Stress Effects Of Free Ammonia In Sludge Fermentation Liquor On Microalgae

Returning the sludge fermentation liquor to the influent wastewater stream increases the nitrogen load of wastewater treatment plant,which is not accord with the concept of resourceoriented treatment.Microalgae-based wastewater treatment could achieve both the wastewater purification and algal biomass production,however,there are problems such as complicated wastewater quality and low growth efficiency of microalgae.Among them,the stress brought by high-strength ammonium wastewater(e.g.sludge fermentation liquor)is one of the most significant factors that limits the efficiency of wastewater treatment by microalgae.In this study,Chlorella sorokiniana was selected as the research object,simulated sludge fermentation liquor was selected as the culture medium,and batch tests with different conditions were designed to explore the actual stress factor to microalgae in sludge fermentation liquor.The stress effects of free ammonia(FA)on microalgae,as well as the physiological response of microalgae to resist stress were further studied.The main findings are as follows:Based on the experiments with factors including ammonium concentration,p H and FA concentration in simulated sludge fermentation liquor,it was found that microalgal biomass(manifested by chlorophyll content in medium)was negatively correlated with the single factor of ammonium concentration or p H,but neither of them was the sole factor inhibiting microalgal growth.However,FA concentration clearly exhibited correlation with the microalgal biomass,with experimental data well fitted by an exponential decay model(On Day 5: y=26.7101e-0.0643x+1.7493,r2=0.96),suggesting that FA was the key stress factor in sludge fermentation liquor to microalgae.The stress effects on microalgae were mainly manifested by reduced growth performance,reduced photosynthesis,as well as wrinkled cell walls.Moreover,microalgae suffering FA stress could recover fast after FA being removed,suggesting the revisability of FA inhibition.Considering the actual situation of real-time changed FA concentration in cultivation system,this study further explored the stress effects of initial and dynamically variational FA concentration on microalgae during cultivation.Except for the reduced growth and photosynthetic performance,high FA concentration also induced the increased production of reactive oxygen species(ROS)and the specific degradation of chlorophyll a in cells.After 24 h of exposure,the highest ROS level of 12.92 FI/1010 cells was observed in the group with highest FA concentration of 110.97 mg/L.Microalgae showed significant physiological response to resist stress,including the timely up-regulation of intracellular superoxide dismutase(SOD)activity(After 24 h of cultivation,the SOD level in microalgae cells grown under 78.34 mg/L of FA was twice as high as that grown under 2.06 mg/L of FA),increased secretion of soluble extracellular polymeric substances(S-EPS;144.13–160.86 mg/g DW),upregulation of the polysaccharide proportion in S-EPS(the ratio of polysaccharide to protein in the group with 110.97 mg/L of FA was 2.6 times higher than that with 2.06 mg/L of FA),as well as the accumulation of cellular carbohydrates(12-14% of DW)and palmitic acid(19.13-34.52%),all of which were supposed to alleviate FA stress through different mechanisms.Moreover,30.37-65.93 mg/L of ammonium nitrogen was removed mainly by algae uptake and evaporation.This study deepened the understanding of the FA stress effects on microalgae,and provided theoretical support for the development of resource-oriented treatment of sludge fermentation liquor based on microalgae cultivation.