Effect And Mechanism Of Characteristic Pollutants On Microalgae Growth And Performance Of Swine Wastewater Treatment

With the development of China’s economy and the continuous improvement of people’s living standards,people’s demand for pig food is increasing daily.Driven by a series of national policies to accelerate the development of the livestock industry,pig farming has shown the characteristics of large-scale and intensive.Serious environmental pollution problems,such as water eutrophication,soil pollution and air pollution,even threaten public health.Therefore,how to efficiently,cost-effectively,sustainably and environmentally manage these large amounts of swine wastewater has become crucial.In recent years,the use of microalgae to treat swine wastewater can meet the requirements of high efficiency,low cost,simple operation and resource recovery,and has become a potential swine wastewater treatment technology.At present,most researches focus on improving the efficiency of microalgae in treating pig wastewater.In order to improve feed utilization,promote pig production and prevent diseases,heavy metals and antibiotics are added to feed as additives.Due to the low absorption rate of pigs and the problem of additive abuse,most of the heavy metals and antibiotics are excreted in the form of feces and urine and enter the wastewater.However,few studies have focused on the performance of microalgae in treating swine wastewater and biomass recovery under the stress of heavy metals and antibiotics,especially the combined stress of heavy metals and antibiotics.In response to the above problems,this thesis takes the typical characteristic heavy metals（Zn（II））and antibiotics（tetracycline antibiotics and sulfonamide antibiotics）in swine wastewater as the target object,and explores the impacts of heavy metals,antibiotics and their combined stress on the performance of microalgae to treat swine wastewater and biological resource recovery,and in-depth elucidate the impact mechanism of combined stress via the analysis of the main physiological and biochemical indicators of microalgae.The content of this paper will help promote the engineering application of microalgae to treat swine wastewater and provide a theoretical basis for realizing the sustainable development of the microalgae treatment wastewater industry.The main content and results are as follows:The Part 1 explored the effect of Zn（II）on the removal of nutrients and the recovery of biological resources in the treatment of pig wastewater by the microalga Coelastrella sp.The results show that the effect of low concentration of Zn（II）（0-1.0mg/L）on the removal of NH₃-N can be ignored,and then increasing the concentration of Zn（II）will reduce the removal of NH₃-N.Through analysis,we find a high correlation between the removal efficiency of NH₃-N and the biomass of microalgae,and the effect of Zn（II）on NH₃-N removal is mainly through affecting the assimilation of NH₃-N by microalgae,and indirectly affecting p H to regulate NH₃-N volatilization.For TP removal,when the Zn（II）concentration is lower than 2.0 mg/L,the TP removal decreases with the increase of the Zn（II）concentration;the subsequent increase in the Zn（II）concentration promotes the TP removal,especially when the Zn（II）concentration increases,especially when the concentration of Zn（II）is 8.0 mg/L,the TP removal efficiency is as high as 77.6%,which is attributed to the formation of Zn₃（PO₄）₂·2H₂O precipitation.In addition,within the range of Zn（II）concentration of0.50-8.0 mg/L,as the Zn（II）concentration increases,the biomass of microalgae decreases,SOD enzyme activity,glutathione and protein content increase,and the cell community appears the reunion phenomenon.The content of chlorophyll a showed a trend of increasing first and then decreasing,reaching a maximum at a Zn（II）concentration of 1.0 mg/L.（Corresponding to Chapter 2 of the article）Part 2 features tetracycline antibiotics（tetracycline（TC）,oxytetracycline（OTC））and sulfonamides（sulfamethazine（SMZ）and sulfamethoxazole（SMO））in swine wastewater.The effect of typical antibiotics on the removal of nutrients and the recovery of biological resources in the anaerobic fermentation broth of the microalgae Coelastrella sp.in the treatment of pig wastewater was studied.The results of the study found that TC and OTC can significantly inhibit the removal of NH₃-N and TP by microalgae,while high concentrations（5000μg/L）of SMZ and SMO can promote the removal of TP by microalgae.TC and OTC can slightly promote the increase of microalgae biomass under the condition of low concentration（50μg/L）.When the concentration exceeds this concentration,the increase of its biomass will be inhibited;SMZ and SMO have no obvious inhibitory effect on the growth of microalgae biomass.It can promote the growth of microalgae in the later stage of experimental culture.When the TC concentration is higher than 5000μg/L,it will significantly inhibit the accumulation of oil in microalgae.Low concentration of OTC（50μg/L）can promote the increase of microalgae fatty acid unsaturation,while high concentration（5000μg/L）of TC and OTC can reduce the unsaturation of microalgae fatty acid.The presence of TC,OTC and SMZ can all promote the accumulation of chlorophyll a in microalgae and show a decrease in accumulation as its concentration increases;however,the presence of SMO inhibits the accumulation of microalgae chlorophyll a,and the inhibitory effect increases with the increase of SMO concentration.In addition,by optimizing the concentration of TC,SMZ and SMO,protein accumulation in microalgae can be maximized.（Corresponding to Chapter 3 of the article）On the basis of the above-mentioned Parts 1 and 2,the Part 3 explored the effect of the microalga Coelastrella sp.on the removal of nutrients in swine wastewater and the recovery of biological resources under the combined pollution stress of Zn（II）and OTC.The results showed that,compared with single Zn（II）and OTC stress,the combined stress of Zn（II）and OTC significantly inhibited the growth of microalgae and the removal of NH₃-N,and the inhibitory intensity increased with the increase of Zn（II）concentration.Meanwhile,under compound stress conditions,compared with Zn（II）,OTC has a more significant effect on the growth of microalgae and the removal of NH₃-N and TP.Compared with single Zn（II）and OTC stress,the combined stress of OTC and high concentration Zn（II）（5.0 mg/L）can significantly promote the accumulation of oil in microalgae,especially when OTC is 50μg/L the maximum accumulation of oil can be obtained,about 0.176 g/g.In addition,compared with single Zn（II）and OTC stress,the combined stress of Zn（II）and high concentration OTC（5000μg/L）can significantly reduce the unsaturation of fatty acid methyl esters in microalgae,which is mainly reflected in the reduction of C18:3 and the increase of C18:1.However,under the same conditions,combined stress will promote the accumulation of protein in microalgae,especially in the presence of high concentrations of OTC（5000μg/L）,the promotion effect is significant.（Corresponding to Chapter 4 of the article）Part 4 further explored to clarify the influence mechanism combined stress of Zn（II）and OTC on the basis of Part 3.The results showed that the complexation between OTC and Zn（II）could occur under compound stress,and Zn（II）and OTC could affect the removal efficiency of NH₃-N by microalgae by affecting the activity of glutamine synthase（GS）.With the increase of Zn（II）concentration and OTC concentration in the solution,the GS activity of microalgae decreased,especially in the high concentration of Zn（II）and OTC combined stress conditions,the GS activity significantly decreased,resulting in a corresponding reduction of NH₃-N removal efficiency.At the same time,under the combined stress environment,a series of biological responses of microalgae to resist toxicity increased the content of adenosine triphosphate（ATP）in microalgae,and increased with the increase of Zn（II）and OTC concentration,which promoted the removal of TP by microalgae.（Corresponding to Chapter 5 of the article）.