Reduction of bulking and foaming sludge by mechanical treatment


A sludge with poor settling properties, which results from massive growth of filamentous bacteria, is described as bulking sludge or as foaming sludge respectively, if foam is formed on the water surface. The reasons for formation of bulking- and foaming sludge are not completely clarified to this day and there are no methods to fight these sludges, which are generally effective, because of the variety of filamentous microorganisms and the large amount of factors, influencing the production of filamentous microorganisms.

Any method aimed at fighting the filamentous growth must eliminate the reasons for this development. In many cases this requires a rebuilding and reorganisation of the process layout. From the implementation of the method to the solution of the problem a lot of time may pass and success cannot be guaranteed.

Seeking a possibility to stop the filamentous growth immediately or to fight the symptoms respectively, leads to new ideas for the process layout. The non-specific method of mechanical comminution of bulking- and foaming sludge presented here, aims at a quick solution of this often urgent problem. At present it cannot be answered if a causal fight against these phenomenons can be replaced in the long term. The usage of mechanical comminution appears reasonable, if reorganisation of the process layout or rebuilding actions are not practicable, have not been carried out yet or have not been successful.

The basic mechanisms of this treatment and the possibility for their use are described in this article. The research presented shows the possibilities for the mechanical treatment of foaming- and bulking sludge and their effects on the wastewater treatment process. The aims are the prevention of foaming problems in the aeration tank and in the digester, as well as the prevention of bulking sludge loss in the final clarification. This can be achieved by comminution of the voluminous flocs of foaming- and bulking sludge to a size of normally formed sludge flocs thus improving their settling properties.

At present, experiences are mainly available from lab-scale tests. The positive experiences in the improvement of settling properties were confirmed in first results obtained in wastewater treatment plants, but the influence on the outlet results and the necessary energy input still is under investigation. Therefore mechanical comminution of foaming- and bulking sludge represents an interesting new technique, whose possible applications have to be confirmed.



The problems caused by filamentous organisms and the positive effect of comminution are shown in principle in Fig.1:

·      On the one hand the filamentous structures can prevent the flocs from settling effectively by causing a voluminous floc structure. There is no chance to get a more compact floc without destroying those structures (Fig.1. left).

·      On the other hand substances, which causes floc flotation because they have a smaller specific density than water, can get caught in the filamentous structure. Especially gas bubbles tend to do so. The gas bubbles can be caught in the floc-network and make the floc float permanently. (fig.1, middle).

·      Mechanical disintegration of bulking- and foaming sludge is used to break up the filament structure. That removes the supporting structure of filamentous organisms from the voluminous floc and gas bubbles are released. The smaller fragments of the floc cause an increased settling velocity and a decreased settled sludge volume (Fig.1, right). These positive effects have been observed for different parts of the wastewater treatment process (aeration tank, final clarification, stabilisation).


Bulking sludge

Filamentous sludge with gas bubbles (scum)

Filamentous sludge after Disintegration

Fig.1:           The comminution effect on filamentous sewage sludge flocs with settling problems shown             schematically


The sludge volume index (SVI) determination of the non-treated and the treated foaming sludges revealed that the non-treated foaming sludges only partly settled in the sedimentation cylinder and a big part remained floating (Fig. 2). After treatment with the high-pressure homogenizer at Dp = 100 bar, the whole sludge settled and the SVI was 125 ml/g. Corresponding results were also obtained with other devices and for two different sludges. The improvement of the sedimentation can be seen clearly in the photographs.


Fig. 2: a) 0 min

 b) 30 min.

a, b: settling of the foaming sludge before (cylinder on the left) and after mechanical treatment (cylinder on the right, high-pressure homogenizer 100 bar), all samples diluted 1:4



Apart from settling problems in the aeration tank, foam problems in digesters are known. These problems are frequently based on filamentous organisms in the activated sludge process. To determine the effect of mechanical treatment on the digestion process batch-tests were carried out.

Fig. 3 shows the obtained digestion results of a test comparing disrupted and untreated sludges. In this test, after one day digestion time only, a floating sludge volume FSVDS = 100% for the non-treated sludge sample was observed. In this case that means that not only the sludge of the aeration tank was floating, but also the seeding anaerobic sludge in addition was forced to float as well. Disintegration using high-pressure homogenizers at 100 bar leads to FSVDS = 0% for short digestion times. Within a digestion time of five days the amount rose to FSVDS = 40%. This is caused by the high gas production within this time and can be seen clearly in the standardized gas production, shown here as well. The increase of FSVDS coincides with the maximum of the gas-production. After a decrease of the daily gas production, FSVDS decreases very fast.

Fig. 3: Floating sludge volume FSVDS in anaerobic batch-tests