Activated Sludge BulkingAuthor(s): H. HeukelekianSource: Sewage Works Journal, Vol. 13, No. 1 (Jan., 1941), pp. 39-42Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25029143 .

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ACTIVATED SLUDGE BULKING*

By H. Heukelekian

Associate, Sewage Experiment Station, New Brunswick, N. J.

A few fundamental facts regarding the activated sludge process will

be helpful for a clearer understanding of the bulking problem. 1. Activated sludge is an aerobic process. The mechanism of this

process is completely inhibited under anaerobic conditions and greatly

impaired under partially aerobic conditions. Passing air through an

aeration tank does not necessarily insure aerobic conditions for several reasons :

(a) Low rate of solution of oxygen in sewage.

(b) Most of the oxygen passes out into the air unused.

(c) The high oxygen demand exerted by the large number of organisms in activated sludge in the presence of food materials.

(d) Inefficiency of the spiral flow aeration system with its short circuit ing and core formation.

The supply of oxygen adequate at one place may be entirely insuffi cient at another, due to variations in the strength of sewage, amount and character of sludge carried, design of the tank, type of air diffusers, etc. In addition, even at a given place a supply of oxygen adequate at one time may be inadequate at another time due to some of the above variables as well as differences in temperature.

It is clear therefore that the amount of air supplied is not a reli able criterion of the maintenance of the aerobic conditions. An ade

quate supply of air may be defined as that amount necessary to meet the demand under the existing conditions leaving some in excess as

residual dissolved oxygen. The requisite residual dissolved oxygen level to insure aerobic conditions throughout the sludge floe cannot be

definitely stated at present. It is true that in a homogenous system such as in the B.O.D. determination or in surface waters, the oxygen consumption rate is not influenced by the dissolved oxygen level within the normal atmospheric range of oxygen tension. Yet it is conceivable that in a heterogenous system such as in the activated sludge, an oxygen

deficiency may exist within the interior of floe while the surrounding water carries small amounts of dissolved oxygen. This is to be at tributed to the high rate of oxygen consumption within the floe and the low rate of oxygen diffusion from the surrounding water into the floe.

Since the vital seat of activity is within the floe, the oxygen tension in the surrounding water should be high enough to establish an oxygen

gradient and provide for the replenishment of oxygen as fast as it is consumed.

* Journal Series Paper, N. J. Agri. Exper. Sta., New Brunswick, N. J., Dept. Water and

Sewage Research.

39

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40 SEWAGE WORKS JOURNAL Jan., 1941

Frequently dissolved oxygen may be entirely absent in the inlet end of the aeration tank due to the high rate of biochemical oxygen utiliza tion. Depending on the length of oxygen deficiency period in the aera tion tank, appreciable changes take place in the character of the sludge.

Subsequent recovery under aerobic conditions maintained in the latter

part of the tank may not be complete. It is also essential that dissolved oxygen should be present at all points in the tank.

2. It can be safely stated that irrespective of whether the initial

phase of clarification is brought about by purely physical-chemical or

biological agencies, the final stage of purification as well as the initial clarification stage is dependent on the floe produced by the microbiolog ical flora. Since activated sludge is not a pure culture of a single or

ganism, but is composed of a variety of different organisms, it is evi

dent that under certain environmental conditions the proper biological balance may be upset, giving rise to the preponderance of certain or

ganisms and a different type of a floe. The change in the biological balance is due to the fact that there are specific differences of optimum environmental conditions even among closely related organisms. Some

of the organisms involved in the activated sludge have been identified, but their specific physiological requirements are not known sufficiently to suppress at all times the activities of the undesirable kinds or to en

hance the activities of the more desirable kinds and thereby to increase

the efficiency of the process. 3. The activated sludge process is suitable only for wastes in which

the concentration of food material is low. When fresh solids are

aerated an aerobic decomposition takes place, but a floe similar to ac

tivated sludge is not obtained. It is also a matter of general observa

tion that large quantities of organic industrial* wastes such as packing

house, dairy, distillery, and sulfite liquor wastes interfere with the

activated sludge process. With high concentrations of food materials

bacterial growth consists of cells dispersed in the liquid phase, while

food materials in dilute condition are more readily utilized by organ isms capable of forming growth aggregates. When food material is

too dilute the only possible efficient microbiological process takes place when the food material is localized in certain areas in higher concen

tration by virtue of surface adsorption. In these localized areas will

also be found a large number of micro-organisms It has been observed

that when abnormally large quantities of a material like sugar are

added to activated sludge not only does the floe become diffuse but the

liquor acquires a turbidity chiefly due to the bacterial cells growing in the liquid phase. Dispersion of the cells might also be caused by the anaerobic condition brought about with such overloading.

The above fundamental principles involved in the activated sludge process have an important bearing on the bulking problem because

there is an intimate relationship between bulking and the purification mechanism. If the purification mechanism is destroyed by the use of

poisons or in the absence of oxygen, bulking will not be noticed. On

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Vol. 13, No. 1 ACTIVATED SLUDGE BULKING 41

the other hand a bulky sludge usually produces the most sparkling effluents. According to our view bulking is a "disease" of the sludge developed in the course of purification of sewage under unfavorable environmental conditions Bulking results when the conditions are not so unfavorable as to destroy the purification mechanism and yet suffi ciently unfavorable so as to bring a shift in the delicate biological bal ance. One of these unfavorable conditions is the inadequate oxygen supply. The inadequacy of the oxygen may be due to any or a combi nation of diverse factors such as have been discussed above. If acti vated sludge was formed by a pure culture of a strictly aerobic organ ism the inadequate supply of oxygen would reflect itself immediately on decreased efficiency of purification. But since there are other or

ganisms present, which probably can tolerate a lower oxygen tension, the actual result of an inadequate oxygen supply is the suppression of the strictly aerobic organisms and the stimulation of organisms with lower oxygen tolerance without an impairment of the purification. Al

though the evidence is not complete it appears from observations given in the literature that zoogleal organisms are strict aerobes and that

Sphaerotilus and other filamentous organisms can grow at a lower oxygen tension. When the oxygen supply is ample in relation to the

demand, zoogleal organisms thrive; Sphaerotilus and other filamentous

organisms are small in number and the sludge settles rapidly. When the oxygen supply in relation to the demand becomes inadequate

Sphaerotilus and other filamentous organisms get the ascendancy and the sludge becomes bulky. Biochemical activities of the latter organ isms bring about the purification of sewage in a way similar to the

zoogleal organisms except with a lower and more efficient oxygen utili zation at the lower tensions. In other words when the sludge is diffuse and filamentous it exposes more surface, which enables the sludge to obtain the limited amount of oxygen present in the medium immediately surrounding it, whereas with a compact sludge the oxygen has to pene trate further into the floe.

The concept of bulking as a "disease" of the sludge suggests the associated idea of an incubation period. Just as in infectious diseases there is a period between the actual infection and the appearance of the symptoms of the disease; the causes of bulking might be operative for a period of time before the actual symptom becomes apparent.

When the disease is so advanced as to make the symptoms obvious it takes considerably more effort to cure it. The failure of bulky sludge to respond quickly to a greater air supply does not disprove the contention that an adequate air supply applied at the critical period prior to the "outbreak of the disease" would have been more effective in checking and preventing the bulking. The slow, cumulative type of bulking, nat urally applies only to sewage not containing appreciable amounts of industrial wastes. Explosive and sudden types of bulking frequently occur when "shots" of industrial wastes are received. Cause and ef fect in this type of bulking usually follow each other so closely that it

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42 SEWAGE WORKS JOURNAL Jan., 1941

is not so mysterious as the more subtle and slow changing kind in the

absence of industrial wastes. In the latter case causative factors are

not so violent but assume an additive and accumulative character over

a comparatively long period of time. Let us assume an activated

sludge plant which does not receive any industrial waste but which is

normally operated without a sufficient safety factor. During the sum

mer months with dry weather flow the strength of sewage increases, the sewage has a high oxygen demand, a lower oxygen saturation value

and a higher biochemical oxygen utilization rate. A greater percent

age of the oxygen supplied goes to the oxidation of the substrate and

a smaller percentage to oxidation of the sludge. The sludge becomes

less and less oxidized, therefore exerting a higher demand, thereby tax

ing the already limited oxyg?nation capacity. This is only one typical case ; other series of circumstances causing bulking could also be cited.

What can the operator do to check bulking? The first maxim is constant vigilance, to keep ahead of the process so that it does not get out of hand making belated measures more or less useless and costly.

Daily examination of the sludge with the microscope will be found valu

able in determining whether the filamentous organisms are becoming

preponderant. Daily dissolved oxygen determinations of the aeration

tank contents with particular attention to the inlet end of the tank, will also be useful. Nothing has been said here about the relation of nitri

fication to bulking but in general the presence of nitrates is an indica

tion of adequate oxygen supply and therefore nitrate determination is

desirable. Naturally the determination of the sludge index day by day will aid in showing the tendency toward sludge bulking.

When it is found by means of the above determinations that the

sludge has started to become bulky the first remedial measure would be to reduce the amount of sludge carried in the aeration tanks, thereby

reducing somewhat the oxygen demand. Simultaneously the quantity of air supplied may be increased. Keeping the sludge in a fresh con

dition by not allowing it to accumulate in the settling tank for any length of time is always a good practice. Further and more drastic meas

ures such as dilution of the sewage with effluent or surface water, chlorination of the sludge to reduce the high initial rate of oxygen de

mand may also be found helpful. However, the value of all these reme

dial measures will be greater the earlier they are attempted. Changes in the basic design of the aeration tanks with the purpose of increasing the efficiency of aeration are beyond the operators' realm but may be

necessary before bulking is controlled. Changes which might help are

prevention of short circuiting and core formation, selection of proper

diffusers, and increasing the air diffusion area in relation to the surface area.

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