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Biological foams in activated sludge plants: Characterization and situation
War. Res. Vol. 25, No. 11, pp. 1399-1404, 1991 Printed in Great Britain. All rights reserved
0043-1354/91 $3.00+0.00 Copyright © 1991 Pergamon Press pie
BIOLOGICAL FOAMS IN ACTIVATED SLUDGE PLANTS: CHARACTERIZATION AND SITUATION R. PUJOL 1@, PH. DUCHENE2~), S. SCHETRITE2 a n d J. P. CANLER 1
tCEMAGREF Lyon, 3 Quai Chauveau, 69336 Lyon Cedex 9 and 2CEMAGREF Paris, 14 Avenue de St Mand6, 750012 Paris, France (First received March 1990; accepted in revised form May 1991)
Abstract--Stable biological foams are a worrying problem for the operation and control of activated sludge plants. The flotation of a fraction of the sludge is related to the metabolism of certain filamentous micro-organisms which synthetize and/or excrete hydrophobic compounds. In France, a survey of 6000 activated sludge plants has shown that 20% of them (87% of these plants which operate by extended aeration) are affected by this problem. Microscopic observations performed on samples collected in 58 selected plants have shown that, among the filamentous germs identified, the most frequent is Microthrix parvicella (45% of cases), followed by type 0675 (26% of cases) and the Nocardioform actinomycetes (14% of cases). Identification criteria for all these germs are documented. The investigation reveals the difficulties encountered by plant operators to control foaming phenomenon. So far, promising results have been obtained either by using the contact zone technique or resorting to chlorination.
Key words--activated sludge, foaming, bulking, filamentous micro-organisms, identification
INTRODUCTION
For the past two decades, the development of stable foams on the surface of aeration basins and settling tanks has become the subject of major concern for activated sludge plant operators. The number of publications dealing with the phenomenon bears witness to the seriousness and universal impact of this problem (Hao et al., 1988). These brownish and viscous foams form a layer whose thickness may reach several decimetres. Their removal is often complicated, both technically and financially speaking. In many cases, a part of the foam is discharged together with the treated water at the overflow of the secondary settling tank. Not only is the aesthetics of the plant, i.e. the visible consequence of the nuisance, often affected by the foaming phenomenon, but also further serious drawbacks and constraints are to be taken into account such as plant operation difficulties (foam withdrawal . . . . ) and possible loss of treatment quality. In France, C E M A G R E F and the leading French companies specializing in the operation and management of activated sludge plants have joined forces in a scientific group called " G I S MOUSSES" to study and test the means currently available to control foaming problem. The present study analyses the preliminary data collected by C E M A G R E F . After detailed characterization of the various types of foams, the French situation is reported, using the results of a study conducted in 1988. Attention is specially focused on the observation of filamentous micro-organisms
which cause foaming. Finally, a preliminary step towards the solution of the problem is proposed, on the basis of results from first experimentations. FOAMING
CHARACTERIZATION
Any relevant approach to the problem entails forming a precise diagnosis that may direct the choice of the appropriate technical action to be taken in order to overcome the difficulties. As a first step, therefore, it is necessary to draft a classification of the types of foams and the conditions under which they Occur.
Foaming during the start-up These foams occur and grow rapidly at the initial start-up of the plant. They are usually whitish and of light weight. They disappear when the activated sludge reaches maturity. Partial degradation of organic matter, in particular surfactants, accounts for this transient phenomenon (Jenkins et al., 1986).
Denitrification foaming The transformation of ammoniacal nitrogen into nitrates is possible in activated sludge plants operating at low loading rates. The presence of nitrates in the settling sludge often induces a phenomenon of denitrification. This results in the release of nitrogen microbubbles which lower the apparent density of sludge and favours its flotation, particularly in the secondary settling tank. The resulting floating foam is generally unstable and can be lowered by squirting water over it (or by mere rainfall); in this case, solutions will depend on overall optimization
1399
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R. PUJOLet al.
of operation parameters of the plant (aeration, recirculation . . . . ).
Surfactant .foaming Massive use of even biodegradable detergents and heavy inflow of colloidal organic matter (e.g. blood) or hydrocarbons are liable to generate foaming on the surface of the basins used for treatment. If such inflow remains occasional, the foaming process may affect the plant for a short time only, or it may persist and, in the long run, cause the development of stable biological foam.
Stable biological foaming Macroscopic criterion. Biological foaming forms very stable floating stacks on the surface of treatment tanks. Aerators fail to destroy them, they simply drift towards the sides of the tank, then rapidly close in again at the centre to form a floating layer of foam in the absence of stirring. In aerobic stabilization tanks, the foam remains stagnant on the surface, increases its concentration and no appreciable reduction in volume with time can be noted. In such foams, the concentration of suspended solids may frequently reach 50 g/l. Despite such high concentrations, dewatering of these products is difficult to achieve, most likely because of the poor quality of sludge and their apparent viscosity. In winter, plant operation difficulties are worse still as the foam may freeze and form blocks liable to interfere with the operation of surface aerators and scrapers. Microscopic criterion. Microscopic studies of biological foam samples show high concentrations of filamentous germs which constitute the common denominator of all such foams. The phenomenon can be defined as a system comprising three components: filamentous microorganisms + gas bubbles + floc particles = stable biological foam. Foam density is about 0.7 (Jenkins et al., 1986), which is (generally) sufficient to prevent wind from blowing the foam away. Air or nitrogen bubbles play a most important part since 0.44 ml of gas is enough to allow flotation of 1 mg of dry matter (Lemmer, 1985). As regards the filamentous micro-organisms which were identified, they are mainly mycobacteria (actinomycetes, genus Nocardia and genus Rhodococcus) and a few germs occurring in association with bulking phenomena (Microthrix parvicella, in particular). Though nutritional conditions are a determining factor in the growth of filamentous germs, the original causes of the presence of certain micro-organisms (e.g. mycobacteria) remain difficult to explain. Also, some mechanical factors (overflow device at the exit of the aeration tank, protection sleeves of *Service of technical aid to wastewater treatment plant managers.
aerators . . . . ) and their effects on the occurrence of the phenomenon should not be ignored. Foaming has often been observed after inflow of surfactants or hydrophobic substances (greases, oil . . . . ), as these constitute a favourable substrate for the growth of filaments (Eikelboom, 1975; Pipes, 1978; Khan and Forster, 1988; Lemmer and Bauman, 1988). The phenomenon is enhanced by the multiplication of these filaments, whose metabolism leads to hydrophobic substances produced internally or secreted by micro-organisms (Lemmer and Kroppenstedt, 1984). These physiological data are confirmed by Vega Rodriguez's observations (1983) which show that a population of Nocardia sp. is readily transferred to the foam (proportion: 70-90%) during the sludge aeration phase. The density of filaments is generally found to be higher in the foam than in the sludge, which is in agreement with the results of Wheeler and Rule (1980). THE SITUATION OF THE PROBLEM IN FRANCE
A survey on the occurrence of foaming was conducted in order to evaluate the importance of the phenomenon in France. For this purpose, a detailed questionnaire was sent to the SATESE* of each French county (department). The level of response to the questionnaire was 88% (i.e. 79 of the 90 letters sent) and the number of activated sludge plants included in the survey was 6013 (about 75% of all French plants). The responses reveal that foaming develops preferentially in the case of activated sludge by extended aeration (87% of cases), regardless of treatment capacity, geographical location or aeration devices. The phenomenon appears to be cyclic or repetitive in 50% of foaming cases. Seasonal influence seems to show greater impact in winter and spring. The total number of plants affected by foaming was 1192, i.e. nearly one plant in five. This figure can be compared with that which was recorded for a previous survey concerning bulking: one plant in four was found to be affected by bulking (Pujol and Canler, 1989). These two biological problems often have similar causes and, as a result, the combination of bulking and foaming frequently occurs in the same plant. The comparison of these results with those from another investigation carried out by Blackbeard et al. (1986) in South Africa indicates a higher number of cases in that country: foaming in 40% of plants, bulking in 32%. The differences between results may be explained by the difficulty in establishing a precise threshold value above which the foaming phenomenon can be said to be a severe nuisance. At the present time, the threshold value remains a rather subjective estimation. Regarding the incidence of bulking, the difference between the two investigations
Foaming in activated sludge can be explained by the reference to a higher sludge volume index (in France SVI = 200 ml/g as against 150ml/g in South Africa) as the characteristic threshold value of bulking. That biological foaming is a major problem in most countries equipped with activated sludge plants is a fact that clearly emerges from these two investigations, complemented by the observations of: Wagner (1984), Chambers and Tomlinson (1982), Jenkins (1986) and Blackall et al. (1988). Filaments prevailing in foams Materials and methods. Information concerning filamentous germs identification, as provided by the questionnaire, proved fairly unreliable and insufficient. For this reason, it was decided to make further observations on a sample of 58 plants selected on the basis of the acuteness of their foaming problem: these obervations were carried out both on foam and sludge. The identification criteria used by C E M A G R E F are those of Eikelboom's determination key (Eikelboom and Van Buijsen, 1983) complemented by Jenkins et al. (1986). They are based on the morphological features of filaments and two staining techniques (Gram and Neisser) and one test (sulphur test). Observations are performed under a phase contrast microscope allowing 500x and 1250x magnifications. The samples studied are collected from newly formed foams, they are light-brown in colour whereas old foams which have long remained on tank surfaces are blackish-brown. The latter constitute a fermented medium that favours the growth of sulphur autotrophic germs such as Thiothrix in particular. Results. If bulking and foaming often occur together in the same plant, microscopic observation shows an essential feature of the filaments linked to foaming. Generally, these germs stick to the microscope slide, they flock, in large numbers, within a single plane and are easily identifiable by the viewer. This particular feature is connected with the presence of hydrophobic metabolic substances. Results are shown in Table !. Main .filaments --Microthrix parvicella is the germ most often observed in the analysed samples, this filament has been emphasized by several authors (Slijkhuis and Deinema in Chambers and Tomlinson, 1982; Table I. Main filamentous micro-organisms in foams Main filamentous micro-organisms
Number of cases
Percent of total
Microthrix part'icella Type 0675 Nocardioform actinomycetes Type 0092
32 15 8 3
55 26 14 5
Associated filaments Type 0041 Type 021 N Thiothrix sp.
29 3 3
1401
Eikelboom and Van Buijsen, 1983; Blackbeard et al., 1986). Results shown in Table l also reveal the presence of type 0675 which, for the first time, appears significantly in a study of this type. Finally, the Nocardioform germs (i.e. various morphologically similar actinomycetes) are prevalent in only 14% of cases. These germs have long been known to be connected with the foaming phenomenon (Pipes, 1978; Dhaliwai, 1979; Hiraoka and Tsumura, 1984; Hao et al., 1986; Sezgin et al., 1988). --Microscopic observations provide further information on the usual criteria used for the recognition of filaments. For example, in the case of Microthrix parvicella, the trichoma often exhibits a kind of membrane enclosing a clear area (magnification > 500 x -contrast phase). These clear areas are fragile and represent the future breaking-points of the filament. Although the "sheath" enclosing the trichoma is not as clearly visible as those of germs such as Sphaerotilus natans, the "no sheath present" feature proposed by Eikelboom (1975) or Jenkins et al. (1986) in the determination key should not be considered as a fundamental standard of identification. For Microthrix p., the best identification criteria remain the following: size of the trichoma (breadth = 0.5-0.8% m; length >200 #m), great flexibility of the filament, absence of adhesions on the trichoma, positive Gram and Neisser staining (blue granules). --Type 0675 is generally shorter in length than Microthrix p., the rectangular cells are often visible inside the sheathed trichoma; the filaments are less flexible and bear adhesions. Neisser staining is generally positive (blue granules) and the filaments cannot always be visualized in one single plane. --The germs classified as Nocardioform are characterized by short and numerous ramifications. However, the differences found in the morphologies of these germs (variable breadth of trichomae, swellings, etc.) are in agreement with the observations of Lemmer and Kroppenstedt (1984) and Kahn and Forster (1988) who, on the basis of physiological tests, have confirmed the multiple forms of actinomycetes present in foam, particularly those belonging to the genus Nocardia and the genus Rhodococcus. Owing to their morphology and their heterogeneous spatial distribution in sludge, Nocardioform germs do not, as a rule, induce notable impairment of sludge settleability (SVI < 200ml/g). This is not true of other filaments identified in sludge. For example, very high indices (SVI of 400-500 ml/g) are possible in the case of Microthrix p. --Regarding type 0092, one mainly observes thin filaments (0.44).7/~m); they are rather stiff and take up a uniform greyish-blue tinge on Neisser staining. The number of cases is limited. Associated filaments
--Type 0041 is frequently found in association with typical filaments found in fresh foams. Its
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density is often weaker in foam than in sludge so that type 0041 is expected to show less flotation ability (its metabolites are probably less hydrophobic). However, when its development in sludge is considerable, it generates very severe bulking. In France, type 0041 is certainly the most representative germ found on extended aeration activated sludge plants. The causes for its development are related to nutritional shortages in the sludge; however, remedies to such deficiencies are made possible by creating a contact zone (Pujol, 1987). - - A s regards Thiothrix and type 021 N filaments identified in some samples, their presence can be related to a more or less advanced state of fermentation or to marked nutritional imbalance in this particular medium of foams which can remain on the surface of treatment tanks for a long time. To summarize, as in the case of bulking, precise identification of the germs involved, together with thorough knowledge of their ecophysiology, will remain the basis for selecting and applying reliable solutions allowing the control of the growth of all these filamentous micro-organisms. Comparison with results reported in the cited literature
To date, few detailed investigations into the foaming phenomenon have been carried out. However, when examining the results from observations performed on sludge in various countries, one notes that they mention the presence of the main germs found in foams (Table 2). Regarding foams, a more precise comparison can be made, using the results of the study conducted in South Africa (Blackbeard et al. 1986): the data were obtained from 37 plants whose foam samples were systematically analysed under the microscope. These results were compared to those of the present French study and are shown in Table 3, referring only to the filamentous types found in both investigations. Microthrix parvicella ranks first in both investigations, which confirms that it is the main causative agent of foaming. Next, actinomycetes (Nocardia sp.) hold nearly the same rank: third in France and second in South Africa. Type 0041 ranks third in South Africa, which suggests that the plants studied were very likely affected by bulking (simultaneous presence of 0041 in foam and in sludge). The rank differences for types 0675 and 0092 may perhaps show the limitations of observation criteria. If the ranks observed for Microthrix p. and actinomycetes are in agreement with the data reported in Table 2. Place of foaming micro-organismsin activated sludge samples Rank classification The Fed. Rep. U.S.A.* Netherlands*Germany* France Microthrix parricella 7 Nocardia sp. I *In Blackbeardet al. (1986).
I
2
I
14
5
3
Table 3. Foamingmicro-organisms:comparisonof France/South Africa Dominantfilamentsin foams France South Africa 1
I
Type 0675 2 Nocardia sp. 3 Type 0092 5 Type 0041" 1 Numbers of plants 58 *In France, foamingassociated filament.
Microthrix part,icella
5 2 1 3 37
the literature, the presence of types 0092 and 0675 (encountered in both investigations) is more surprising. These two filaments should appear less hydrophobic than the typical filaments (Microthrix p. and actinomycetes) observed in stable foams. One of the identification criteria, sticking to the microscope slide, is less characteristic. The growth conditions of these germs may correspond to particular media inducing lower hydrophobic substances. Contribution to the control o f biological f o a m i n g
In our investigation, the questionnaire inquired also about the curative actions undertaken to control the problem. Examination of answers to this question revealed that technical measures had been taken in only 16% of cases, with an average success rate that did not exceed 30%. Such figures show the complexity of the problem and difficulty for the skilled persons who are in charge of the plant operation to find suitable and efficient solutions alone. Besides the emergency modifications of usual plant operation (e.g. aeration, recirculation, sludge withdrawal...), the solutions which proved most successful are shown in Table 4. These results are not surprising, since the contact zone technique (Eikelboom cited in Chambers and Tomlinson, 1982; Pujol and Boutin, 1989) allows control of the development of certain filamentous micro-organisms resistant to nutritional deficiencies: results obtained from biological foams suggest that this category of germs includes Microthrix p. and types 0041, 0675 or 0092. This solution, therefore, appears promising since it acts directly on certain causes of filamentous growth. The other solutions which proved efficient are based on chemical reagent injections. Overall, chlorine treatments have yielded good results. However, the use of toxic agents involves close supervision and steady control of sludge quality. In economic terms, the addition of reagents means extra operating costs. As a consequence, applying these methods will be considered when acute difficulties occur over short Table 4. Effectivenessof the technicalsolutions Successful in % Numberof of cases cases Mixingzone 73 11 Oxidant addition 66 9 Anti-foamingaddition 57 7 Coagulant addition 46 28
Foaming in activated sludge periods of time. They have no effect on the causes of foaming, and foaming may recur as soon as the chemical treatment is discontinued. Foaming induced by Nocardia sp. can generally be treated with reasonable success by reducing the suspended solids concentration in the aeration tank. It seems, however, that this solution is not successful for all foams caused by actinomycetes. In the present state of knowledge, any strategy to be applied to fight the phenomenon must be considered with extreme caution. As in the case of bulking, only a methodological approach, based on microscopic follow-up examinations of sludge and the good knowledge of a specific plant, is a sound guarantee for the efficiency of technical solutions. Further research work should soon make it possible to have more precise ideas about what must be done to handle the present complex situation. CONCLUSIONS Stable biological foams create a worrying problem for the operation and management of activated sludge plants. Foaming occurs as a consequence of the growth of certain filamentous micro-organisms whose metabolism induces the formation of hydrophobic internal compounds or excretions which, in association with air-bubbles, negatively affect the apparent density of particles and promote their flotation on the surface of tanks. The origins of the development of such filaments are complex; at the present stage of knowledge, the most important ones seem to be: --nutritional conditions within the bacterial microenvironment, --introduction of hydrophobic substances in wastewater, --certain mechanical aspects (aeration, stirring). In France, a survey of over 6000 plants showed that 20% of activated sludge installations are affected by foaming chronically or periodically (50% of cases). Activated sludge plants operating in extended aeration (i.e. 87% of cases) are faced with this problem. Furthermore, biological foaming and sludge bulking are frequently found together on the same plant. Microscopic analyses performed on foam samples collected in 58 selected plants revealed that Microthrix parvicella is the most frequent germ (55% of cases), followed by type 0675 (26%) and "Nocardioform" germs (14%). Morphological criteria for the identification of filaments (e.g. for Microthrix parvicella) were refined and the variety of actinomycetes forms was confirmed. Besides actinomycetes, the other germs responsible for foaming are characterized by: --sticking to the microscopic slide, (i.e. they can therefore be visualized in the same plane),
1403
--Neisser positive granules, --small breadth trichoma ( < 1/~m), --flexible shape and septa not really visible. A preliminary examination of solutions currently employed to control foaming showed the difficulties encountered by plant operators in coping with a complex biological problem: only 30% of successful results for all treated cases. Promising methods such as the contact zone technique and chlorination provide better results. On-going research should soon provide a sound basis for efficient solutions to this type of problem. REFERENCES
Blackall L. L., Harbers A. E., Greenfield P. F. and Hayward A. C. (1988) Actinomycete scum problems in Australian activated sludge plants. Wat. Sci. Technol. 20, 493-495. Blackbeard J. R., Ekama G. A. and Marais G. R. (1986) A survey of filamentous bulking and foaming in activated sludge plants in south of Africa. Wat. Pollut. Control 1, 90--100. Chambers B. and Tomlinson E. J. (1982) Bulking of Activated Sludge: Preventative and Remedial Methods. Horwood, Chichester. Dhaliwal B. S. (1979) Nocardia amarae and activated sludge foaming. J. Wat. Pollut. Control Fed. 51, 344-350. Eikelboom D. H. (1975) Filamentous organisms observed in activated sludge. Wat. Res. 9, 365-388. Eikelboom D. H. and Van Buijsen H. J. J. (1983) Microscopic Sludge Investigation Manual, 2nd edition. TNO,
Delft, The Netherlands. Hao O. J., Strom P. F. and Wu Y. C (1988) A review of the role of Nocardia like filaments in activated sludge foaming. Wat. Sth Afr. 14, 105-110. Hiraoka M. and Tsumura K. (1984) Suppression of actinomycete scum production: a case study at Senboku wastewater treatment plant, Japan. Wat. Sci. Technol. 16, 83-90. Jenkins D., Richard M. G. and Daigger G. J. (1986) Manual on the Causes and Control of Activated Sludge Bulking and Foaming. Ridgelines Press, Lafayette, Calif.
Khan A. R. and Forster C. F. (1988) Biosurfactant production by Rhodococcus rubra. Envir. Technol. Lett. 9, 1349-1360. Lemmer H. (1985) The ecology of scum causing actinomycetes in sewage treatment plants. Wat. Res. 20, 531-535. Lemmer H. and Bauman M. (1988) Scum actinomycetes in sewage treatment plants---Part 2. The effect of hydrophobic substrate. Wat. Res. 22, 761-763. Lemmer H. and Kroppenstedt R. M. (1984) Chemotaxonomy and physiology of some actinomycetes isolated from scumming activated sludge. Syst. appL Microbiol. 5, 124-135. Pipes W. O. (1978) Actinomycetes scum production in activated sludge processes. J. Wat. Pollut. Control Fed. April, 628-634. Pujol R. (1987) Maitrise du foissonnement des boues activ~es: biosorption et zones de contact. Approche m~thodologique. Th~se de doctorat INSA, Lyon, France. Pujol R. and Boutin P. (1989) Control of activated sludge bulking: from the lab to the plant. Wat. Sci. Technol. 21, 717-726. Pujol R. and Canler J. P. (1989) Le foisonnement des boues activ~es:situation du probl~me en France. Technol. Sci. Meth. 89, 19-24.
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Sezgin M., Lechevalier M. P. and Karr P. R. (1988) Isolation and identification of actinomycetes present in activated sludge scum. Wat. Sci. Technol. 20, 257-263. Vega Rodriguez B. A. (1983) Quantitative evaluation of Nocardia spp. presence in activated sludge. Thesis, Civil Engineering, University of California, Berkeley, Calif.
Wagner F. (1984) Studies on the causes and prevention of bulking sludge in Germany. War. Sci. Technol. 16, 1-14. Wheeler D. R. and Rule A. M. {1980) The role of Nocardia in the foaming of activated sludge: Laboratories studies. Presented at the Georgia Water and Pollution Control Association Annual Meeting, Savannah, GA.
0043-1354/91 $3.00+0.00 Copyright © 1991 Pergamon Press pie
BIOLOGICAL FOAMS IN ACTIVATED SLUDGE PLANTS: CHARACTERIZATION AND SITUATION R. PUJOL 1@, PH. DUCHENE2~), S. SCHETRITE2 a n d J. P. CANLER 1
tCEMAGREF Lyon, 3 Quai Chauveau, 69336 Lyon Cedex 9 and 2CEMAGREF Paris, 14 Avenue de St Mand6, 750012 Paris, France (First received March 1990; accepted in revised form May 1991)
Abstract--Stable biological foams are a worrying problem for the operation and control of activated sludge plants. The flotation of a fraction of the sludge is related to the metabolism of certain filamentous micro-organisms which synthetize and/or excrete hydrophobic compounds. In France, a survey of 6000 activated sludge plants has shown that 20% of them (87% of these plants which operate by extended aeration) are affected by this problem. Microscopic observations performed on samples collected in 58 selected plants have shown that, among the filamentous germs identified, the most frequent is Microthrix parvicella (45% of cases), followed by type 0675 (26% of cases) and the Nocardioform actinomycetes (14% of cases). Identification criteria for all these germs are documented. The investigation reveals the difficulties encountered by plant operators to control foaming phenomenon. So far, promising results have been obtained either by using the contact zone technique or resorting to chlorination.
Key words--activated sludge, foaming, bulking, filamentous micro-organisms, identification
INTRODUCTION
For the past two decades, the development of stable foams on the surface of aeration basins and settling tanks has become the subject of major concern for activated sludge plant operators. The number of publications dealing with the phenomenon bears witness to the seriousness and universal impact of this problem (Hao et al., 1988). These brownish and viscous foams form a layer whose thickness may reach several decimetres. Their removal is often complicated, both technically and financially speaking. In many cases, a part of the foam is discharged together with the treated water at the overflow of the secondary settling tank. Not only is the aesthetics of the plant, i.e. the visible consequence of the nuisance, often affected by the foaming phenomenon, but also further serious drawbacks and constraints are to be taken into account such as plant operation difficulties (foam withdrawal . . . . ) and possible loss of treatment quality. In France, C E M A G R E F and the leading French companies specializing in the operation and management of activated sludge plants have joined forces in a scientific group called " G I S MOUSSES" to study and test the means currently available to control foaming problem. The present study analyses the preliminary data collected by C E M A G R E F . After detailed characterization of the various types of foams, the French situation is reported, using the results of a study conducted in 1988. Attention is specially focused on the observation of filamentous micro-organisms
which cause foaming. Finally, a preliminary step towards the solution of the problem is proposed, on the basis of results from first experimentations. FOAMING
CHARACTERIZATION
Any relevant approach to the problem entails forming a precise diagnosis that may direct the choice of the appropriate technical action to be taken in order to overcome the difficulties. As a first step, therefore, it is necessary to draft a classification of the types of foams and the conditions under which they Occur.
Foaming during the start-up These foams occur and grow rapidly at the initial start-up of the plant. They are usually whitish and of light weight. They disappear when the activated sludge reaches maturity. Partial degradation of organic matter, in particular surfactants, accounts for this transient phenomenon (Jenkins et al., 1986).
Denitrification foaming The transformation of ammoniacal nitrogen into nitrates is possible in activated sludge plants operating at low loading rates. The presence of nitrates in the settling sludge often induces a phenomenon of denitrification. This results in the release of nitrogen microbubbles which lower the apparent density of sludge and favours its flotation, particularly in the secondary settling tank. The resulting floating foam is generally unstable and can be lowered by squirting water over it (or by mere rainfall); in this case, solutions will depend on overall optimization
1399
1400
R. PUJOLet al.
of operation parameters of the plant (aeration, recirculation . . . . ).
Surfactant .foaming Massive use of even biodegradable detergents and heavy inflow of colloidal organic matter (e.g. blood) or hydrocarbons are liable to generate foaming on the surface of the basins used for treatment. If such inflow remains occasional, the foaming process may affect the plant for a short time only, or it may persist and, in the long run, cause the development of stable biological foam.
Stable biological foaming Macroscopic criterion. Biological foaming forms very stable floating stacks on the surface of treatment tanks. Aerators fail to destroy them, they simply drift towards the sides of the tank, then rapidly close in again at the centre to form a floating layer of foam in the absence of stirring. In aerobic stabilization tanks, the foam remains stagnant on the surface, increases its concentration and no appreciable reduction in volume with time can be noted. In such foams, the concentration of suspended solids may frequently reach 50 g/l. Despite such high concentrations, dewatering of these products is difficult to achieve, most likely because of the poor quality of sludge and their apparent viscosity. In winter, plant operation difficulties are worse still as the foam may freeze and form blocks liable to interfere with the operation of surface aerators and scrapers. Microscopic criterion. Microscopic studies of biological foam samples show high concentrations of filamentous germs which constitute the common denominator of all such foams. The phenomenon can be defined as a system comprising three components: filamentous microorganisms + gas bubbles + floc particles = stable biological foam. Foam density is about 0.7 (Jenkins et al., 1986), which is (generally) sufficient to prevent wind from blowing the foam away. Air or nitrogen bubbles play a most important part since 0.44 ml of gas is enough to allow flotation of 1 mg of dry matter (Lemmer, 1985). As regards the filamentous micro-organisms which were identified, they are mainly mycobacteria (actinomycetes, genus Nocardia and genus Rhodococcus) and a few germs occurring in association with bulking phenomena (Microthrix parvicella, in particular). Though nutritional conditions are a determining factor in the growth of filamentous germs, the original causes of the presence of certain micro-organisms (e.g. mycobacteria) remain difficult to explain. Also, some mechanical factors (overflow device at the exit of the aeration tank, protection sleeves of *Service of technical aid to wastewater treatment plant managers.
aerators . . . . ) and their effects on the occurrence of the phenomenon should not be ignored. Foaming has often been observed after inflow of surfactants or hydrophobic substances (greases, oil . . . . ), as these constitute a favourable substrate for the growth of filaments (Eikelboom, 1975; Pipes, 1978; Khan and Forster, 1988; Lemmer and Bauman, 1988). The phenomenon is enhanced by the multiplication of these filaments, whose metabolism leads to hydrophobic substances produced internally or secreted by micro-organisms (Lemmer and Kroppenstedt, 1984). These physiological data are confirmed by Vega Rodriguez's observations (1983) which show that a population of Nocardia sp. is readily transferred to the foam (proportion: 70-90%) during the sludge aeration phase. The density of filaments is generally found to be higher in the foam than in the sludge, which is in agreement with the results of Wheeler and Rule (1980). THE SITUATION OF THE PROBLEM IN FRANCE
A survey on the occurrence of foaming was conducted in order to evaluate the importance of the phenomenon in France. For this purpose, a detailed questionnaire was sent to the SATESE* of each French county (department). The level of response to the questionnaire was 88% (i.e. 79 of the 90 letters sent) and the number of activated sludge plants included in the survey was 6013 (about 75% of all French plants). The responses reveal that foaming develops preferentially in the case of activated sludge by extended aeration (87% of cases), regardless of treatment capacity, geographical location or aeration devices. The phenomenon appears to be cyclic or repetitive in 50% of foaming cases. Seasonal influence seems to show greater impact in winter and spring. The total number of plants affected by foaming was 1192, i.e. nearly one plant in five. This figure can be compared with that which was recorded for a previous survey concerning bulking: one plant in four was found to be affected by bulking (Pujol and Canler, 1989). These two biological problems often have similar causes and, as a result, the combination of bulking and foaming frequently occurs in the same plant. The comparison of these results with those from another investigation carried out by Blackbeard et al. (1986) in South Africa indicates a higher number of cases in that country: foaming in 40% of plants, bulking in 32%. The differences between results may be explained by the difficulty in establishing a precise threshold value above which the foaming phenomenon can be said to be a severe nuisance. At the present time, the threshold value remains a rather subjective estimation. Regarding the incidence of bulking, the difference between the two investigations
Foaming in activated sludge can be explained by the reference to a higher sludge volume index (in France SVI = 200 ml/g as against 150ml/g in South Africa) as the characteristic threshold value of bulking. That biological foaming is a major problem in most countries equipped with activated sludge plants is a fact that clearly emerges from these two investigations, complemented by the observations of: Wagner (1984), Chambers and Tomlinson (1982), Jenkins (1986) and Blackall et al. (1988). Filaments prevailing in foams Materials and methods. Information concerning filamentous germs identification, as provided by the questionnaire, proved fairly unreliable and insufficient. For this reason, it was decided to make further observations on a sample of 58 plants selected on the basis of the acuteness of their foaming problem: these obervations were carried out both on foam and sludge. The identification criteria used by C E M A G R E F are those of Eikelboom's determination key (Eikelboom and Van Buijsen, 1983) complemented by Jenkins et al. (1986). They are based on the morphological features of filaments and two staining techniques (Gram and Neisser) and one test (sulphur test). Observations are performed under a phase contrast microscope allowing 500x and 1250x magnifications. The samples studied are collected from newly formed foams, they are light-brown in colour whereas old foams which have long remained on tank surfaces are blackish-brown. The latter constitute a fermented medium that favours the growth of sulphur autotrophic germs such as Thiothrix in particular. Results. If bulking and foaming often occur together in the same plant, microscopic observation shows an essential feature of the filaments linked to foaming. Generally, these germs stick to the microscope slide, they flock, in large numbers, within a single plane and are easily identifiable by the viewer. This particular feature is connected with the presence of hydrophobic metabolic substances. Results are shown in Table !. Main .filaments --Microthrix parvicella is the germ most often observed in the analysed samples, this filament has been emphasized by several authors (Slijkhuis and Deinema in Chambers and Tomlinson, 1982; Table I. Main filamentous micro-organisms in foams Main filamentous micro-organisms
Number of cases
Percent of total
Microthrix part'icella Type 0675 Nocardioform actinomycetes Type 0092
32 15 8 3
55 26 14 5
Associated filaments Type 0041 Type 021 N Thiothrix sp.
29 3 3
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Eikelboom and Van Buijsen, 1983; Blackbeard et al., 1986). Results shown in Table l also reveal the presence of type 0675 which, for the first time, appears significantly in a study of this type. Finally, the Nocardioform germs (i.e. various morphologically similar actinomycetes) are prevalent in only 14% of cases. These germs have long been known to be connected with the foaming phenomenon (Pipes, 1978; Dhaliwai, 1979; Hiraoka and Tsumura, 1984; Hao et al., 1986; Sezgin et al., 1988). --Microscopic observations provide further information on the usual criteria used for the recognition of filaments. For example, in the case of Microthrix parvicella, the trichoma often exhibits a kind of membrane enclosing a clear area (magnification > 500 x -contrast phase). These clear areas are fragile and represent the future breaking-points of the filament. Although the "sheath" enclosing the trichoma is not as clearly visible as those of germs such as Sphaerotilus natans, the "no sheath present" feature proposed by Eikelboom (1975) or Jenkins et al. (1986) in the determination key should not be considered as a fundamental standard of identification. For Microthrix p., the best identification criteria remain the following: size of the trichoma (breadth = 0.5-0.8% m; length >200 #m), great flexibility of the filament, absence of adhesions on the trichoma, positive Gram and Neisser staining (blue granules). --Type 0675 is generally shorter in length than Microthrix p., the rectangular cells are often visible inside the sheathed trichoma; the filaments are less flexible and bear adhesions. Neisser staining is generally positive (blue granules) and the filaments cannot always be visualized in one single plane. --The germs classified as Nocardioform are characterized by short and numerous ramifications. However, the differences found in the morphologies of these germs (variable breadth of trichomae, swellings, etc.) are in agreement with the observations of Lemmer and Kroppenstedt (1984) and Kahn and Forster (1988) who, on the basis of physiological tests, have confirmed the multiple forms of actinomycetes present in foam, particularly those belonging to the genus Nocardia and the genus Rhodococcus. Owing to their morphology and their heterogeneous spatial distribution in sludge, Nocardioform germs do not, as a rule, induce notable impairment of sludge settleability (SVI < 200ml/g). This is not true of other filaments identified in sludge. For example, very high indices (SVI of 400-500 ml/g) are possible in the case of Microthrix p. --Regarding type 0092, one mainly observes thin filaments (0.44).7/~m); they are rather stiff and take up a uniform greyish-blue tinge on Neisser staining. The number of cases is limited. Associated filaments
--Type 0041 is frequently found in association with typical filaments found in fresh foams. Its
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density is often weaker in foam than in sludge so that type 0041 is expected to show less flotation ability (its metabolites are probably less hydrophobic). However, when its development in sludge is considerable, it generates very severe bulking. In France, type 0041 is certainly the most representative germ found on extended aeration activated sludge plants. The causes for its development are related to nutritional shortages in the sludge; however, remedies to such deficiencies are made possible by creating a contact zone (Pujol, 1987). - - A s regards Thiothrix and type 021 N filaments identified in some samples, their presence can be related to a more or less advanced state of fermentation or to marked nutritional imbalance in this particular medium of foams which can remain on the surface of treatment tanks for a long time. To summarize, as in the case of bulking, precise identification of the germs involved, together with thorough knowledge of their ecophysiology, will remain the basis for selecting and applying reliable solutions allowing the control of the growth of all these filamentous micro-organisms. Comparison with results reported in the cited literature
To date, few detailed investigations into the foaming phenomenon have been carried out. However, when examining the results from observations performed on sludge in various countries, one notes that they mention the presence of the main germs found in foams (Table 2). Regarding foams, a more precise comparison can be made, using the results of the study conducted in South Africa (Blackbeard et al. 1986): the data were obtained from 37 plants whose foam samples were systematically analysed under the microscope. These results were compared to those of the present French study and are shown in Table 3, referring only to the filamentous types found in both investigations. Microthrix parvicella ranks first in both investigations, which confirms that it is the main causative agent of foaming. Next, actinomycetes (Nocardia sp.) hold nearly the same rank: third in France and second in South Africa. Type 0041 ranks third in South Africa, which suggests that the plants studied were very likely affected by bulking (simultaneous presence of 0041 in foam and in sludge). The rank differences for types 0675 and 0092 may perhaps show the limitations of observation criteria. If the ranks observed for Microthrix p. and actinomycetes are in agreement with the data reported in Table 2. Place of foaming micro-organismsin activated sludge samples Rank classification The Fed. Rep. U.S.A.* Netherlands*Germany* France Microthrix parricella 7 Nocardia sp. I *In Blackbeardet al. (1986).
I
2
I
14
5
3
Table 3. Foamingmicro-organisms:comparisonof France/South Africa Dominantfilamentsin foams France South Africa 1
I
Type 0675 2 Nocardia sp. 3 Type 0092 5 Type 0041" 1 Numbers of plants 58 *In France, foamingassociated filament.
Microthrix part,icella
5 2 1 3 37
the literature, the presence of types 0092 and 0675 (encountered in both investigations) is more surprising. These two filaments should appear less hydrophobic than the typical filaments (Microthrix p. and actinomycetes) observed in stable foams. One of the identification criteria, sticking to the microscope slide, is less characteristic. The growth conditions of these germs may correspond to particular media inducing lower hydrophobic substances. Contribution to the control o f biological f o a m i n g
In our investigation, the questionnaire inquired also about the curative actions undertaken to control the problem. Examination of answers to this question revealed that technical measures had been taken in only 16% of cases, with an average success rate that did not exceed 30%. Such figures show the complexity of the problem and difficulty for the skilled persons who are in charge of the plant operation to find suitable and efficient solutions alone. Besides the emergency modifications of usual plant operation (e.g. aeration, recirculation, sludge withdrawal...), the solutions which proved most successful are shown in Table 4. These results are not surprising, since the contact zone technique (Eikelboom cited in Chambers and Tomlinson, 1982; Pujol and Boutin, 1989) allows control of the development of certain filamentous micro-organisms resistant to nutritional deficiencies: results obtained from biological foams suggest that this category of germs includes Microthrix p. and types 0041, 0675 or 0092. This solution, therefore, appears promising since it acts directly on certain causes of filamentous growth. The other solutions which proved efficient are based on chemical reagent injections. Overall, chlorine treatments have yielded good results. However, the use of toxic agents involves close supervision and steady control of sludge quality. In economic terms, the addition of reagents means extra operating costs. As a consequence, applying these methods will be considered when acute difficulties occur over short Table 4. Effectivenessof the technicalsolutions Successful in % Numberof of cases cases Mixingzone 73 11 Oxidant addition 66 9 Anti-foamingaddition 57 7 Coagulant addition 46 28
Foaming in activated sludge periods of time. They have no effect on the causes of foaming, and foaming may recur as soon as the chemical treatment is discontinued. Foaming induced by Nocardia sp. can generally be treated with reasonable success by reducing the suspended solids concentration in the aeration tank. It seems, however, that this solution is not successful for all foams caused by actinomycetes. In the present state of knowledge, any strategy to be applied to fight the phenomenon must be considered with extreme caution. As in the case of bulking, only a methodological approach, based on microscopic follow-up examinations of sludge and the good knowledge of a specific plant, is a sound guarantee for the efficiency of technical solutions. Further research work should soon make it possible to have more precise ideas about what must be done to handle the present complex situation. CONCLUSIONS Stable biological foams create a worrying problem for the operation and management of activated sludge plants. Foaming occurs as a consequence of the growth of certain filamentous micro-organisms whose metabolism induces the formation of hydrophobic internal compounds or excretions which, in association with air-bubbles, negatively affect the apparent density of particles and promote their flotation on the surface of tanks. The origins of the development of such filaments are complex; at the present stage of knowledge, the most important ones seem to be: --nutritional conditions within the bacterial microenvironment, --introduction of hydrophobic substances in wastewater, --certain mechanical aspects (aeration, stirring). In France, a survey of over 6000 plants showed that 20% of activated sludge installations are affected by foaming chronically or periodically (50% of cases). Activated sludge plants operating in extended aeration (i.e. 87% of cases) are faced with this problem. Furthermore, biological foaming and sludge bulking are frequently found together on the same plant. Microscopic analyses performed on foam samples collected in 58 selected plants revealed that Microthrix parvicella is the most frequent germ (55% of cases), followed by type 0675 (26%) and "Nocardioform" germs (14%). Morphological criteria for the identification of filaments (e.g. for Microthrix parvicella) were refined and the variety of actinomycetes forms was confirmed. Besides actinomycetes, the other germs responsible for foaming are characterized by: --sticking to the microscopic slide, (i.e. they can therefore be visualized in the same plane),
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--Neisser positive granules, --small breadth trichoma ( < 1/~m), --flexible shape and septa not really visible. A preliminary examination of solutions currently employed to control foaming showed the difficulties encountered by plant operators in coping with a complex biological problem: only 30% of successful results for all treated cases. Promising methods such as the contact zone technique and chlorination provide better results. On-going research should soon provide a sound basis for efficient solutions to this type of problem. REFERENCES
Blackall L. L., Harbers A. E., Greenfield P. F. and Hayward A. C. (1988) Actinomycete scum problems in Australian activated sludge plants. Wat. Sci. Technol. 20, 493-495. Blackbeard J. R., Ekama G. A. and Marais G. R. (1986) A survey of filamentous bulking and foaming in activated sludge plants in south of Africa. Wat. Pollut. Control 1, 90--100. Chambers B. and Tomlinson E. J. (1982) Bulking of Activated Sludge: Preventative and Remedial Methods. Horwood, Chichester. Dhaliwal B. S. (1979) Nocardia amarae and activated sludge foaming. J. Wat. Pollut. Control Fed. 51, 344-350. Eikelboom D. H. (1975) Filamentous organisms observed in activated sludge. Wat. Res. 9, 365-388. Eikelboom D. H. and Van Buijsen H. J. J. (1983) Microscopic Sludge Investigation Manual, 2nd edition. TNO,
Delft, The Netherlands. Hao O. J., Strom P. F. and Wu Y. C (1988) A review of the role of Nocardia like filaments in activated sludge foaming. Wat. Sth Afr. 14, 105-110. Hiraoka M. and Tsumura K. (1984) Suppression of actinomycete scum production: a case study at Senboku wastewater treatment plant, Japan. Wat. Sci. Technol. 16, 83-90. Jenkins D., Richard M. G. and Daigger G. J. (1986) Manual on the Causes and Control of Activated Sludge Bulking and Foaming. Ridgelines Press, Lafayette, Calif.
Khan A. R. and Forster C. F. (1988) Biosurfactant production by Rhodococcus rubra. Envir. Technol. Lett. 9, 1349-1360. Lemmer H. (1985) The ecology of scum causing actinomycetes in sewage treatment plants. Wat. Res. 20, 531-535. Lemmer H. and Bauman M. (1988) Scum actinomycetes in sewage treatment plants---Part 2. The effect of hydrophobic substrate. Wat. Res. 22, 761-763. Lemmer H. and Kroppenstedt R. M. (1984) Chemotaxonomy and physiology of some actinomycetes isolated from scumming activated sludge. Syst. appL Microbiol. 5, 124-135. Pipes W. O. (1978) Actinomycetes scum production in activated sludge processes. J. Wat. Pollut. Control Fed. April, 628-634. Pujol R. (1987) Maitrise du foissonnement des boues activ~es: biosorption et zones de contact. Approche m~thodologique. Th~se de doctorat INSA, Lyon, France. Pujol R. and Boutin P. (1989) Control of activated sludge bulking: from the lab to the plant. Wat. Sci. Technol. 21, 717-726. Pujol R. and Canler J. P. (1989) Le foisonnement des boues activ~es:situation du probl~me en France. Technol. Sci. Meth. 89, 19-24.
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Sezgin M., Lechevalier M. P. and Karr P. R. (1988) Isolation and identification of actinomycetes present in activated sludge scum. Wat. Sci. Technol. 20, 257-263. Vega Rodriguez B. A. (1983) Quantitative evaluation of Nocardia spp. presence in activated sludge. Thesis, Civil Engineering, University of California, Berkeley, Calif.
Wagner F. (1984) Studies on the causes and prevention of bulking sludge in Germany. War. Sci. Technol. 16, 1-14. Wheeler D. R. and Rule A. M. {1980) The role of Nocardia in the foaming of activated sludge: Laboratories studies. Presented at the Georgia Water and Pollution Control Association Annual Meeting, Savannah, GA.