Influencing Factors Of Microalgal Culturing In Helotism Wastewater Treatment System

Environmental problems are very serious in our country: on the one hand, large amount of wastewater discharge in rivers without treatment and reasonable reuse; On the other hand, with the intensification of the greenhouse effect, the reduction CO₂ emission and the application of CO₂ fixing method are in urgent. Once microalgae can be used in wastewater treatment system, it can absorb CO₂ while the purification of wastewater. It is able to effetely achieve wastewater recycling and CO₂ emission reduction requirements.This thesis has used some experiment method (single factor experiment method, uniform experiment method, etc.) to investigate various environmental factors, on the growth of algae in artificial wastewater and real wastewater, including light, nutrients, pH, CO₂ enrichment, etc. This study also set up a laboratory scale experiment, to verify the previous results and investigated the pollutants removal in various experimental conditions.The conclusions of this thesis are as follows:1) The primary sedimentation tank effluent is more suitable for algae growth than the secondary sedimentation tank effluent. Firstly, this is because the primary sedimentation tank effluent enriches more nutrients which are more suitable for algae uptake. Secondly, primary sedimentation tank effluent contains more bacteria, which can promote the growth of the algae.2) Algae can release a large number of active oxygen, which can make the DO up to 20 mg·L^-1. Once aeration, due to aeration stripping effect, the final dissolved oxygen will reduce to 7-9 mg·L^-1. And the greater the aeration intensity, the lower the DO in the solution is.3) The enrichment of CO₂ can facilitate on algal growth significantly. Low concentration enrichment of CO₂ can increase algae production significantly, but high concentrations of CO₂ enrichment will inhibit on algal growth. Recommended enrich 2% (0.004 v·v-1·min^-1) CO₂4) Light intensity and photoperiod have complex interaction effect on algae growth, and also affected by the water depth, light transmission and other factors. In the initial cultivation, because of a good translucent of liquid algae, the effect of SS is not significant on the algae growth. But when the algae grow into the stable period, the maximum cell density will be restricted by the SS concentration. Greater SS concentration is, the lower the maximum cell density can reach.5) Urea as a nitrogen source is the most conducive to algae growth. In addition, high concentrations of inorganic nitrogen inhibit the growth of algae. At 20mg·L^-1 85mg·L^-1 range, the higher the concentration of inorganic nitrogen, the slower growth of algae. In a typical wastewater phosphorus concentration range (2mg·L^-18mg·L^-1), algae grow the fastest when the phosphorus concentration is 4mg·L^-1. pH can significantly affect on the growth of algae. The best pH for Scenedesmus dimorphus growth is 7.5. There is not significant interaction between phosphorus concentration and the pH value.