Research On Advanced Wastewater Treatment Based On Microalgae Technology

It is the best way to solve problems of urban water shortage and water pollution by sewage advanced treatment and reuse,which has become rising demand in our country.Large-scale microalgae are cultured in sewage,which could simultaneously realize sewage purification and convert pollutants（e.g.,nitrogen and phosphorus）in sewage into microalgal biomass.However,microalgae cultivation in sewage is still in the laboratory research stage.The effect of low-concentration sewage on microalgae growth and sewage purification is unclear.In terms of technology,after microalgae purified sewage,there are problems of difficult separation of algae and high harvest cost.Economical and convenient harvesting technology is urgently needed.Thus,we investigated the change of operation mode and the development of reactor structure,which affect the microalgal yield and sewage purification.Effects of different microalgae harvesting techniques were evaluated.Furthermore,microalgae harvesting methods were optimized,which could promote the engineering application of purification sewage based on microalgae culture.Firstly,the microalgae production and wastewater treatment effect in Flat Photobioreactor（F-PBR）were studied under the conditions of batch and semi-continuous.In batch mode,the maximum microalgae concentration and yield were0.216 g/L and 18 mg L^-1 d^-1,respectively.After 19 d,the TN and TP concentrations in the reactor were 2.8 mg/L and 0 mg/L,respectively（corresponding to nitrogen and phosphorus removal were 88.98%and 100%,respectively）.Hence,batch culture proved the purification potential of microalgae for low-concentration sewage.To verify the feasibility of engineering application of this technology,microalgae yield and sewage purification effect were studied in semi-continuous mode.The results showed that 100%purification of phosphorus could be completed within 1 d after fresh sewage was replaced.The high ratio of nitrogen and phosphorus adversed to long-term microalgae growth.Microalgae yield and sewage purification effect were decreased with increasing periods of culture.To alleviate the restriction of low-concentration sewage and influent quality fluctuation on the microalgae growth,continuous flow mode was proposed to improve influent nutrient load in the reactor.However,the continuous flow F-PBR reactor is easy to cause the loss of microalgae with the effluent.By adding a membrane module into the reactor,Membrane photobioreactor（MPBR）was presented.When HRT=2 d,the maximum microalgae concentration and yield in MPBR reactor were 22 g/L and 23mg L^-1 d^-1,respectively.The average removal of COD,NH₄⁺-N,TN and TP in MPBR reactor reached 44.8%,91.1%,45.87%and 98.56%,respectively.Hence,MPBR reactor not only promotes the continuous microalgae growth but also has a better sewage purification effect.By adding carriers into the MPBR reactor,Suspended Attached Microalgae Cultivation Photobioreactor（SAMC-PBR）was constructed.The maximum microalgae concentration and yield in SAMC-PBR reactor were 0.378 g/L and 33 mg L^-1 d^-1,respectively.The average removal of COD,NH₄⁺-N,TN and TP in SAMC-PBR reactor reached 46.54%,95.88%,50.25%and 98.91%respectively.Thus,SAMC-PBR reactor has better performance than MPBR reactor.In the process of algae-water separation,the effects of membrane separation,gravity precipitation and filter harvesting were studied.Membrane separation has the better effect,which could achieve 100%microalgae harvesting.However,membrane module need to clean once a day.Its operating expenses are higher than other harvesting techniques.With regard to microalgae harvest costs,costs of gravity precipitation harvesting are the lowest,95.8%-98.2%algae could be harvested after 24 hours.The filter material was replaced by superfine fiber cotton,which could have the strong ability to capture microalgae.When the packing density is 0.015 m²/L,filter（superfine fiber cotton as the filter material）could harvest 99.9%microalgae after running for 2 h;its harvesting effect is similar to membrane separation and its running time is less than gravity precipitation.Thus,filter（superfine fiber cotton as the filter material）is optimal microalgae harvesting technology.