Performance Of Using Algal-bacterial Bioreactors For Synthetic Domestic Wastewater Treatment

The aeration cost can account for up to 50%?70%of the total operation cost of a conventional wastewater treatment plant.Biological wastewater treatment using activated sludge is energy intensive,algal-bacterial consortia can be an alternative method of treatment.Applying algae and bacteria can generally achieve better bioreactor performance than applying bacteria alone.Algae and bacteria are applied to treat domestic wastewater,and 2?10 days are generally required for pollutant removal.The high hydraulic retention times(HRTs)demonstrated that the performance of the algal-bacteria reactors were extremely low,thus indicating that algal-bacterial bioreactors should be further improved.A algal-bacterial consortium was composed of activated sludge and algae from the wall of a secondary settler.Three photo membrane bioreactors(PMBR)and a photo sequencing batch biofilm reactor(PSBBR)were used for synthetic domestic wastewater treatment.The effects of four continuous flow reactors on the treatment of domestic sewage were analyzed,the impacts of sludge residence time(SRT)on the performance of the PMBR reactor were investigated,the activities of the microorganisms in reactors were tested,the concentrations of the signaling molecules in the reactor and the effect of the detected signaling molecule on the activity of the algae were measured.The results can provide theoretical guidance for the application of the algal-bacterial consortium in domestic wastewater treatment.The main conclusions are as follows:? Experiments were conducted to investigate the performance of PMBR(reactor 1,R1)for domestic wastewater treatment.A low light intensity,200 ?mol/m2/s,was applied in a PMBR.The results showed that ammonium was almost completely removed and the removal efficiency of chemical oxygen demand was 90%when the HRT was as low as 24 h and mechanical aeration was not applied.Ammonium-oxidizing bacteria(AOB)and algae approximately equally shared the ammonium in the reactor.Phosphate removal rate was approximately 30%.A light intensity of up to 600 ?mol/m2/s did not inhibit algal activity.The complete removal of ammonium resulted in a decline in the chlorophyll a concentration.Nevertheless,the reactor perforamance remained stable.Heterotrophic bacteria,autotrophs(AOB,NOB),algae and phosphate-accumulating organisms(PAOs)coexisted and functioned in the reactor.The filter of the PMBR at a transmembrane pressure as low as 4 kPa.Considering the stable pollutant removal performance and significant reduction in HRT,this PMBR has the potential to be applied in wastewater treatment.? The impacts of sludge retention time(SRT)on the performance of algal-bacterial reactors were evaluated.Photo membrane reactors R2 and R3 were applied at SRTs of 10 days and 20 days,respectively.A low light intensity,200 ?mol/m2/s,and a low hydraulic retention time,8 hours,was intentionally applied for showing the effect of SRT.Results showed that biomass concentration did not exceed 2 g/L.Without external oxygen supply,the chemical oxygen demand(COD)removal efficiencies were approximately 60%and 50%at the SRTs of 20 days and 10 days,respectively.Ammonium removal efficiencies were approximately 50%in the two reactors.Slight aeration increased the COD and ammonium removal efficiencies to approximately 90%and 100%respectively in the reactor 3(R3),whereas the improvements in the reactor 2(R2)were less.Ammonium oxidation bacteria(AOB)were effectively washed out by applying the SRT of 10 days,whereas much more AOB were detected in the reactor 3(R3).The presence of phosphate accumulation organisms in continuously illuminated reactors was proved by high throughput sequencing.Strikingly,the reactor 3(R3)showed higher phosphate removal efficiency at the conditions that more AOB,which competed for ammonium against algae,were detected in the reactor 3(R3).Therefore,improving phosphate removal by washing out AOB via SRT control is not feasible.Furthermore,the SRT also had significant impacts on algal ecology.Microscopic examination showed that Oscillatoria sp.,the most active algae,dominated in the reactors,whereas the low SRT washed out Chlorella sp.,Diatoms sp.,Closterium sp.Generally,experimental results suggested that SRT of 20 days(R3)showed better reactor performance.? Experiments were conducted to investigate the performance of a novel algal-bacterial biofilm reactor(reactor 4,R4)with three dimensional carriers for domestic wastewater treatment.The experimental light intensity was 200 ?mol/m2/s,HRT was 24 h,12 h and 8 h respectively.Results showed that a HRT of 12 hours ensured 90%removal of organic matters and ammonium,and phosphate removal efficiency was around 3%.Decreasing the HRT to 8 hours significantly deteriorated the reactor's pollutant removal efficiencies and increasing the HRT to 24 hours did not improve these efficiencies.Organic matter and ammonium removal rates resulting from oxygen released by algae were 70%and 50%of the rates when dissolved oxygen concentration was maintained at 2 mg/L,respectively.Ammonium removal rate was approximately 10390 mg NH4+-N/m2/d while the reactor was aerated and 5000 mg NH4+-N/m2/d when the reactor was illuminated only,which were significantly higher than reported nitrification rates.Moreover,Oscillatoria sp.dominated in the biofilm,while other rapid growing algae such as Chlorella sp.and Scenedesmus sp.almost disappeared.Finally,heterotrophic bacteria,autotrophs(AOB,NOB),phosphate accumulation bacteria,and algae coexisted and functioned in the reactor.These results showed that an algal-bacterial biofilm reactor with a low reactor footprint was developed.Compared with the algal-bacterial biofilm reactor(R4),the algae in the suspended algal-bacterial consortium(R1,R3)had higher community diversity.The PMBR reactor(R1)and the PSBBR reactor(R4)could effectively remove the COD and ammonium with a HRT of 24 hours,in which the R1 was more efficient and more stable.However,For the HRT of 8 hours,both of the R3 and R4 could not efficiently remove the the pollutants.Among current known chemical signals,urocanic acid was the only chemical signal that was detected in R1 and R4.The concentration of urocanic acid in R1 and R4 was 0.5 ?g/L and 19.5 ?g/L,respectively,which were much lower than reported functional concentrations.It was found that the concentration of uric acid as low as 0.5 ?g/L could significantly reduce chlorophyll a concentration and further inhibited the algal activity.Inhibiting the secretion of uremic acid by bacteria may promote the growth of algae and thus improve the performance of the reactor.