Reject water is a high-strength ammonium wastewater produced in the sludge dewatering process in urban WWTPs. This effluent is usually mixed with the influent of the WWTP to be treated in the conventional water line. However, different studies have demonstrated that the specific and separated treatment of reject water is more convenient than its recycle. Among the proposed treatments, biological processes are the most convenient from both economic and ecological points of view. Biological nitrogen removal of reject water can be performed by (i) the classical nitrification–denitrification, (ii) full nitritation–denitritation and (iii) partial nitritation (PN)–Anammox which is the most novel process and ensures nitrogen removal through an autotrophic process. As a pretreatment of the anammox reactor, the PN reactor has to achieve an effluent ratio of nitrite/ammonium around 1.3, which is the stoichiometric ratio required by Anammox.
PN reactors are usually operated without advanced control loops, as only DO control is usually implemented. The effluent with the required nitrite/ammonium ratio is achieved thanks to the bicarbonate/ammonium ratio of the reject water, which typically contains the stoichiometric alkalinity required to oxidize around 50% of the inlet ammonium. However, the treatment of wastewaters without the proper bicarbonate/ammonium ratio or some fluctuations of influent ammonium and alkalinity concentrations could strongly affect the nitrite/ammonium ratio of the effluent and therefore it could disturb the Anammox process.
Process control is widely recognized as essential to ensure successful reactor operation under different influent conditions in PN systems. Main control options recommended consider flow adjustment, influent total inorganic carbon (TIC) control and base/bicarbonate dosing in the reactor. Flow adjustment is a feasible option because a large number of sludge dewatering systems in WWTP work only part of the day and hence reject water storage is already available. In this scenario, the development of new PN systems with a specific control loop is a requirement to produce a proper effluent for Anammox treatment from any high-strength ammonium wastewater, independently of its bicarbonate/ammonium ratio.
To this aim, we have developed novel automatic control loops able to maintain a specific ammonium concentration in the effluent of both, activated sludge reactors or granular biomass reactors under continuous operation. In both cases, ammonium control loops manipulate the influent flow-rate to obtain a more reliable system able to treat reject water at high rates and obtaining an effluent suitable for a subsequent Anammox reactor. Both nitrifying systems have been tested at pilot-scale treating real reject water during several months.
We collaborate in this field with several municipal WWTP operators: Aigües de Barcelona (AGBAR) and Depuración de Aguas del Mediterráneo (DAM).