Study On Denitrification Performance Of Marine Recirculating Aquaculture Wastewater Based On Agricultural Waste And Artificial Polymer

Nitrate nitrogen(NO3~--N)accumulation is one of the most important factors affecting water quality and aquatic growth in recirculating aquaculture systems(RAS),and the removal of nitrate from RAS wastewater is essential for the efficient and green development of the mariculture industry.Denitrification technology has demonstrated excellent nitrogen removal capability in other wastewater treatment studies,but denitrification performance can be limited by the high salinity and low carbon to nitrogen ratio(C/N)characteristics of marine recirculating aquaculture wastewater.Using an external carbon source to solve the problem of insufficient carbon sources for denitrification in low C/N water is a feasible approach.However,there are few studies on the application of denitrification technology in marine recirculating aquaculture wastewater,and it is difficult to determine whether the carbon source used for freshwater denitrification can maintain its carbon source release capacity and denitrification performance in marine.To address this scientific problem,this study prepared four composite carbon sources as denitrification applied carbon sources using two agricultural wastes:reed straw(RS)and corn cob(CC)and two artificial polymers:polycaprolactone(PCL)and poly-3-hydroxybutyrate-hydroxypropionic acid(PHBV)to construct a denitrification system using a column reactor with marine recirculating aquaculture wastewater as the research object.By investigating the carbon release capacity and denitrification performance of the composite carbon sources in the mariculture wastewater.And the effects of three influencing factors:influent NO3~--N concentration,hydraulic retention time(HRT)and temperature(T)on the denitrification performance of denitrification system were investigated.The actual denitrification system was constructed with the optimal carbon source and optimal influencing factors to investigate the actual denitrification capacity of the denitrification system and the denitrification mechanism of denitrification in marine recirculating aquaculture wastewater.In order to provide a solution to the problem of low C/N in marine recirculating aquaculture wastewater,the application of denitrification technology is limited to purifying the wastewater efficiently.It also explored the feasibility of a column denitrification system constructed by composite carbon source to be applied in the actual project of marine recirculating aquaculture,in order to provide a reference for promoting the effectiveness of marine recirculating aquaculture.The specific results are as follows.1.CC+PCL is considered an ideal carbon source for denitrification of marine recirculating aquaculture wastewater due to its more stable carbon release performance and higher denitrification efficiency.The carbon release performance of the carbon sources indicated that the four composite carbon sources had high carbon release capacity.The results of energy dispersive x-ray spectroscopy,scanning electron microscopy and Fourier transforms infrared spectroscopy indicated that the CC+PCL biochar release performance was more stable.The denitrification performance of the different carbon source systems varied,with CC+PCL having the highest denitrification efficiency(78.97%),followed by CC+PHBV(59.99%),RS+PCL(45.93%),and RS+PHBV(44.80%).The carbon sources in the reactors were taken for high-throughput sequencing and real-time fluorescence quantification(q PCR)after the experiment,and the results showed that the highest abundance of the phylum变形菌门was found in CC+PCL.Q-PCR results showed that key enzyme genes for the denitrification process were detected in all carbon sources of the four systems.2.A composite carbon source consisting of CC+PCL can be used as a carbon source for marine denitrification systems.The results of three influent NO3~--N concentration gradient experiments showed that the influent NO3~--N concentration had a small effect on denitrification performance.High-throughput sequencing of carbon source biofilms in different systems at the end of the experiment showed that higher influent NO3~--N concentrations(40 mg/L)adversely affected community species diversity and species evenness.Q-PCR results indicated that high influent NO3~--N concentration significantly inhibited the expression of denitrification functional genes.Therefore,an influent NO3~--N concentration of 30mg/L is a more suitable influent NO3~--N concentration.The results of the denitrification performance of different HRT systems proved that the lower HRT was not conducive to the rapid removal of NO3~--N and caused the problem of NH4~+-N accumulation.High-throughput sequencing results showed that 5 h HRT was more beneficial for increasing the species richness of the denitrification system.Q-PCR results showed that several denitrification genes(nar G,nir K,nir S and nor B)were elevated with decreasing HRT.Therefore,an HRT of 5 h was the optimal HRT.low temperature(T=20°C)affected the efficient removal of nitrate,resulting in residual NO2~--N concentration and reduced utilization of SCFAs.the most abundant clade in both T=30°C and T=25°C was变形菌门,and the most abundant clade in the system at T=20°C was拟杆菌门.The low temperature was not favorable for the expression of denitrifying functional bacteria,and the need for nitrogen functional genes could be satisfied at T=25°C.Increasing the temperature to 30°C did not significantly enhance the nitrogen removal performance.Therefore,a temperature of 25°C was most favorable for efficient nitrogen removal and microbial growth in this system.3.The column reactor with CC+PCL as the applied carbon source showed efficient nitrogen removal results in the wastewater treatment of the actual marine recirculating aquaculture-fish wastewater(small pilot system)and marine recirculating artificial wetland aquaculture-shrimp wastewater(medium experimental system),which proved its feasibility in the practical application of marine aquaculture wastewater.CC+PCL was used as an additional carbon source for the column reactor denitrification system,and the denitrification system was used to remove the high concentration of NO3~--N accumulated in the marine recirculating aquaculture-fish(small pilot system).The actual results showed that the time for the denitrification system to adapt to the reconstituted system under the condition of HRT=5 h was very short(1 d-3 d),and it was able to maintain the efficient nitrogen removal(NRE>99%),and the experimental process could ensure the normal growth of fish was ensured during the experiment.The results of carbon source sampling in the post-test system using macro-genome sequencing methods showed that the phylum with the highest species abundance was变形菌门(50.76%).Genes for denitrification processes were detected in carbon source biofilms at different heights of the denitrification system.The most abundant pathway in the denitrification system of aquaculture-fish wastewater was amino acid transport and metabolism(7.86%),and the next most abundant pathways included energy metabolism(7.66%),cell growth(6.60%),and carbon metabolism(6.53%).The column denitrification system with CC+PCL as an additional carbon source also showed efficient nitrogen removal in the application of purification of marine recirculating aquaculture-shrimp wastewater.With an initial HRT of 5 h,the denitrification system can quickly achieve the removal of NO3~--N from recirculating aquaculture-shrimp wastewater(5 d:NRE>90%)and maintain a high NRE status(99.15%-99.67%).The results of biofilm macro genetic sequencing within the denitrification system of recirculating aquaculture-shrimp wastewater were similar to those of aquaculture-fish wastewater denitrification,with变形菌门(58.57%)being the most abundant phylum in the system.The results of functional gene metabolic pathways were the same as those of the aquaculture-fish wastewater denitrification system,with amino acid transport and metabolism(7.64%)being the most abundant pathway in the aquaculture-shrimp wastewater denitrification system.Multiple results show that in practical applications,the composite carbon source prepared by CC+PCL can obtain high removal efficiency and qualified other effluent indexes when used for denitrification of NO3~--N in marine recirculating aquaculture wastewater.It also ensures the efficient nitrogen removal and stable operation of the actual marine recirculating aquaculture system and has the potential to be applied to the actual marine aquaculture wastewater denitrification treatment.