Records dating from 1962 show that swimming was banned in the lower streams of Hangang-daegyo while deformed fish with crooked spine were reported, testifying to the severe water pollution in Han River which Seoul, with a population approaching 3 million, was facing. In a 1963 survey, BOD values reached 241mg/L in Cheonggyecheon and 124mg/L in Jungnangcheon. In addition, the seasonal rain typical of the Korean climate was contributing to frequent flooding.
A need for sewage treatment in Seoul was furthermore highlighted as the 1960s saw the beginning of an explosive population influx, and industrialization started to operate in full scale in the 1970s. Modernized sewerage propagated throughout Europe after mid-19th century, as the dumping of human waste into streams along with other causes led to urban problems such as a raging cholera epidemic; while Seoul did not experience the same problems, concerns nevertheless arose regarding similar risks.
Seoul's sewer system back then was rather incomplete, inclined towards anti-flood measures such as rainwater exclusion. Under the circumstances, water quality improvement in Han River was prioritized, and large-capacity sewage treatment plants were constructed in a short span of time.
In Western Europe sewers were first constructed in order to allow the complete discharge of sewage into streams, with sewage treatment plants built subsequently. Japan, launching its sewerage plans after Europe, reflected on its predecessor and conducted both the installation of sewers and the construction of sewage treatment plants simultaneously. On the other hand, sewage treatment plants in Seoul were built before an adequate sewer system was; therefore, due to faulty wastewater inflow to treatment plants, domestic sewage could not be sufficiently treated. Only after the entire sewerage system was developed, including sewer installation, did the city see significant improvements in sewage issues.

The expansion of sewage treatment facilities and improved eco-friendliness

The first sewage treatment plant in a 3 million-population era
 
The first attempt to construct sewage treatment facilities in Seoul materialized in 1962. Since the country had neither the financial resources nor the technology to construct a large-scale sewage treatment facility, foreign credit was brought in: the project was funded by a loan from Agency for International Development and led by Daniel, Mann, Johnson & Mendenhall. The plan for Cheonggyecheon Sewage Treatment Project was devised in 1962; the loan request plan was submitted to the Ministry of Construction in 1964 and the government-guaranteed Loan Application was submitted to USOM/K in 1965. Following the Loan Guarantee passing the National Assembly in 1966, the $3.5 million AID loan agreement was signed in 1967, and in the same year, a technical service agreement was signed with D.M.J.M. The project design was approved in 1969, and 1970 saw the construction initiated after the design approval by USAID. However, inflation led to shortages of funds, and a $2.8 million additional loan was signed in 1974. The construction saw its completion in 1976, six years after work initiation. After 14 years that passed between the planning and completion of the first sewage treatment facility in Seoul, the city finally saw the historic beginning of the sewage treatment era.Cheonggyecheon Sewage Treatment Plant(currently Jungnang Water Reclamation Center, Plant No.1), with a capacity of 150,000 ㎥/day, was capable of processing domestic sewage generated by 1.3 million people in the 5,600ha Cheonggyecheon watershed area. The wastewater with an average BOD of 330mg/L reached 19mg/L or less after being treated and was discharged onto Jungnangcheon, before its inflow into Han River's mainstream. The next facility to be constructed was Jungnangcheon Sewage Treatment Plant, and ground was broken for it in 1975. Financed with ₩4 billion in loans from English bank Gradley Brandt and ₩6.2 billion domestic loan, the total budget summing up to ₩10.2 billion, the plant's treatment capacity amounted to 210,000 ㎥/day. Unlike Cheonggyecheon Sewage Treatment Plant, the new plant featured a self-generating capacity of 1,400kW, 60 percent of its required electrical power, using gases produced in the treatment process.

The expansion of sewage treatment facilities
 
The Han River Development Project led to another series of comprehensive sewerage development projects. Led under the agenda for an advanced capital and in preparation for the 1986 Asian Games and 1986 Summer Olympics in Seoul, the project had for goal a comprehensive and multi-purpose development of Han River. Since its launch in September 1982, the project involved 11 domestic construction companies and a total budget of ₩413.3 billion. The Han River Development Project saw its completion in September 1986. The urgency of water conservation in Han River, increasingly recognized along the course of the project, resulted in the establishment of the Sewer Maintenance Master Plan and the consequent additional sewage treatment plant construction. Measures were taken against rapid population growth- and industrial development-induced water pollution in Han River and water bodies where discharge happens such as Jungnangcheon and Tancheon. The Sewer Maintenance Master Plan devised in 1984, and the 1992 amendment of the plan, applied to the entire area of Seoul and included an objective for total sewage coverage by the year 2001.
In order for these plans to be realized, Seoul was divided into four treatment areas circa the 1988 Summer Olympics. Plants corresponding to Jungnang and Tancheon areas, located upstream, were assigned the biological secondary treatment and Gayang(currently Seonam Water Reclamation Center) and Nanji treatment plants were assigned primary treatment, hence the opening of a genuine sewage treatment era. Consistent facility expansion afterwards resulted in a total capacity of 3.71 million ㎥/day in December 1995 corresponding to 73 percent of total sewage quantity, and through another expansion initiated in 1998 aiming for complete sewage treatment the capacity of 5.81 million ㎥/day was achieved.

Greening of sewage treatment facilities
 
By covering the upper part of the Water Reclamation Centers and turning those spaces into eco-friendly parks, the local population have been provided with resident-friendly resting spaces.

Improvement of sewers

The combined sewer system
 
86 percent of Seoul is served by combined sewers. Sewer facilities construction in Seoul had for initial purpose the rapid exclusion of stormwater in order to prevent urban flooding, rather than the provision of infrastructure for the city's future development; therefore, sewer maintenance in frequently flooded lowlands and areas with faulty drainage was a priority.In addition, sewer deterioration caused problems such as sewer damage and faulty connections, which in turn led to excessive inflow of unidentified water such as groundwater, rainfall, river water and tap water. Due to the inflow of stormwater, groundwater and such, along with the quotidian sewage, the total inflow reaching treatment plants amounted to 5.09 million ㎥/day by 1995, while the total processable facility capacity was a mere 3.71 million ㎥/day, hence the water treatment facilities were overloaded. Sewage exceeding facility capacity underwent only primary treatment and bypassed secondary treatment plants, inevitably being simultaneously discharged with water which did get subjected to secondary treatment and leading to serious water pollution issues in the discharge areas.

Sewer maintenance for flood prevention
The average annual precipitation of Seoul over the past 50 years (1962-2011) amounts to 1,429.5mm; its average annual precipitation over ten years has increased by 184.2mm, from 1,498.4mm (1962-1971) to 1,613.7mm (2002-2011). In addition, while the average number of days with more than 80mm rainfall and high potential incident rate from 1962 to 2011 was 3.0 days, the average over a decade showed an increase of 0.7 days and, therefore, an increase of the city's flood damage after its urbanization process, from 3.2 days in 1962-1971 to 3.9 days in 2002-2011. Facilities that make up a city's drainage system include banks, gutters, sewers, detention basins, retention basins and drainage pumping stations. In the process of urbanization, existing natural streams are sometimes reduced or turned into culverts; as the extent of impermeable area increased by 40 percent in 2010 compared to 1962, natural groundwater flow may alter into a fast overflowing on the surface. Such a quantitative increase of surface overflow and the acceleration of peak flow velocity in turn increase the peak runoff, resulting in stream depletion and shorter time of concentration. In pursuance of diminished flood damage, the city of Seoul is conducting a sewer maintenance in order to resolve problems such as drainage system overload, lack of adequate catchment facilities, flooding of underground facilities such as the subway, faulty natural flow, and water level rise in rivers and basins. Retention basins and stormwater storage tanks are under construction as effective measures against exceptional situations when river flood caused by water level rising coincides with urban flood caused by localized heavy rain and faulty drainage of street water. In preparation against flood damage caused by localized heavy rain, the city extended in 2009 the standards for considering the average probable rainfall amount, applying to its sewer system's stormwater exclusion capacity: from 10 years for main sewers and 5 years for branch sewers to 30 years for main sewers and 10 years for branch sewers.

A smart sewerage system using advanced information technology

Constant maintenance and efficiency enhancement of sewerage systems are also important tasks, for which the city has introduced state-of-the-art information systems to sewer maintenance and enhancing citizen services.

Seoul currently boasts a 100 percent connection rate to the sewerage system and by the end of 2013 possesses sewage treatment facilities with a total capacity of 4.98 million ㎥/day. The total length of sewers by the end of 2012 amounts to 10,487km.

Swimming in Han River made possible by improved water quality

 
Seoul has currently achieved 100 percent connection of its population to the sewerage system. The total treatment capacity, originally 5.81 million ㎥/day, was reduced to 4.98 million ㎥/day by the end of 2013 due to the conversion into advanced treatment facilities with higher nitrogen and phosphorus treatment efficiency. The total sewer length is maintaining 10,487km in 2012. With the introduction of advanced sewage treatment for nitrogen and phosphorus removal since 1996, the water quality in Han River was further improved; BOD in midstream and downstream was reduced to as low as 3mg/L or less, which makes swimming possible.

Water quality improvement in tributaries 

Water quality improvement in Jungnangcheon
 
Even after the achievement of 100 percent sewerage connectivity (population covered by sewage treatment facilities and sewers) in 1998, the water in Jungnangcheon was considerably polluted with BOD value exceeding 20mg/L. Advanced treatment was introduced in 2011 to Jungnang Water Reclamation Center, resulting in much improved water quality. Furthermore, as the upstream area of Jungnangcheon water system such as Uijeongbu reached 90 percent or more sewerage-connected population and introduced advanced sewage treatment facilities, the current BOD is maintaining 10mg/L or less.

Water quality improvement in Tancheon
 
Even after the achievement of 100 percent sewerage connectivity (population covered by sewage treatment facilities and sewers) in 1998, the water in Tancheon was considerably polluted with BOD value exceeding 20mg/L. As the upstream area of Tancheon water system such as Seongnam expanded its facilities' capacity and introduced advanced sewage treatment facilities, and advanced treatment was introduced in 2011 to Tancheon Water Reclamation Center, the water quality has considerably improved with BOD maintaining 10mg/L or less.

Comprehensive efforts for the reduction of flood damage during localized heavy rains
 
In addition to sewer maintenance, stormwater-overflow-reducing facilities for facilitating the exclusion of flood discharge and disaster-preventing rainwater detention facilities which can serve as temporary storage of flood discharge have been installed. Construction standards for flood vulnerable areas have been provided that makes it mandatory to install stormwater reservoirs of seepage water, to secure the flooding depth of buildings and to secure antiflooding facilities for underground constructions. Comprehensive flood countermeasures are being applied including the relocation in urban watersheds, inducement to water-resistant construction, expansion of detention facilities, assigning disaster prevention functions to public facilities (such as parks and greenery, roads, schools, civil offices), improved ground coverage through reduced impermeability and supply of disaster prevention equipment (e.g. flood barriers, backflow prevention equipment).
 
Communication channels that provide citizens with disaster-related information include general broadcast systems, local announcements, social network services, billboards, and such; these are elaborated by enhanced optional location-based notification services and the establishment of situation phases for civil action and evacuation. Citizen-participatory safety management involving smart information and communication technologies and community mapping is encouraged. The establishment of a citizen-led disaster prevention system including real-time citizen reporting system and counter-flood systems in the private sector is also underway. Long-term measures such as the introduction of rainfall radar will allow the identification and forecasting of various weather phenomena, hence making it possible to prevent and respond to natural disasters. Flood prediction maps according to precipitation is also provided as an active disaster control measure.

The Sewerage Act, enacted in 1966, provided a legal base for sewer administration. As sewerage-related tasks greatly increased, the Sewerage Division under the Construction Management Bureau underwent a large-scale reorganization by being altered to the newly established Sewerage Administration Division, consolidated with the Water Management Division and raised to the status of Sewage Management Bureau in 1976. In addition, the 1982 Han River Development Project provided efforts for the enhanced use of Han River and improved water quality in it. The Ordinance on Sewerage Use announced in 1983 laid the legal basis for the collection of public sewerage user fee. The four sewage treatment plants currently in use include Jungnang, Nanji, Tancheon and Seonam Water Reclamation Centers. The official name has been changed from Sewage Treatment Plant to Water Reclamation Center in 2006, in an effort to rebranding the plants from locally unwanted facilities to water production facilities. Sewage treatment facilities have been under direct municipal operation of Seoul until the IMF financial crisis in 1997 led to a general restructuring of the government and the consequent restructuring of organizations affiliated to the city in 1999, resulting in a decision to gradationally privatize sewage treatment plants. The Government Organization Act and The Act on Public-Private Partnerships in Infrastructure provided the legal grounds for such moves. As part of its restructuring efforts, the Metropolitan Government of Seoul has been conducting a discussion led by the Administrative Reform Committee and the Administrative Reform Working Committee on the reorganization of affiliated agencies including the treatment plants since 1998. Among the four Water Reclamation Centers, Tancheon and Seonam have been outsourced in 2000 and 2001 respectively. The decision to outsource Nanji Water Reclamation Center, based on the 2008 reorganization plan for the Establishment of Organizations with Efficiency and Competitiveness which put emphasis on the active contracting-out of outsourceable facilities and the efficiency of organization management, was eventually foundered. Consequently Nanji and Jungnang were left under direct management and partial contracting-out. In order to secure the financial resources for sewerage management, sewerage user fee has been collected since 1984 according to the causer pays principle. The administrative organization supervising Seoul's sewerage facilities is led by the Water Quality Management Bureau under the Urban Safety Office, which directs a total of four divisions each consisting of five or seven teams: Water Quality Management Division, Sewerage Treatment Planning Division, Sewerage Treatment Facilities Division and Stream Management Division. Among these, the Sewerage Treatment Planning Division and Sewerage Treatment Facilities Division are directly related to sewer management. Among the four Water Reclamation Centers of Jungnang, Nanji, Tancheon and Seonam, Jungnang and Nanji Water Reclamation Centers are under direct municipal management with the exception of the outsourced sludge treatment facilities, and Tancheon and Seonam Water Reclamation Centers are completely outsourced.

Large precipitation differences along the year due to the characteristics of the rainy season

The total sewer length of Seoul by 2010 was 10,280 km, among which combined sewers occupy 86 percent. In case of rainfall, untreated pollutants exceeding the sewerage facility capacity(Q+2mm/hr) are discharged and causing river water pollution, imposing difficulties in achieving water quality objectives. Korean climate is characterized by a concentrated precipitation; 74 percent of the average yearly precipitation happens between June and September, intensifying flood damage in wet seasons. On the other hand, precipitation is scarce during dry and normal seasons, causing long-term accumulation of pollutants which are discharged to the river in high concentration during rainfall: the overflow from combined sewers are increasingly affecting river water pollution. In areas with combined sewer systems, pollutants are discharged through storm outfalls in the form of CSOs(Combined Sewer Overflows) during rainfall, causing major social problems such as fish kills. Large amounts of hazardous substances including sewer sediments, organic matter, bacteria, oil and heavy metals are included in CSOs, of which the direct runoff onto streams in a short period disrupts the water quality and the ecosystem. Meanwhile, the citizens' need for improved water quality is also increasing and putting emphasis on the importance of water management, but most streams suffer water pollution in public bodies of water caused by CSOs due to initial rainwater in combined sewers.
In response to this matter, the city has established and is operating an integrated management of streams and watersheds in order to process nonpoint pollution and overflow discharged onto streams in case of rainfall. The overflow water quality standard as suggested by the Sewerage Act, under 40mg/L, applies to the administrative standard for CSOs in storm outfalls alongside streams; CSOs exceeding 40mg/L are stored and processed. In addition, routine processes such as sewer pipeline dredging, rainwater management, water cleaning of roads and unidentified water reduction measures are carried out in watersheds in order to reduce the emission of CSOs.

Lack of awareness and faulty construction of separate sewer systems

Areas in Seoul served by separate sewer systems include large-scale residential development projects such as Sanggye, Junggye, Mokdong, Gaepo and Garak and cover a total of 351 ㎢. However, lack of awareness regarding drainage facilities in construction clients, contractors and superintendents along with the lack of depth in the installation of public sewer pipelines is causing difficulties and reluctances to the direct connection of sewer pipelines in drainage facilities. Such problems lead in turn to faulty connections between sanitary sewers and stormwater sewers, causing sewage inflow in stormwater sewers; in order to intercept and process such sewage, storm outfall chambers are installed at discharge points of stormwater sewers. In order to prevent the confusion or misconnection of sanitary sewers and stormwater sewers in separate sewer areas, construction standards require sanitary sewers to have a dark brown surface or dark brown tape or paint covering its upper part. Public promotion to residents for the correction of existing misconnections according to the causer pays principle has been unsuccessful despite 5-8 years of enforcement, due to realistic difficulties.

Insufficient financial support for the sewerage system

Recent changes in climate resulted in different rainfall characteristics, causing sewer flooding in urban areas of Seoul and, therefore, calling for continuous flood countermeasures such as capacity-increasing sewer maintenance projects and construction of sewage detention facilities. In addition, as the government's regulations regarding TMDL, total phosphorus, simple sewage treatment of discharged sewage untreated by public treatment facilities during rainfall, CSOs countermeasure and such are growing, water quality control at terminal disposal facilities is becoming a bigger challenge. Continuous investment of additional financial resources is needed in order to meet these requirements. However, on the contrary, recent years have seen diminishing funding for sewerage facilities, making even the adequate maintenance of existing sewerage a difficult task. Since the current sewerage user fee is extremely low, falling short of actual treatment costs, some financial improvements may be made possible to some extent by the appropriate realization of sewerage user fee. To do so, evaluation of original asset and reappraisal of total cost are needed as is establishment of an independent sewerage budget that will allow the sewerage services to switch to a local public enterprise accounting, based on the Local Public Enterprises Act.
 

Seoul holds 36 rivers defined by law including Han River, five miscellaneous rivers and other brooks; the water quality in these streams are being maintained with the objective to enable swimming, in order to provide the citizens with ample water environment. Through past water quality management efforts including the achievement of 100 percent sewerage-connected population and the reduction of water pollution, the pollution load flowing into rivers has diminished, and water quality has been greatly improved. The functions of a sewerage system include flood prevention via urban rainwater exclusion, sewage carrying, improvement of living environment by conversion into flush toilets, water preservation in public bodies of water such as rivers, lakes and seas as well as health environment and sanitation. As it is an infrastructure indispensable to people's environmental rights, the necessity for sewerage maintenance is ever increasing. When considering runoff characteristics of Seoul as the urban city it is, dominance of ​​impermeable area directly connected to the drainage system can be noticed, which along with changes in rainfall patterns influences the planning of urban drainage facilities. When approaching sewage issues, the close relationship they have with urban planning and land use must be taken into account. Long-term variations in rainfall patterns due to macroscopic changes such as climate change must also be prepared against in advance, in order to prevent disasters such as flooding. Under the objective of creating a comfortable and safe living environment, Seoul is proposing the next prospect in order to construct a future-oriented sewerage system which promotes intimacy with citizens, ecological preservation, advanced technology, security and resource recycling.