A process for the nitritation of ammonia in an aqueous medium comprising: contacting the aqueous medium with ammonia-oxidizing nitrifier microorganisms under aerobic conditions at a temperature of from 0°C to 50°C wherein: the nitrifier microorganisms are provided in a biocatalyst comprising: a. a solid structure of hydrated hydrophilic polymer defining an interior structure having a plurality of interconnected major cavities having a smallest dimension of between 5 and 100 microns and a Hydration Expansion Volume, which is calculated in volume percent, of 1000 or more, and b. a population of nitrifier microorganisms substantially irreversibly retained in the interior structure, said population of microorganism being in a concentration of about 60 grams or more per liter based upon the volume defined by the exterior of the solid structure when fully hydrated, wherein the microorganisms maintain their population as substantially stable, for a time sufficient to oxidize at least a portion of the nitrogen in the ammonia to nitrite anion to provide a nitrite-containing effluent and an off-gas, wherein at least 40 atomic percent of the nitrogen in the ammonia is converted to nitrite anion, wherein a sufficient amount of biocatalyst is used such that less than about 0.25 atomic percent of the bioconverted ammonia nitrogen is emitted as nitrous oxide into the off-gas, and nitrate anion is produced in an amount less than 10 atomic percent of the ammonia nitrogen converted. contacting the aqueous medium with ammonia-oxidizing nitrifier microorganisms under aerobic conditions at a temperature of from 0°C to 50°C wherein: the nitrifier microorganisms are provided in a biocatalyst comprising: a. a solid structure of hydrated hydrophilic polymer defining an interior structure having a plurality of interconnected major cavities having a smallest dimension of between 5 and 100 microns and a Hydration Expansion Volume, which is calculated in volume percent, of 1000 or more, and b. a population of nitrifier microorganisms substantially irreversibly retained in the interior structure, said population of microorganism being in a concentration of about 60 grams or more per liter based upon the volume defined by the exterior of the solid structure when fully hydrated, wherein the microorganisms maintain their population as substantially stable, a. a solid structure of hydrated hydrophilic polymer defining an interior structure having a plurality of interconnected major cavities having a smallest dimension of between 5 and 100 microns and a Hydration Expansion Volume, which is calculated in volume percent, of 1000 or more, and b. a population of nitrifier microorganisms substantially irreversibly retained in the interior structure, said population of microorganism being in a concentration of about 60 grams or more per liter based upon the volume defined by the exterior of the solid structure when fully hydrated, wherein the microorganisms maintain their population as substantially stable, for a time sufficient to oxidize at least a portion of the nitrogen in the ammonia to nitrite anion to provide a nitrite-containing effluent and an off-gas, wherein at least 40 atomic percent of the nitrogen in the ammonia is converted to nitrite anion, wherein a sufficient amount of biocatalyst is used such that less than about 0.25 atomic percent of the bioconverted ammonia nitrogen is emitted as nitrous oxide into the off-gas, and nitrate anion is produced in an amount less than 10 atomic percent of the ammonia nitrogen converted.