Multi-objective Optimization Of Ultrasonic Parameters For Microcystis Aeruginosa Removal And Study On The Characteristics Of Inactivated Algae Cells

Microcystis aeruginosa has strong vitality and reproductive capacity and is the most common and representative speices on cyanobacteria blooms.And Microcystis aeruginosa can secrete microcystins（MCs）with hepatotoxicity,neurotoxicity,skin irritation and nephrotoxicity,which would ause serious harm to human health.Ultrasonic technology is a new environmental friendly technology developed recently,it has the advantages of easy to operate,reacting fast,and no chemicals added.Although the ultrasonic technology on algae removal had been extensively investigated,the effect of ultrasonic parameters on algae organic matter（algal organic matter;AOM）release had been poorly studied.The selection of ultrasonic parameters directly determines the energy consumption level of ultrasonic technology,so the ultrasonic parameters should be optimized reasonably to reduce safety risks and energy consumption.The main conclusions of this research were summarized as follows:（1）The single factor experiments were designed to investaged the effects of ultrasonic time（X₁）,ultrasonic power density（X₂）and ultrasonic frequency（X₃）on the removal of algae cells and the values of UV₂₅₄、SUVA（specific ultraviolet absorbance;SUVA）、DOC（dissolved organic carbon;DOC）.The results showed that ultrasonic treatment could remove M.aeruginosa effectively,and the maximum removal rate was78.3%in this research.The removal rates were proportional to ultrasonic time,power density and frequency.The values of SUVA,DOC and UV₂₅₄ of algae solution increased significantly in 10 minutes after ultrasonic treatment,indicating that the ultrasound would cause some intracellular and extracellular organic matter released to the water.However,the values of SUVA,DOC and UV₂₅₄54 of algae solution decreased slightly after 48 hours,presumably because the inactivated algal cells and the high-viscosity extracellular polysaccharide on the outer surface adsorbed and flocculated organic matters during precipitation,and some of the organic matter sunk to the bottom with the inactivated algae cells.The damaged algae cells continuously absorbed nutrients to regain activity,so the concentration of organic matters decreased in 48h.（2）Response surface methodology was used to explore the importance of ultrasonic frequency,ultrasonic time and ultrasonic power density to the algae removal rateρand UV₂₅₄.The results showed that,for the algae removal rate,the order of significant degree for each factor was:ultrasonic frequency>ultrasonic time>ultrasonic power density.For the UV₂₅₄ of algal liquid,the order of significant degree for each factor was:ultrasonic power density>ultrasonic frequency>ultrasonic time.At the same time,the second-order response models ofρand UV₂₅₄ were obtained with the ultrasonic frequency,ultrasonic time and ultrasonic power density as variables.（3）To improve the algae removal rate,reducing energy consumption and reducing the UV₂₅₄54 of algal liquid,the multi-objective optimization of the ultrasonic system was carried out by using NSGA-II genetic algorithm and Matlab software,and a good Pareto frontier surface and a series of well-distributed,versatile Pareto optimization solutions are obtained.It was verified that the Pareto optimal solutions were reliable and could provide guidance for practical engineering.（4）After ultrasound treatment,the algal cells dispersed into a single cell from the cluster state,cell wall and cell membrane damage and fold.The internal organelles of algal cells become fuzzy and disorderly,some of them atrophy or disappear,and the structure was destroyed obviously.The extracellular polymer enriched on the extracellular surface becomes loose,and some extracellular organics enter the water body,resulting in the increase of organics in the water.Ultrasonic role of free radicals on algal cell oxidative damage,cell membrane lipid peroxidation products MDA content increased dramatically,at the time of 48 h reached 18.7 mg/L,6.3 times that of the initial value,algal cell antioxidant system is not enough to resist external damage,the content of SOD and CAT showed a trend of decrease then increase first,the activity of algal cells to reduce and even death.Hydroxyl radicals produced by ultrasound in water are the basis of sonochemistry,and their quantity is directly proportional to ultrasonic time and power density.Within a certain frequency range,the number of hydroxyl radicals is positively correlated with the frequency.When the frequency exceeds a certain value（740kHz）,the ultrasonic cavitation effect decreases and the yield of·OH decreases.Algae fluid three-dimensional fluorescence detection results showed that the soluble microbial metabolites（SMPs）,aromatic protein material and humic acid substance is the main component of algal organic solution ultrasonic action will lead to rising concentrations of organic matter in water,especially soluble microbial metabolites and classes of humic acid substances content increased significantly.The increase of organic matter concentration is proportional to the ultrasonic power density.Therefore,improper control of ultrasonic parameters has certain safety risks.