Changes in Relative Abundance of Microbial Groups Involved in Nitrogen Removal in the Anammox‒Partial Nitrification Reactor System at Increase in Ammonium Nitrogen and COD Load

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This is the study of efficiency of anaerobic reject water treatment and of changes in the relative abundance of the major microbial groups involved in nitrogen removal at increase in nitrogen and COD (chemical oxygen demand) load. A two-reactor laboratory system carried out anammox and partial nitrification, with the biomass immobilized on a brush carrier and water recycling. Nitrogen concentration in the water varied from 50 to 250 mg N-NH4/L, while COD varied from 50 to 1650 mg О2/L. Efficiency of the process was assessed by monitoring the concentrations of N-NH4, N-NO2, N-NO3, COD, dissolved oxygen, and pH in the inflowing and treated sludge liquor. The efficiency of nitrogen removal was 75‒90%. Active development of anammox bacteria and first step nitrifiers was observed in the anammox reactor, while almost no second step nitrifiers were present. Elevated content of organic matter in sludge liquor was shown to cause a significant decrease in the water purification degree due to lower abundance of anammox bacteria and the first step nitrifying Nitrosomonas species, while the abundance of the second step nitrifying Nitrospira species increased. The major microbial groups involved in nitrogen removal were better represented in biofilms than in free-floating sludge (flocs and granules). While the size of the granules and flocs did not change at increased load, their color changed from gray-red to a darker one.