Abstract
Simultaneous nitrification-denitrification (SND) is, in theory, a key advantage of aerobic granular sludge systems over conventional activated sludge systems. But practical experience and literature suggests that SND and thus total nitrogen removal are limited during treatment of municipal wastewater using AGS systems. This study thus aims at quantifying the extent and understanding the mechanisms of SND during treatment of municipal wastewater with aerobic granular sludge (AGS) systems. Experiments (long-term and batch-tests) as well as mathematical modelling were performed. Our experimental results demonstrate that SND is significantly limited during treatment of low-strength municipal wastewater with AGS systems (14–39%), while almost full SND is observed when treating synthetic influent containing only diffusible substrate (90%). Our simulations demonstrate that the main mechanisms behind limited SND are (1) the dynamics of anoxic zone formation inside the granule, (2) the diffusibility and availability of electron-donors in those zones and (3) the aeration mode. The development of anoxic zones is driven by the utilisation of oxygen in the upper layers of the granule leading to transport limitations of oxygen inside the granule; this effect is closely linked to granule size and wastewater composition. Development of anoxic zones during the aerobic phase is limited for small granules at constant aeration at bulk dissolved oxygen (DO) concentration of 2 mgO2 L−1, and anoxic zones only develop during a brief period of the aerated phase for large granules. Modelling results further indicate that a large fraction of electron-donors are actually utilised in aerobic rather than anoxic redox zones – in the bulk or at the granule surface. Thus, full SND cannot be achieved with AGS treating low strength municipal wastewater if a constant DO is maintained during the aeration phase. Optimised aeration strategies are therefore required. 2-step and alternating aeration are tested successfully using mathematical modelling and increase TN removal to 40–79%, without compromising nitrification, and by shifting electron-donor utilisation towards anoxic redox conditions.
Highlights
Introduction van Loosdrecht andBrdjanovic, 2014 In aerobic granular sludge (AGS) systems, mass transfer is limited by diffusion, which leads to concentration gradients of electron-donors/final acceptors within the granules
Average effluent NO3À concentrations below 4 mg NO3eN LÀ1 were measured for AGS system fed by 100%-VFA synthetic WW only, while values of 5e15 mg NO3eN LÀ1 were measured in the effluents of the complex synthetic, primary effluent and raw WW AGS systems, respectively
Our long-term experiments indicated that limited simultaneous nitrification and denitrification (SND) is observed for AGS systems treating municipal WW, while full total nitrogen (TN) removal via SND is representative of systems fed with VFA only
Summary
Introduction van Loosdrecht andBrdjanovic, 2014 In AGS systems, mass transfer is limited by diffusion, which leads to concentration gradients of electron-donors/final acceptors within the granules. Values ranging from 10 to 100% are reported for different dissolved oxygen (DO) concentrations in the bulk liquid (Kocaturk and Erguder, 2016; De Kreuk et al, 2005; Lochmatter et al, 2013). For those systems, high SND efficiencies of more than 75% are typically observed for DO values below 4 mgO2 LÀ1 (Fig. 1). It is crucial to understand the extent and mechanisms of SND in order to further optimise TN removal of AGS systems
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