Towards energy-positive urban wastewater treatment plants

For the achievement of sustainable (energy-neutral or even energy-positive) wastewater treatment plants the use of Anammox for sewage treatment has been proposed. The performance of one-stage nitrogen removal of pretreated municipal nitrogenous wastewater has been tested with SBR as a first approach. In many of the studies, the known weak point of those trials is that nitrite-oxidizing bacteria (NOB) developed in the long term operation, triggering the production of nitrate, and decreasing importantly the N-removal performance with Anammox.


A two-stage N-removal system operating in continuous mode could be thought also as an appealing solution for sewage treatment. In the past, poor results were reported, and little attention had been paid to partial nitritation with biofilm reactors either because such a process was thought difficult to be maintained in the long term or because trials yielded not the expected results. However, our research group has developed stable nitritation in biofilm reactors operating in continuous mode for the specific treatment of the reject water and other types of rich ammonium wastewaters at temperatures over 20ºC. The success of such a treatment relies in the use of a control strategy to maintain the adequate ratio between oxygen and ammonium concentrations in the reactor bulk liquid, as to repress NOB activity in the biofilm. However, when applying such a strategy to the mainstream, two different challenges could be outlined: (i) the partial nitritation reactor would need to produce the adequate ratio between ammonium and nitrite concentrations as to feed a subsequent anammox reactor and (ii) to the best of our knowledge, a stable partial nitritation reactor (achieving an effluent with a nitrite/ammonium ratio of 1), with floccular, attached or granular biomass, at temperatures lower than 15 ºC and treating low-strength ammonium wastewaters has not been reported.

In this research field, we would like to demonstrate the feasibility of a two-stage system reactor operating in continuous mode at low temperatures (up to 10ºC) to perform the autotrophic BNR in the mainstream of a urban WWTP. Our system consists of: (i) a nitrifying airlift granular reactor followed by (ii) an Anammox UASB reactor. Currently, we are working at lab-scale and low temperatures in both reactors with excellent results of removal efficiency and nitrogen loading rates applied. In a few months, we expect to scale-up our system to a pilot-scale about one cubic meter.

We are collaborating in this field with a municipal WWTP operator: Depuración de Aguas del Mediterráneo (DAM) and with the Department of Biotechnology of Delft University of Technology (The Netherlands). 

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