Study On The Efficiency Of Microalgae-Bacteria Symbiosis System In Treating Livestock And Poultry Breeding Wastewater

The livestock and poultry industry maintains large-scale,intensive development.As more and more livestock and poultry breeding wastewater rich in nitrogen,phosphorus and other biological nutrients is discharged into the surrounding ecosystem,serious environmental pollution has been caused.This underpins an imperative need for energy-efficient,easy-to-operate wastewater treatment technologies that reach the applicable discharge standard of wastewater for livestock and poultry breeding.To this end,microalgae and activated sludge were co-cultivated at 5:1 to establish a stable and efficient microalgae-bacteria symbiosis system.Control variables were used to evaluate the effects of light-dark（L/D）ratio,aeration intensity,and Microalgae-bacteria retention time（Mb RT）on the nutrient absorption rate of the microalgae-bacteria symbiosis system and the growth rate of microalgae.Further,an orthogonal experiment with a 33factorial design was conducted to determine the optimal operating conditions of the microalgae-bacteria symbiosis system.And carry out the pilot test under the optimal operating conditions.To evaluate the effectiveness of the microalgae-bacteria symbiosis system in livestock and poultry breeding wastewater treatment and calculate the percentages of nitrogen removal by microalgae and bacteria,respectively.Main conclusions are described as follows:1)The effectiveness of biological nutrient removal（BNR）by the microalgae-bacteria symbiosis system exposed to different L/D cycles.The experimental results showed that in the 12:12h plus 24:0 h L/D cycles,the bioreactor exhibited higher TP and NH₄⁺-N removal efficiency compared with in 12:12h and 24:0 h L/D cycles,with the TP removal efficiency increased by 7.17%and 2.49%and the NH₄⁺-N removal efficiency by 8.11%and 2.97%,respectively;the COD removal efficiency was up to 86.94%,demonstrating desired BNR performance.2)Operations of the microalgae-bacteria symbiosis system were evaluated with the aeration intensity of 4,6,and 13.3 L/min,respectively.It was found that the COD and TP removal efficiency was improved as the aeration intensity increased.Over-aeration adversely affected the photosynthesis process by stimulating the reaction between ribulose 1,5-bisphosphate（Ru BP）and oxygen（i.e.,carboxylation of the Calvin cycle）catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase（Ru Bis CO）with an increased level of dissolved oxygen（DO）in the environment.When the aeration intensity was 6 L/min,a desired NH₄⁺-N removal efficiency was achieved,4.01%higher than at 4L/min and 4.74%higher than at 13.3 L/min.Therefore,the aeration intensity that applies to realistic wastewater treatment should be determined after adequate consideration of NH₄⁺-N removal efficiency and energy consumption.3)The microalgae-bacteria symbiosis system was evaluated when operating at varying Mb RT.According to the experimental results,while a shorter Mb RT（10 d）indicated higher COD and NH₄⁺-N removal efficiency,representing an increase by 1.70%and 4.18%compared with the Mb RT of 20 d and by 1.48%and 2.26%compared with the Mb RT of 30 d.4)An experiment with an L₉（3³）orthogonal array was conducted to determine the optimal operating conditions of the microalgae-bacteria symbiosis system.The quality of effluents was evaluated by measuring the percentages of COD,TP,and NH₄⁺-N removed from 9 bioreactors.Results of the orthogonal experiment were processed by the software Minitab to calculate measures of variability,including range and variance,by which the optimal operating conditions were determined:L/D cycles=12:12 h plus 24:0 h,aeration intensity=6 L/min,and Mb RT=10 d.5)Under the optimal operating conditions,trial runs were performed in photobioreactors,and a parallel experiment was designed with activated sludge to investigate the performance of the microalgae-bacteria symbiosis system with regard to low-loaded livestock and poultry breeding wastewater treatment.Effluents from the photobioreactors became stable 6 days after the initiation of the experiment,and the COD,TP,and NH₄⁺-N removal efficiency reached 93.33-94.67%,95.06-98.60%,and 72.50-82.27%,respectively.Compared with the activated sludge system,the microalgae-bacteria symbiosis system was more efficient in livestock and poultry wastewater management.In terms of energy consumption,the microalgae-bacteria symbiosis system used 0.547 k Wh of electric power per day,and 68.357 k Wh of electric power per ton for the system to degrade per ton of Wastewater,demonstrating lower energy consumption compared with the activated sludge system.In other words,under the given conditions,the microalgae-bacteria symbiosis system is an ideal bioreactor characterized by minimum energy consumption,maximum economic benefits and greatest pollutant removal performance.Additionally,the analysis of nitrogen removal by the microalgae-bacteria symbiosis system and the activated sludge system 6 days after the initiation of the experiment revealed that in the microalgae-bacteria symbiosis system,the nitrogen removal efficiency of microalgae and bacteria was 122.25 mg/L and 14.47 mg/L,respectively,accounting for 89.41%and 10.59%of the total nitrogen removal efficiency,which further demonstrated that the microalgae-bacteria symbiosis system achieved efficient nitrogen removal depending on nitrogen assimilation by microalgae.