On-site Zero-Food Waste in High-rise Building Complex of 1451 Households in Seoul, Korea

With the global trend of increasing high-rise residential buildings, handling, collecting and recycling of household food waste have become significant challenges. A simple, convenient and advanced system of the on-site zero-food waste has been developed to address these issues. The system consists of two components: a food waste cutting and transfer devices installed under every sink in all units and a set of modules, a double central automated modules for usual and a single manual module for emergency installed in the building's basement. Good dried compost is produced automatically and hygienically through the module sets and the compacted processes of solid-liquid separation, thickening, dewatering, and bio-drying are built in the module. The recovered compost has a low moisture content, allowing for long-term storage without spoilage or odor. A fully sealed automated system and remote monitoring capabilities ensure a hygienic environment and convenient management. For reference, even this system is installed in an underground space and operates 24 hours a day, 365 days a year, but it is operated cleanly and automatically, and it is expected to have a synergistic effect on the sustainable behaviors in the adjacent community room of recyclables separation.

 

Background

In Korea, following the introduction of the volume-based waste fee system in 1996, stricter waste management policies, including the ban on direct landfilling (2005) and ocean dumping (2013) for food waste mandated separate food waste collection.
While this significantly improved chronic solid waste management issues, the dynamic increase in the supply of high-rise residential buildings driven by economic development presented new challenges for food waste separation and recycling.
As an integrated solution to these challenges, we developed a zero-food waste system utilizing centralized and automated biomass recovery.

 

The Impacts of Food Waste and Food Waste Separation

Reducing food waste at its source through separate collection improves the quality of remaining household waste, facilitating sanitary management and promoting resource recovery.
Local governments in Nordic countries, including Sweden, are collecting household food waste through pipelines to biogas plants.
These two systems - the volume-based waste fee system and separate food waste collection - have given the chance to the municipal solid waste management from open dumping and incineration to a sustainable waste management policy in Korea. Over 90% of separated food waste is transported to livestock feed, compost, and biogas production facilities, making the remaining household waste more readily available for physical separation and thermal treatment. 
However, the mass recycling system of household food waste required a long-terms of vehicle collection and transportation often requires often leads to the generation of secondary wastes and chronic challenges due to deterioration in the quality of food waste raw materials and recycled products.

 

Pilot study of automatic solid recovery system for food waste

This pilot study was conducted in a small, multi-family apartment building. The system's basic components - a cutting and transfer device, a solids-liquid separator, and a solids recovery device - were developed and adopted to the existing domestic sewer pipe lines of apartment.

Purposes and Methods of the Pilot Study 

The purpose of this pilot study was to improve the inconvenient and unsanitary food waste separation and collection system and to develop a on-site centralized and automatic separation system for high-rise residential buildings. 
To achieve this, we secured basic data necessary for performance improvement and application of devices, including the cut and transfer device, solids-liquid separator, and solid recovery process.
For investigation of the impacts of effluent on the public sewer system and process efficiency we analyzed the quality and quantity of the effluent and the basic quality of the recovered solids.  
This system was monitored for three seasons and ten months, including the analysis of food waste, sewage, recovered solid, resident awareness, and etc.

 

The Results of Pilot Study 

1. Among all waste streams subject to separate collections, 70% of residents answered have suffererred the inconveniance for existing food waste separation system.  
2. The increase in sewage after the installation of this system was negligible, considering the daily sewage generation per capita over 400L/c.d in Seoul.
3. The BOD₅, SS, and n-hexane levels in the effluent were within the medium-strength concentration range of typical urban sanitary sewage.
4. Therefore, no significant changes in the quantity and quality of the effluent were expected after installation of the pilot study system.
5. The food waste reduction effect was estimated at 84.0% after the system application, despite the short investigation period during the pilot study. 
6. 87% of respondents cited the convenience of using the system, but some elder residents expressed the inconveniences due to the lack of experiences using the devices. 
7. 83% of respondents reported being unaware of the electricity and water bill increases.
8. Two times results of solid discharge rates for the effluent quality analysis with a model and actual system by two designated testing institutes were 17.2 and 18.7%, respectively, which were confirmed to be within the government standard of 20%.
9. Therefore, the problems could be solved by applying this system:

• Unpleasant and unsanitary issues around food waste management 
• Many chronic issues by mandatory and conventional food waste separate collection and recycling system, especially secondary waste generated from food waste recycling facilities  
• Issues on increasing the capacity of anaerobic digestion tanks: efficiency and risk due to environmental constraints and operational sensitivity

 

Development of full-scale zero-food waste system in high-rise residential buildings 

Goals

Responding to the rapidly increasing global trend of high-rise residential buildings and the need for sustainable development Goals to ensure a clean environment for future generations, we sought to develop a hygienic, convenient, and innovative zero-food waste emission system.

 

This system is designed to be centralized, automated, integrated, and compacted for maximum efficiency and minimal footprint in high-rise residential buildings. Furthermore, the recovered solids are dried using natural energy from their own fermentation process, ensuring their potential as fresh, safe, and high-quality compost and other bio-resources. Ultimately, this system aims to achieve on-site zero- food waste management without the chronic problems of existing food waste recycling systems.

 

Improvement of full-scale system using convergence technologies

While the pilot study yielded many important results, the prototype system was not suitable for actual application in high-rise apartments or for technology marketing. Therefore, functional, structural, and performance improvements were made to the basic concept, adding appropriate features. The mechanical, biological, and IC processes were redesigned to incorporate ergonomic and energy-saving concepts. Furthermore, the moisture content of the recovered solid biomass of 65-70% in the pilot study was not suitable for on-site use as is and can cause microbial spoilage and odor. Therefore, we modified the microbial fermentation conditions to a bio-drying process that utilizes the internal heat of decomposition, eliminating the need for external energy. As a result, the temperature within the process increased to approximately 40-50°C, preventing the growth of pathogenic bacteria and ensuring sufficient heat generation for solid drying. Furthermore, by considering the airflow rate (AFR), a key variable in the bio-drying function, the moisture content of the recovered solids was reduced to less than 15%.

 

Furthermore, in the pilot study, the horizontal process was redesigned into a vertical system structure, saving space and energy. Integration and ICT capabilities were added for automated and unmanned management of the entire system, and compact modularization enhanced various conveniences.

 

Installation and Operation of full-scale system for high-rise residential buildings

 

_{Figure 1. Layout of zero-food waste system for high-rise residential building} 

 

This system was installed building-by-building in a complex of nine 30 to 35 story buildings, totaling 1,451 units. Each unit's dedicated space beneath the kitchen sink in 1,451 cutting and conveying devices, and each building's basement houses one set of nine module sets. Each building has a system as illustrated in Figure 1.

As shown in the figure, of the three module sets, the two modules on either side are always on standby, 24 hours a day, 365 days a year. During dishwashing, the cut and transfer devices are activated at the sink, transporting the shredded food waste along with water into the module. The central manual module is not normally used, but is designed for emergency use with supervision. The automatic modules allow for intermittent and continuous operation. The automatic module houses the solid-liquid separation and concentration process, the dewatering process, the bio-drying process, and a temporary storage room for recovered solids (see Figure 2). Each process is connected to a linkage device so that each process can operate automatically, either individually or simultaneously. This is connected to the central control room and the manager's cell phone according to the signal, so that the central and remote managers can check and take action anytime and anywhere, eliminating the need for a resident manager.

 

_{Figure 2. Flow chart(a) and pictures of module inside(b) and bio-drying process(c)(d)}

 

Operation of full-scale system for high-rise residential buildings

Food waste fed into each household's cut and transfer device is cut to an appropriate size and discharged within 30 seconds by a multi-stage impeller with a small amount of water. It is then transferred to the underground module  through the vertical drain pipes and transferred to the automatic modules. These modules operate intermittently but continuously 24 hours a day, as needed. If one module fails, the other modules continue to operate.

If a process interruption occurs, a failure indicator for a specific component appears on the LCD and a notification is simultaneously sent to the manager's mobile phone, enabling prompt repairs. In the event of an abnormality, such as excessive food waste generation or a breakdown of the cut and transfer device within the household, an emergency manual module can be activated under the manager's supervision. This response system includes monitoring and emergency phone lines, automatic remote notifications to relevant personnel, and an integrated management server at the central disaster prevention center.

Figure 3 shows the quality of module effluent collected randomly over a single day on Tuesday, November 7, 2023, and Sunday, May 19, 2024, approximately two years and two and half years after the start of operation. Although some differences were observed between weekends and weekdays and significant fluctuations occurred during sleeping hours and dinnertime, the average water quality was within the range of typical municipal wastewater treatment plant influent.

 

_{Figure 3. Effluent water qualities of a module set in a high-rise residential building sampled on Nov.7(Tue), 2023 and May.19(Sun), 2024}

 

Figure 4 shows photographs of recovered solids (compost) with a moisture content of 15% or less collected on September 6, 2023, stored at room temperature for 1, 70, 140, and 280 days. It was confirmed that there was no changes in the external appearance and internal sensory odor.

 

 

_{Figure 4. Photos of recovered biomass stored at room temperature for 280 days, 140 days, 70 days, and 1 day since September 6, 2023}

 

Table 1 presents the analysis results of the solids recovered sampled from the module based on legal standards for compost and livestock feed. The solids excellently comply with all compost standards and have a satisfactory maturity level, demonstrating excellent compost quality. Furthermore, the solids not only meet the feed standards, but also contain significantly lower levels of harmful substances such as salt and heavy metals than the standards. No microbial indicators were detected, demonstrating excellent safety as compost and feed. Therefore, the recovered solids can be evaluated as excellent compost and are believed to be harmless when consumed by soil organisms and wildlife.

 

_{Table 1. Comparison of the Analysis Results of the Solids Recovered from the Module with Legal Standards for Compost and Livestock Feed}

 

Conclusions

In response to the global trend of rapid increasing in high-rise residential buildings and toward a low-carbon and circular economy, we developed a system that achieves zero food waste in Seoul, South Korea. This system is divided into two main parts within a high-rise residential building: the upper part is a cut and transfer device installed under each unit's kitchen sink, and the lower part is a set of 3 modules that integrated and compacted processes of solid-liquid separation, concentration, dewatering, bio-drying, and storage. These two parts are connected by the kitchen sewer pipes. The upper part resembles a conventional food disposer type, which discharges the waste into the public sewer system. However, its function is different to cut food waste into appropriate sizes and transfer to modules by discharge pressure to facilitate horizontal conveyance. The lower module sets receive the cut and transfered food waste and recover the solid through mechanical separation and drying by biological fermentation. The final dried recovered solid has a moisture content of less than 15%, making them suitable for long-term storage at room temperature. The high-quality compost is recovered in ready-to-use condition and can be utilized in safe and fresh at anytime at-site and anywhere due to long storaging and transportation are available.This system mechanically separates fresh, nutrient-rich solids from food waste immediately after generation and then undergoes biological fermentation and drying processes, producing high-quality compost in a short period of time. The resulting compost can be used immediately on-site and can be stored for long periods without spoilage, ensuring safe and fresh use in anytime. Furthermore, the effluent from the modules, discharged into the public sewer, was considered to have no quantitative or qualitative impacts on the municipal sewerage treatment system. And despite being installed underground, this system is operated hygienically and without odors, which is expected to have a synergistic effect in promoting sustainable behaviors among residents in nearby recycling and specific waste separation room.

Finally, the system has been implemented in nine high-rise residential complexes with 1,451 units, achieving on-site zero food waste for more than three years to date.

 

Contact

Hyena Co., Ltd
5F, 22-14, Industrial-ro 8beon-gil
KOR - Bucheon-si

phone:+82 70 8798 5710

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