Construction And Operation Of Biofilm Reactor For Purification Of Biogas Slurry By Microalgae

With the rapid development of livestock industry,biogas engineering to realize the utilization of livestock manure is becoming more and more popular in China.A large amount of biogas slurry will remain after biogas production from livestock manure by anaerobic fermentation.Biogas slurry is a kind of high concentration organic wastewater with the characteristics of high ammonia nitrogen and high turbidity.The traditional wastewater treatment process cannot effectively purify biogas slurry to meet the discharge standard,however,microalgae can consume nitrogen and phosphorus in biogas slurry to meet their own growth,and in this process,they can generate biomass energy such as oil and other high value-added products.Therefore,the combination of microalgae cultivation and biogas slurry purification can realize the resource utilization of biogas slurry and obtain high-value biomass products,which has a very bright application prospect.In this paper,the purification of biogas slurry by microalgae is the main research target.Starting from the bottleneck problem of purification of biogas slurry by microalgae,the concentration gradient of ammonia nitrogen was gradually increased by changing the dilution ratio of biogas slurry,and then microalgae were domesticated to obtain high ammonia nitrogen tolerance.Then,based on the newly obtained microalgae species,a ladder micro algae biofilm reactor was proposed to overcome the high turbidity limit of biogas liquid and solve the problem of high energy consumption of algal water separation.The influence of different substrate materials on the membrane forming characteristics of microalgae was studied,and the effect of light intensity and flow rate on the removal efficiency of nitrogen and phosphorus in biogas liquid was clarified.Finally,a series system of continuous treatment of biogas slurry was constructed to realize the continuity and stability of the ladder biofilm reactor.The mechanism of removal of pollutants from biogas slurry and the regulation method of discharge up to standard were explored.The main results are as follows:(1)The average diameter of microalgae increased by 40.7%compared with that before domestication.With the gradual increase of ammonia nitrogen concentration,the removal rate of ammonia nitrogen increased from 27.98 mg/L/d to 82.66 mg/L/d,which indicated the tolerance of microalgae to ammonia nitrogen was gradually improved.There is a phenomenon of"luxury absorption"of phosphorus during the growth of microalgae,so in the 60%biogas slurry and 80%biogas slurry domestication experiments,the removal efficiency of total phosphorus reached 100%.With the initial ammonia nitrogen concentration of biogas slurry was 678 mg/L,microalgae could grow normally in the raceway pond,the final removal efficiency of ammonia nitrogen and total phosphorus reached 100%,which confirmed that the domesticated microalgae could tolerate high ammonia nitrogen and had excellent nitrogen and phosphorus removal performance.(2)When determine the suitable substrate for microalgae membrane formation,the biomass productivity of Reactor based on cotton was 7.66±0.184 g/m~2/d after running for 7 days,which was 1.90～2.85 times of the other four materials(coral velvet,filter cloth,filter cotton and linen),the water absorption was 3.2±0.17 g/g,the HRT was0.85±0.050 h/m,and the removal efficiency of ammonia nitrogen and total phosphorus was 100%in 5 and 3 days.In general,cotton is the most suitable substrate for microalgae formation.The effect of different light intensity and flow rate on the treatment of biogas slurry by ladder microalgae biofilm reactor was studied.When the light intensity was 15000 Lux,the maximum biofilm biomass productivity was 7.02±0.044 g/m~2/d,and the removal efficiency of nitrogen and phosphorus in biogas slurry was 100%.In the initial stage of biofilm formation,it takes one day for microalgae in the bottom suspension to adhere to the biofilm substrate at low flow rate(10 mL/min),and two days at medium flow rate(15 mL/min)and high flow rate(20 mL/min),indicating that the biomass of biofilm formation on the substrate was independent of the flow rate,but the time of biofilm formation increased with the increase of flow rate.After the microalgae film formation in the reactor reached a stable state,the microalgae biofilm density decreased from 70.03 g/m~2 in the initial position to 23.73 g/m~2 in the end position along the flow direction at low flow rate(10 mL/min),and from 63.41g/m~2 to 12.33 g/m~2 at high flow rate(20 mL/min).It shows that the effect of flow shear force will make the density of microalgae biofilm decrease along the flow direction,and the larger the flow,the more obvious the decrease of biomass.Under different flow rates,the removal efficiency of ammonia nitrogen and total phosphorus reached 100%.However,the removal period of ammonia nitrogen was shorter at high flow rate,and the removal period of total phosphorus was shorter at low flow rate.(3)In the experiment of using sequencing batch operation method to run ladder microalgae biofilm reactor for a long time,the biofilm density reached 52 g/m~2 after three cultivation cycles(21 days)and keep stable in the next 14 days.In the process of five batch purification of biogas slurry,the removal efficiency of total phosphorus in biogas slurry was 100%,and the lowest removal rate of ammonia nitrogen was89.3%,the export concentration of nitrogen and phosphorus met the Discharge standard of pollutants for livestock and poultry breeding(GB 18596-2001).When the density of biofilm was stable,the concentration of microalgae at the outlet of the reactor was 0 mg/L,which proved that the structure of biofilm in the reactor was stable,and the effective separation of microalgae and biogas slurry was realized by forming a membrane.In the series system,the Fv/Fm value of microalgae in reactors 1 and 2was stable at 0.56,and the Fv/Fm value of microalgae in reactor 3 was stable at 0.66,which proved that the photosynthetic potential of microalgae was not affected during the continuous treatment of biogas slurry.Finally,the removal efficiency of ammonia nitrogen and total phosphorus at the outlet of the system reached 100%,which achieved the goal that nitrogen and phosphorus could be completely removed by a single cycle of biogas slurry flowing through the reactor.