Study On Nutrient Removal From Secondary Effluent And Water Purification Using Ulva Meridionalis

Cultivation of micro-algae utilizing secondary effluent for advanced treatment from wastewater treatment plants can achieve both purposes of wastewater purification and biomass recovery.However,the harvest of micro-algae after cultivation requires additional costs.Compared to micro-algae,Macrophytes with large individual biomass can be easy harvesting.Hence,this work focuses on a macrophyte Ulva meridionalis(U.meridionalis)with rapid growth rate,in order to examine the feasibility of nutrient removal from secondary effluent and evaluate biomass production possible at different salinity conditions.The biomass production and nutrient removal efficiency for U.meridionalis were assessed using a synthetic culture medium combined with artificial seawater.Laboratory batch cultivation under different initial nitrogen and phosphorus concentrations were conducted at temperature of 25±0.5?and at optimal pH(7.85±0.1)with a 12:12 L:D cycle using a cool white fluorescent light(18000 lux).To assess the growth performance of U.meridionalis under different nitrogen and phosphorus concentrations,specific growth rate was calculated based on the growth curve.Results showed that the removal rate of total inorganic nitrogen and phosphate by U.meridionalis significantly increased under high nutrient loading.The maximum biomass and specific growth rate were 0.679 g/L and 0.796 d~-11 under the conditions that nitrogen and organic phosphorus concentrations were 4.02 mg/L and 0.125 mg/L,respectively.Cultivations were performed using secondary effluent and artificial seawaterforwastenutrientcapturingandwastewaterpurification.U.meridionalis grew well and absorbed nitrogen and phosphorus in secondary effluent.The removal rate of nitrogen and phosphorus from wastewater is positively correlated with the growth of U.meridionalis.The better growth of U.meridionalis,the higher removal rate of nitrogen and phosphorus.Meanwhile,the removal efficiency of nitrogen and phosphorus by U.meridionalis is also related to the concentration of nutrients and the ratio of nitrogen and phosphorus in wastewater.The maximum daily nitrate and phosphate removal efficiencies were 70.3%and 64.8%under the conditions that total nitrogen and phosphate concentrations were 1.48 mg/L and 0.21 mg/L,respectively.And the maximum daily nitrate and phosphate removal efficiency were 74.9%and 67.7%under the conditions that total nitrogen and phosphate concentrations were 4.26 mg/L and3.04 mg/L,respectively.Salinity has significant impact on biomass production and nutrient removal because it is a key factor for the growth.There is no significant difference for macrophyte growth and inorganic nitrogen uptake between 100%and 60%salinity.However,the absorption and utilization of phosphorus using U.meridionalis had great influence.Under the condition of low salinity,the removal efficiency of phosphorus from wastewater by U.meridionalis decreased.Therefore,considering the economic cost,mixture of secondary effluent and seawater having a mixing ratio of 2:3 may be feasible for macrophyte cultivation.Mass balance analysis revealed that 50%~70%of total nitrogen in secondary effluent was absorbed by U.meridionalis for its growth,and other parts were converted into other forms of nitrogen through some processes,such as,part of ammonia nitrogen will change into ammonia volatilization and emission into the air due to the p H change of culture medium by U.meridionalis growth.The results of this study concluded that U.meridionalis was effective on secondary effluent advanced treatment,and further to be an alternative technology for offshore municipal wastewater advanced treatment.