Study On Biostability And Control Measures Of Drinking Water In Water Distribution System Under Water Source Switch

With the increase of water demand and the improvement of water quality requirements,it's common to import water sources.Differences in water quality from different water sources ultimately may cause certain impacts on water supply pipelines,causing water quality instability problems such as "red water","yellow water",and bacterial sloughing etc.The stability of drinking water distribution system under water source switching is vital to ensure safe water supply,especially the biostability.Therefore,this paper explores the biostability of drinking water distribution systems after switched to a high-quality water source.Adenosine triphosphate(ATP)based measurement was proposed to rapid evaluate biostability.This paper focus on stability differences of different pipes material and flow rates,biofilm structure and microbial community.The control mechanism with residual chlorine was explained,and the difference in the mixing ratio of different water sources was clarified.The theory of microbial stability of water supply pipe network is improved.These were important for ensuring the biological stability of drinking water under multi-source water supply.The main conclusions are as follows:Based on the relationship between ATP and biomass in actual pipe network,it is proposed that microbial stability can be quickly evaluated based on ATP dynamic limits and changes.ATP can sensitively reflect biomass difference between water sources,and quickly evaluate the stability of water source switching.Especially,when the level of AOC and culturable bacteria number are similar in proposed and old water sources,which can make up for the deficiencies and misjudgments of conventional evaluation methods such as AOC and HPC.The bacterial sloughing large in the first 7 days after switch,then the biomass gradually decreased,and a new stable state was reached again after 35 days.The stability of different pipes differs greatly,the influence of PE pipe and plastic steel pipe is small,while the ductile iron pipe and cast iron pipe have poor stability.The structure of the biofilm changed,and the biomass also decreased differently between different pipes.PE pipes and plastic steel pipes with smooth surfaces had less shedding after switching,followed by ductile iron pipes with rough surfaces.There is a large amount of biomass shedding on cast iron pipes with more pipe scales and nodules on the surface.Using high-throughput sequencing,the difference of structure of the biofilm community were observed,mainly because the dominant bacterial genera changed.In general,the switch has better water quality safety in the future,because the abundance of pathogenic and chlorine-resistant bacteria will reduce.The flow rate and the mixing ratio of different water sources have a greater effect on the biostability under water source switch.When the flow velocity increases from 0.3m/s to lm/s,the ATP content increases significantly indicating poor stability.After 12 hours stagnation,the bacteria will regrow causing water quality deteriorate.Therefore,large changes of hydraulic conditions should be avoided.The pipe network should be flushed before actual switching water sources.Directly,100%switching to new water source,the ATP and biomass are smaller than other mixing ratios,and the biomass increase smallest after 24h.Therefore,it is recommended to directly switch to the new water source.Residual chlorine could kill the microorganisms shedding under water source switch.The initial dosage of 0.4 mg/L residual chlorine and before the chlorine decayed to 0.05 mg/L,the chlorine could effectively control the growth of microorganisms.After actually switch the new water source,the water treatment plant can consider adding biological filters and slow filters to reduce the AOC level in the finished water to improve the biological stability.In addition,it is necessary to replace the less stable pipes,and replace the cast iron pipes with more stable pipes such as PE or plastic steel pipes to ensure the stability of drinking water distribution system.