Study On The Application Of External Anaerobic Reactor For Water Treatment Of Recirculating Aquaculture

The industrialized recirculating aquaculture mode has many advantages,such as water saving,land saving,simple culturing operations,easy control of the culturing environment and so on.Therefore,it has been widely accepted and popularized in the vast aquaculture area.However,because of the lack of special denitrification modules and equipments in the modern culture system,the accumulation of nitrate（NO₃^--N）will exist after the system is run and used for a long time.Therefore,it is necessary to develop an economical,low-cost,highly efficient and sustainable denitrifying equipment to compensate the deficiency of present water treatment system and technology.At present,the domestic researches on anaerobic denitrification are mostly executed in the laboratory,however the study on aquaculture production scale about anaerobic denitrifying technology is relatively less.This study used a self-designed anaerobic denitrification device,with Clinoptilolite as the filler,adopting packed-bed bioreactor mode,connecting it to the bottom of the primary biofilter,naturally cultivating biofilm and working by the way of constantly running water,using the organic compound originally deriving from the aquaculture wastewater from the biofilter as the denitrifying carbon source,by the means of controlling and changing the reactor’s effluent flow rate to investigate the nitrogen removal effect under different Hydraulic Retention Time（HRT）;revealing the relationship between HRT,dissolved oxygen（DO）and denitrifying effect,as well as the relationship between the amount of the organic matter attached to different filler layers in the reactor,the denitrifying effect of these layers and the whole denitrification effect of the reactor.This study can provide directions and improvement measures to the following up researches carried out continually on anaerobic denitrification,providing experiences and references to the researches which develop a highly economical,efficient and sustainable anaerobic denitrifying reactor in the production finally.The main contents and results of this study are as follows:（1）Before the experiment,the reactor running 70 days for cultivating biofilm,the experiment takes 60 days,separately detecting six groups of water quality criteria which are sampled from the reactor’s influent,sampling and effluent ports under different flow rates（642.2 L/h、612.4 L/h、342.1 L/h、24.71 L/h、10.48 L/h、5.397L/h,their corresponding HRT are 0.2115 h、0.2542 h、0.5359 h、7.430 h、17.52 h、34.04 h）.The experiment takes 10 days for each HRT respectively.（2）During the experiment,the pH,temperature and salinity ranges from6.52⁷.40,20.80℃²2.54℃,30.48³1.21.The changing range of TOC/TN in the whole experiment is 0.6084².483,dominating in 1.0¹.5.The changes of pH,temperature,salinity,COD and TOC/TN have no obvious affect on the denitrifying effect of the reactor’s each sampling and effluent ports.（3）With the increase of HRT,the average DO of the reactor drops sharply,which is not related to the fluctuation of the influent DO.When the HRT is more than0.5359 h,the effluent DO drops to 0.5mg/L around or below,the reactor presenting denitrifying effect overall.The reactor has the effect of removing inorganic nitrogen（IN）and total nitrogen（TN）at all experimental period.As HRT increasing,the removal rates of IN and TN both firstly increase and then decrease.The ammonium（NH₄⁺）and nitrite（NO₂^-）removal rates increase slightly,then decrease significantly and then rise,presenting an anti "S" shape.The nitrate removal rate first increases and then decreases,from accumulation to removal.The overall treatment effect of the reactor experiences a transformation from nitrification to denitrification;when HRT is 17.52 h,the reactor’s nitrogen removal efficiency is the best,and the removal rate of NO₃^-reaches 77.48%.As HRT extends continually,the nitrogen removal efficiency decreases.If sorting the nitrate removal rates from small to big,we can conclude:when the nitrate removal rate reaches a certain value,then ammonium and nitrite begin to accumulate.The higher the nitrate removal rate is,the more nitrite and ammonium accumulate,and nitrite begins to accumulate first.（4）When HRT is low（HRT<0.5359h）,ammonium and nitrite are the main forms of nitrogen removed from the reactor,and ammonium is the majority.while higher（HRT>7.430h）,nitrate takes up the major nitrogen being removed.（5）As the HRT increasing,the layer beginning Denitrifying and the main denitrification segment begin to move down,and finally concentrate to the filler between the influent and the sampling port 1.In addition,the difference in the amount of feculence adhered to the filler in the reactor leads to the different denitrification effect in different layers of the reactor;with the increase of HRT,the denitrification efficiency of the reactor’s upper filler decreases,and there is a slight rise in the upper outlet’s water quality criteria.（6）Clinoptilolite has the deodorizing function on aquaculture wastewater,and organic compound attached and deposited on the filler also loses its original stink smell.