Study On The Effect Of First Flush Collection In Terms Of Reducing Rainwater Runoff Pollution Load In Chemical Industrial Parks

The pollution load of rainwater runoff in industrial area is so high that may cause severe and sudden water pollution accidents.The construction of first flush rainwater system for chemical industrial parks is an important approach to reduce the risk of rainwater pollution.Thus,it is very essential to carry out the study on the effect of first flush collection in terms of reducing pollution load to improve the first flush rainwater system in chemical industrial parks.Supported by relevant research projects in Shanghai Chemical Industry Park,the paper analyzes the pollution characteristics of rainwater in the chemical industrial park from three different perspectives: water quality from rainwater outfalls,water quality from rainwater switch well and water quality sampled in the field.The number of chemical enterprises that do long term online monitoring of first flush are very limited,the paper chooses a typical chemical enterprise(INVISTA Nylon Chemical Co.)that meets the conditions as the research subject.Based on more than 35 thousand monitoring data of the rainwater switch well in INVISTA from 57 rainfall,the paper studies on the relationship between the first flush collection volume and the rainwater quality standard-reaching rate under different collection control modes,the impact of the first flush rainwater system and the first flush collection volume on the rainwater management cost of chemical enterprises,in the hope of providing references and suggestions for chemical enterprises to optimize their first flush rainwater system.The main research conclusions and results are as follows:(1)The analysis of first flush collection by rainfall depth show that,when the first flush collection depth is set under 30 mm,the gain of improving the first flush collection depth is relatively high.The chemical enterprises that apply the recommended first flush collection depth according to the national standard(20mm ～30mm)are able to meet the need of reducing the pollution load of rainwater nicely,while continuing increasing the first flush collection depth after 30 mm benefits less.(2)The analysis of first flush collection by rainfall duration show that,In the first 4 hours after the rainfall,the standard-reaching rate of rainwater runoff increases from 48.91% to 56.97% only,thus the effect of having a small increase in the first flush collection time is not that noticeable.If the chemical enterprises apply the recommended first flush collection time from the national standard,the pollution load of rainwater runoff is reduced to a certain extent and the standard-reaching rate of rainwater runoff is improved,but the risk of water environment pollution still exists.Although relevant national standards provide both first flush collection modes,the analysis results show that the amount of first flush collected by rainfall duration would be influenced by the characteristics of the rainfall process,and have poor adaptability,thus it is recommended that chemical enterprises collect first flush by rainfall depth in practice to achieve better effect of reducing the pollution load of rainwater.(3)The analysis of INVISTA’s rainwater management cost under different first flush collection depths show that in the range of 5mm～15mm,the effect of increasing the first flush collection depth on the cost of polluted rainwater treatment is relatively small;in the range of 0～25mm,increasing the first flush collection depth would significantly reduce the frequency of exceeding the water quality standard at the rainwater outfall.After INVISTA collects 15 mm first flush rainwater,the total amount of polluted rainwater is expected to decrease by30.51%,the frequency of exceeding water quality standards at the rainwater outfall is reduced by 37.70%,and the rainwater management cost of INVISTA is expected to be reduced by 179,900 yuan(34.80%)annually.Therefore,if chemical enterprises manage first flush runoff well,the rainwater management cost would be reduced significantly,and the risk of water pollution would be reduced at the same time.