Bioaugmentation is a bioremediation approach involving the seeding of the competent microbial consortia to improve nitrogen removal from wastewater. The discharge of nitrogen-saturated wastewater into freshwater results in cultural eutrophication and huge outbreak of harmful algal bloom. The purpose of this study was to relate the population size of bioaugmented ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) with nitrification performance in an experimental up-flow biological aerated filter (UBAF) for domestic wastewater treatment. Fluorescent in situ hybridization (FISH) was used to enumerate the portions of AOB and NOB in UBAFs. The FISH method disclosed that nitrifying bacteria represented 31.3% of the total population. The AOB (16.6%) outcompeted NOB (14.6%) after 15 days of bioaugmentation. Additionally, AOB/NOB ratio (1.13) showed a higher nitrification process. The average effluent from experimental UBAF exhibited high nitrogen and nutrients removal, the averaged removal efficiencies for NH₄⁺-N, NNO₃-, COD, BOD₅ and phosphate were 85.2%, nitrate accumulation (2.2 times), 67.3%, 75.6%, and 58.3%, respectively. Therefore, an increase in the number of nitrifying microorganisms confirms the efficacy of microbial seeding and correlates with the physical-chemical data on bioconversion of nitrogenous compounds. However, on day 35, there was a disparate distribution pattern of the nitrifying bacteria (AOB, NOB) in the experimental UBAF, which in turn declined the nitrification performance.

The study demonstrated the importance of monitoring the shifts of the nitrifying bacterial population in UBAF. Moreover, the bioaugmentation of nitrifying bacteria in UBAF packed with cost-effective and eco-friendly ceramsite could be a promising alternative technique for the domestic wastewater treatment.