< MODULE 4 >

We have discussed the drinking water system of Seoul and let’s talk about the waste water and urban drainage system of Seoul from now.

The city has 5.6 million cubic meters per day of waste water treatment capacity, and it has 10,000 kilometers of sewer line.

The sewer line of Seoul goes like this. The rain water and wastewater goes into the combined sewer line together, and that wastewater goes to the wastewater treatment plants.

When we have much rain and the amount of waste water goes over three times of the capacity of treatment plant, then the overflow goes directly into the river.

87 percent of the sewer line of Seoul is combined system. Again, in the combined system, the rain water and wastewater goes together into the combined sewer line.

13 percent of the sewage system is separated system. In separate system the rainwater is separated from wastewater. It’s very eco-friendly, but considering the precipitation pattern of Seoul, it goes like this.

The two third of annual precipitation is concentrated in two month, so in usual time we don’t need to use the separate system. So the combined system is much more efficient and economic under the condition of Seoul.

As we discussed before, the Seoul has eight drinking water districts but for wastewater Seoul has four district and four wastewater treatment plants.

Two covers the Northern part and two wastewater treatment plants cover the Southern part of Seoul.

And as we discussed before, the total capacity of wastewater treatment is 5.6 million cubic meters per day and we have only four wastewater treatment plant in Seoul, which means one plant has a huge size.

The smallest one is 900,000 cubic meter per day of capacity. This is the second largest wastewater treatment plant in Seoul, you can compare the size of plants with the buildings and communities around the plant.

It’s huge. Most of water demand comes from domestic, the house, as the city has little factories and farms. When we make an average of flow of the Han River, it was 600 CMS, cubic meter per second during 2004 and 2013.

If the water flow of Han River is 600 cubic meters per second, 6 percent of the water, Han River water, is taken by the city to make drinking water and 9 percent including the rain water, storm water, 9 percent of water is returned to the Han River.

We had two years of consecutive drought this year so we had problems of the water resources. It was 600 CMS but it’s decreased to 200 and 100 and 80 and last month we had 50 CMS.

In that case we take 70 percent of Han River to make drinking water, so we have to be prepared for the next drought. Fortunately it’s recovered to 80 CMS right now but we have to think about the water resources for drought.

The water reuse rate of Seoul is only 3.9 percent. As the water reuse rate is so low, of 3.9 percent, the city wanted to increase the reuse rate with the restoration of sustainable water cycle.

It has plans to increase the water reuse rate from 3.9 percent to 14.4 percent in 2020. Rain water harvesting will be increased by 1.2 times in 2020 so the city promotes the citizens to install the rainwater harvesting infrastructure, including passageway pot and green roof and green harvesting barrel.

If the citizens install that kind of systems in their building or house, the city support 90 percent of the capital cost for the system. The city implemented another approach called Low Impact Development rule. It means if we construct the building or site then it has to have a mandatory storm water quota.

For instance if it is public construction site, public building, they had to have six millimeter of storm water retention facilities. If it is park, they had to have 7.5 millimeter storm water retention facilities. And the city also has planned to increase the greywater reuse by 1.6 times by 2020.

Another reuse item category is wastewater reuse and city has plan to increase it by 1.7 times by 2020, but the using the recycled water has problems because we had to install new infrastructure to deliver the recycled the water to the citizens or to the customers.

Fortunately, we have a new community development plan near the largest wastewater treatment plant in Seoul located at the western part of the Seoul. We call them Magok area. This is a newly developed area and fortunately the wastewater treatment plant is located near the community.

This is the largest wastewater treatment plant in Seoul.  So the recycled water from that plant can be provided to the community as toilet flushing water or the gardening and washing water

Drinking water concerns about drought, while the urban drainage system concerns about flood.  So let's talk about the flood control system of Seoul. The city increased the monitoring systems, strengthen the monitoring systems related with the flood such as precipitation, and they increased the number of observatory from 217 to 224.

Also we installed the water level prediction and alarming systems. Seoul has 17 storm water retention basins right now, but the city wanted to increase it triple by 2017. As we discuss before the sewer line of Seoul is combined sewer line system.

So we have combined sewer overflows with much rain so the city wanted to increase the CSO, the combined sewer overflows control basins by triple by 2019.