Treatment Of Aquaculture Wastewater By Microalgae Membrane Culture And Characteristics Of Membrane Pollution

With the increasing demand for aquatic products,aquaculture is also developing rapidly.Due to the decomposition of feed and animal excreta in the water body,aquaculture wastewater containing nitrogen and phosphorus will be produced,and the direct discharge will cause irreparable consequences to the surrounding environment.At present,the urgent problem of aquaculture industry is to find a wastewater treatment technology with low cost and high pollutant removal rate to realize the deep purification of aquaculture wastewater.The existing treatment technology of aquaculture wastewater is adjusted according to the traditional methods,but the disadvantage is that the removal of nitrogen and phosphorus pollutants in water is not complete,and the resource recycling can not be realized.In this paper,nitrogen and phosphorus and other pollutants in aquaculture wastewater are used as nutrients,and microalgae are used to absorb nitrogen and phosphorus to achieve the purification effect of deep nitrogen and phosphorus removal.At the same time,algae cells can be harvested to produce biodiesel and other products,so as to realize the resource treatment of aquaculture wastewater.In this paper,a traditional cylindrical photobioreactor(PBR)was constructed,under the condition of batch culture,Chlorella vulgaris and Scenedesmus obliquus were cultured with aquaculture wastewater respectively.Due to the low content of nitrogen and phosphorus in the water,microalgae could not grow to a higher concentration.After 30 days of cultivation,Chlorella achieved a higher biomass of 0.27 g/L than Scenedesmus obliquus,the removal rates of nitrogen and phosphorus were 90.96%and 93.56%respectively,and the oil content of Chlorella vulgaris was 38%.The results show that microalgae culture can achieve the deep purification of aquaculture wastewater.Compared with Scenedesmus obliquus,Chlorella vulgaris has better adaptability to aquaculture wastewater and water purification ability.However,due to the low content of nitrogen and phosphorus in aquaculture wastewater and the limited nutrients,the microalgae biomass can not be continuously improved,and the nutrient removal rate and microalgae culture rate are greatly limited.In addition,the traditional batch culture method can not supply nutrients for microalgae continuously,which makes microalgae unable to grow continuously and achieve the purpose of efficient removal of nutrients.At the same time,the traditional batch culture method can not separate the algal liquid,which is easy to cause the loss of suspended microalgae cells.In the second stage,on the basis of the first stage,the membrane technology was introduced into the microalgae treatment process of aquaculture wastewater,and a membrane photobioreactor(MPBR)was constructed.Under the condition of continuous inflow and outflow,Chlorella vulgaris was concentrated and cultured by membrane method with aquaculture wastewater.The membrane module is added in the reactor to separate the algae liquid,so that the Chlorella vulgaris cells in the effluent can be completely intercepted in the reactor,so as to separate the algae liquid in the culture process,at the same time,the hydraulic retention time(HRT)and biomass retention time(BRT)of microalgae in the reactor can be controlled to ensure that microalgae can be cultured efficiently and nitrogen,phosphorus and other nutrients can be removed in depth.Compared with HRT=12 h,the removal efficiency of nutrients and microalgae concentration of MPBR reactor with HRT=24 h were better.After 22 days of cultivation,the maximum growth of microalgae in MPBR reactor with HRT=24 h was 0.53 g/L,and the daily maximum removal rates of nitrogen and phosphorus were 75.42%and 84.60%.The results show that extending HRT in a certain range can improve the microalgae production capacity of MPBR reactor,and is conducive to the full utilization of nutrients in wastewater,so as to realize the deep and stable removal of nitrogen,phosphorus and other nutrients in aquaculture wastewater.In the third stage,the effect of adding organic carbon sourced sodium acetate on the concentration cultures and nutrient removal of Chlorella pyrenoidosa in MPBR was further studied under the condition of reactor HRT=24 h.When the concentration of sodium acetate was 1 g/L,the maximum biomass of Chlorella pyrenoidosa were 1.03 g/L,it was twice as high as the MPBR without sodium acetate,the removal efficiency of nitrogen and phosphorus was 93.93%and 88.55%respectively.At the same time,the membrane fouling characteristics of membrane modules in the MPBR reactor were simultaneously studied by microalgae metabolites analysis,membrane fouling analysis,membrane resistance distribution,and microscopic observation of membrane structure.The results showed that the main component of membrane resistance of MPBR was internal resistance,and protein was the main component of membrane pollutants.At the same time,higher concentrations of nutrients or carbon sources can promote the efficient cultivation of microalgae,but at the same time increase the release of algae organic matter,which forms membrane fouling on the membrane module and aggravates the membrane fouling to a certain extent.In conclusion,Chlorella vulgaris has stronger growth ability and purification ability than Scenedesmus obliquus.MPBR can better solve the problem that PBR can’t separate algal liquid.Continuous in and out water culture in MPBR can obtain higher microalgae biomass yield and deeper removal of nutrients than traditional batch culture,and the effect is the best when HRT=24 h.On this basis,adding organic carbon sourced sodium acetate can improve the growth rate of microalgae and biomass production,and realize the deep purification of water.In addition,the resistance of membrane module in MPBR is mainly internal resistance,which is related to the growth of microalgae in the reactor.