Proceedings

Low cost SBR N&P removal through load selection

Holm, N.C.1 and Parnowska, M.2, 1LimnoTec Abwasseranlagen GmbH (SH+E), 2SH+E Group, UK

(free)

Abstract Innovative wastewater treatment processes were developed for high efficiency N&P removal, control strategies, fault detection and sludge floc selection in addition to bio-augmentation processes aimed at the reduction of sludge volume indexes (SVI) with the help of online sensors. In this paper, several examples of new or changed process strategies for full-scale wastewater treatment plants (WWTP) are presented. Two of these processes consider new bio-augmentation strategies with successful SVI reductions. The results will be presented and discussed, especially with regards to N&P removal, process control and stability, applicability and effects upon overall treatment performances.
Keywords bio-augmentation, biological P elimination, denitrification, nitrification, SBR, sludge volume index (SVI)
Introduction
The activated sludge process, which was first used in England at the beginning of the 20th century (Arden and Lockett, 1914), has become a huge success story, based mainly on an effective separation of sludge and water in the downstream sedimentation/secondary clarification basin.
Separation is based on specific sedimentation properties of the activated sludge flocs. These occur automatically after a certain run-in phase, even without inoculation with activated sludge from an existing plant, because all particles which do not sediment are washed out of the system. If this selective pressure towards sedimentation did not exist, for example, such as in MBR plants, none or very few flocs would form.
The classical activated sludge process is therefore already based on a pronounced biomanipulation.
Many further bio-manipulations have been developed up to the current time in many variations in order to “breed” new required characteristics such as nitrification, biological P elimination, reduction of the sludge volume index (SVI) etc., into the activated sludge.
This papers deals in particular with some of these required characteristics (nitrification, denitrification and biological P elimination) in SBR systems. Several new SBR process concepts are presented, focussing mainly upon the enhancement of denitrification and the biological P elimination. But all these processes need space/time in the form of higher sludge ages and huge amounts of MLSS (mixed liquor suspended solids). Because the possible MLSS concentrations are determined by the sludge volume index (SVI), a reduction of the SVI will be especially advantageous.
Despite this inherent activated sludge (AS) process characteristic, many WWTPs suffer from a high SVI, eventually causing bulking sludge and sludge loss via the final clarifier. For this reason many variations of further biological or technical manipulations have been developed up to the current time in order to “breed” a reduced SVI into the AS. The extent of the problem has been shown during the course of many research studies. In the context of an extensive international study, 42 of 70 industrial WWTPs investigated faced problems caused by bulking sludge in the secondary final clarifier (Eikelboom and Geurkink, 2001). 63 % of all WWTPs in England and 25 % in France had this problem of bulking sludge (Wanner, 1994). During an investigation of Bavarian WWTPs (Bauer, 2003) 14 % of all WWTPs with extended aeration showed an SVI of more than 150 ml/g which generally is considered the threshold for bulking sludge. Another 19 % evidenced an SVI of more than 135 ml/g, which resulted in the fact that these WWTPs sometimes also suffer from bulking sludge.
Various process technology aspects have been investigated for their effects on the SVI. Some of them – like anoxic or anaerobic selectors – seem to be at least partly favourable, but a lot of exceptions were found. Organic and inorganic additives are often used to suppress bulking sludge, floating sludge and foam, but the efficiency varies from plant to plant. The same was reported on the efficiency of a new “multi-component additive” (Seka et al., 2001), which was less efficient for the Nostocoida filamentous organisms and caused a significantly higher sludge production. Positive effects on the SVI were obtained by adding the divalent cations Ca and Mg (Sobeck and Higgins, 2001). However, realisation in full-scale hardly seems feasible because of the large quantities of chemicals that would be needed.
The realisation of the feast/famine principle at SBR (Sequencing Batch Reactor) plants (Irvine and Busch, 1979), which can be achieved by pronounced shock loadings and plug flow concepts at conventional continuous plants, ranks among the most stable positive factors. Further positive factors are strictly anaerobic conditions in anaerobic-basins, oxygen concentrations > 1.5 mg/l in the aeration basins/areas/phases and ammonium concentrations in the effluent < 1.0 mg/l in addition to aluminium precipitants (Martins et al., 2003). However, many contradictory large-scale examples do exist.
All the procedures referred to above aim at creating automated and controlled process conditions which indirectly have a positive selection effect on the fast-settling floc fractions. This is possible in principle, because the AS in all the plants consists of a mixture of flocs with different settling rates (Schmid et al., 2003).
This paper presents some new and altered processes which aim at creating special selection strategies. In most cases, these strategies have a direct negative selection effect on slow-settling floc fractions. This is achieved in some cases by selecting alternative waste activated sludge (WAS) withdrawals, which ensure a high proportion of the slow-settling fraction within the AS is selectively withdrawn with the WAS.

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