Assessment of autotrophic nitrogen removal in a membrane bioreactor as mainstream post-treatment unit

The growing pressure of mankind on water resources and its surrounding areas has been a serious problem since the last decades. Nutrients being dumped with no control generate eutrophication issues, which jeopardize both aquatic life and human uses. State of the art technology for nitrogen removal is sometimes too expensive or too complex for developing countries to afford, for which innovative treatments with reduced operation expenses and the possibility of cost recovery are essential. An autotrophic process for nitrogen removal has been targeted as a potential candidate for this, namely Anammox. The former is a process carried out by a recently discovered group of anaerobic bacteria, able to produce N2 gas using ammonium and nitrite as substrates. However, nitrifier denitrification, a process carried out by ammonia oxidizing organisms under oxygen stress conditions may also take place under similar conditions, leading to emissions of nitrous oxides and other nitrogenous gases that are of environmental concern due to their undesirable green house gas potential. Yet, they have significant commercial applications (for example, the use of N2O in industry and medicine). To assess the applicability of the autotrophic nitrogen removal processes, a lab-scale membrane bioreactor (MBR) was used, in which a low strength ammonium synthetic wastewater, free of organics, was supplied to the MBR. The reactor was operated at room temperature (20-23 oC), with an SRT of 15 days, under varying pH and dissolved oxygen (D.O.) conditions. These last two were also used for inhibition of nitrite oxidizers, enriching the microbial community with ammonia oxidizers, fact that was verified by fluorescent in situ hybridization tests. The experiment was carried out in three phases. The first one (day 0 to 71) comprised the enrichment of the AOO culture by controlling pH and D.O. An important gap between the TN in the influent and effluent was detected after D.O. decrease below 0.19 mgO2/L. Phase II (day 71-93), intended to control the NH4+/NO2-ratio to feed a prospective Anammox seed, but this failed, for which a change in the oxygenation strategy was implemented. On phase III (day 93-119), the nitrogen gap was greatly increased yet Anammox was not linked to it. Nitrogen removal rates of 70% from the liquid phase were obtained, consequence of nitrifier denitrification, process which was at the same time pointed out as responsible for the inhibition of both free cell and granular Anammox. From the off-gases, only N2O could be measured, but it could not account for the entire gap of missing nitrogen. NO and N2, which are also linked to nitrifier denitrification, were believed to be produced in significant amounts. Besides from the lower nitrogen load, the effluent was solids and microbe-free, making it potentially reusable. Nitrifier denitrification was therefore identified as a mainstream post treatment process with a potential high impact on the nitrogen removal field, not only because it could introduce significant savings in oxygen pumping costs, but also because of its potential to recover nitrogen as a commercial valuable product, provided the emissions are handled in a safe manner