Abstract
The present study aims at optimising the nitrification and denitrification phases at intermittently aerated process (activated sludge) removing nitrogen from municipal wastewater. The nitrogen removal performance recorded at 22 intermittently aerated plants was compared to the results obtained from the simulations given by the widely used ASM1. It is shown that simulations with a single value for the heterotrophic yield with any electron acceptor over-predict the nitrate concentration in the effluent of treatment plants. The reduction of this coefficient by 20% for anoxic conditions reduces the nitrate concentration by 10 g N·m-3. It significantly improves the accuracy of the predictions of nitrate concentrations in treatedeffluents compare to real data. Simulations with dual values (aerobic and anoxic conditions) for heterotrophic yield (modified ASM1) were then used to determine the practical daily aerobic time interval to meet a given nitrogen discharge objective. Finally, to support design decisions, the relevance of a pre-denitrification configuration in front of an intermittently aerated tank was studied. It is shown that when the load of BOD5 is below the conventional design value, a small contribution of the anoxic zone to nitrate removal occurs, except for over-aerated plants. When plants receive a higher load of BOD5, the modified ASM1 suggests that the anoxic zone has a higher contribution to nitrogen removal, for both correctly and over-aerated plants.
Highlights
Due to the ability of most heterotrophic micro-organisms to utilise nitrate as electron acceptor in anoxic conditions, intermittently aerated processes can achieve a significant nitrate conversion into N2 gas
From the data observed on the 22 full-scale plants operated at food-to-micro-organism ratio (F/M) ratio < 0.07 kg BOD5∙kg MLVSS·d)-1, it is shown that the pre-denitrification layout does not provide higher daily average denitrification rates than those observed with the single tank configuration
This modification lowers the predicted nitrate concentration in treated water by 10 g NO3-N·m-3 for simulations of over-aerated plants. Once this modification is implemented in ASM1, simulations allow to determine the optimal daily aerobic time range to maintain both NH4+-N concentrations below 2 g N·m-3, and the sum of NH4+-N and NO3--N concentrations below 10 g N·m-3
Summary
Due to the ability of most heterotrophic micro-organisms to utilise nitrate as electron acceptor in anoxic conditions, intermittently aerated processes can achieve a significant nitrate conversion into N2 gas (denitrification). ISSN 1816-7950 = Water SA (on-line) aeration periods to maintain low-growth rate nitrifying bacteria (FNDAE_25, 2002; Choubert et al, 2005a) These technical rules provide a nitrogen discharge in accordance with the objectives of the European Directive 91/271/EEC (1991) requiring a high nitrogen removal efficiency from wastewaters above a temperature of 12°C: a total nitrogen discharge objective below 15 gN·m-3 is required in treated effluent. Recent studies have shown that some of the default parameter values proposed in ASM1 needed to be reconsidered to correctly predict nitrogen removal performances (Dold et al, 2005; Choubert et al, 2008) These results influence the optimal operating conditions like the aeration time. The performances of both layouts were compared, and the relevance of the predenitrification design was critically assessed for different F/M ratios
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