Multiple Electron Donors-Based Denitrification Processes For Nitrate-contaminated Water Treatment

Nitrate?NO₃^-?has been a major pollutant in the ecosystem with the long-term emphasis on economic development while the neglect of environmental protection.High NO₃^-concentration in water has adverse effects on human and animal health.Based on previous studies on denitrification technology,this study was carried out to select effective electron donors,and develop denitrification reactors for different NO₃^--contaminated water.This study provides a theoretical foundation and technical support for NO₃^--contaminated water remediation.Batch experiments were carried out to compare denitrification performance of different agricultural wastes as carbon sources for groundwater remediation,including rice washing drainage?RWD?,maize stalks,poplar leaves and sawdust.RWD-based denitrification resulted in a high nitrate removal rate.Results showed that RWD not only can be used as a carbon souce but also as a bacterial source for denitrification.Both of the bacterial community structure and chemical composition of dissolved organic matter in the system using RWD as bacterial source were simple.Specific comparison down to the genus level showed the dominant denitrifying bacteria of Thiobacillus,Anaerolineaceae and Methylophilaceae in the seeding sludge.Ideonella,Cloacibacterium and Enterobacter were dominant after the RWD addition in the inoculated system,while Stenotrophomonas and Enterobacter were dominant genera when RWD as sole bacteria source in the un-inoculated system.An upflow zeolite-based biofilm reactor?UZBR?with rice washing drainage?RWD?as carbon source was proved to successfully improve NO₃^--contaminated denitrification efficiency and simultaneously treat RWD in the process.At optimum conditions?RWD/synthetic nitrate-contaminated groundwater=1/5.5?v/v??,nitrate removal efficiencies were>95%and TOC concentration in RWD decreased by>80%in both inoculated and un-inoculated systems.Side-by-side microcosms were used to compare methanol,fish waste?FW?,woodchips?WC?,elemental sulfur?S⁰?and a combination of woodchips and sulfur for synthetic marine recirculating aquaculture systems?RAS?water denitrification.The highest denitrification rate was obtained with methanol,followed by FW,S⁰,eucalyptus mulch,and eucalyptus mulch with sulfur.Significant differences were observed in denitrification rate for different wood species,pine>oak>eucalyptus.Wood-sulfur heterotrophic-autotrophic denitrification?WSHAD?resulted in a higher denitrification rate and lower SO₄^2-generation and alkalinity consumption than SOD alone and a lower COD release than WC alone,minimizing secondary pollution.Sulfur-based intermittently operated submerged denitrification reactors?sulfur-IOSDR?was used for nitrate removal from a pilot-scale marine RAS.Advantages of this novel reactor configuration include high denitrification rates and low carry-over of organic carbon and excess biomass to the RAS.In studies with synthetic RAS water at varying hydraulic residence times?HRTs?,denitrification rates varied between 84 and942 g N/?m³·d?and increased linearly with decreasing HRT.After introduction of marine fish,nitrogen species,TSS and S^2-in the RAS was maintained at a healthy level by setting up an appropriate HRT?12 h?for sulfur-IOSDR when producing marine fish?Poecilia sphenop?.On average,7%of the feed nitrogen was assimilated or utilized by the fish,6%was removed by passive denitrification,60%was removed by the sulfur-IOSDR,and 27%was discharged by sampling and solids removal.To lower the accumulation of SO₄^2-in the fish tanks,WSHAD performance for the marine RAS water treatment was investigated in the IOSDR?WS-DR?.WS-DR presented high denitrification rate and low SO₄^2-production.NO₃^-removal efficiencies were 90.3%±4.3%and denitrification rates were 102.6±8.5 g N/?m³·d?,and NO₃^-concentrations in fish tanks were kept at 56.8±2.7 mg NO₃^--N/L.SO₄^2-production(4.25±2.35 mg SO₄^2-/mg NO₃^--N)was lower than the sulfur autotrophic denitrification theoretical value(7.54 mg SO₄^2-/mg NO₃^--N),and there is no SO₄^2-accumulation in fish tanks.In the WS-DR,heterotrophic denitrification was dominant?>50%?when wood released adequate organic carbon,while sulfur autotrophic denitrification was dominant?80%?when organic carbon released from wood was inadequate.Wood in the WS-DR continuously released organic carbon,which followed the first-order kinetics,and the release attenuation rate constant of soluble COD and readily biodegradable COD were 0.271 and 0.403 d^-1,respectively.This study proved the effectiveness of RWD as carbon and bacterial sources for NO₃^--contaminated groundwater denitrification.Sulfur-based or a combination of wood and sulfur-based IOSDR can maintain water quality in the marine RAS at healthy levels by setting up an appropriate HRT for IOSDR,providing a new approach for different NO₃^--contaminated water remediation,including freshwater and saline water.