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Aerobic granulation in Sequencing Batch Reactor for industrial wastewater treatment

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Granulation is the process in which biomass forms discrete, well defined, smooth and dense pellets, without any application of support material. Aerobic granulation, similar as anaerobic granulation, could lead to very compact high performance reactors with elevated biomass accumulation and low sludge production. To date this process is not widely investigated and few is known on its applicability to full scale wastewater treatment.

The aim of this study was first to test this process on synthetic wastewater and confirm results presented in literature. In a second step real industrial wastewater was also tested and the performances of the system were studied: Applying real industrial wastewater is essential in order to screen the value of the aerobic granules process. Initial focus was put on removal of organic pollutants and destined loading.

Two pilot plants were operated as sequencing batch bubble columns. The reactors consist of cylindrical columns with a working volume of 40L. Initial phase definition of a SBR cycle is as follows: inlet phase (4 minutes), aeration phase (3 hours), settling phase (3 minutes) and discharge phase (2 minutes). A volume exchange rate of 60%, resulting to a hydraulic retention time of 5 hours, is applied. Acetate is used as carbon source for synthetic wastewater. The industrial effluent originates from the pharmaceutical industry and exhibits a biodegradability of 80%.

Focus was put on the feasibility of the process and on its operational procedure. Main parameters necessary to form aerobic granules in SBR are: high shear stress, short settling time, alternation of feast and famine periods and short hydraulic retention time.

For both wastewater, the pilot plants are seeded with conventional activated sludge. A rapid formation of aerobic granules is observed along with a high COD removal rate. Granules characteristics are similar with the two types of wastewater and according to visual observation, it seems that filamentous bacteria are part of the granule structure.

With synthetic wastewater high organic loads were successfully tested: 7-8kgCOD/m3.d, and a very good dissolved COD removal rate was reached : 95%. With industrial wastewater a dissolved organic load of 5.5kgCOD/m3.d was applied, resulting in a COD removal rate of 80% (the 20% left is the refractory part of the COD). However, a relative high concentration of suspended solids in outlet causes a lower performance of total COD removal. Biomass detachment seems to play a non negligible role in the current set-up. This illustrates that this process treats soluble COD, but all particulate pollutants should be removed via appropriate pre-or post-treatment. Furthermore, it emphasis that the operation of the reactors still have to be optimised in order to reduce biomass detachment to a minimum.

This study focuses preliminary on COD removal, but literature reveals also its potential for N and P removal. First results with real industrial wastewater demonstrate the feasibility of this innovative process. Ongoing investigation are aiming at optimisation of the operation with regard to granule stability and performance as well as to economic aspects.
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Document Type: Research Article

Publication date: 2003-01-01

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