Characterization and quantification of anammox start-up in UASB reactors seeded with conventional activated sludge

• The anammox start-up was systematically characterized and quantified.
• The major bioprocesses in start-up were comparatively discussed and quantified.
• Reaction and growth characteristics during start-up were carefully illustrated.
• Nitritation sludge was an excellent inoculum to start up anammox process.
• A novel mixing inoculation strategy was proposed.

The start-up of anammox process inoculated with anaerobic granular sludge and nitritation sludge were carefully investigated in two upflow anaerobic sludge blanket (UASB) reactors. Three major bioprocesses related in anammox start-up i.e. anammox, denitrification and cell lysis, were quantified in both reaction and microbial growth aspects in the four consecutive phases namely cell lysis phase, lag phase, activity elevation phase and stationary phase. The results showed that anammox was negligible when cell lysis was predominant in the reactors; however, the density of anammox bacteria objectively increased in this phase due to the biomass reduction caused by hydrolysis. During lag phase, poor anammox was detected; while denitrification and cell lysis were the two leading reactions. Anammox performance of the reactors was significantly enhanced in activity elevation phase. Denitrification continuously weakened in stationary phase and anammox gradually became the dominant reaction. Nevertheless, heterotrophic denitrification still predominated throughout the start-up course due to the extremely low growth rate of anammox bacteria. Results also suggested that the inoculation strategy using nitritation sludge as the main inoculum commixing with anaerobic granular sludge could contribute to the relatively shorter start-up time and better reactor performance. Such an alternative may prove effective strategy for full-scale application of anammox process.