Construction Of Algae-bacteria Co-cultivation System For Enhancing Algae Harvest And Accelerating Wastewater Treatment

Microalgae as potential bioresources received extensive concern.However,microalgae harvesting technology is a bottleneck in the development of microalgae biotechnology.Bioflocculation is a potential method to harvest algae,reducing cost and energy.On one hand,bioflocculation method using bioflocculation bacteria can harvest algae and avoid the negative effects of traditional flocculants such as metal ions.On the other hand,the bioflocculation bacteria promote wastewater degradation as the result of axenic algae generally showing low wastewater treatment capability.In this work,co-culture conditions were optimized to weaken the negative competition between the microalgae and bioflocculation bacteria by response surface methodology(RSM),enhancing harvesting efficiency and lipid content.And we proved algae-bioflocculation bacteria co-cultivation(AB)in synthetic wastewater,aiming to enhance algae harvest,promote algae biomass,accelerate wastewater treatment and reveal the mechanism of algae bioflocculation.Axenic microalgae exhibited poor harvesting,as expressed by a flocculation efficiency of 0.2%.This work optimized the co-culture conditions of microalgae and bioflocculation bacteria in synthetic wastewater using response surface methodology,aiming to enhance microalgae harvesting and lipid content.Three significant process variables: inoculation ratio of bacteria and microalgae(B/C),initial glucose concentration(Cglucose),and co-culture time(T),were proposed in the RSM model.Fvalues of 3.98/8.46 and R2 values of 0.7817/0.8711 both indicated a reasonable prediction by the RSM model.The results showed that microalgae harvesting efficiency reached 45.0–50.0%,and the lipid content was over 21.0% when co-cultured with bioflocculation bacteria under the optimized culture conditions of inoculation ratio of bacteria and microalgae of 0.20–0.25,initial glucose concentration of less than 1.5 mg/L and co-culture time of 9–14 d.We investigated the effect of algae harvesting efficiency and wastewater treatment under different hydraulic conditions.The results showed that the algae harvesting efficiency was 86.65%,algae biomass was 7108.00 mg/L and chemical oxygen demand(COD)removal was 77.96% when co-cultured condition was under the higher hydraulic condition(150 rpm/min).We also proved algae-bioflocculation bacteria cocultivation(AB)in synthetic wastewater under higher hydraulic condition,aiming to enhance algae harvest,promote algae biomass and accelerate wastewater treatment.The results showed that the harvesting efficiency of AB achieved to 88.72% only in 24 hours co-cultivation as compared to 4.54% achieved with purified algae cultivation(PA).Algae biomass of AB raised to 7406.06 mg/L and 8205.00 mg/L in the co-cultivation time of 24 hours and 48 hours respectively.The COD and total nitrogen(TN)removal efficiency of 91.66 % and 52.34% in AB during 48 hours cultivation were much higher than that of PA owing to the presence of bacteria.Scanning electron microscopy(SEM)observed the structure of algaebioflocculation bacteria granules and algae-bioflocculation bacteria flocs.SEM images indicated that EPS and bacteria had relevance with algae aggregation.Extracellular polymeric substances(EPS)measurement showed that polysaccharide and protein content in AB were 257.89 mg/L and 60.92 mg/L by the end of 24 hours cultivation.Tryptophan and tyrosine were the dominant fluorescent components of EPS in AB by three-dimension excitation emission matrix fluorescence spectroscopy(EEM)measurement.Fourier transform infrared(FT-IR)results observed that the change of glucosidic bond and amine group associated with polysaccharide and protein.Highthroughput sequencing analysis indicated that Bacillus and Sphingobacterium increased the secretion of tryptophan and rhamnose respectively.Brevundimonas secreted protein and polysaccharide for algae flocculation.The comprehensive results further supported that aromatic protein(tryptophan and tyrosine)and polysaccharide of extracellular polymeric substances played key roles in harvesting algae in AB.This work provided new insights into microalgae harvesting and cost-effective microalgal bioproducts and confirmed the promising prospect of introducing bioflocculation bacteria into microalgae bioenergy production.