Seoul City operates a total of 4 sewage treatment facilities; Jungnang, Nanji, Tancheon, and Seonam, and produces an average of 1,770 tons of sewage sludge daily. Seoul's sewage sludge treatment methods can be divided into two categories: self-treatment, which involves drying, incineration, and other methods within the sewage treatment facilities, and consignment treatment, which involves outsourcing to external companies. Since 2015, Seoul City has been promoting the ’self-sufficiency of sewage sludge treatment’ plan, which aims to treat all the sewage sludge generated in Seoul through self-treatment and recycle it for energy generation. Due to the promotion of self-sufficiency in sewage sludge treatment, the self-treatment rate in Seoul has steadily increased, from about 45% in 2017 to about 62% in 2020. On the other hand, the consignment treatment rate was about 14% in 2020, which is very low compared to the consignment treatment rates of approximately 72% in Busan, 51% in Gyeonggi Province, and 45% in Incheon. Seoul City has been promoting the expansion of private sector treatment (the  consignment treatment), and as a result, the consignment treatment rate increased to about 25% in 2022. However, the rate is still lower than the consignment treatment rates of the other cities and regions. Moreover, with the recent implementation of the coal-fired power generation phase-out policy, the amount of sewage sludge being taken into power plants has been steadily decreasing. Consequently, Seoul City's strategy for self-sufficiency in sewage sludge treatment, which aims to treat all sewage sludge and recycle it for fuel generation, can no longer ensure continuous and stable sewage sludge treatment. Therefore, Seoul City urgently needs to establish a stable sewage sludge treatment plan that can accommodate future environmental conditions and policy changes through diversification of treatment methods (expanding recycling and resource utilisation treatment) and expansion of private sector treatment (the consignment treatment) for self-treated sewage sludge.

To explore ways to expand private sector treatment of Seoul's sewage sludge, a survey on the problems related to processing conditions and institutional improvements, and support measures for expanding private processing was conducted. The results showed that the major problems hindering the expansion of private treatment of Seoul's sewage sludge were demand-side issues, problems with outsourcing contract methods, and issues related to managing the amount of sewage sludge generated. Therefore, to expand the private treatment of sewage sludge, it was suggested that Seoul City needs to support the activation of the use of recycled products of sewage sludge, such as the mandatory use of a certain proportion of Seoul City's sewage sludge recycled products, and institutional improvements such as expanding long-term contracts for sewage sludge outsourcing. In addition, this study proposes a system for sharing information on the amount of sewage sludge generated in Seoul and the distribution of recycled products as an alternative solution to comprehensively address the major problems in private sewage sludge treatment. Through the sharing of information, the system can facilitate the connection and supply contract between the demand side (private treatment companies) and the necessary sewage sludge quantity, thereby increasing the use of recycled products.

This study examined various sewage sludge recycling options, including productisation (construction materials, activated carbon, etc.), cementation (cement substitute materials, cement auxiliary fuels), composting (fertilizers, compost), soil treatment (crushed soil, soil improvement agents, artificial soil), and fuel conversion (solid fuel products, fuel for thermal power and combined heat and power plants). The characteristics and application conditions of each sewage sludge recycling option were evaluated based on these five categories. Based on the review of the applicability of various sewage sludge recycling options, the current conditions of sewage sludge treatment in Seoul, along with consideration of various changing conditions, this study analysed alternative methods for diversifying sewage sludge treatment in Seoul. Considering the current situation and conditions of sewage sludge treatment in Seoul, it is necessary to prioritise the safety and environmental aspects of treatment that take into account the treatment technology and facility level, the generation of harmful substances during the treatment process, and the circularity of resources over the economic, accommodative, and latent aspects of sewage sludge treatment. It has been found that promoting diversified sewage sludge treatment approaches, such as cement solidification, utilisation as fuel in power plants, and reuse as construction materials, would be appropriate.