Pollution control of outfall of rainwater-sewage confluence in old town

Huang Yongqiang Shi Kai Zhu Yan Liu Rongping

(School of Environmental and Safety Engineering， University of Jiangsu， Zhenjiang 212013， China)

T he ancient canal flows across the old town of Zhenjiang city from east to west［1］.There are several problems in its drainage system.For example，the drainage facility is old and many sewer lines are connected in a wrong way.As a result， much more serious pollution is caused.In order to improve the water quality of the ancient canal，the government has renovated the drainage pipeline network ofthemiddleancientcanalsince 2011［2］.There are 25 outfalls at the middle ancient canal.By field investigations，24 of them could be transformed to the distribution system of rain and sewage，while the pipeline network of the eastern catchment area of old railway bridge outfall was not qualified to the condition［2］.In order to control the pollution of rainwater-sewage confluence at the outfall of old railway bridges，we set up a rainwater-sewage confluence pollution control demonstration project at the old railway bridge outfall，and did some research on the effect of the project［3］.

The eastern catch ment area of the old railway bridge outfall lies in the semi-urban area，and the area of catchment is about 1.2 × 106m2.The design treatment flow rate is under the flow of torrential rain considering that the frequency of torrential rain is rather low and its processing is uneconomical［4］.The runoff coefficient of suburb areas was 0.6， and the upper limit of 12 h overflow quantity was 10 000 to 22 000 m3［2］.According to the monitored data，the sewage quantity of the outfall in dry days is about 780 m3/d.The measured quantity of overflow through the outfall on rainy days is about 1 153 to 16 547 m3per time［3］.Its pollution concentration is high［5］.In the area， the project team carried out the overflow pollution control demonstration project.The processing scale of the project is 2 600 m3/d.

The research purpose is to control the pollution of rainwater-sewage confluence at the outfall of old railway bridges in Zhenjiang，so as to protect the ancient canal which receives the confluence.In the research field，Jiang et al.［6］made a study on domestic urban rainwater runoff pollution，and found that the planning of the urban drainage system should integrate the requirement of urban rainwater pollution control and make arrangements on the counter measures and land use.Zhang［7］made a study on the combined sewer overflow pollution of Jizhuangzi drainage system in Tianjin.By collecting the data of the drainage pipelines and pumping stations，combined with the specific characteristics of rainfall in Tianjin，the combined sewer system of Jizhuangzi was established.Xu［1］proposed the combined sewer overflows pollution prevention and treatment techniques，which were fit for Zhenjiang.

In this paper，on the basis of detecting and analyzing the water yield and water quality of the runoff of rainwater-sewage confluence，a combined technology which contains four processes for lowering the pollution load is developed，and then the processing effects of the project are analyzed.

1 Treatment Process of Project

1.1 Process overview

The acreage of the processed area is about 1.2 ×106m.During the rainy days the overflow rate is ceaselessly changing， so the treatment method cannot be single［7］.In order to reduce the pollutant content which would be discharged into the ancient canal，the whole pollution treatment process is made up of four independent continuous processes，which can help to achieve the general objective of the overflow pollution control project［2］.Four treatment processes， respectively， are as follows:

Process 1Unpowered overflow pollution treatment equipment+ecological moderative pollutant-reduction system.

Process 2Accurate hydrocyclones+bidirectional rotational flow-precipitate integrated purifier+adsorbent clean-up bed［8］.

Process 3Meshed rotational flow treater［9］.

Process 4Accurate hydrocyclones+rotational flow magnetism defecator+adsorbent clean-up bed.

1.2 Introduction of four processes and flow chart

Four processes are detailedly introduced，including the processing efficiency of several parameters.

1.2.1 Process 1

Process 1 contains two consecutive parts.The first is unpowered overflow pollution treatment equipment.Its design processing capacity is 40 m3/d，and the processing efficiency(removal rate)is 40%to 60%of SS.

The second is an ecological moderative pollutant-reduce system.Its design processing capacity is 100 m3/d，and the processing efficiency(removal rate)is above 40%of COD，above 20%of NH3-N，and above 20%of TP.The process flow diagram of process 1 is shown in Fig.1.

Fig.1 Technology process 1

1.2.2 Process 2

Process 2 contains three consecutive parts.The first is accurate hydrocyclones.Its design processing capacity is 500 m3/h，and the processing efficiency(removal rate)is 50%to 70%of SS.

The second is bidirectional rotational flow-precipitate integrated purifier.Its design processing capacity is 40 m3/h，and the processing efficiency(removal rate)is 70%to 80%of SS，and 20%to 30%of COD.

The third is an adsorbent clean-up bed.Its design processing capacity is 40 m3/h，and the processing efficiency(removal rate)is above 40%of COD，above 20%of NH3-N，and above 20%of TP.The process flow diagram of process 2 is shown in Fig.2.

Fig.2 Technology process 2

1.2.3 Process 3

Process 3 contains one piece of equipment which is a meshed rotational flow treater.Its design processing capacity is 1 000 m3/h，and the processing efficiency(removal rate)is 30%to 50%of SS.The process flow diagram of process 3 is shown in Fig.3.

Fig.3 Technology process 3

1.2.4 Process 4

Process 4 contains three consecutive parts.The first is accurate hydrocyclones.The second is a rotational flow magnetism defecator，and the third is an adsorbent cleanup bed.

The design processing capacity of process 4 is 40 m3/h，and the processing efficiency(removal rate)is 60%to 80%of SS，and 30%to 40%of COD.The process flow diagram of process 4 is shown in Fig.4.

Fig.4 Technology process 4

1.3 Actual picture of the project

The combined technology is composed of the four processes and it was put into operation at the outfall of old railway bridges in 2011.The actual picture of the project is shown in Fig.5.

Fig.5 Actual picture of overflow pollution control project

2 Operation Effect of the Project

After construction and debugging of the project，it was put into operation.By means of monitoring the processing effect of five rain times，it is found that the effect of reducing the overflow pollution of rainwater-sewage confluence of the processed area is good.During the five rains，the inflow and outflow of the system was monitored to obtain several parameters，including COD，NH3-N， TP and SS［10］.The results and the calculated removal rate are listed in Tabs.1 to 4.

Tab.1 System removal rate of COD

Tab.2 System removal rate of NH3-N

Tab.3 System removal rate of TP

Tab.4 System removal rate of SS

The implementation of the project can effectively control the overflow pollution of the confluence of outfall，and the pollution load which can be discharged into the ancient canal is reduced.

In order to further realize the proposed regularity of the project during rainy days，researchers monitored the momentary data of four parameters during a time of rain，and then figured out the momentary removal rate.The results are listed in Tab.5.

Tab.5 System momentary removal rate

With the extension of rainy time，the results indicate that the removal rates of four parameters are continuously increased.The pollution load of overflow is significantly lower than that before.

3 Conclusion

The outfall overflow pollution control project can effectively reduce the pollution load of various contaminative parameters of rainwater-sewage confluence.We mainly made two groups of monitoring.In the first， we sampled and monitored five rains.For each rain， we figured out one removal rate of pollution parameters including COD，TP，NH3-N and SS.In the second，we sampled one rain and figured out a series of momentary removal rates of pollution parameters.

From the monitor results，it is easily found that the positive effect of the project is obvious，which drastically reduces the content of COD，TP，NH3-N and SS of the rainwater-sewage confluence.Besides， the process can control the overflow quantity of the rainwater-sewage confluence during rainy days，reduce the pollutant con-centration which would be discharged into the ancient canal，and effectively protect the water environment of the ancient canal.The model of the pollution control project can be used as a reference for other similar river pollution control［11］.

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