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Floc characteristics were studied at a full scale activated sludge treatment plant with a unique process solution incorporating pre-denitrification with post-nitrification in nitrifying trickling filters. Since greater nitrogen removal is achieved when more secondary settled wastewater is recirculated to the trickling filters, the secondary settlers are always operated close to their maximal capacity. The flocculation and settling properties are therefore crucial and have an effect on the overall plant performance. Since the plant is operated at a short sludge age, these properties change quickly, resulting in variable maximal secondary settler capacity. The dynamics in floc structure and microbial community composition were studied and correlated to the secondary settler performance. Fluorescence in situ hybridisation was used to investigate the microbial community structure and their spatial distribution. The floc structure could to some extent be related to the flocculation and settling properties of the sludge. Even small differences had an influence suggesting that colloidal properties also play a significant role in determining the floc properties. No correlation between microbial community composition and settling properties could be established with the group-specific probes investigated. [ABSTRACT FROM AUTHOR]

Secondary settling dynamics at maximal capacity were investigated at a full scale wastewater treatment plant which utilizes a unique process solution incorporating pre-denitrification with postnitrification in nitrifying trickling filters. Since nitrogen removal is greater when more secondary effluent is recirculated to the trickling filters, the secondary settlers generally operate at close to their maximal capacity. The settling and flocculation properties of the activated sludge are therefore a major capacity-determining factor for plant operation. Due to the short sludge age, the flocculation properties, with respect to both thickening and clarification, can change quickly. The dynamics in these changes were studied and the factors that determine the maximal settling capacity were assessed. Solids flux curves were constructed from batch settling tests and compared with the actual maximal settling capacities. [ABSTRACT FROM AUTHOR]