In this lesson we will learn the following:
- What it means for filamentous bacteria to be in the wastewater treatment system.
- What is bulking and foaming and how to take care of this problem.
In addition to the online lecture, read chapter 7 in Wastewater Microbiology.
Filamentous bacteria serve as the backbone of floc formation. Sludge settles most efficiently when it contains a moderate number of filaments which provide structure for the floc and aid in the stripping of the water column. The floc cannot form properly if there are too few filaments, and the floc cannot settle properly if there are too many. The filamentous bacteria are analyzed in two ways: their effect on floc structure and their abundance. In small amounts, they are quite good to a biomass, but in large amounts they can cause many problems.
There are a number of types of filamentous bacteria which proliferate in the activated sludge process. Filamentous organisms perform several different roles in the process, some of which are beneficial and some are detrimental. When filamentous organisms are in low concentrations they serve to strengthen the floc particles. This effect reduces the amount of shearing in the mechanical action of the aeration tank and allows the floc particles to increase in size. Larger floc particles are more readily settled in a clarifier. Larger flow particles settling in the clarifier also tend to accumulate smaller particulates (surface adsorption) as they settle, producing an even higher quality effluent. Conversely, if the filamentous organisms reach too high a concentration, they can extend dramatically from the floc particles and tie one floc particle to another (interfloc bridging) or even form a filamentous mat of extra large size. Due to the increased surface area without a corresponding increase in mass, the activated sludge will not settle well. This results in less solids separation and may cause a washout of solid material from the system. In addition, air bubbles can become trapped in the mat and cause it to float, resulting in a floating scum mat. Due to the high surface area of the filamentous bacteria, once they reach an excess concentration, they can absorb a higher percentage of the organic material and inhibit the growth of more desirable organisms.
Filamentous bacteria have many positive aspects, such as:
- They are very good BOD removers.
- They add a backbone or rigid support network to the floc structure.
- Helps the floc structure to filter ut fine particulate matter that will improve clarifier efficiency.
- They help the floc to settle if in small amounts.
- They reduce the amount of "pin" floc.
They also have severyal negative aspects, such as:
- They can interfere with separation and compaction of activated sludge and cause bulking when predominant.
- They can affect the sludge volume index (SVI) (Sludge Volume Index).
- They can cause poor settling if dominant.
- They can fill up a clarifier and make it hard to settle, causing TSS (Total Suspended Solids) carryover.
- They can increase polymer consumption.
- They can increase solids production and cause solids handling costs to increase significantly.
Since the introduction of continuous-flow reactors, sludge bulking has been one of the major problems affecting biological waste treatment. There are several problems regarding solid separation in activated sludge. Often in industrial and municipal activated sludge processes a nutrient deficiency may occur. The nutrients that are usually deficient in these processes are either nitrogen or phosphorus. This deficiency results in the production of nutrient deficient floc particles, loss of settleability, and possibly a billowy white or greasy gray foam on the surface of the aeration tank.
During a nutrient deficiency, the bacteria within th floc particles remove soluble BOD from the wastewater. However, when nitrogen or phosphorous is deficient, the soluble BOD is not degraded but it is stored within the floc particles as an exocellular polymer-like material. This slimy material interferes with settling and may cause foam upon aeration.
Bulking is a problem consisting of slow settling and poor compaction of solids in the clarifier of the activated sludge system. Filamentous bulking is usually caused by the excessive growth of filamentous microorganisms. Bulking is caused by the overgrowth of filamentous bacteria in activated sludge. These bacteria are normal components of activated sludge flocs but may outcompete the floc-forming bacteria under specific conditions.
Name of Problem Cause of Problem Effect of Problem Dispersed growth Microorganisms do not form flocs but are dispersed, forming only small clumps or single cells Turbid effluent. No zone settling of sludge Sime (jelly) Viscous bulking Microorganisms are present in large amounts of extracellulr slime Reduced settling and compaction rates. Virtually no solids separation, in severe cases in overflow of sludge blanket from secondary clarifier. Pin floc (or pinpoint floc) Small, compact, weak, roughly spherical flocs are formed, the larger of which settle rapidly. Smaller aggregates settle slowly. Low sludge volume index (SVI) and a cloudy, turbid effluent. Bulking Filamentous organisms extend from flocs into the bulk solution and interfere with compaction and settling of activated sludge. High SVI; very clear supernatant. Rising sludge Denitrification in secondary clarifer releases poorly soluble N2 gas, which attaches to activated sludge flocs and floats them to the secondary clarifier surface. A scum of activated sludge forms on the surface of the secondary clarifier. Foaming/scum formation Caused by (1) nondegradable surfactants and (2) the presence of Nocardia and sometimes (3) the presence of Microthrix parvicella Foams float large amounts activated sludge solids to the surface of treatment units. Foam accumulates and putrefies. Solids can overflow into secondary effluent or overflow onto walkways.
Sludge settleability is determined by measuring the sludge volume index (SVI), which is given by:
where SV = volume of settled sludge after 30 min (mL/L); and MLSS = mixed liquor suspended solids (mg/L).
The sludge volume index is expressed in mL per gram and is thus the volume occupied by one gram of sludge. A high SVI (>150 mL/g) indicates bulking conditions, whereas an SVI below 70 mL/g indicates the predominance of pin (small) floc. Based on the relationship between floc-forming and filamentous bacteria, three types of flocs are observed in activated sludge: normal flocs, pin-point flocs, and filamentous bulking.
Normal flocs: A balance between floc-forming and filamentous bacteria results in strong flocs that keep their integrity in the aeration basin and settle well in the sedimentation tank.
Pin-point flocs: In these flocs, filamentous bacteria are absent or occur in low numbers. This results in small flocs that do not settle well. The secondary effluent is turbid despite the low SVI.
Filamentous bulking: Filamentous bulking is caused by the predominance of filamentous organisms. The filaments interfere with sludge settling and compaction.
Filamentous bacteria have a higher surface-to-volume ratio than that of their floc-forming counterparts, which helps them survive under low oxygen concentration and low nutrient conditions. Filamentous bacteria are able to predominate under low dissolved oxygen, low F/M, low nutrient conditions or high sulfide levels. However, it appears that low F/M is the predominant cause of bulking in wastewater treatment plants. These differences between filamentous and floc-forming bacteria can be exploited to control filamentous bulking in activated sludge.
Types of Filamentous Microorganisms
Some 20 to 30 types of filamentous microorganisms are known to be involved in activated sludge bulking. A survey of bulking activated sludge plants in the U.S. has revealed that approximately 15 major types of filamentous microorganisms are responsible for bulking, one of the most predominant being Nocardia, which is responsible for foaming.
The application of conventional techniques for the identification of filamentous microorganisms is both difficult and time-consuming. Other problems are their slow growth and difficulties in obtaining pure cultures from activated sludge samples. Thus, filamentous microorganisms were first characterized by microscopic examination, mostly with a phase contrast microscope. For such identification, the following characteristics should be obtained. Filament shape, size and shape of the cells, branching, motility, presence of a sheath, presence of epiphytic bacteria on sufaces, size and diameter and presence of granules. Other tests can help detect the presence of these microorganisms, such as gram stain and Neisser stain techniques.
The dichotomous key is used to identify filamentous microorganisms.
Factors Causing Filamentous Bulking
Filamentous microorganisms are normal components of the activated sludge flocs. Their overgrowth may be due to one or a combination of the various factors. High carbohydrate wastes appear to be conducive to sludge bulking. Carbohydrates composed of glucose, maltose, and lactose support the growth of filamentous bacteria. Some filaments appear to be favored by redily biodegradable organic substrates, such as alcohols, volatile fatty acids and amino acids, while others are able to use slowly biodegradable substrates.
Low substrate concentration (low F/M ratio) appears to be the most prevalent cause of filamentous bulking. At low substrate concentration filamentous microorganisms have a higher substrate removal rate than that of floc-formers, which prevail at high substrate concentrations.
Sludge Loading and Sludge Age
The relationship between sludge loading and sludge age depends on whether the reactor is a completely mixed or plug flow system. In completely mixed systems, increasing sludge loading leads to a decrease of SVI and thus to a decrease of filamentous microorganisms. Some filamentous microorganisms occur over a wide range of sludge age (MCRT: mean cell retention time) values while others occur only at low or high values. The optimum pH in the aeration tank is 7.0-7.5. pH values below 6.0 may favor the growth of fungi and cause filamentous bulking. In laboratory activated sludge units, bulking caused by the excessive growth of fungi occurred after 30 days at pH 4.0 and 5.0.
The growth of certain filamentous bacteria is favored by relatively low dissolved oxygen levels in the aeration tank. A substrate overload in the tank may induce oxygen deficiency. Aeration tanks should be operated with a minimum of 2 mg O2/L to avoid a predominance of specific filamentous microorganisms. Deficiences in nitrogen, phosphorus, iron or trace elements may cause bulking. Some filamentous microorganisms display a high affinity for nutrients. Increased temperature supports the growth of filamentous bacteria associated with low dissolved oxygen concentrations. Moreover, there is a tendency of Microthrix parvicella to e the dominant filamentous microorganism during the winter season.
According to a sludge bulking hypothesis, activated sludge consists of three categories of "model" microorganisms: (1) fast-growing zoogleal type microorganisms; (2) slow-growing filamentous organisms with high substrate affinity; and (3) fast-growing filamentous organisms with a high affinity for dissolved oxygen. Intermittent feeding pattern creates favorable conditions for the development of nonfilamentous microorganisms that have high substrate uptake rates during periods of high substrate concentration and a capacity to store reserve materials during periods of starvation (endogenous metabolism).
Another hypothesis on filamentous bulking is based on the ability of filamentous bacteria to denitrify nitrate to only nitrite with no accumulation of toxic nitric oxide by the cells. This gives a competitive advantage over floc-forming bacteria.
Control of Sludge Bulking
Filamentous bacteria can be controlled by treating the return sludge with chlorine or hydrogen peroxide to selectively kill filamentous microorganisms. This approach is based on the fact that filamentous microorganisms protruding from the flocs are more exposed to oxidants, whereas most of the floc-forming microorganisms embedded inside the flocs are protected from the lethal action of the oxidants. Bulking control by chlorination was proposed over 50 years ago and this practice is probably the most widely used cost-effective and short-term method for controlling filamentous bacteria. Chlorine may be added to the aeration tank or to the return activated sludge (RAS). The method of choice is the addition of chlorine to the RAS line as chlorine gas or sodium hypchlorite about three times per day. Chlorine concentration should be 10-20 mg/L (concentrations greater than 20 mg/L may cause deflocculation and formation of pin-point flocs). However, chlorination is sometimes unsuccessful in bulking control.
Hydrogen peroxide is generally added tot he RAS at concentrations of 100-200 mg/L. However, as shown for chlorine, excessive levels of hydrogen peroxide can be deleterious to floc-forming bacteria. In addition to its role as an oxidizing agent, hydrogen peroxide may also act as a source of oxygen in the aeration tank. Ozone was also proposed for curing filamentous bulking.
Synthetic organic polymers, lime, and iron salts may be added to the mixed liquor to improve bridging between the flocs and thus promote sludge settling. However, the addition of lime and iron salts increases the solids load, and the use of polymers is costly. Although treatment with polymers and coagulants leads to an immediate improvement in sedimentation, their effect is of short duration because they exert no adverse effect on filamentous microorganisms.
Foaming in Activated Sludge
A brief review of activated sludge foams and their causes is given in the table below. Use of microscopic examination can readily diagnose most of these, particularly when filaments are involved.
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.
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 slaghterhouse, 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. 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 plant's solids inventory is present in the non-circulating foam.
Control Methods for Filamentous Foaming
The three filaments that cause foaming all grown 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.
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 Microthrix 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 Microthrix 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 dec doesn't cause them without the other two factors.
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 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. parvicella 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 avoided.
Return sludge chlorination has not eliminated Nocardia, although it often helps, due to Nocardia's growth mostely 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. Here it is important to remember that Nocardia cells float, dead or alive, due to their hydrophobic (water fearing) 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 controled, 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.
Filamentous bacteria serve as the backbone of floc formation. Sludge settles most efficiently when it contains a moderate number of filaments which provide structure for the floc and aid in the stripping of the water column. The floc cannot form properly if there are too few filaments, and the floc cannot settle properly if there are too many.
During a nutrient deficiency, the bacteria within th floc particles remove soluble BOD from the wastewater. However, when nitrogen or phosphorous is deficient, the soluble BOD is not degraded but it is stored within the floc particles as an exocellular polymer-like material. This slimy material interferes with settling and may cause foam upon aeration. Bulking is a problem consisting of slow settling and poor compaction of solids in the clarifier of the activated sludge system. Filamentous bulking is usually caused by the excessive growth of filamentous microorganisms. ludge settleability is determined by measuring the sludge volume index (SVI), which is given by:
A high SVI (>150 mL/g) indicates bulking conditions, whereas an SVI below 70 mL/g indicates the predominance of pin (small) floc.
Filamentous bacteria can be controlled by treating the return sludge with chlorine or hydrogen peroxide to selectively kill filamentous microorganisms.
Chlorine concentration should be 10-20 mg/L (concentrations greater than 20 mg/L may cause deflocculation and formation of pin-point flocs). However, chlorination is sometimes unsuccessful in bulking control. Hydrogen peroxide is generally added tot he RAS at concentrations of 100-200 mg/L. However, as shown for chlorine, excessive levels of hydrogen peroxide can be deleterious to floc-forming bacteria.
Numerous measures for controlling foams in activated sludge have been proposed. Several of these cures are not always successful and have not been rigorously tested under field conditions. The control measures proposed include chlorination of foams, increase in sludge wasting, use of biological selectors, reducing air flow in the aeration tank, reducing pH and oil and grease levels, addition of anaerobic digester supernatant, water sprays, antifoam agents, physical removal and use of antagonistic microflora.
There is no lab associated with this lesson.
Answer the questions in the Lesson 8 Quiz. When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.