Startup Strategies for Mainstream Anammox in Moving Bed Biofilm Reactors (MBBRs)

Partial denitrification/anammox (PdNA) is a biological nitrogen removal technology with significant carbon and aeration savings when compared with conventional nitrification/denitrification. Yet despite these benefits, the use of PdNA in mainstream wastewater treatment remains limited. One of the main barriers to implementation of anammox-based technologies is the slow growth rate of anammox (AMX), which results in a long startup time. To accelerate startup, the typical approach to commissioning AMX-based processes, specifically used for sidestream partial nitritation/AMX, is with biomass augmentation, which is practically unrealistic for full-scale mainstream applications. Thus, this study evaluated startup strategies for mainstream PdNA without AMX inoculation in moving bed biofilm reactors (MBBRs) with two simultaneous experiments. In one experiment, an MBBR was started using IFAS carriers with a preliminary biofilm and no external carbon dosing or AMX biomass inoculation. The feed was controlled to 20°C and included mainstream conditions of nitrite and ammonia controlled to the stoichiometric requirements for AMX growth. After only 84 days of operation, AMX activity was confirmed in the reactor with evidence of activity a few weeks before testing. In the second experiment, four reactors were started with either virgin carriers or integrated fixed-film activated sludge (IFAS) carriers with a preliminary biofilm of heterotrophs and nitrifiers. The reactors were fed mainstream levels of ammonia and nitrate with a temperature control target of 20°C and one reactor of each carrier type was dosed with carbon in the form of either glycerol or methanol. Carbon dosing was based on a feedback proportional-integrative-derivative (PID) control loop with a nitrate residual of 1-1.5 mgNO3-N/L. Of the four reactors, the preliminary biofilm carrier reactor dosed with glycerol achieved AMX activity first after 224 days of operation, but it was determined this was likely limited by synthetic feeding for the first 184 days. These results, along with other recent PdNA work, suggest that growth of AMX on biofilm carriers can be established in mainstream conditions in 50-100 days, depending on media selection and carbon source. Ultimately, this research will help utilities understand methods for starting up mainstream PdNA MBBRs from scratch and make this technology more accessible.