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Mechanical treatment and microscopic investigation of bulking activated sludge
2000 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Bulking sludge is one of the most common problems in wastewater treatment plants with activated sludge process included. The causes of bulking sludge are countless and complex and many factors which influence the composition of sludge micro-organisms, and the occurrence of bulking sludge is often difficult to explain. Therefore, there is no general combat method that could be applied for every case of bulking. It seems difficult to predict the effect of one measure and literature recommendations are sometimes ambiguous and contradictory. In this work the possibilities to combat an activated bulking sludge, caused by filamentous organisms, by employing mixing and ultrasound were investigated in batch and continuous experiments. The aim was to improve the settling properties of the sludge and eliminate the symptoms of bulking. Mechanical treatment of bulking sludge is a new method, which shows fast results and can be applied for every case of bulking. The sludge was microscopic examined with the purpose to identify and assess the amount of filamentous organisms, to recognise changes of the sludge structure and composition due to the mechanical treatment, and to enable a prompt intervention at the first signs of bulking. Batch experiments were carried out to investigate adjustments of the devices and experimental conditions. One type of mixer and two different ultrasound devices (20 and 41.9 kHz) were used. Continuous experiments were accomplished with mixer and ultrasound at 20 kHz to test the combat by mechanical treatment in an activated sludge process. Two 9 L reactors completely mixed with integrated sludge separation were used. At the time of treatment by mixing or ultrasound one of the reactors was connected to a 3 L vessel where the activated sludge was treated twice daily. The other reactor was run as reference. In all batch experiments with mixing, the settling properties were not improved, but rather deteriorated. However, in the continuous experiments it was shown that mixing could very well be used to combat activated bulking sludge caused by Sphaerotilus natans. The successful treatment was initiated when a large amount of filamentous organisms were observed in microscope but the sludge still exhibited proper settling. The sludge was treated at a rotation velocity of 20500 rpm for five minutes, corresponding to an energy input of 7.2 kWh/m3. When this treatment with mixing was applied at very high values of SVI, i.e. at very strong bulking, the treatment had no effect on the settling properties. Both ultrasonic batch devices were able to combat the growth of filamentous organisms, at frequency 20 kHz better than at 41.9 kHz. At 20 kHz and middle intensity, the SVI value was decreased from over 200 mL/g to below 150 mL/g in only a few minutes. This adjustment was initially employed in the continuous experiment, but the energy input was to low for combating bulking sludge. As the sludge was treated for five minutes at a frequency of 20 kHz and middle intensity, corresponding to an energy input of 6.6 kWh/m3, the filamentous bulking was successfully combated. Fungi were the dominating organisms at this moment, and an increase of the pH value actually combated these fungi in both reactors. However, other types of filamentous organisms took over in the reference reactor and the SVI values soon exceeded 1000 mL/g, whereas the ultrasonic treatment prevented new filaments from being established in the sludge and the proper settling properties remained in this reactor. In this work it was shown that mixing or ultrasound could be used to combat filamentous bulking without destroying the structure of the sludge. No significant decrease of COD removal was noticed. Nevertheless, the results also showed that a mechanical treatment does not always improve the settling properties possibly because of the different nature and composition of activated sludge from different treatment plants.

Place, publisher, year, edition, pages
Keyword [en]
Technology, bulking activated sludge, mechanical treatment, mixing, ultrasound, filamentous organisms, microscopic examination
Keyword [sv]
URN: urn:nbn:se:ltu:diva-57368ISRN: LTU-EX--00/153--SELocal ID: e0bb0fb8-bfed-4eea-b420-c876a205cc4fOAI: diva2:1030755
Subject / course
Student thesis, at least 30 credits
Educational program
Civil Engineering programmes 1997-2000, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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