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.
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.
Aerobic granular sludge was developed as an alternative technology to conventional activated sludge processes for the treatment of wastewater. The morphological structure of aerobic granular sludge (i.e. high particle diameter and density) provides two main advantages if compared to activated sludge processes: (i) the ability of settling faster, which ease the retention of biomass; and (ii) the existence of substrate profiles across the granule radius, which allows simultaneous aerobic, anoxic and anaerobic processes into the same granule. Both characteristics contribute to reduce the required reactor capacity producing more compact designs, or to treat higher wastewater loading rates, when compared to conventional activated sludge systems. Furthermore, since aerobic granules can be separated from wastewater in the same reactor vessel, external settling units become unnecessary.