This study assessed the simultaneous nitrification and denitrification processes and remaining organic matter removal from anaerobic reactor effluent treating wastewater in a single reactor. A structured-bed reactor, with polyurethane foam as support media, was subjected to intermittent aeration and effluent recirculation. Aerated/non-aerated periods varied in the range of 2/1-1/3 h. The chemical oxygen demand (COD) in the effluent remained between 26 and 42 mg L throughout all the aeration conditions. Aeration periods of 1/2 h removed 80 and 26% of Total Kjeldahl Nitrogen and Total Nitrogen, respectively. A low solid production was observed during the 300 days of operation, resulting in a solid retention time of 139 days. The results indicate that the non-aerated periods generated alkalinity that favored nitrification, maintaining low COD concentrations in the effluent. The structured bed reactor presented a low solid production and effluent loss below 20 mgSSV L, similar to concentrations obtained in secondary decanters.

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Simultaneous nitrification and denitrification (SND) is a process that can remove both nitrogen and organic matter in a single unit. Several bench-scale studies show that the structured bed reactors (STBR) subjected to recirculation and intermittent aeration have achieved a good performance for SND treating different types of wastewater. Thus, this study took a step forward and evaluated the efficiency and stability of treating domestic sewage in a pilot-scale STBR. COD removal efficiencies higher than 87% were achieved in the whole experimental period. The highest Total-N removal efficiency was approximately 74 ± 7% by adopting a hydraulic retention time (HRT) of 47.2 h and intermittent aeration (2 h aerated and 1 h non-aerated). The setup of the aeration system was an important mechanism to ensure the optimal balance between nitrification and denitrification in a pilot-scale system.

A novel foam aerated biofilm reactor (FABR) was evaluated to remove nitrogen and COD from synthetic wastewater. This new reactor configuration establishes a counter-diffusion biofilm like the well-known membrane aerated bioreactor (MABR), however using polyurethane foam as the support material and aeration system by air diffuser. It is especially well suited for wastewaters with low COD/N ratios. In FABR, a polyurethane foam sheet separates the aerobic compartment, which received the aeration, and the anoxic compartment, which received the effluent. Foams sheets with thicknesses of 10, 5 and 2 mm and synthetic wastewater with COD/N ratios of 5 and 2.5 were evaluated. The 2 mm thick foam reactor did not show good biomass adherence and, therefore, did not show N removal efficiency. The 5 and 10 mm reactors, in both COD/N ratios, showed an average total nitrogen removal efficiency from 36 to 46%. The denitrification efficiency was 100% throughout the experimental period.Lower values of polyurethane foam thickness and COD/N ratio did not provide a higher nitrification rate, as expected. Throughout the operation, biomass growth was associated with a decrease in nitrification efficiency, which was restored after biomass removal. It was concluded that the greatest limitation of the system is associated with mass transfer resistance in the biofilm and not with the thickness of the foam layer. The increase in this resistance occurred due to the biomass growth inside the foam, leading to pore clogging and creating barriers to efficient mass transport. A mathematical model was developed to describe the mass transport in the foam. The effective diffusivity and porosity of the biofilm were determined experimentally and incorporated into the model. The value of the effective diffusivity proved to be more significant for the results of the model. Gradients of oxygen, nitrate, nitrite and ammonia concentration through the biofilm were measured using microsensors and compared with the values calculated by the model. It was concluded that the foam fiber and bacterial cell complex can be considered as a biofilm, as long as it is considered adequate effective diffusivity, which depends on biomass growth within the foam pores.

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