Seoul, the capital of Korea, has had a revision of water policy for the city as a whole due to a successful rainwater harvesting (RWH) project and looks set to provide the impetus for increased interest in RWH in other Korean municipalities.
Built on a 6.25 hectare site originally owned by Konkuk University, the project answers the classic problem confronting municipal authorities in highly developed urban areas. Development usually means previously permeable surfaces are ‘hardened up’, leading to high runoff rates during storms.
The project had its beginnings in a proposal from the university to sell what was then a playing field for residential and commercial development. This was to be no small development – as constructed it consists of a department store and four apartment blocks, each of between 35 and 57 storeys. Overall there are 1310 apartments providing accommodation for between 4000 and 5000 people.
Construction began in late 2003 and was completed in April 2007. Each block has a plan area of 1500m2.Rainwater from all four roofs is piped to the RWH basement under the 35 storey block where there are three separate storage tanks each of 1000 m3 capacity.
The first tank acts as protection against storm runoff and is therefore normally kept empty in the storm season.
Runoff from drains beneath the greened areas surrounding the apartment blocks is also routed to the first tank. Overflows can be released to the storm sewer and the tank can be drained to the sewer when storm peaks have passed, but is more usually emptied usefully for irrigation of the greened areas. Only roof water discharges to the second tank; a cleaner input is required because the design function here is beneficial use – for flushing the public toilets around the complex, for irrigating the greened and planted areas and for cleaning paved surfaces. Irrigation provides the main demand, which can rise to 200m3 per day. To assure output quality the inlet line here is fitted with a self-cleaning filter although, by way of comparison, the turbidity of the settled stored water in the first tank is said to average between 0 and 1.5 Nephelometric Turbidity Units (NTU) even without such treatment. Mooyoung says this is mainly due to a helpful characteristic of the site, the top 1.5m being a fine sand so that rainfall or irrigation water reaching the under drains is already very well filtered. Further quality improveent by sedimentation takes place within the tanks, which are provided with internal baffle walls to prevent short circuiting and where retention time can be up to ten days. Storage in the third tank can be used in the same way as for the second tank but is also intended to provide for emergencies such as fire fighting and is therefore more usually kept full.
A cost comparison exercise on conventional and RWH systems in Seoul City indicated that the energy required to treat and deliver a cubic metre of tap water is 1.8244kWh, with most of this being in energy for transmission.
For grey water (treated on site) this would reduce to 1.1177kWh per cubic metre whereas, according to the Mooyoung calculation, the same volume of rainwater, needing no transport or treatment, can be delivered for a mere 0.0012kWh, the energy needed to pump the water from the rainwater storage tank only.
