Evaluation of the suppressive effect of intermittent aeration on nitrite-oxidising bacteria in a mainstream nitritation-anammox process

Abstract: An alternative to conventional removal of nitrogen through autotrophic nitrification and heterotrophic denitrification is autotrophic nitritation-anammox. The anammox bacteria oxidise ammonium directly to nitrogen gas with nitrite as an electron acceptor. Total autotrophic removal of nitrogen in the mainstream would bring wastewater treatment plants closer to being energy self-sufficient as it would allow for a significant reduction of aeration and an increased chemical oxygen demand reduction in the pre-treatment. An increased chemical oxygen demand reduction by mechanical treatment would potentially generate a greater biogas yield in the subsequent anaerobic digestion of the sludge. Nitritation-anammox processes have been successfully implemented over the world for treatment of ammonium rich sludge liquor of higher temperatures, while the feasibility of a mainstream implementation is still under evaluation. Lower ammonium concentrations, lower operating temperatures and better effluent quality represent the main challenges considering this energy autarkic treatment technique. Terminating nitrification at nitritation, i.e. favouring ammonia-oxidising bacteria while supressing nitrite-oxidising bacteria, is vital for a functioning nitritation-anammox process. This study aims to evaluate the suppressive effect of intermittent aeration on nitrite- oxidising bacteria while sustaining anammox activity by ex-situ batch tests in a pilot-scale moving bed biofilm reactor at Sjölunda Wastewater Treatment Plant in Malmö, Sweden. The pilot plant consists of one reactor treating sludge liquor and two mainstream reactors, connected in series, receiving effluent from a high-loaded activated sludge plant. The batch test showed a slight decrease of nitrite-oxidising bacteria activity when the reactors were intermittently aerated. Some loss in activity is expected as oxygen supply is decreased when aeration is switched from continuous to intermittent. Furthermore, the decrease coincided with an increased organic carbon loading favouring fast growing heterotrophic bacteria. The decrease in nitrite-oxidising bacteria activity can thereby be coupled with an increased competition for dissolved oxygen and space with heterotrophic bacteria. The suppression of nitrite-oxidising bacteria was not selective as results indicate a decrease in ammonia-oxidising bacteria activity as well. The nitrogen removal rate was decreased during the study while the potential anammox activity was stable in the mainstream and increased in the sludge liquor reactor. This indicates that the anammox bacteria are not hampered but rather that the availability of nitrite, i.e. the activity of ammonia-oxidising bacteria, is the limiting factor of the process.