The Effect Of MgCl₂ And AlCl₃ On The Yield Of Activated Sludge And Microbiological Mechanism

The sludge yield coefficient refers to the amount of sludge produced by the activated sludge degradation unit organic matter（BOD）,which is a key indicator for measuring the amount of excess sludge in the biological treatment system.Numerous studies have shown that there is a correlation between metal ion and sludge yield coefficients.Although metal ions show an inhibitory effect on activated sludge at higher concentrations,a large number of preliminary experiments have shown that certain metal ions,such as Aluminum and Magnesium,can stimulate the increase of sludge yield coefficient in a certain low concentration range.If the relationship between the metal ion and the sludge yield coefficient is found,the sludge yield coefficient can be reduced by changing the metal ion content in the wastewater to achieve the purpose of sludge reduction.Magnesium is commonly found in natural water bodies.Aluminum enters the sewage system through various production and life links.Therefore,Magnesium and Aluminum are common in the sewage system.Therefore,this paper is based on the influence of the concentration changes of Magnesium chloride（MgCl₂）and Aluminum chloride（AlCl₃）on the properties of activated sludge,apparent yield coefficient of the sludge(Y_obs)and extracellular polymer（EPS）.MgCl₂ and AlCl₃ were added to the SBR reactor to investigate the effect of MgCl₂ or AlCl₃ on the excess sludge production of the SBR system and the microbial community structure of the activated sludge（under the action of AlCl₃）.The results showed that the concentration of MgCl₂ was 5-10 mg/L,ν_q（sludge growth rate）,ν_m（volatile sludge growth rate）and Y_obs（sludge yield coefficient）increased with the increasing of MgCl₂ concentration.When the concentration of MgCl₂ was 10 mg/L,the maximum values ofν_q,ν_m and Y_obs were 716.33 mg/L·d,900.5mg/L·d and 1.8485 mgMLVSS/mgBOD,respectively.When the concentration of MgCl₂ was higher than 10 mg/L,ν_q,ν_m and Y_obsbs began to decrease.When the concentra-tion of MgCl₂ was less than the concentration of 10 mg/L that caused the maximum sludge yield,the average removal rate of COD in reacter（R1）was higher.When the concentration of MgCl₂ was 5 mg/L,the maximum COD removal rate of 83.64%was obtained.The concentration of AlCl₃ was within 5-15mg/L,ν_m and Y_obsbs increased with the increasing of AlCl₃ concentration,and reached the maximum at 15mg/L.The maximum values ofν_m and Y_obs was 821.43mg/L·d and 1.6283 mgMLVSS/mgBOD,respectively.When the AlCl₃ concentration was higher than 15 mg/L,ν_m and Y_obsbs began to decrease.When the concentration of AlCl₃ was less than the concentration of15 mg/L of the maximum sludge yield,the average removal rate of COD in the reactor（R2）reactor was higher.When the AlCl₃ concentration was 5 mg/L,the maximum COD removal rate of 90.34%was obtained.In summary,if the concentration of MgCl₂ is less than 10 mg/L and the concentr-ation of AlCl₃ is less than 15 mg/L,the maximum sludge yield coefficient and the inhibition of microorganisms can be avoided.At the same time as achieving a higher COD removal rate,the purpose of sludge reduction is achieved.The highest COD removal rate was obtained when the concentration of AlCl₃ and MgCl₂ was 5 mg/L.The COD removal rate under the action of AlCl₃ was 90.34%,which was greater than 83.64%under the action of MgCl₂.Therefore,compared with MgCl₂,the content of AlCl₃ is more critical in the pursuit of minimum sludge production and the higher COD removal rate.High-throughput sequencing technology was used to detect and analyze the microbial community structure diversity under the action of AlCl3.The results showed that the abundance of activated sludge flora was higher under the action of 5 mg/L AlCl₃,while the concentration of AlCl₃ at 135 mg/L inhibited some microorganisms in the activated sludge.At the Phylum level,Proteoba-cteria adapted to AlCl₃ and can multiply rapidly.Low concentrations of AlCl₃（5-35mg/L）had a stimulatory effect on Patescibacteria.At the Class level,AlCl₃ had the effect of stimulating the growth of Alphaproteobacteria.AlCl₃ inhibited the growth of Gammaproteobacteria.At the Genera level,the addition of AlCl₃ had a strong inhibito-ry effect on the genus such as Tolumonas and Aeromonas.However,5 mg/L of AlCl₃had a significant promoting effect on the genus such as f_uncultured_gamma_proteob-acterium_Unclassified and f_Saccharimonadaceae_Unclassified.AlCl₃ had little effect on the number of microbial groups at the Phylum,Class and Order levels,but had significant effects on Families,Genera and Species,with the greatest impact on the number of microbial groups at the Genus level.Most of the bacteria in the activated sludge were suitable for rapid growth in 5 mg/L of AlCl₃,which might be the reason why the COD removal rate is maximized at this time.