Research And Optimization Of Biological Nitrogen Removal Using PBS For RAS Wastewater Treatment

In-organic nitrogen i.e., ammonia, nitrite and nitrate removal was essential for the zero-discharge and sustainable operation of recirculating aquaculture system (RAS). Biological denitrification is considered as an effective way for RAS nitrate removal. This study evaluated the heterotrophic denitrification using poly (butylene succinate) (PBS) as carbon source and biofilm carrier for real RAS wastewater treatment. The effects of varied operation condition (influent type, salinity, nitrate loading) of anoxic up-flow fixed bed denitrification reactor on nitrate removal performance and microbial community were investigated. Through intermittent aeration, simultaneous ammonia and nitrate removal was achieved in the air lift intern-loop sequencing batch reactor and clarified the metabolism pathways of ammonia and nitrate. The main conclusions of this experiment are drawn as follow:(1) Under anoxic operation condition, PBS had the advantage of fast acclimation and real RAS wastewater had more potential than simulated wastewater for nitrate removal. When increasing the influential nitrate loading, high efficiencies were achieved after several days acclimation. The higher nitrate volumetric removal rates of 0.53±0.19 kgNO3--N·m-3·d-1(salinity,0‰) and 0.66±0.12 kg NO3--N·m-3·d-1 (salinity,25‰) were achieved for real RAS wastewater treatment, and nitrite concentration was maintained below 1 mg·L-1. In addition, the reactor of salinity 25‰ exhibited more stable nitrate removal efficiency, but caused adverse effects such as excessive effluent dissolved organic carbon (DOC), sulfate reduction and dissimilation nitrate reduce to ammonia (DNRA) activity. The degradation of PBS was further confirmed by SEM and FTIR analysis.(2) In airlift inner-loop sequencing batch reactor, the electron transportation to SRB and DNRA could be inhibited through alternant aerobic/anoxic strategy, which was beneficial for RAS practice. Besides, no common DNRA bacteria and low SRB abundance were detected by Illumina high throughput sequencing analysis.(3) Simultaneous ammonia and nitrate removal were achieved through intermittent aeration treatment. The long-term reactor performance showed that the average ammonia removal and denitrification rates of 47.35±15.62 gNH-N·m-3·d-1 and 0.64±0.14 kgNOs-N·md-1 could be achieved, while no obvious nitrite (0.70±0.76 mg·L-1) accumulation was detected. Besides, the dissolved organic carbon (DOC) in effluents was maintained at 148.38±39.06 mg·L-1. Based on the cycle experiments and further batch tests, ammonia might be removed mainly through heterotrophic nitrification, while denitrification activity of nitrate and nitrite reductions was enhanced through aeration treatment.(4) Illumina sequencing revealed that salinity and DO have significant influence on microbial community formation. The abundance and species of functional denitrification and degradation microflora might be the primary cause of varied reactor performance.