (the optimum) in the RAS line. The needed

frequency is a function of the relative growth rates and efficiencies of kill of filamentous and

floc-forming organisms. Success has been achieved at frequencies as low as one per day but

not less, however, this is plant specific.

In plants with long aeration basin hydraulic residence times (industrial waste plants), the daily

solids flux in the RAS line is generally too low for successful bulking control using chlorine at this

point alone. Here, most success has been achieved using multiple chlorine addition points such

as the RAS line(s) and the final clarifier(s) in combination.

A target SVI value (or other sludge settling measure) must be set and chlorine applied only

when this value is exceeded. This is determined by trial-and-error at each plant. It should be

remembered that chlorination controls filament extension from the floc surface and merely

reduces the symptoms of bulking. Filaments will regroup rapidly, often with a vengeance, after

termination of chlorination since the cause of the bulking has not been addressed.

Signs of over chlorination are a turbid (milky) effluent, a significant increase in effluent TSS, a

loss of the higher life forms (protozoa), and a reduction in BOD removal. It is normal to see a

small increase in effluent suspended solids and BOD5 when using chlorine for bulking control.

Microscopic examination of the activated sludge during chlorination is recommended to control

chlorine application. Chlorine effects on filaments include, in order: a loss of intracellular sulfur

granules (in those filaments that have these); cell deformity and cytoplasm shrinkage; and finally

filament breakup. For sheathed filaments, the sheath is not destroyed by chlorine. Here, sludge

settle ability remains poor until the sheaths are washed out of the system by sludge wasting,

which can take 1-2 sludge ages. Chlorine use should be stopped when only empty sheaths

remain and not continued until the SVI falls, which can result in over chlorination. As a general


observation, chlorination should be stopped when about 70% of the cells are damaged or

missing in a filament.

One argument to chlorine use in bulking control is the possibility of the production of

chlorination by-products. This is unlikely since the chlorine is short-lived in activated sludge

(minutes) and applied at a low dosage (lower than used in effluent disinfection). Chlorine cannot

be used if waste constituents react with chlorine to form by-products such as petrol-chemical or

phenol wastes.

Long Term Control Methods

Long term measures include activities such as: control of influent waste septicity (organic acids

and H2S); nutrient additions (industrial waste systems only); changes in aeration; and changes in

biomass concentration or changes in waste feeding pattern.

These control measures will be expanded upon below. In addition, control of foaming problems

will be addressed at the end of this paper.

Low Dissolved Oxygen Problems

In general, the rate of BOD removal is near maximum at 1.0 mg/L dissolved oxygen (DO)

concentration, while the rate of nitrification is near maximum at 2.0 mg/L DO. However, the

actual DO concentration within the biological flocs is less than that measured in the bulk solution

around the flocs, due to oxygen use as it penetrates into the flocs.

Low aeration basin DO leads to bulking by several filaments: S. natans, type 1701 and H.

hydrossis. The DO concentration needed to control these filaments is not a constant, rather, is a

function of the organic loading rate (F/M) of the system (Palm et al., 1980). At F/M values of

about 0.5 or less, a DO concentration of 2.0 mg/L usually controls these filaments. However,

at higher F/M values a DO value of greater than 2.0 mg/L may be needed. This is due to the

need to keep the floc interiors aerobic, and this is more difficult at higher F/M values where the

OUR of the sludge is high. The DO concentration in the bulk solution around the flocs has to be

high enough to maintain an aerobic floc interior. Since oxygen moves into the floc by diffusion,

its bulk concentration needs to be high enough to reach the floc centers before becoming

depleted. A bulk solution DO concentration of 4.0 mg/L or more has been needed to prevent

these filaments in some industrial wastewater systems operated at high F/M values of >0.5.

Note that raising the MLSS concentration causes a reduction in the system F/M and OUR, and

this change can alleviate oxygen limitation within the flocs and control the low DO filaments.

Low DO filaments have been eliminated from many systems by an increase in the MLSS



Control of low DO bulking is by raising the aeration basin DO concentration, if possible, or by

raising the aeration basin MLSS concentration to decrease the F/M (both should be done

concurrently). Note that this action is opposite to what intuition directs -- to reduce the MLSS

concentration, since less biomass needs less oxygen (wrong! - the F/M is actually increased at

lower MLSS concentration). An increase in the RAS rate may also be beneficial, as this brings

biomass back to the aeration basin where it helps lower the F/M.

A common experience is that it takes a higher aeration basin DO concentration to "cure" low

DO bulking than to prevent it in the first place. Often, a short term bulking control option is

used, most often chlorination, to control this bulking problem.

Wastewater Septicity and Organic Acids

Septicity is the term used to describe the condition where the wastewater becomes anaerobic

and anaerobic bacteria ferment organic materials to organic acids such as acetic, propionic,

butyric and valeric acids. Sulfate reducing bacteria also convert sulfate to hydrogen sulfide at

this condition. A septic wastewater thus contains a relatively high amount of organic acids and

hydrogen sulfide.

A number of filaments grow on organic acids and some hydrogen sulfide (type 021N, Thiothrix

I and II, type 0914 and Beggiatoa). Observation of these filaments with intracellular sulfur

granules is a tip-off of a septicity problem and high hydrogen sulfide concentration. An organic

acid concentration of >100 mg/L and a sulfide concentration of >1-2 mg/L usually causes an

overgrowth of these bacteria.

Septicity can occur ahead of the plant, in the collection system, or can occur in the treatment

plant. Common locations of septicity in a collection system include lift stations, force mains and

long, stagnant lines. Influent septicity is usually indicated by odors (sulfide or "rotten egg" smell),

a dark color to the wastewater, and corrosion.

High amounts of organic acids and sulfides also occur in septage. These filaments may occur

due to a high loading of septage. Some industrial wastewaters also contain a high amount of

organic acids, such as wastewater from pickling and textile dyeing operations.

Septicity can also occur in the treatment plant. Common locations of septicity include poorly

aerated or poorly mixed equalization basins; septic primary clarifiers; poorly mixed aeration

basins; septic final clarifiers; and septic sludge processing side-stream returns. A common cause

of septicity is the use of a primary clarifier as a sludge thickening tank or return of waste

activated sludge to a primary clarifier.

Septicity can be tested for by analyzing the various basin influents and effluents for their organic

acid content, using the distillation and pH titration method in Standard Methods (the same test

as used for anaerobic digester operation). An organic acid concentration >100 mg/L is high

and would account for the growth of these filaments. Hydrogen sulfide can also be tested for


using one of the readily available HACH Chemical Co. test kits. A hydrogen sulfide

concentration >1-2 mg/L causes the growth of type 021N and Thiothrix I and II.

Note that some of the filaments now listed as septicity filaments (i.e. type 0961, type 0581 and

type 0092) were previously listed as low F/M filaments. It has been learned that these filaments

are actually caused by septicity and organic acids, but these grow slowly and only occur at a

lower F/M. Low F/M is not their cause, only where they occur.

There is some selection of these filaments according to the type of organic acids present. Type

021N and Thiothrix I and II prefer simple organic acids such as acetic, propionic and butyric

acids. Type 0581 and type 0092 appear to prefer higher carbon number and more complex

organic acids. For example, type 0092 is a particular problem when the wastewater contains

citric acid from industry.

Influent wastewater septicity can be treated by pre-aeration (which releases odors), chemical

oxidation (chlorine, hydrogen peroxide, or potassium permanganate), or chemical precipitation

(ferric chloride). Septicity in the collection system can be prevented by addition of sodium

nitrate as an “oxygen source” (commercially available as Bioxide).

If the influent wastewater septicity cannot be reduced, then the aeration basin can be configured

to allow better treatment of organic acids and sulfides. The organic acid concentration and

sulfide concentration in contact with the biomass can be reduced by using completely-mixed or

step-fed aeration basin conditions. A plug flow aeration basin configuration or a sequencing

batch reactor is the worse case for this condition.

Low F/M Problems and Selectors

Four filaments -- type 0041, type 0675, type 1851 and type 0803 -- are specifically caused by

low F/M conditions, usually below an F/M of 0.15, and corresponding longer sludge age. Their

specific mechanism of successful competition is not known. These may simply be slow growing

and occur only at longer sludge age associated with lower F/M. These may also grow on

particulate BOD, which would be used after the more readily degradable soluble BOD is

exhausted. It has also been suggested that these filaments compete successfully due to a low

endogenous maintenance energy requirement.

Control of low F/M bulking can be achieved by reducing the aeration basin MLSS

concentration and increasing the F/M (manipulating the "M" component). Lowering the MLSS

concentration may not be suitable for many plants as this may cause the loss of nitrification and

increase waste sludge production. Any change in operation that effectively increases the

substrate concentration available to the activated sludge and introduces batch or plug-flow

characteristics to the aeration basin, even on a short-term basis, will help combat low F/M

bulking. These include: compartmentalization of aeration basins; fed-batch operation;

intermittent feeding of wastes; and use of a selector. These latter methods do not reduce the

MLSS concentration in the system. Incidentally, step feeding of wastes, recommended for low

DO bulking, can lead to low F/M bulking, so this may need to be changed.


Filamentous bulking by the low F/M filaments is most common in completely-mixed aeration

basin systems at low aeration basin substrate (BOD) concentration. Intermittently-fed and

plug-flow systems are more resistant to this type of bulking. This observation has lead to the use

of selectors where the RAS and the influent wastewater mix for a short time prior to the main

aeration basin.

A selector is a mixing basin or channel where RAS and influent wastes mix prior to the aeration

basin. Selector design is empirical at this time. Successful examples involve a 15-30 minute

contact time of the RAS and influent waste; are aerated; and achieve at least an 80% removal of

soluble BOD5 through the selector. Several newer designs are either operated anoxic (no free

oxygen but nitrate present) or anaerobic, however, these are too new to state their general

usefulness. Design and operation of selectors for filament control is beyond the scope of this

paper, and the interested reader is directed to Jenkins et al. (1993, 2003) for further


A selector can be too large or too small in size to properly function. The goal is to provide a

short term, high substrate condition which favors certain floc-formers but which discourages

filaments. These floc-formers appear to rapidly store BOD as cellular storage products in the

selector, which they use later for growth in the main aeration basin (they pack their own "lunch

bags" in the selector). If the selector is too large, the substrate concentration achieved may not

be high enough to encourage these special floc-formers and discourage filaments. If too small,

insufficient time may be available for substrate uptake and storage. Also, a selector that is too

small may cause the floc-formers to shunt carbonaceous substrate to exocellular polymer that

can increase the SVI of the sludge ("slime bulking") and pose problems in waste sludge

dewatering. The best approach is to try several selector sizes, using a larger basin or channel

with movable baffles or exit gates.

Selectors are specific tools to combat low F/M filaments and are not needed by all plants.

There have been many instances of inappropriate selector use where they actually made the

problem worse, for example, where bulking was caused by low DO, nutrient deficiency or


Nutrient Deficiency

Nitrogen and phosphorus can be growth limiting if not present in sufficient amounts in influent

wastewater, a problem with industrial wastes and not domestic wastes. In general, a

BOD5:N:P weight ratio in the wastewater of 100:5:1 is needed for complete BOD removal.

Other nutrients such as iron or sulfur have been reported as limiting to activated sludge, but this

is not common.

Signs of nutrient deficiency include: filamentous bulking by several specific filaments (see Table

2); a viscous activated sludge which exhibits significant polysaccharide ("slime") when "stained"

with India ink; and foam on the aeration basin which contains a high amount of polysaccharide

(which has surface active properties). One check for nutrient deficiency is to be sure that at


least 1.0 mg/L total inorganic nitrogen (TIN = ammonia plus nitrite plus nitrate) and 0.5 - 1.0

mg/L ortho-phosphorus (soluble phosphorus) remain in the effluent at all times. The best

location to test for nutrient residuals is the feed from the aeration basin to the final clarifier(s).

Sometimes nutrients are released from the sludge at endogenous conditions in the final

clarifier(s), falsely elevating the effluent nutrient concentrations.

If needed, nutrients should be dosed to the incoming wastewater or the aeration basin.

Nitrogen sources include: anhydrous ammonia, urea, and ammonium salts ((NH4)2SO4,

NH4Cl or NH4NO3). Both ammonia and nitrate are nitrogen sources for growth. Phosphorus

sources include: H3PO4, Na2PO4 and (NH4)2PO4 (among others).

In systems treating mixed domestic and industrial wastes, only total inorganic nitrogen (TIN) and

soluble ortho-phosphorus should be used to calculate nutrient availability. Organically

combined nitrogen and phosphorus (Kjeldahl nitrogen and total phosphorus) may not be

hydrolyzed fast enough by the microorganisms in the activated sludge to keep pace with BOD

use. Also, the nutrient addition rate should match the influent BOD strength as much as

possible, as short term BOD spikes can cause an aeration basin to become nutrient limited for

short time periods (which can cause bulking) even though the 24-hour average BOD:N:P ratio

is satisfactory.

Foaming Control

Three filaments cause foaming: Nocardia, M. parvicella and type 1863. All of these filaments

grow on grease and oil, and these can become a problem when grease and oil are high in

amount in the influent wastewater. Systems that lack primary clarification (the main grease and

oil removal mechanism) appear to suffer more foaming problems. Communities with enforced

grease and fat ordinances appear to suffer less from foaming problems. Also, disposal of

septage, which contains substantial grease and oil content, to small activated sludge systems has

been associated with foaming problems.

Note that Nocardia here is used as a group name rather than a specific species. Recent work

has shown that a number of actinomycetes can cause foaming and include Nocardia amarae,

N. pinensis, N. rhodochrus and other Nocardia-like species. These are often collectively

referred to as the Nocardioforms, or the foam-causing actinomycetes.

Nocardia and M. parvicella also occur at a longer sludge age. The sludge age at which these

filaments can be controlled is a function of the wastewater temperature, being lower at higher

temperature. Nocardia appears to be favored at higher aeration basin temperatures and M.

parvicella at lower aeration basin temperatures. Nocardia can usually be controlled by a

sludge age below 6-8 days and M. parvicella at a sludge age below 8-10 days at moderate

wastewater temperatures. However, many plants have had to reduce the sludge age to less than

2 days for Nocardia control, and this may be inconsistent with other process goals, such as

nitrification or sludge handling capability.


A third factor in the growth of Nocardia and M. parvicella is septicity or low oxygen

conditions. Note that the combination of grease and oil, longer sludge age, and septicity or low

oxygen conditions is needed for these filaments to overgrow the system and cause foaming. In

this regard, Nocardia and M. parvicella can be considered “low DO filaments”, although low

DO per sec doesn’t cause them without the other two factors.

Nocardia and M. parvicella appear to grow better on unsaturated fatty acids in comparison to

saturated fatty acids. A change in the US diet from saturated to unsaturated fatty acids is one

reason why foaming by these bacteria is more prevalent today than it was 20-30 years ago.

Also, anaerobic bacteria break down fatty acids by first modifying them to an unsaturated form.

This may be why septicity is one of the causes for these bacteria, providing them with a source

of unsaturated fatty acids.

Type 1863 differs in growing at a low sludge age, usually less than 3-4 days. It indicates a high

amount of grease and oil and a young sludge condition. Many type 1863 foaming episodes have

been caused by a reduction in primary clarification when units were removed from service for

repair or cleaning and grease and oil concentration increased in the aeration system.

Control of Nocardia and M. parvicvella foaming is difficult. Chemical antifoam agents have not

proven generally effective, probably because these act on chemical surfactants and not on a

solids-stabilized foam. Many plants reduce aeration to control foaming, but process

performance may suffer if oxygen becomes limiting. Further, low oxygen-induced bulking may

occur when this is done.

Physical control of foams is most widely practiced using enlarged surface scum traps and

forceful water sprays (often containing 50 mg/L chlorine). Many foams reach problem levels

because they build up on these surfaces and are not removed. Foam should be removed

entirely from the system and not recycled back into the plant, for example, into the headworks.

Foam disposal into aerobic or anaerobic digesters can result in foaming there, so this should be


Return sludge chlorination has not eliminated Nocardia, although it often helps, due to

Nocardia's growth mostly within the activated sludge flocs where it isn't readily contacted by

chlorine. Also, much of the Nocardia may be present on the aeration basin surface and this

doesn’t go through the RAS line to see chlorine. RAS chlorination is more useful for foams

caused by M. parvicella.

Many anaerobic digester foaming incidents may be attributed to treatment of

Nocardia-containing waste activated sludge. A nationwide survey in 1981 by the American

Society of Civil Engineers revealed that as many as half of the anaerobic digesters in use had

experienced foaming at one time or another. It was recently reported that 54% of 26 California

activated sludge plants surveyed had recently experienced anaerobic digester foaming (Van

Niekerk et al.,JWPCF 59:249, 1987). Here, it is important to remember that Nocardia cells

float, dead or alive, due to their hydrophobic cell surface. Even though Nocardia are strict

aerobes, their cells are readily floated and cause foaming even under anaerobic conditions.


Nocardia and M. parvicella are controlled by addressing all three causative factors above. A

reduction in the grease and oil content of the wastewater is needed, either through source

control or improved operation of the primary clarifier (if present) to better remove grease and

oil. These filaments are usually controlled by a reduction in the system sludge age as given

above. Septicity, if present, needs to be controlled, and the aeration basin DO concentration

should be raised. Note that higher aeration causes more foam formation, due to the physical

action of more air present. Many operators reduce aeration when foaming occurs to reduce the

foam, but this only causes more filament growth in the long term.


Most activated sludge upsets and loss of process control are caused by one of several

microbiological problems which include poor floc formation, pin floc, dispersed growth,

filamentous and slime bulking, filamentous foaming, zoogloeal bulking, nitrification and

denitrification problems and toxicity. Use of the microscopic examination and the OUR test are

invaluable tools in troubleshooting the activated sludge process. Once the cause of the problem

or upset is known, specific remedies appropriate for the problem can be used. Short term

control methods such as chlorination are often used to quickly stop a bulking problem.

However, the best approach is to investigate the long-term control methods suitable for the

problem that is occurring to achieve trouble free operation.


Eikelboom, D.H and van Buijsen, H.J.J., Microscopic Sludge Investigation Manual, TNO

Res. Inst. for Env. Hygiene, Delft, The Netherlands, 1981.

Jenkins, D., M. G. Richard and G.T. Daigger, Manual on the Causes and Control of

Activated Sludge Bulking and Foaming, 2nd Ed., Lewis Publishers, Boca Raton, FL, 1993;

3rd Ed., 2003.

Activated Sludge Microbiology, M.G. Richard, Water Environment Federation, Alexandria,

VA, 1989.

Palm, J.C., D. Jenkins and D.S. Parker, Relationship Between Organic Loading, Dissolved

Oxygen Concentration and Sludge Settleability in the Completely-Mixed Activated Sludge

Process, J. Water Poll. Control Fed. 52:2484, 1980.

Walker, I. and M. Davis, The Relationship Between Viability and Respiration Rate in the

Activated Sludge Process, Water Research: 575, 1977.

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