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Financial support was provided by the Spanish Ministerio de Medio Ambiente y Medio Rural y Marino (Project Exp. 007/RN08/03.1). Altres ajuts: MAPAMA/007/RN08/03.1 Growth in production and use of nanoparticles (NPs) will result increased concentrations of these in industrial and urban wastewaters and, consequently, in wastewater-treatment facilities. The effect of this increase on the performance of the wastewater-treatment process has not been studied systematically and including all the microbial communities involved in wastewater treatment. The present work investigates, by using respiration tests and biogas-production analysis, the inhibitory effect of four different commonly used metal oxide (CeO₂ and TiO₂) and zero-valent metal (Ag and Au) nanoparticles on the activity of the most important microbial communities present in a modern wastewater-treatment plant. Specifically, the actions of ordinary heterotrophic organisms, ammonia oxidizing bacteria, and thermophilic and mesophilic anaerobic bacteria were tested in the presence and absence of the nanoparticles. In general, CeO₂ nanoparticles caused the greatest inhibition in biogas production (nearly 100%) and a strong inhibitory action of other biomasses; Ag nanoparticles caused an intermediate inhibition in biogas production (within 33-50%) and a slight inhibition in the action of other biomasses, and Au and TiO₂ nanoparticles caused only slight or no inhibition for all tested biomasses.

A ratio control strategy has been used to demonstrate the feasibility of this automatic control procedure for the achievement of stable full and partial nitritation. The control strategy assured constant ratio between the dissolved oxygen (DO) and the total ammonia nitrogen (TAN) concentrations in the bulk liquid of aerobic granular sludge reactors operating in continuous mode. Three different set-ups with different reactor capacities were used (3, 110, and 150 L). High strength synthetic wastewaters and reject water were tested with similar performance. Achieved nitrogen loading rates ranged between 0.4 and 6.1 kgN·m−3·d−1, at temperatures between 20°C and 30°C. Granular sludge and nitritation were stable in the long term continuous operation of the reactors. Suitable stable effluent for Anammox has been obtained using the desired TAN setpoint (i.e. 50% of influent ammonium oxidation). An existing biofilm model developed incorporating the implemented control loops and validated in a previous publication was used to investigate the effects of the ammonium concentration of the influent and the biofilm density on the achievement of full nitritation. The model demonstrated how sludge recirculation events led to a stable and significant increase of the biomass concentration in the reactor, which in turn resulted in the achievement of high nitrogen loading rates, due to the action of the control strategy. The model predicted an enhancement of stable full nitritation at higher ammonium concentrations in the influent. Poor influence of the biofilm density in the achievement of full nitritation was predicted with the model.

The use of different carbon sources (ethanol, acid-fermented primary sludge centrate, acid-fermented secondary sludge centrate, glycerol and landfill leachate) in heterotrophic denitrification from nitrite (denitritation) was studied in a sequencing batch reactor, operated without pH control. Complete denitritation of a high-strength nitrite wastewater was achieved using these organic carbon sources with the exception of fermented secondary sludge centrate. Loading rates around 0.2 g N L −1 d −1 were obtained for glycerol, landfill leachate and ethanol after a short start-up period of 20 days. The maximum specific nitrite removal rate of 0.25 g N g −1 VSS d −1 was achieved for glycerol, while values between 0.13 and 0.17 g N g −1 VSS d −1 were obtained using ethanol, landfill leachate and fermented primary sludge centrate. The COD/N ratio consumed varied between 3.0 for ethanol and 8.8 for landfill leachate. The denitritation rates and the required COD/N ratio for each carbon source are reported for the first time – they can be used for the scale-up of the denitritation process.

Funding: AGAUR and ACC1Ó (Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya), 2010VALOR0096. The authors thank Eduard Moliné (DAM) for helpful discussions during the course of the research. M. Pijuan also acknowledges the Ramon y Cajal research fellowship (RYC-2009-04959) provided by the Spanish Government. Nitrous oxide (N₂O) and methane emissions were monitored in a continuous granular airlift nitritation reactor from ammonium-rich wastewater (reject wastewater). N₂O emissions were found to be dependent on dissolved oxygen (DO) concentration in the range of 1-4.5 mg O₂/L, increasing within this range when reducing the DO values. At higher DO concentrations, N₂O emissions remained constant at 2.2% of the N oxidized to nitrite, suggesting two different mechanisms behind N₂O production, one dependent and one independent of DO concentration. Changes on ammonium, nitrite, free ammonia and free nitrous acid concentrations did not have an effect on N₂O emissions within the concentration range tested. When operating the reactor in a sequencing batch mode under high DO concentration (>5 mg o₂/L), N₂O emissions increased one order of magnitude reaching values of 19.3+/-7.5% of the N oxidized. Moreover, CH₄ emissions detected were due to the stripping of the soluble CH4 that remained dissolved in the reject wastewater after anaerobic digestion. Finally, an economical and carbon footprint assessment of a theoretical scaled up of the pilot plant was conducted.

An automated sequencing batch reactor operation based on online measurement of the ammonium concentration was investigated as a tool for improving the start-up of a nitrifying granular airlift reactor. The effectiveness of this start-up procedure was verified with the characteristics of the developed granular sludge but also the improvement of the start-up was confirmed when comparing with the results achieved with two continuous-mode start-up strategies. Once a stable granular biomass was obtained, the reactor started to operate in continuous mode during more than 100 days, maintaining the characteristics of the granular biomass and achieving a nitrogen loading rate of 1.75 g N L−1 d−1. The intermittent recirculation of small flocs of nitrifying biomass was explored as an alternative to increase the biomass concentration in the reactor and consequently, to increase the treated loading rate.