HRT Optimization Of Microalgae-Bacteria Symbiotic Membrane Photobioreactor For Advanced Treatment Of Swine Wastewater

As Microalgae-bacteria membrane photobireactor（MB-MPBR）is an improved design of membrane bioreactor,which exhibits characteristics such as efficient simultaneous removal of carbon,nitrogen,and phosphorus,biomass resource recovery,stable operation,and small footprint.Membrane can effectively separate HRT and SRT to solve the biomass scour issue.The microalgae-bacteria consortia in the system could reduce aeration energy consumption,and the appropriate aeration can form shear force,which could reduce the adsorption and deposition of pollutants on the membrane surface and prolong the membrane operational cycle.The treatment effects depend on the optimization of various operating conditions and operating parameters,including influent wastewater characteristics,hydraulic residence time（HRT）and sludge residence time（SRT）and so on.Among these factors,HRT is the key factor affecting pollutants removal and biomass yeild.Different HRTs have direct effects on the contact time between microorganisms and substrate as well as the mass transfer process.Moreover,HRT also has great impact on membrane fouling conditions of MB-MPBR.Longer HRT would provide more preferable environment for the growth of microalgae-bacteria consortia,leading to relatively slight membrane fouling conditions.Therefore,it is very important to select a suitable HRT.In this experiment,the systematic properties of different HRTs in three groups（Group A=12 h,Group B=18 h,Group C=24 h）were compared and analyzed.The nutrients loads in the three groups were inversely proportional to HRT,and the durations taken for each group to reach the same particle size were proportional to HRT.The microalgae weights of Group A,B and C at Day 5 to Day 7 were 8 times the initial microalgae inoculation amount.Combined with dry weight analysis,the activated sludge concentration in Group A,B and C were 6.38±0.12 g/L,4.05±0.2g/L,and 2.78±0.13 g/L respectively,which were 6.4 times,4 times,and 2.9 times the initial inoculation amount.It was proved that,the total biomass yeild decreased with the increase of HRT.HRT had little effect on COD removal of the three groups.COD removal rates of all groups could achieve around 92%,and Group B and C could maintain high COD removal rates during the 30 days operational duration.Microalgae in the system could exceed their excessive phosphorus uptake,and their phosphorus uptake capability was higher than that of bacteria at the same biomass.The phosphorus removal rates were stable between 60%and 70%after Day 11.The NH₄⁺-N removal in the system was also significant,and the nitrogen contained in the reactor was removed by the assimilation and absorption of microalgae-bacteria symbionts.The average removal rate of NH₄⁺-N in all groups were around 90%with the best removal rate of91.75±1.12%（Group B）.In summary,MB-MPBR system exhibited relatively high treatment effects on carbon,nitrogen and phosphorus at different HRTs.Combined with the normalized flux of mixture EPS and its linear fitting results,it was found that the fitting results of mixtures in three groups were standard blockage>middle blockage>complete blockage>filter cake layer blockage.Combined with the analysis of three-dimensional fluorescence,Fourier transform infrared spectroscopy（FTIR）,contact angle and other methods,from the EPS intercepted by membrane,Group A showed most severe membrane fouling conditions,while Group B and Group C were relatively slighter.The main substances causing the membrane fouling were the soluble EPS and fulvic acid（FA）substances.FTIR was used to analyze the mixtures of each group and the EPS components obtained.Membrane fouling were mainly caused by polysaccharides and proteins-like substances.The contact angle of membrane surface in Group B at Day15 was less than 55°,indicating strong hydrophilicity and obvious mitigation of membrane fouling.In summary,the removal effects of C,N and P in MB-MPBR system were relatively stable among all groups.Group B showed the best removal effect on NH₄⁺-N.The membrane fouling conditions of Group A was most severe,while relatively slighter for Group B and Group C.The main contributors were polysaccharides and proteins.On the premise of ensuring the treatment effect of wastewater,A shorter HRT can save the cost,considering the membrane fouling conditions of Group A and the unstable microalgae-bacteria system,HRT at 18 h was thought to be the best choice.