Urban wastewater

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.

Enhanced Biological Phosphorus Removal (EBPR) is considered as the most economical and environmentally sustainable alternative to prevent eutrophication from phosphorus discharges. Most of the reported WWTP configurations for simultaneous C/N/P removal have an aerobic zone before the settler which may result in some nitrate presence in the external recycle and consequently, in the anaerobic zone. This presence is one of the most reported causes of EBPR failure in real WWTP and, despite its significance, the processes involved have not been fully understood yet.

Current legislation for WWTP establishes stricter removal of organic matter and nutrients, especially nitrogen and phosphorus. Hence, new WWTPs must be designed and other already existing must be retrofitted to achieve the required discharge limits. Mathematical models allow a correct description of the processes observed in the WWTP and hence are a great help for these tasks. For instance, the IWA ASM2d is able to describe the behavior of the observed biological removal of COD, N and P in a common WWTP.

Wastewater treatment by Sequenced Batch Reactor (SBR) is a very versatile technology. Reaction and settling processes are carried out in the same reactor, allowing a highly configurable and adaptable wastewater treatment system. Our research group has been working since year 2000 with SBR configurations for removal of organic matter, nitrogen and phosphorus. We have 15 years of expertise in SBR configurations for Enhanced Biological Phosphorus Removal (EBPR). Classic Ana/Aer configuration for P-removal and novel Ana/Anox/Aer or Ana/Anox cycles have been widely tested in different projects.

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.

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