Study On Activated Carbon Adsorption And Biofilter Advanced Treatment For Microalgal Extracellular Organic Matter

In recent years,the rapid development of industrialization and urbanization has led to the discharge of a large quantity of industrial and municipal wastewater containing nitrogen and phosphorus nutrients into the water,causing serious eutrophication of natural water bodies.Microalgae,as microorganism that can efficiently remove nitrogen and phosphorus,rapidly fix CO2 and produce biomass through photosynthesis,have a wide application prospect in the advanced treatment of nitrogen and phosphorus in wastewater in the future.But in the actual process of microalgal cultivation,microalgae secrete a mass of extracellular organic matter in wastewater.The extracellular organic matter can not only cause serious bacterial contamination,but also deteriorate the water quality of microalgal effluent and inhibit the growth of microalgae,which severely impact the stable operation of the nitrogen and phosphorus advanced treatment facilities.Based on the bacterial contamination in microalgal cultivation,deterioration of water quality and inhibition of microalgal growth caused by microalgal extracellular organic matter,the granular activated carbon was selected for treatment of extracellular organic matter secreted by typical microalgae(Scenedesmus obliquus)in advanced wastewater treatment.The granular activated carbon could adsorb the extracellular organic matter and bacteria to mitigate the bacterial contamination during microalgal cultivation.Besides,the biodegradation of extracellular organic matter by activated carbon biofilter achieved the biological stabilization and stable re-cultivation of the effluent from microalgal treatment.The main contents and conclusions were as follows:(1)The adsorption capacity of six common commercial adsorbents to extracellular organic matter was compared and granular activated carbon was screened out as the optimal adsorbent for the treatment of extracellular organic matter.In microalgal cultivation with only bacterial contamination,bacteria could make use of microalgal extracellular organic matter for growth,resulting in massive microalgal lysis in stationary phases,where the final biomass of microalgae decreased from 0.45 g L-1 in blank group to 0.06 g L-1.However,the addition of granular activated carbon could efficiently mitigrate the bacterial contamination,where the final biomass of microalgae increased from 0.06 g L-1 to 0.46 g L-1.The mechanism of mitigation of bacteria contamination was further investigated and revealed that activated carbon could adsorb microalgal and bacterial extracellular organic matter and reduce secretion of bacterial extracellular organic matter,which posibble decreased the concentration of algicidal extracellular substances in culture medium.At the same time,the adsorption of bacteria by activated carbon weakened the growth competition or inhibition between microalgae and bacteria,effectively mitigating microalgal lysis and water quality deterioration.(2)For the significant increase in biochemical oxygen demand caused by extracellular organic matter,the activated carbon biofilter was applied to biodegradate the extracellular organic compounds and reject microalgal cells to achieve the biological stabilization of microalgal effluent.The hydraulic retention time and influent quality were studied the performance of activated carbon biofilter.Under 8 h optimal hydraulic retention time,the removal rate of microalgal cell and extracellular organic matter reached to 33%and 52%respectively.For the influent with low biological availability,the removal rate of extracellular organic matter fell to 30%,while the biological availability of effluent was further reduced to about 0.15 mg L-1,which suggested that the effluent was in good biological stability.The bacterial species in the biofilter was characterized further.The results showed that the bacterial community was dominated by Proteobacteria(79.6%)and Bacteroidetes(10.3%).Proteobacteria played an important role in the carbon cycle,the nitrogen cycle,and the sulfur cycle in biological membrane.Proteobacteria can not only anaerobically fermented carbohydrates and proteins in the extracellular organic matter,but also bioferment microalgae biomass to remove microalgal cell.(3)The growth of microalgae,secretion of extracellular organic matter,bioutilization of inorganic carbon,TN and TP were study to reveal the effect of microalgal effluent and biofilter effluent on the recultivation of microalgae.The microalgal effluent significantly inhibited the growth of microalgae and bioutilization of nitrogen and phosphorus,where the biomass decreased by 30%and the removal of TN and TP decreased by 14%and 39%respectively.Although biofilter treatment showed no improvement in mitigating the inhibitory effect of microalgal effluent on microalgal growth,but could effectively improve the bioutilization of TN and TP,where TN and TP removal rate reached 98%and 98%respectively,far higher than that of 85%and 61%in the raw microalgal effluent,which was of great significance in the increase of protein content in microalgal cells and the enhancement of photosynthesis.(4)For the design of biofilter unit,the sedimentation tank effluent from microalgal advanced treatment pond was designed as the biofilter influent,and the maximum inflow was set as 5000 m3 d-1.The biofilter effluent was required to meet the criterion of biological stability in drinking water,namely BDOC below 0.25 mg L-1.The volume of activated carbon filter bed in biofilter was 2150 m3 and its height and effective area were 3m and 717 m2 respectively.The actual size of filter pool was 30 m in diameter and 4.5 m in height.The rotating distributor was used to spray water.Finally,the construction drawing of biofilter was accomplished.