Study On Biological Nitrogen Removal By Using BDPs For RAS Wastewater Treatment

The removal of inorganic nitrogen,including total ammonia nitrogen（TAN）,nitrite nitrogen（NO₂^--N）and nitrate nitrogen（NO₃^--N）,is an essential part of sustainable development in recirculating aquaculture system（RAS）.The TAN and nitrite in water can be converted into nitrate by biological nitrification.Solid-phase denitrification is an effective way to remove nitrate.However,traditional solid-phase denitrification systems,using biodegradable polymers（BDPs）as denitrifying carbon sources,have some problems,such as high cost of materials and high concentration of dissolved organic carbon（DOC）.In this study,poly（butylene succinate）（PBS）and low-cost bamboo powder（BP）were mixed to develop a new cheap solid carbon source（PBS/BP）for nitrate wastewater treatment in RAS.In order to control the DOC release rate of PBS/BP,low-temperature freezing treatment was carried out.The effects of different operating conditions such as salinity,hydrautic retention time（HRT）and freezing treatment of carbon source on denitrification efficiency and microbial community were explored.In addition,bioflocc technology is another effective method to control inorganic nitrogen for wastewater.To solve the problem of high effluent DOC in solid-phase denitrification,a new CDBFT system（Comnined denitrification and biofloc technology）was designed in this study,using poly（3-hydroxybutyrate-co-3-hydroxyvalerate）（PHBV）as carbon source.The feasibility of combined denitrification and biofloc process along with the influence of LED illumination on denitrification performance were studied.The results of the study are as follows:（1）PBS/BP freshwater denitrification reactor can obtain a quick start-up without external inoculation.The maximum volumetric removal rate of nitrate was 0.68±0.03kg NO₃^--N/（m³·d）at HRT of 2 h.In addition,the release kinetics model of PBS/BP carbon source obeys Fickian diffusion law.Freezing treatment can slow down the release rate of carbon source and prolong its service life.The biodegradability of PBS/BP was well characterized by scanning electron microscopy（SEM）and fourier transform infrared spectroscopy（FTIR）.Illumina high-throughput sequencing showed that Acidovorax and Alicycliphilus were the main denitrifying bacterum in freshwater denitrification reactor.（2）Compared with the freshwater reactor,the seawater denitrification reactor started slowly,but showed more efficient denitrification efficiency in the stable stage.The maximum nitrate removal rate was 0.83±0.11 kg NO₃^--N/（m³·d）at HRT of 2 h.Illumina high-throughput sequencing showed that Simplicispira and Pseudomonas were the main denitrifying bacterum in seawater denitrification reactor.RDA（Redundancy analysi）between microbial community and environmental factors showed that freezing treatment was positively correlated with effluent nitrate and effluent nitrite,but negatively correlated with effluent DOC and effluent TAN;Salinity was negatively correlated with DOC in effluent and positively correlated with nitrite in effluent;HRT was positively correlated with DOC and TAN in effluent,but negatively correlated with nitrate and nitrite in effluent.（3）The CDBFT systems,with and without blue LED lights,have achieved good performance in the treatment of aquaculture wastewater.Both systems have undergone simultaneous nitrification and denitrification in denitrification system（DES）.The removal rates of total nitrogen were 86.18%and 89.49%for photoless and photovoltaic groups,respectively.For the whole system,the removal rates of total nitrogen in the photoless and photovoltaic groups were further improved to 87.45%and 92.23%respectively.The DES played a major role in the removal of nitrogen,while the biofloc system（BFS）could further reduce DOC in effluent and improved the nitrogen removal performance of the whole system.Compared with the photoless group,the photovoltaic system obtained a better nitrogen removal performance with the algae synergy.