Study On The Effects Of Nitrate On Bacterial Community Diversity And Iron Stability In Water Supply Pipes

After long-term use corrosion and scaling occur in water supply pipes due to the effects of chemistry and microorganisms.When the water supply conditions change,the steady state balance between water and pipe scales may be destroyed,resulting in the iron release.In this paper,the effects of nitrate on the physicochemical characteristics,bacterial community diversity,iron release and nitrogen conversion of pipe scales were investigated by using cast iron and steel pipes for study.Moreover,based on aerobic denitrifying bacteria the effect of microorganism on iron release in water supply pipes were studied.The main contents include the following aspects:（1）By using scanning electron microscopy（SEM）,X-ray fluorescence spectrometer（XPF）,X-ray diffraction analyzer（XRD）,and X-ray photoelectron spectroscopy（XPS）,the effects of increasing nitrate concentrations on the physical and chemical characteristics of pipe scales in water supply pipes were analyzed.There were similar layered structures in the scales of cast iron and steel pipes.The original scales in cast iron pipes and steel pipes had large gaps and loose texture.After increasing the nitrate concentrations,the inner layer of steel pipe scales became small spherical structure,and was protected by needle-like and reticulated structure materials.The inner layer of cast iron pipe scales was densely spherical,and protected by wire structure.In addition,increasing the nitrate concentration makes the outer layers of both pipe scales denser,especially cast iron pipe scales.The main components of the scale of the two pipes were Fe and O.The main components of the original cast iron and steel pipe scale wereα-Fe OOH,Fe₂O₃,Fe₃O₄,γ-Fe OOH and Fe SO₄.After increasing the nitrate concentration,the inner layer of steel pipe pipe scale is free ofγ-Fe OOH,and the inner layer of iron pipe scale is mainly Fe SO₄ and Fe₃O₄.This shows that the composition of pipe scales was influenced by the increase of nitrate concentration in water,and the release of iron was inhibited due to the increase of the content ofα-Fe OOH and Fe₃O₄.At the same time,the proportion of Fe3+oxide states and metal oxides in the scale of the two pipes increased.（2）By using biological high-throughput sequencing,the effects of increasing nitrate concentration on the bacterial community diversity and species composition of water pipe scales were analyzed.The richness of bacterial species in pipe scales was increased with increasing the concentration of nitrate will change the richness of microbial species in steel and cast iron pipes.The average richness of proteobacteria in steel pipes increased by 6%,the average richness of proteobacteria in cast iron pipes decreased by 4.20%,and Nitrospirae content decreased significantly,by 10.80%.At the same time,the bacterial community diversity on cast iron and steel pipe scale was reduced.（3）The effects of nitrate on iron release from water supply pipelines were studied through static simulation experiments.Increasing the nitrate concentration can suppress iron release from water supply pipes,but as the nitrate concentration rises,the iron release suppression effect of steel pipes gradually weakens.When the nitrate concentration is 30mg/L,the total iron content in the effluent of the cast iron pipe is the lowest at 0.11 mg/L,which is 90.27%lower than that under the tap water condition;when the nitrate concentration is 10 mg/L,the total effluent of the steel pipe The lowest iron content was0.64 mg/L,which was 85.84%lower than that under tap water.The main reason is that denitrification reactions have occurred in cast iron pipes and steel pipes,but due to the difference of denitrifying bacteria in different pipes,the content of nitrous nitrogen in cast iron pipes is higher,and the ammonia nitrogen content in steel pipes was higher.（4）The aerobic denitrification medium was used to screen the aerobic denitrifying bacteria of the actual scale of the water supply pipeline,and a dynamic simulation reactor was established to analyze the effect of aerobic denitrifying bacteria on the iron release from the pipeline.The results show that aerobic denitrifying bacteria have an inhibitory effect on iron release from steel pipe and cast iron pipe scale.Under the condition of"20mg/L nitrate+aerobic denitrifying bacteria",the iron release inhibition effect in the cast iron pipe is the best.The total iron concentration in the effluent is 0.14 mg/L,which is lower than that under the condition of sterile water.99.44%.With the increase of nitrate concentration,the inhibitory effect of iron release in steel pipe weakened.The inhibitory effect was best under the inflow condition of"10 mg/L nitrate+aerobic denitrifying bacteria",and the total iron concentration in the effluent was 0.19.mg/L,which is 91.56%lower than that under sterile water.（5）The denitrification products of aerobic denitrifying bacteria in different pipes are different.Ammonia nitrogen and nitros nitrogen are produced in cast iron pipes at the same time,the highest ammonia nitrogen content is 0.99 mg/L,and the highest nitrite nitrogen content is 0.05 mg/L;while the main product of denitrification in steel pipes is ammonia nitrogen,which has a maximum content of 1.28 mg/L.