Screening Of Yeast Strain With High Phosphorus Removal Efficiency And Its Phosphate Metabolism Characteristics

With the rapid development of industrialization,agricultural modernization and urbanization,the nutrients discharged have been increased,resulting in serious eutrophication of water bodies in recent years.Water eutrophication is a global problem of water environmental pollution,and it is also one of the most challenging problems in water protection.However,the excessive discharge of phosphorus is the main cause of this problem,so the effective removal of phosphorus is an urgent problem,which is of great significance to the purification of water.Compared with chemical phosphorus removal,biological phosphorus removal has the advantages of energy saving and environmental protection,such as low operating costs and low sludge production,so it has been widely used.In biological phosphorus removal system,phosphorus-accumulating bacteria are considered to play a major role,while yeast,as a single-cell eukaryotic microorganism,is often neglected in wastewater treatment system,and there is little research on phosphorus removal.In fact,the diversity of yeast population in wastewater treatment system is very rich,and yeast has the characteristics of osmotic pressure resistance,high temperature resistance and high metabolic efficiency.It has a good enzyme system in vivo and can adapt to various special environments.Therefore,studying the basic rules of phosphorus metabolism of yeast,especially expounding the key regulatory factors of phosphorus metabolism of yeast and their rules of action,and defining the ways of phosphorus removal can provide a new pathway for biological phosphorus removal.In this study,six dominant yeasts isolated and purified from A/O alternating biofilm system in the laboratory were activated and enriched,and then inoculated into the synthetic wastewater medium for treatment.It was found that these six strains of yeast had certain phosphorus removal capacity.The specific growth rate of strain BZ was 0.11 h^-1,and the amount of phosphorus removal per unit dry weight per unit time was 1.246 mgP/?h.g yeast?,which is highest among the six yeasts.Considering the growth rate and phosphorus removal efficiency,BZ was finally selected as a high-efficiency phosphorus removal yeast.What is more,BZ can be identified as Trichosporon asahii by morphological characteristics,physiological and biochemical characteristics and molecular biological identification.The phosphorus removal ability of yeast BZ was investigated by beaker tests at different initial phosphorus concentration.The results showed that the phosphorus removal rate of yeast decreased with the increase of phosphorus concentration.When the phosphorus concentration was less than 5 mg/L,the removal rate of phosphorus by yeast could reach100%,while the removal rate could still reach 70%when the phosphorus concentration was10 mg/L.At the same time,the growth kinetics and phosphorus removal kinetics of yeast were fitted,which accorded with Monod equation and Michaelis-Menten equation respectively.The kinetic equations werem???.The effects of temperature,pH,types of carbon sources,concentration of nitrogen sources,ratio of carbon to phosphorus,dissolved oxygen and trace elements on the growth and phosphorus removal efficiency of yeast BZ were analyzed,and the suitable conditions for phosphorus removal by yeast BZ were confirmed.In the temperature range of 15³5°C,yeast can maintain a high phosphorus removal efficiency,the optimum temperature is about 25°C,and the phosphorus removal rate reached more than 65%.The tolerance range of yeast BZ to pH is wider.When pH is 5⁸,the growth and phosphorus removal rate of yeast have few differences,and they all have a high phosphorus removal rate.When the optimum pH is 6,the phosphorus removal rate is 67%.Different carbon sources have significant effects on the growth and phosphorus removal of yeast.The best carbon source of yeast BZ is the mixture of glucose and sodium acetate,and phosphorus removal rate reached 68.6%.High ammonia nitrogen concentration can promote the growth of yeast BZ and increase the phosphorus removal rate.At the initial ammonia nitrogen concentration of 0⁵0 mg/L,the biomass and phosphorus removal rate of yeast BZ increased with the increase of ammonia concentration.With the increase of C/P,the more abundant carbon source,the larger the biomass of yeast and the higher the phosphorus removal rate were.When C/P was less than 10,the growth and phosphorus removal efficiency of yeast were significantly inhibited,and when C/P was greater than 75,phosphorus removal rate reached more than 90%.Under the conditions of0.5⁵ mg/L dissolved oxygen,yeast can grow normally and maintain a high phosphorus removal rate,which can reduce aeration and save cost.Compared with the case with trace elements,the maximum OD₆₀₀ only decreases by 0.026,while the phosphorus removal rate decreases from 68.6%to 57.0%without trace elements.Based on the analysis of the mass balance of phosphorus in the culture system,the enrichment of phosphorus by yeast and the distribution of intracellular and extracellular phosphorus,the transfer process of phosphorus from the main body of liquid phase to yeast cells was preliminarily discussed.It was found that in the whole culture system,when the yeast BZ reached the maximum biomass,the phosphorus removal reached the maximum.The removal of phosphorus?about 70%?by yeast cells was higher than that?about 30%?by EPS.In addition,positive phosphorus accounted for about 30%of total phosphorus in extracellular EPS and 20%of intracellular positive phosphorus in bacteria.EPS mainly acts as a phosphorus storage and transfer station.The uptake and transformation of phosphorus by cells could be the main phosphorus removal pathway in the yeast BZ phosphorus removal system.There are mainly sugars and proteins in EPS,and the change of glycogen may be related to secreting EPS by yeast under eutrophication and consuming EPS under starvation.By analyzing the three-dimensional fluorescence characteristics of EPS,protein-like substances are the main fluorescent substances in EPS during BZ phosphorus removal process.In the yeast phosphorus removal system with C/P of 40,there were glycogen and PHA in the intracellular polymer.The ATP required by yeast to degrade phosphate come from the hydrolysis of PHA.External carbon source is mainly converted into glycogen by yeast as the main energy storage material.Scanning electron microscopy?SEM?and energy dispersive spectroscopy?EDS?analysis showed that the yeast was rod-shaped.And it was speculated that the surface of yeast might be encapsulated by calcium magnesium phosphate after the reaction.Trichosporon asahii,a high-efficiency phosphorus-removal yeast screened in this paper,has strong phosphorus removal ability for synthetic wastewater culture medium.The pathway and mechanism of phosphorus removal were further clarified.It filled the gap in the field of phosphorus removal by yeast,enriched and deepened the theory of biological phosphorus removal,and provided theoretical reference for developing new technologies of phosphorus removal from wastewater.