Analysis of Rainfall Runoff Pollution and Split-Flow on Urban

Pavement

Yongqiang Huang

, Cancan Liu and Wenliang Zhao

The School of Environmental and Safety Engineering, Jiangsu University, Zhenjiang, 212013, China.

Email:

yqhuang@ujs.edu.cn

Keywords: Rainfall runoff, water quality, rainfall utilization, initial split-flow

Abstract: The pollution situation and the variation characteristics of the water quality of rainfall runoff in urban area

were analyzed. Water quality monitoring results indicated that, during the early period of rainfall, the

maximum concentrations of suspended solid, chemical oxygen demand, ammonia nitrogen and total

phosphorus in the water on the urban pavement reached 410 mg/L, 282.4 mg/L, 11.18 mg/L and 1.2 mg/L,

respectively. This means that the initial rainfall runoff pollution is serious. Additionally, with the increase of

rainfall duration, the pollutant concentration gradually decreased and stabilized eventually. Finally, the

engineering simulation experiment of split-flow of the initial rainfall showed that the reduction rate of

chemical oxygen demand, total phosphorus, suspended solid, and ammonia nitrogen on the pavement

rainfall could reach 48.15%, 51.77%, 56.91% and 33.45% when the amount of initial rainfall split-flow was

3 mm. Based on the field observations and theoretical analyses, the initial rainfall discarding strategy is

recommended as a primary treatment of rainfall reuse system to improve water quality under different

rainfall patterns. These findings are vital for the rainfall utilization in the construction of Sponge City

Project.

1 INTRODUCTION

In recent years, the urban water logging and

drinking water pollution caused by emergencies

have occurred frequently. Rainfall could be used as

drinking water resource or alternate source in case of

sudden pollution. Some developed countries such as

Germany (Thilo and Uwe, 1999), the United States

and Japan (Wang et al., 2011) attach great

importance to the recycling of rainfall; they have

carried out the rainfall utilization project and

formulated a series of policies and regulations, and

also established a relatively perfect system of

rainfall utilization (Zhao and Niu, 2001). Compared

with the developed countries abroad, rainfall quality

in China is obviously poorer. It is due to the serious

air pollution and the scouring of the atmosphere in

the process of rainfall. And there are still many

deficiencies in China's rainfall resource utilization

and management system. Therefore, China has put

forward the construction of "sponge city".

Zhenjiang City has made certain achievements

on district transformation as well as waterfront park

construction based on the low impact development

concept (Wang et al., 2014). The population in

university is dense and the domestic water demand

of university is great. Therefore, the trial of rainfall

collection and utilization in universities can

effectively reduce the water pollution caused by

anti-emergencies. It can also make more effective

designs and modifications on the transformation of

the original conditions (Zhao, 2012). Based on the

new campus of Jiangsu University, this paper

analyzed the water quality characteristics of rainfall

runoff on pavement and carried out the engineering

simulation of split-flow. And this research provided

basic data and technical support for pavement

surface rainfall utilization.

170

Huang, Y., Liu, C. and Zhao, W.

Analysis of Rainfall Runoff Pollution and Split-Flow on Urban Pavement.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 170-175

ISBN: 978-989-758-342-1

Table 1: Campus rainfall and monitoring time.

number Monitoring date early sunny

days

(day)

Pavement runoff

time

Rainfall

duration

(min)

Rainfall

amount

(mm)

Rainfall

intensity

(mm/min)

1 2017.5.29 2 10:45 76 1 0.013

2 2017.6.16 1 14:23 155 11.7 0.075

3 2017.6.25 4 15:57 20 4 0.200

4 2017.7.6 3 8:56 390 6 0.015

5 2017.7.16 5 8:51 300 8 0.027

6 2017.8.7 12 16:20 60 1.5 0.025

7 2017.8.8 1 17:00 64 16.9 0.264

8 2017.8.20 1 18:59 42 2.8 0.067

9 2017.9.29 4 8:31 50 1.1 0.022

10 2017.11.12 5 11:21 >480 18.5 <0.039

2 MATERIAL AND METHODS

2.1 Sampling

A double-edged fine-toothed comb on the ground in

front of the library of Jiangsu University was taken

as a sample site to analyze 10 times of different

rainfall. When the rainfall on the road formed

runoff, we collected the runoff and started the

timing. We took some sample at regular intervals

and recorded time of each sample based on the

rainfall intensity. The initial sample interval of

runoff was short. With the increase of rainfall

duration, the sample interval gradually increased.

Table 1 shows the rainfall monitoring information.

2.2 Determination of Indicators

In general, the pollutants in rainfall are mainly

suspended matter, organic pollutants, nitrogen,

phosphorus and other nutrient elements. Thus

suspended solid (SS), chemical oxygen demand

(COD), ammonia nitrogen (NH

-N) and total

phosphorus (TP) were determined as the indicators

and analyzed.

SS is an important index for evaluating the

sensory properties and pollution of water body, and

SS concentration is determined by weighing method.

COD indicates the relative content of organic matter

in water, and is determined by dichromate oxidation

method. NH

-N is a nutrient in water and can cause

water eutrophication. It is the main oxygen

consumption pollutant in water. Concentration of

-N is determined by nessler's reagent

colorimetric method. TP is the main factor of

eutrophication in water body to evaluate the degree

of water pollution. And TP concentration was

measured with ultraviolet-visible spectrophotometry

after potassium persulfate digestion at 121℃ for 30

min.

2.3 Experiment of Initial Split-Flow on

Pavement

Sansalone et al. (1997) proposed that the pollution

load of 20% of the initial rainfall runoff accounted

for 80% of the total rainfall in the study of road

runoff. In the research on initial split-flow on

pavement rainfall, He Junchao (2015) and others

pointed out that when the runoff ratio was about

30% to 40%, the corresponding proportion of

pollution load was about 42% to 70%. Chew (he et

al., 2004) and others found that under the conditions

of the rainfall intensity being 0.1 mm/min, the

pollution load in the initial 20 minutes accounted for

42% of the whole rainfall, and the pollution load in

later 20 minutes only accounted for 10%. It could be

seen that the pollutants in the runoff were mainly

concentrated in the initial runoff. Therefore, rainfall

in the early stage was supposed to be discarded, and

then the rainfall could be collected, stored, treated

and reused.

The main purpose of the initial discarding is: (1)

To avoid bringing a large amount of sediment into

the rainfall treatment structure, and to reduce the

difficulty of cleaning and maintenance at the later

stage; (2) To protect the rainfall storage and to

ensure that the rainfall quality in the later treatment

process is relatively stable; (3) To avoid collecting

and treating the rainfall which is in very small

amount with high pollution, and to protect storage

Analysis of Rainfall Runoff Pollution and Split-Flow on Urban Pavement

171

structures and subsequent treatment equipment (Liu

and Fu, 2004).

2.4 Initial Split-Flow Calculation

Because the initial rainfall split-flow is affected by

many factors such as the nature and size of the

catchment, the degree of pollution, the subsequent

treatment system and the balance of water flow,

there has not been a unified formula so far. This

paper determined the actual split-flow according to

the pollutant load needed to control.

Some scholars (Charbeneau and Barrett 1998; He,

2011) have found an exponential relationship

between pollutant concentration and cumulative

runoff in rainfall, and the following relations are

given:

eCC

⋅−

(1)

Where C

is the concentration of contaminant in

runoff as runoff starts t minutes, mg/L. C

is the

pollutant concentration in rainfall as runoff forms,

mg/L. K

is the comprehensive scouring coefficient

with rainfall as the variable. And it is related to the

surface water quality, rainfall intensity and pollutant

trait, mm

-1

. h is the cumulative rainfall within t

minutes of runoff, mm.

dhCY

∫

(2)

dhCY

∫

(3)

Where H is the total rainfall. C

is the

concentration of pollutants in the rainfall. As the

runoff starts for t minutes, the pollutant

concentration is C

. Y

is the pollution load of the

runoff of different rainfall. Y

is the total runoff

pollution load generated by rainfall.

dhC

⋅−

−

===

∫

(4)

Where L is the pollutant migration rate during the

runoff scouring process. It is also known as

pollution load rate.

3 RESULTS AND DISCUSSION

3.1 Variation of SS Concentration in

Rainfall Runoff on Pavement

Figure 1 shows the variation of SS concentration in

the pavement rainfall with the rainfall duration. The

concentration of SS decreased sharply in the early

stage of rainfall, and then decreased slower. After

20-30 minutes, the precipitation of SS was basically

below 30 mg/L, which met class VI surface water

standards (

GB3838-2002). As shown in Table 1,

the amount and intensity of rainfall were small and

the rainfall time was short on May 29. Hence, the

overall curve of SS concentration was high, and it

fluctuated. Comparing the two times of rainfall on

June 16 and June 25, we could find that the decline

trend of SS concentration curve on June 25 was

steeper than on June 16 in the first 5 minutes. With

the increase of rainfall duration, the stabilization

trend of SS concentration on June 25 was faster.

Therefore, the greater the intensity of rainfall was,

the stronger the effect of scouring and dilution to

contaminants was.

Figure 1: Pavement rainfall SS process along with the

change of rainfall duration.

3.2 Changes of Cod Concentration in

Rainfall Runoff on Pavement

Figure 2 shows the relationship between the rainfall

COD concentration and the rainfall duration. It

could be seen that the initial concentration of COD

in rainfall runoff was high. The concentration of

COD decreased significantly in initial 10 minutes.

Then the concentration of COD was basically below

20 mg/L after 20 minutes. In contrast of the number

of early sunny days before the two times of rainfall

on August 7 and August 20, it was found that the

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172

longer the interval between the two times of rainfall

was, the higher the concentrations of the initial

rainfall contaminants were. It indicated that the

pollutants would accumulate over time.

Figure 2: Pavement rainfall COD process along with the

change of rainfall duration.

3.3 Variation of Ammonia Nitrogen

Concentration in Rainfall Runoff

on Pavement

Figure 3: Pavement rainfall NH3-N process along with the

change of rainfall duration.

Figure 3 shows the relationship between rainfall

ammonia nitrogen concentration and rainfall

duration. It could be seen that there was a significant

downward trend of ammonia nitrogen concentration

in runoff at the beginning, and then the ammonia

concentration tended to be stable. Similarly, when

the rainfall duration was short, and rainfall intensity

was very small, the road surface runoff ammonia

nitrogen concentration curve was high. In addition,

the descending rate of early ammonia nitrogen

concentration was very fast, then the descending rate

gradually slowed down, and finally it tended to be

stable.

3.4 Variation of TP Concentration in

Rainfall Runoff on Pavement

Figure 4 shows the relationship between the rainfall

TP concentration on the pavement and the duration

of the rainfall. It could be seen that the concentration

of TP in rainfall runoff dropped sharply in 0-10

minutes. Then the curve tended to be gentle after 20

minutes. Compared with the curve of August 8, the

curve of August 7 was significantly higher. It was

because the interval was 12 days. Additionally, the

temperature of summer was high and there was no

staff pouring the road. So when it began to rain on

August 7, the TP concentration of initial rainfall

runoff was up to 1.2 mg/L. With the process of the

rainfall, TP concentration gradually decreased, but it

was still higher than the concentration in other

rainfall. It was because the rainfall intensity on that

day was very small. Furthermore, the TP

concentration on August 8 significantly decreased

after the scouring. Thus, the initial concentration of

rainfall runoff pollutants was low, and the

concentration of pollutants during the final

stabilization process was also low. Above all, the

time required to stabilize was short.

Figure 4: Pavement rainfall TP process along with the

change of rainfall duration.

3.5 Determination of Split-Flow in

Rainfall Runoff

The initial scouring of COD and SS of pollutants in

rainfall runoff was the strongest, which indicated

that the pollution load transferred by the initial

runoff was the largest. Thus, the migration rate of

COD and SS were used to determine the split-flow.

Analysis of Rainfall Runoff Pollution and Split-Flow on Urban Pavement

173

As shown in Figure 5, the COD and SS

concentrations in the initial runoff were high on

June 16, 2017. With the accumulation of rainfall, the

COD and SS concentrations gradually decreased.

Figure 5: COD, SS scour curve of road runoff.

The curve-fitting expressions of COD and SS

were y = 171.23e

-0.2789x

and y = 479.57e

-0.2817x

respectively, and they were consistent with the

general relationship between the change of

pollutants and the cumulative rainfall shown in

equation (1). The correlation coefficient R

of the

measured curve and the fitting curve that reflected

the variation of COD

concentration during the

rainfall was 0.8418, and the correlation coefficient

concerning the variation of SS during the rainfall

was 0.9645. And the fitting degree was good, which

indicated that the main pollutants in the road rainfall

runoff and the cumulative rainfall were

exponentially related.

According to equation (4), the pollutant loading

rate of COD and SS were calculated, and the

pollutant loading rate L was expressed by the

arithmetic mean of them. Table 2 lists the

relationship between the loading rate of main

pollutants and the rainfall amount.

Table 2: Relationship between main pollutant loading rate

of road runoff and rainfall.

Rainfall

amount (mm)

1 2 3 4 5 6 7 8 9 10

Road pollutant

loading rate

(

)

25.4 44.6 59.1 70.1 78.3 84.6 89.3 92.9 95.6 97.6

It could be seen from Table 2 that the pollution

load rate in the initial runoff flow with the amount

of 1 mm is 25.40%. With the accumulation of

rainfall, the pollution load rate gradually increases,

but the increasing rate decreases. When the amount

of the rainfall reaches 3 mm, the pollutant loading

rate is about 60%. Therefore, when the initial

rainfall is discarded, the pollutant loading rate is

about 60%.

Some studies have pointed out that the pollutants

in pavement runoff are strongly affected by the

previous number of sunny days (Li et al., 2008). It

could be seen from Table 1 that the rainfall on June

16 was only one day from the last rainfall, indicating

that the accumulation of pollutants was relatively

slow. In addition, the proportion of contaminants in

the initial runoff was large (Ding et al., 2011).

Hence in the demonstration area, if the amount of

the initial split-flow on pavement surface is 3 mm,

this scheme could remove more than 60% of the

pollutant load in the same rainfall intensity, which

met the requirement of Zhenjiang Sponge City

Construction Project. It could also alleviate the

burden on subsequent rainfall treatment facilities. In

summary, the amount of initial split-flow at the

demonstration site has been designed to be 3 mm.

3.6 Simulation of Initial Discarding

Engineering

The object of the simulation was selected at a

rainfall outlet on the ground in front of the library of

Jiangsu University. And two times of rainfall on

January 3 and January 4 were discarded. The rainfall

runoff area was about 400 m

, and the split-flow

volume was designed to be about 1.2 m

Table 3. Changes of water quality in the engineering

simulation test of initial rainfall discarding.

Date Water

Sample

COD

(mg/L)

-N

(mg/L)

2018.1.3

Sample 1 51.51 5.614 0.109 86

Sample 2

39.39 3.736 0.054 46

2018.1.4

Sample 3

81.82 5.842 0.141 123

Sample 4

42.42 4.788 0.068 53

In this experiment, the runoff rainfall flowing

through the rainfall port was collected into the split-

flow vessel by laying a tarpaulin at the selected

rainfall port, and the rainfall characteristics were

recorded synchronously within a rainfall tank. The

pavement was made of impervious material like

asphalt and tiles. Therefore, once runoff formed on

the road surface, the runoff could be replaced by the

rainfall. After the formation of the runoff, and the

amount of rainfall recorded within the rain barrel

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174

reached 3 mm, it was deemed that the amount of the

runoff reached 3 mm which was the amount of

preset split-flow. Then we closed the inlet of the

container and marked the road surface rainfall with

chlorine bottles as the water sample 2. The initial

split-flow in water containers was marked as the

water sample 1. Table 3 shows the pollutant

concentrations of two water samples.

According to the calculated formula of the pollutant

reduction rate:

100(%).Re

−

rateduction

(5)

The results show that the reduction rates of COD

in the initial rainfall runoff could reach 23.53% and

48.15%, the reduction rates of TP in the initial

rainfall runoff could reach 50.46% and 51.77%, the

reduction rates of SS in the initial rainfall runoff are

46.51% and 56.91%, and the reduction rates of NH

N in the initial rainfall runoff could reach 33.45%

and 18.04% when the amount of split-flow are 3

mm. Therefore, it is an economical alternative to use

the initial disposal as a primary treatment method

for road rainfall reuse system.

4 CONCLUSIONS

(1) The initial runoff pollution was serious, and

the highest concentrations of SS, COD, NH

-N and

TP in the initial runoff were 410 mg/L, 282.4 mg/L,

11.18 mg/L and 1.2 mg/L. With the increase of

rainfall duration, the concentrations of pollutants

gradually reduced, and finally stabilized. In addition,

the decreasing trend of the concentrations of SS,

COD, NH

-N and TP were related to the number of

sunny days, rainfall and rainfall intensity.

(2) According to the pollution load rate

calculation, when the initial rainfall split-flow was

designed to be 3 mm, more than 60% of the

pollutant load on rainfall road runoff could be

removed, which was the goal of Sponge City

Construction in China.

(3) Aiming at the engineering simulation of the

university, the results of the engineering simulation

on pavement showed that the reduction rates of

COD, TP, SS, NH

-N on the pavement rainfall could

reach 48.15%, 51.77%, 56.91% and 33.45% when

the amount of initial split-flow was 3 mm. It

indicated that the initial rainfall disposal could be

used as primary treatment of pavement rainfall reuse

system in rainfall utilization.

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