| The direct discharge of organic wastewater is one of the main factors that lead to black and odorous water.Anaerobic digestion,the anaerobic granular sludge bed technology in particular,is favored in the field of environmental engineering because of its high volumetric efficiency,reusable energy,less excessive sludge and other advantages.However,many industrial organic wastewaters,such as paper mill wastewater,generally contain calcium,and often cause the calcium salt precipitation in anaerocic granules,seriously affecting the activity of anaerobic granules and greatly limiting the function of anaerobic reactor.In this paper,the calcified anaerobic granule was characterized,the deactivation of calcified anaerobic granule was explained,and the Double Circulation Anaerobic Sludge(DCAS)bed reactor was developed,whose performance verified the feasibility of strategy “reusing floation sludge + discharging calcium salt via calcified anaerobic granule”.The major results are as follows: 1.The calcified anaerobic granular sludge was characterized by observing the content and distribution of calcium salts in anaerobic granules.1)The methanogenic activity of calcified anaerobic granules was significantly lower than that of normal anaerobic granules(control).The results showed that the specific volumetric methanogenic activity(SVMA)of calcified anaerobic granules was 3.52 m L·m L-1·h-1,which was significantly lower than that of control(decrease by 13.09%),under the condition of simulating the influent COD concentration of the anaerobic internal circulation(IC)reactor in the paper mill.2)The physicochemical characteristics of calcified anaerobic granules were significantly different from those of normal anaerobic granules(control).The results showed that the VSS/TSS of calcified anaerobic granules was 0.351,which was only 39.2% of control;The calcium content was 234.49 mg/g TSS,which was 10.79 times of control;The phosphorus content per unit biomass was 5.88 mg/g VSS,which was 2.08 times of control.The porosity of calcified anaerobic granules was 0.607,which was 13% less than that of control;It is worth mentioning that the macropores(diameter > 10 ?m)almost disappeared.3)The large part of calcium salts in the calcified anaerobic granules was precipitated in the form of aggregates and the small part was distributed all the zone.The results showed that calcium salts mainly existed as calcium nuclei in the center of granules in the form of calcite;A small amount of Ca-P salts(size of 100 ?m)was deposited on the surface of granules.2.The deactivation mechanism of calcified anaerobic granule was revealed by investigating the biological and physicochemical properties.1)The kinetic characteristics of calcified anaerobic granules were significantly different from those of normal anaerobic granules(control).The results showed that the maximum specific biomass methanogenic activity(SMA)and maximum SVMA of calcified anaerobic granules were 69.42 m L/(g VSS h)and 7.52 m L/(m L h),respectively,which were higher than those of control(45.87 m L/(g VSS h)and 5.89 m L/(m L h)).The half saturation constant of calcified anaerobic granules to COD was 8.95 g/L,which was 2.51 times as high as that of control,it means that the substrate affinity decreased greatly.2)The microbial community characteristics of calcified anaerobic granules were significantly different from those of normal anaerobic granules(control).The results of q PCR showed that the specific volume gene mcr A abundance of calcified anaerobic granules was 1.80×109 copies/m L,which was significantly higher than that of control(5.04×108 copies/m L).The results of high-throughput sequencing showed that the relative abundance of Methanosaeta in the calcified anaerobic granules was 89.2%,which was significantly lower than that of control(99.5%).3)The mass transfer characteristics of calcified anaerobic granule were significantly different from those of normal anaerobic granule(control).The mass transfer model analysis showed that the methanogenic activity of calcified anaerobic granules was 13% lower than that of control when the activity of anaerobic granules was limited by substrate diffusion,which was consistent with the experimental result of methanogenic activity(COD = 6.6 g/L).4)The deactivation of calcified anaerobic granule was aggravated with the calcium accumulation.The ash accumulation model analysis showed that the relationship between SVMA and ash content of calcified anaerobic granule presented a convex curve;The ash content of calcified anaerobic granule(0.649)decreased SVMA by 15.39%;With the elevation of ash content,the decrease of SVMA was accelerated.3.The feasibility of stratege “reusing floation sludge + discharging calcium salt via calcified anaerobic granule” was verified through the successful operation of Double-Cycle Anaerobic Sludge(DCAS)bed reactor.1)A new type of Double Circulation Anaerobic Sludge(DCAS)bed reactor was developed and operated successfully.The results showed that the performance of DCAS reactor was similar to that of EGSB reactor(control);However,the loss rate of effluent sludge was 45% lower than that of control;The average diameter of anaerobic granules in the top of sludge bed(about 2000 ?m)was larger than that of control(about 1000 ?m).2)The direct construction unit of anaerobic granule and its binding mode were found.The abrasion test and the SEM observation of anaerobic granules from the DCAS reactor showed that the 100 ?m structure was the direct construction unit(subparticle)of anaerobic granules.The subparticle was made up of zoogloea which originated from individual functional bacteria bound by EPS,in turn the subparticles formed the anaerobic granules via filamentous bacteria network.3)The secondary granulation of anaerobic sludge was proposed.The mature granules released the inner biogas and cracked to produce the secondary structure.Meanwhile,functional bacteria grew to produce the secondary structure as well.Both of the secondary structures constituted the suspended sludge in the reactor.The suspended sludge was trapped by filamentous bacteria network to form flocs.The flocs were circulated in the reactor to glanulate.After the new biofilm grew on the surface,the anaerobic granules became mature.4)The feasibility of stratege “reusing floation sludge + discharging calcium salt via calcified anaerobic granule” was analyzed in the case of DCAS reactor.The results showed that the DCAS reactor increased the net sludge growth rate to 3.3 times of that before the optimization.The proportion of Ca CO3 emission to the total deposition rose from 42% to 100% theoretically through the granular sludge discharge.The stratege “reusing floation sludge + discharging calcium salt via calcified anaerobic granule” was effective to control the impact of high calcium on the performance of anaerobic reactor and ensure the sustainablility of efficient performance. |
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