The potential of ultrasound treatment for sludge reduction during the activated sludge process was assessed. Batch and dynamic disintegration studies were completed using an activated sludge pilot-scale plant fed with settled sewage that was comprised of two 1.2 m3 lanes, operated as a test and control. A 1 kW ultrasound system was integrated into the test lane of the pilot-scale plant to allow continuous in-line ultrasound treatment of the return activated sludge (RAS). Seven dynamic trials were completed over an 8-month period, treating from 1.7 to 12.5% of the RAS with energy densities between 42 and 168 kJ L-1. During the batch disintegration studies, it was observed that ultrasound treatment at 42 to 186 kJ L-1 caused floc breakage and sludge solubilisation. Floc size was reduced by 88% at 42 kJ L-1 while the degree of soluble COD release increased almost linearly from 11 to 36 % between 42 to 168 kJ L-1. A change in the biological activity was observed only at 168 kJ L-1 with an 8.5 % increase in the specific oxygen consumption of the treated RAS samples in comparison to the untreated ones. During the dynamic studies, a 20% degree of sludge reduction was observed treating 10% of the RAS at 42 kJ L-1. At these operational conditions, there was no significant difference in the total COD and nitrogen removal between the control and test lanes. However, a 5.5-fold increase in the capillary suction time and a 3.6-fold increase in the specific resistance to filtration in the RAS from the test lane indicated a detrimental impact on dewaterability. Increasing the energy input, by treating 12.5% of the RAS at 84 kJ L-1, did not result in a significant increase in sludge reduction, which indicates that there might be limits to the degree of reduction achievable with the ultrasound treatment. Results from the dynamic studies suggested that lysis-cryptic growth was the main mechanism behind the observed sludge reduction. Based on modelling, lysis-cryptic growth could justify 98 % of the sludge reduction observed. Cost analysis results indicated that sludge reduction by ultrasound disintegration was not currently economically viable unless the cost for sludge treatment and reuse increased to £ 961 from the reference price of £ 423 per tonne of dry solids.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:501021 |
| Date | January 2008 |
| Creators | Minervini, Davide |
| Contributors | Cartmell, Elise ; Parsons, Simon |
| Publisher | Cranfield University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://dspace.lib.cranfield.ac.uk/handle/1826/4085 |
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