Performance And Adaptation Mechanism Of Microalgal-bacterial Granular Sludge Under Light-deficient Conditions

Microalgal-bacterial granular sludge(MBGS)process was a wastewater treatment technology that uses the mutualism between microalgae and bacteria as the driving force of light energy,but there were usually insufficient light intensities such as cloudy and rainy days outdoors.Little research has been done on the ability of MBGS to adapt to light-deficient environments.We studied the feasibility of MBGS system in treating municipal wastewater under the condition without enough light,in the aspects of pollutant removal performance,characteristics of MBGS and analysis of microbial community,providing theoretical basis and technical assistance for further engineering application of MBGS process.(1)The MBGS system was adaptable to continuous weak light(20μmol/m~2/s)and dark conditions,and could operate for a week and half a month,respectively.During the first a week of operation,weak light promoted the granule size growth and extracellular polymer production of MBGS.Firmicutes became the dominant bacteria in complex organics,which might promote the decomposition of complex organic matter under the condition of light-deficient.(2)The MBGS system could be maintained for about 12 days under 12 h/12 h weak light(20μmol/m~2/s)-dark conditions.Among them,the appearance of Tychonema＿CCAP＿1459-11B(secreted anatoxin)made microalgae and bacteria turn from mutualism to confrontation,leading to the collapse of MBGS system.By changing the light intensity,the relative abundance of microbial community significantly increased and MBGS system gradually recovered.In addition,the activity of microalgae and bacteria was related to the change of DO concentration and PH.(3)Further research was carried out in the outdoor weather mainly cloudy and rainy days(average light intensity 34.3μmol/m~2/s)to realize the effective operation of MBGS for about 12 days,and found that it had a good treatment effect on the actual sewage.The increase of biomass and EPS concentration improved the performance of MBGS and increased the granule size,respectively.Outdoor conditions could significantly promote the growth of Chloroflexi and Bacteroidetes,which was helpful to the decomposition and absorption of pollutants by MBGS.It was found that the abundance of genes encoding Psb A,Psb D,Psb E,Psb J,Psb P,Psb27,Psb28-2,Psa C,Psa E,Psa L,Psb X,Pet B,Pet A,and Pet E all increased in the light reaction.In the Calvin cycle,the gene abundance encoding Rubisco decreased but other genes increased,and the genes abundance of CAM cycle increased.This further indicates that MBGS ensure the completion of photosynthesis through other means in outdoor rainy weather.