Research On Characteristics Of Rainwater Quality In Tianjin And Technology Of Drinking Water Preparation From Roof Rainwater

Rainwater is a valuable natural resource.Rainwater harvesting and utilization are effective way to alleviate water shortage and achieve sustainable management of water resource.In this study,the quality of natural rainwater and roof runoff which collected in the campus of Tianjin University was analyzed,and the coagulation-precipitation-microfiltration process was used to purify the roof runoff,in order to provide reference for the practical application of drinking water preparation from rainwater.An automatic device was developed for rainfall monitoring,recording and sampling,the natural rainwater sample at certain interval could be collected with the device.Rainwater quality was affected by many meteorological factors,such as season,antecedent dry intervals,rainfall depth,rainfall intensity,rainfall duration,etc.The quality of natural rainwater was obviously better than that of the roof runoff.The first flush occurred in both natural rainwater and roof runoff,and initial abandoning measures could be taken to improve the overall water quality during rainwater collection.Lead was the main heavy metal pollutant in the rainwater.When the rainfall was about 2 to 3 mm,COD_Mn of natural rainwater can meet the Standards for Drinking Water Quality（GB 5749-2006）.In addition,the quality of natural rainwater could also indirectly reflect the air quality.Two acid rain events were monitored during this study,which were attributed to SO₂ and NO_x emissions from industrial enterprises.Jar tests were carried out to determine coagulant and the dosage,as well as the process condition.Ferric chloride was used as a coagulant to treat the roof runoff in this study,and the dosage（calculated as Fe）was 6 mg/L.The suitable process conditions for the coagulation-settling were G value of 200 s^-1in the mixing stage,G value of 20s^-1 in the flocculation stage.Lab-scale tests were conducted on the purification of the roof runoff collected in the spring and the summer.The experimental results showed that the influent UV₂₅₄ had a good linear correlation with COD_Mn（R²=0.92）.The main water quality indexes of treated water and 28 water quality indexes monitored by a third party all met the requirements of the national standards.The membrane fouling rates of the three batches of lab-scale tests were 7.52×10^-3（h·m）^-1,22×10^-3（h·m）^-1,3.92×10^-3（h·m）^-1,respectively.After physical and chemical cleaning,the membrane specific flux recovered well.The contribution of filter cake fouling resistance was smaller than the intrinsic resistance of the membrane.The experimental results showed that the coagulation-precipitation-microfiltration combined process was efficient for treatment of the roof runoff with good treated water quality.The membrane fouling was controllable.It provides a technological base for drinking water preparation from rainwater and has great engineering application significance.