Closed-loop control of ammonium concentration in nitritation: convenient for reactor operation but also for modeling.

A mathematical biofilm model was developed to describe nitritation in aerobic granular reactor operating in continuous mode. The model includes the automatic closed-loop control of ammonium concentration in the effluent. This is integrated in a ratio control strategy to maintain the proportion between the dissolved oxygen (DO) and the total ammonia nitrogen (TAN) concentrations in the reactor effluent at a desired value. The model was validated with a large set of experimental results previously reported in the literature. The model was used to study the effect of DO and TAN setpoints on the achievement of full nitritation, as well as to establish the appropriate required range of the DO/TAN concentration ratio to be applied. Nitritation at 20 °C was tested experimentally and simulated with the model. Additionally, the importance of controlling the TAN concentration was highlighted with different scenarios, in which periodic disturbances were applied mimicking a poor control situation.

Bioaugmentation as a tool for improving the start-up and stability of a pilot-scale partial nitrification biofilm airlift reactor.

The effectiveness of bioaugmentation in the improvement of the start-up of a biofilm airlift reactor to perform partial nitrification was investigated. Two identical biofilm airlift reactors were inoculated. The non-bioaugmented reactor (NB-reactor) was inoculated with conventional activated sludge, whereas the bioaugmented reactor (B-reactor) was seeded with the same conventional activated sludge but bioaugmented with nitrifying activated sludge from a pilot plant performing full nitritation under stable conditions (100% oxidation of influent ammonium to nitrite). The fraction of specialized nitrifying activated sludge in the inoculum of the B-reactor was only 6% (measured as dry matter). To simplify comparison of the results, operational parameters were equivalent for both reactors. Partial nitrification was achieved significantly faster in the B-reactor, showing a very stable operation. The results obtained by fluorescence in situ hybridization assays showed that the specialized nitrifying biomass added to the B-reactor remained in the biofilm throughout the start-up period.

Applying ratio control in a continuous granular reactor to achieve full nitritation under stable operating conditions.

A ratio control strategy was implemented in a continuous granular airlift reactor to achieve and maintain 100% partial nitrification to nitrite (i.e., full nitritation). The control strategy was designed to maintain a constant ratio between the dissolved oxygen (DO) and the total ammonia nitrogen (TAN) concentrations (DO/TAN concentration ratio) in the reactor bulk liquid. The experimental results demonstrated the feasibility of full nitritation of a high-strength ammonium wastewater with a granular reactor operating in continuous mode, when implementing a suitable control strategy. The effect of the DO/TAN concentration ratio on partial nitrification was fast and reversible, upon switching from complete to partial nitrification, despite the presence of nitrite-oxidizing bacteria (NOB) in the granule. Even at a DO concentration as high as 7 mg of O(2) L(-1), full nitritation was obtained, decoupling the achievement of partial nitrification in continuous granular reactors from low DO concentrations. Inhibition of NOB by free ammonia was found to contribute poorly to the achievement of partial nitrification. An extremely high volumetric nitrogen loading rate was achieved (6.1 g of N L(-1) day(-1) at 30 °C), demonstrating that very compact reactors are applicable to nitrogen removal via nitrite.

Benefits of carbon dioxide as pH reducer in chlorinated indoor swimming pools.

Carbon dioxide is seldom used as pH reducer in swimming pools. Nevertheless it offers two interesting advantages. First, its use instead of the usual hydrochloric acid avoids the characteristic and serious accident of mixing the disinfectant with that strong acid, which forms a dangerous chlorine gas cloud and, second, it allows the facility to become slightly a depository of that greenhouse gas. This work introduces the experience of using CO(2) as pH reducer in real working swimming pools, showing three more advantages: lower chlorine consumption, lower presence of oxidants in the air above the swimming pool and a diminished formation of trihalomethanes in the swimming pool water. Experiments lasted 4years and they were run in three swimming pools in the Barcelona area, where the conventional system based upon HCl and a system based upon CO(2) were consecutively exchanged.