The Microecological Mechanism Of Water Quality Change Induced By Stagnation And Organic Matter In Drinking Water Pipelines

The quality of indoor piped drinking water is closely related to human health.Due to the inevitable stagnation and other problems,resulting in the deterioration of water quality,a large number of bacteria breeding,biological stability damage and other events occur frequently.Therefore,the safety of drinking water quality has attracted much attention.However,fungi,as an important component of water microorganism,enter the drinking water pipe network when fungi outbreak occurs in water source reservoir due to the fact that the existing water treatment technology cannot completely remove them.It is of practical application value to explore the proliferation characteristics of fungi during drinking water stagnation and its influence on the water quality and microbial stability of pipeline network.This study took indoor piped drinking water as the research object,combined with different retention time,to study the impact of source reservoir fungal outbreak on piped drinking water quality and original microbial ecology.The seasonal succession characteristics of drinking water quality induced by overnight detention and fungal proliferation in actual drinking water pipes were investigated.The effects of sudden fungal pollution on water quality,bacterial count,bacterial activity and population structure of piped drinking water were revealed,which provided theoretical basis and scientific basis for drinking water safety.The principal findings of this investigation are as follows:（1）The fungal proliferation characteristics of overnight water bodies in drinking water pipelines in different seasons were studied.The results showed that overnight stagnation would increase the water temperature,and the free chlorine level was lower than the level required by the Drinking Water Sanitation Standard（GB 5749-2022）.The concentrations of total nitrogen,nitrate and total organic carbon decreased after stagnation,and the average concentration showed a trend of first increasing and then decreasing with seasonal changes.The concentration of ammonia nitrogen increased after stagnation,but the seasonal difference was not significant.Fe concentration increased significantly after stagnation.Due to serious aging of pipes in some sampling points,the total iron concentration of the water in detention exceeded the national standard limit,reaching up to 0.715 mg/L.The content of fungi in water increased significantly after stagnation,with the greatest variation in winter and the lowest difference occurring in spring.The quantity of fungi in stagnant water increased by 2,2.6,5.3 and 10.8 times compared with that in fresh water,respectively.The results of a correlation analysis revealed that the concentrations of free chlorine and total residual chlorine were always negatively correlated with temperature.The temperature in fresh water had a negative correlation with the total number of fungi,while the temperature in stagnant water had a positive effect on the total number of fungi.The findings offer a theoretical framework for investigating the temporal and spatial fluctuation of drinking water quality induced by overnight stagnation and the seasonal proliferation characteristics of fungi.（2）The influence of exogenous fungi（10⁵ cells/m L）on the quality of drinking water and microbial ecology was further explored.The results showed that the addition of fungal spore solution accelerated the attenuation of chlorine concentration in water,and the concentrations of TN,NO₃^--N and TOC showed a decreasing trend with the stagnant time,while the concentrations of NO₂^--N in Alternaria group（AG）and Penicillium group（PG）increased by 49 and 75 times,respectively.In addition,the total number of bacterial cells in AG and PG increased significantly,reaching the peak of 1.83×10⁶ and 1.79×10⁶ cells/m L after 48 h stagnation.The addition of fungal spore fluid promoted the metabolic capacity of carbon sources of bacteria,and the promotion effect of spore fluid of AG was more significant.AG had a strong ability to utilize amino acids,esters and amines,while methyl pyruvate and other esters in PG were easy to be utilized,but not easy to utilize monosaccharides.Proteobacteria were dominant in AG,while Bacteroidetes and Firmicutes were dominant in PG.SEM results showed that the AG was positively correlated with water quality,bacterial count and cell activity,while metal was negatively correlated with AG.The results provide a scientific basis for exploring the microecological response mechanism under sudden fungal pollution.（3）Further research was done on the alterations in water quality and microecology brought on by exogenous fungal organics under various stagnation settings.The results showed that under the invasion of fungal organic matter,the concentrations of TN and NO₃^--N in water decreased continuously with the stagnant time,and the concentration of NO₂^--N increased by 160 and 75 times after stagnation.The organic matter composition was more diverse.Trichoderma group（TG）contained fulvic acid and humic acid,and Fusarium group（FG）contained humic acid,fulvic acid and proteinoid,etc.The bacterial concentration increased first and then decreased with the stagnant time,and the peak bacterial cell number in TG and FG was 1.54×10⁶ and 1.77×10⁶ cells/m L,respectively.The two fungal organic matters had different effects on the carbon source metabolism of bacteria in drinking water.TG promoted the carbon source metabolism of bacteria,while FG had certain inhibitory effect.Ochrobactrum sp.and Acinetobacter sp.were the most abundant species in TG,while Bradyrhizobium sp.and Azonexus sp.were the dominant species in FG.The results of RDA and structural equation model showed that the addition of fungal organic matters changed the effect of metals on bacterial community structure,and the effect on bacterial population changed from positive to negative correlation.This study provides a new way to study the microecology of drinking water under the condition of fungal organic matter invasion.