Developing a Centralized Automatic Biomass Recovery System for Zero-Food Waste in High-Rise Apartment

Abstract

An advanced waste management system has been developed specifically for high-rise residential buildings, aiming to achieve zero-food waste emissions through efficient biomass recovery. In response to the global trend of increasing high-rise construction, this system includes two main components: a food waste cutting and transfer device installed under household sinks, and a centralized automatic module located in the building’s basement. The system efficiently handles solid-liquid separation, dehydration, and bio-drying, converting food waste into high-quality biomass with low moisture content that is suitable for long-term storage. Fully automated and equipped with remote monitoring capabilities, the system ensures a sanitary and convenient environment for residents. Successfully implemented in two large-scale apartment complexes in Korea, it has operated seamlessly for over three years without causing any inconvenience to residents. Additionally, the system has encouraged greater resident participation in sustainable waste management practices, such as the separation of recyclable and special wastes in communal sorting rooms. The implementation of this integrated system is proposed as a viable technical solution for sustainable waste management in high-rise buildings, aligning with global trends in eco-friendly urban development and addressing the challenges posed by climate change.

1. Introduction

Humans have built high-rise buildings for various reasons, and the height of buildings has various meanings. The centers of high-rise buildings, which began for religious reasons in ancient times, moved from North America to Asia and the Middle East in modern times. Currently, high-rise buildings and apartments exist not only in both large and regional cities [1]. In Korea, the supply of apartments and high-rise buildings is increasing dynamically, given economic development and social change. Owing to urban redevelopment and new city construction over the past 20 years, seeing skylines change owing to high-rise apartment complexes is not rare, not only in metropolitan areas but also in local cities. National housing statistics show that, among newly allocated homes, apartments increased from the 60s in 2010s to 82.0% in 2022 [2].

Meanwhile, the problems associated with the management of solid waste in today’s society are complex because of the quantity and diverse nature of waste, the development of sprawling urban areas, funding limitations for public services in many large cities, the impacts of technology, and emerging limitations in both energy and materials. Moreover, households are the subject of the second of the six functional elements in the solid waste management system, which includes waste handling and separation, storage, and processing at the source. Waste handling and separation involves the management of wastes until they are placed in storage containers for collection and also encompasses the movement of loaded containers to the point of collection, which is an important step in the handling and storage of solid waste at the source [3]. Therefore, the second functional element of waste management is closely related to housing structures and waste collection systems.

In Korea, a mandatory food waste separation system enacted 20 years ago, motivated by a volume-based waste fee system enacted 30 years ago, has had a groundbreaking impact on household waste management. Among separable household waste items, food waste is the most difficult to separate. Therefore, appropriate measures must be established at the household, local government, and national levels in accordance with changes in the economy, society, and the environment. Although introducing an innovative mandatory separate collection system has quickly brought the country’s waste management level beyond that of developing countries, methods of responding to food waste according to the path appropriate for the continued improvement of living standards and social change must also change. Considering the chronic problems of food waste from the source to the recycling market, as well as the characteristics of a high-rise apartment society, an integrated solution using a zero-food-waste system through centralized and automatic biomass recovery with mechanical-biological treatment (MBT) and information and communication technology (ICT) convergence technology is proposed for next sustainable high-rise apartments. The number of people who want to live in high-rise apartments with a pleasant view is increasing, but none want to separate and store food waste at home or meet neighbors carrying bags of food waste in closed hallways or elevators. Zero-food waste emissions may be the only solution.

2. Impacts of food waste and separate food waste collection system

The world generates 0.74 kg of waste per capita-day, yet national waste generation rates fluctuate greatly from 0.11 to 4.54 kg per capita-day, and waste generation volumes generally correlate with income levels and urbanization rates. The largest waste category is food and green waste, accounting for 44% of global waste at the international level, more than 50% of the waste in low- and middle-income countries, and 32% of the waste in high-income countries. However, the amount of organic waste is widely comparable in absolute weight terms because of the larger amounts of packaging and because of other categories such as income levels. Globally, more than 95% of lower-income countries, more than 80% of middle-income countries, and approximately 40% of waste are disposed off in open dumping and landfills. In other words, the rates of open dumping and landfilling decrease as a country’s income level increases [4].

The U.S. EPA investigated the environmental impacts and contributions to a circular economy of 11 common pathways to manage wasted food, from source reduction to composting to landfill. In its report, the term ‘wasted food’ was used to encompass both excess food and food waste, and two methodologies—life cycle assessment (LCA) and circularity assessment—were employed to evaluate these pathways. Scale emphasizes the importance of preventing and diverting food waste from sewer/wastewater treatment, landfills, and controlled combustion (i.e., incineration) pathways. In addition to its impact on the environment, food waste has negative social and economic impacts; therefore, integrated measures are required [5].

Governments in increasingly more countries and localities are promoting separate events to reduce food waste or processing and recycling byproducts through separate collection. When food waste is reduced at its source through separate collection, the quality of the remaining household waste improves, leading to easier hygiene management and resource recovery.

In 2023, New York City implemented the largest composting program in the United States through a curbside composting program with a simple, universal weekly collection of leaves, yard waste, food scraps, and food-contaminated paper products for cleanliness and sustainability [6]. San Francisco launched the nation’s first and largest municipal food waste composting collection program in 2002, covering both commercial and residential sectors and setting the city as a model green city [4].

The pipeline collection of ground food waste from households to biogas plants has been performed by local governments in Sweden and other Nordic countries [7]. In Burkina Faso, decentralized organic waste management across households can considerably reduce the burden on formal waste collection and disposal infrastructure while improving food security through agricultural benefits and creating opportunities for citizens to generate income from waste [4].

Two systems of volume-rate waste fees for household waste and separate collection of food waste have become motivations for reforming the concept of municipal solid waste management in Korea from open dumping and burning to sustainable waste management policies. Approximately 60% of household waste is separated for recycling, and over 90% of separately collected food waste is transferred to facilities for feedstock of animal feed, compost, and biogas [8]. Separate collection of food waste has made residual household waste easier and more beneficial for physical separation and thermal treatment. However, owing to rapid changes in global issues and the social style of living, this study aimed to report on technology developed for integrated measures through zero-food waste for the social changes of high-rise apartments and a carbon reduction society.

3. Pilot study of automatic solid recovery system for food waste

3.1 Purpose and scope of pilot study

This study assessed a drainage pretreatment system equipped with a solid-liquid separation system and dehydrator, and 240 of the 283 apartment households (participation rate of 88%) participated in three pre-set installation sites. The purpose of this pilot study is as follows, and the connection structure with the sewerage facility is shown in Figure 1.

1. Resolve citizen inconvenience caused by the unhygienic separate collection of food waste and introduce a food waste source treatment and resource recovery system with a treatment method suitable for various residential environments.

2. By using the plumbing facilities of existing apartments (apartments and villas), kitchen sewage with crushed food waste is separated into solids and liquids, which are recovered and used as resources. The sewage is then discharged through a filter into a sewer pipe.

3. Review cutting and transfer devices, solid-liquid separators, and resource recovery devices, among others, and obtain basic data for establishing institutional standards, such as deriving design data.

Figure 1.

Concept of automatic biomass recovery system and public sewer in pilot study.

3.2 Materials and methods

The system of pilot study consisted of a cutting and transfer device, solid-liquid separator, resource recovery device (dehydrator), and pluming. Monitoring of the system included the identification of food waste generation characteristics before and after system installation, kitchen sewage discharge characteristics, performance, possibility of recycling recovered solids, resident awareness, and adaptability to indoor piping and public sewers.

As for a basic piping plan, the system was connected to an existing indoor horizontal branch pipe, underground horizontal pipe, and kitchen sewage vertical pipe, in accordance with the standard specifications and building codes recommended by the Ministry of Land, Transport and Maritime Affairs. Transparent observation pipes and cleaning ports were installed in the transverse pipe for obstruction observation and cleaning.

The cutting and transfer device had a processing capacity of 1.0 L/time and consisted of a main body, automatic tap water supply, and control switch. A shredding method was a combined cutter and hammer mill with a discharge pressure of 1.5 m-H₂O by rotating a multi-stage impeller to prevent blockage of the transverse branch pipe. Automatic and manual operation types existed, and the safety of each device was considered in its design.

The solid-liquid separator had a repeated stroke of a pair of filters linked to a water level sensor and a controller based on environmental settings. A solid slurry transfer pump and screw press are operated in conjunction, and a discharge pump and makeup water circulation pump are operated using a water level sensor. The self-diagnosis functions were displayed on an LCD monitor. It consisted of a submersible solid transfer pump and a pair of filters equipped with submersible transfer pumps. Filter regeneration uses the centrifugal force generated by rotating the filter at high speed to separate and regenerate solids attached to the filter medium. The water level sensor detected the regeneration cycle according to the flow rates of the inflow and discharge water. In addition, dissolved oxygen was generated via aeration during regeneration, creating aerobic conditions that did not produce odors under anaerobic conditions. Dual solid-liquid separation filters were installed such that, even if one filter failed, the other filters operated normally, and the filtration function was always maintained. The treatment capacity was set to less than 100 kg/day in accordance with Annex 3 of the Enforcement Decree, Waste Management Act of Korea [9].

The resource recovery device was a screw dehydration-type device equipped with various functions, such as self-failure diagnosis, environmental settings, freeze prevention, temperature and humidity control, and weight integration. Dual units for safety were installed in the event of a malfunction. These consisted of two screw presses, an electronic scale, an LCD monitor, a system controller, and a recovered solid material container.

System monitoring was conducted for 10 months before and after system installation, and at least three seasons (winter, spring, and summer) were evaluated to determine seasonal suitability. Before installing the system, a survey was conducted to investigate the basic characteristics of kitchen sewage. BOD₅, SS, pH, n-hexane extractable material, T-N, and T-P were analyzed to determine the water quality of the kitchen sewage.

After installing the system, the characteristics of the discharged sewage, the possibility of recycling the recovered biomass, system performance, residents’ awareness (based on a survey), and adaptability to indoor piping and public sewage were investigated. Key evaluations were conducted according to the certification standards of public certification bodies. The process and pictures of the prototype biomass recovery system installed in the three apartments for the pilot study are shown in Figure 2.

Figure 2.

Prototype biomass recovery system installed in pilot study.

3.3 Summary of results and discussion

A pilot study on the food waste reduction effect of a centralized resource recovery system was performed by monitoring for 6 months after system installation. The results and discussion are summarized as follows.

1. By reviewing the survey results and reference data, it was found that 30 L/capita-day of the unit generation amount of kitchen sewage was reasonable before system installation, and 5 L/capita-day of the unit’s added amount after system installation was also reasonable, which is the same as the recommended water amount in the user manual of the cutting and transfer device. When considering the total daily amount of sewage generation, which is 441 L/capita-day in Seoul [10], the amount of 5 L/capita-day added by cutting and transfer devices is negligible.

2. The water qualities of the effluents before (n = 8)/after (n = 9) system installation were 335/283 mg/L for BOD₅, 221/113 mg/L for SS, and 113/44 mg/L for n-hexane extractable material in the Y apartment. Moreover, the following values were obtained in the final test after the system installation: 256.3 mg/L for BOD₅, 142.5 mg/L for SS, and 91.4 mg/L for n-hexane extractable material. Both cases were within the medium-to-strong concentration ranges of typical sanitary wastewater [11] and became lower after system installation. For reference, a single device performance test was conducted to evaluate the basic performance of the system under simulated food waste and test conditions (food waste discharge of 200 g/person-day, kitchen sewage amount of 35 L/person-day; 500 persons). The results of effluent water quality were 166.2 mg/L for BOD₅/L and 170.6 mg/L for SS, which were lower than the pilot study. That is, contrary to concerns, it was confirmed that the centralized automatic biomass recovery system could somewhat improve the effluent water quality in the pilot study.

3. A survey on the food waste reduction effect before and after system installation in the pilot study was performed using measured data for food waste collected after system installation and statistical data from the year before system installation. The results showed a 34.1% reduction in the first month after system installation, a 55.3% reduction in the second month, and an average of 70% reduction from the third to the fifth month. Considering that 84.0% of the households participated in the pilot study, it was estimated that the waste reduction effect due to system installation reached a maximum of 84.0%, even in the first pilot study. Therefore, there was a highly positive effect on achievable the 100% reduction of food waste in further steps.

4. Mass balance analysis was conducted on the solid recovery system in this pilot study to calculate the amount of total solids (TS) discharged into the sewer and the solid biomass recovered from the system. The solid recovery rate was calculated to be 54.6% in the first analysis 3 months after system installation, which increased to 78.7% in the fifth analysis after 5 months, and the TS discharged into the sewer showed a maximum of 46.7% and a minimum of 9.1%. Because biomass recovery and discharge rates vary depending on the composition of food waste and the sampling time, long-term monitoring is necessary to obtain reliable data. Photographs of the biomass recovered during the pilot study are shown in Figure 3. The moisture content was in the range of 65–70%. The salt content was 0.18–0.09%, which was within the legal standards of 2.0% set in the fertilizer process standards [12], 0.4–0.5% for pig feed, and 0.15–0.37% for chicken feed [13].

5. An awareness survey of residents was conducted twice before and after installation regarding the cutting and transporting device installed in each household’s kitchen. The overall response rate was 51% before installation and 84% after installation, which indicates that interest increased after installation. Respondents were women (70%), people over forties (83%), and households over three people (82%). More participation from residents who had higher needs for it were answered in the survey. Among all types of waste subject to separate collection, the most inconvenient waste is food waste (70%), followed by bulky waste (19%), residual waste subject to volume-rate system (7%), and recyclable waste (4%). The most inconvenient aspect of the food waste separate collection system was the discomfort and hygiene issues when handling for sorting and storing food waste (59%), followed by the process of carrying it by hand to the collection point (17%). Respondents who separate and discharge food waste every day are 23%, every 2 days, 28%, and every 3 days, 32%. In other words, 84% of respondents were experiencing the inconvenience of regularly separating and discharging food waste at intervals of less than 3 days. Regarding this, 87% of respondents answered about the convenience of using the cutting and transfer device installed in each household’s kitchen, but some appealed to the inconveniences caused by lack of experience with the machine. When respondents were asked about increases in utility rates, 83% of respondents said they had not noticed increases in electricity and water rates.

6. As a result of considering the feasibility of introducing the system along with a mechanical evaluation to improve the system performance, this system can be an alternative that can solve the problem of convenience for residents regarding a separate collection system of food waste. It can also be a fundamental solution to the problems of food wastewater on land treatment due to the lack of problems with system operation, such as sewer pipe sediment, noise, and odor. A win-win effect may be expected by utilizing the advantages of the automatic biomass recovery system within the limit of not harming the sewage environment and also taking advantage of the carbon reduction effect due to the increase of biomass circularity and zero-food waste emissions. As for the food waste problem, it could be possible to build a sustainable food waste management for high-rise apartment complexes.

Figure 3.

Recovered biomass samples in the pilot study.

4. Development of full-scale zero-food waste system in high-rise apartments

Various policies to save food and reduce food waste have had many positive effects on waste management. However, to solve the appropriate chronic food waste problems, as well as economic and social changes, technological development is required. Therefore, not only the source reduction but also the waste disposal stage, optimal sustainable waste management with zero landfills and zero incineration through innovative technology is required. In this section, the development of technology that can be applied to sustainable high-rise apartments by solving chronic problems in the separation and recycling stages of food waste sources [14] by improving the pilot study results is described.

4.1 Goals and tasks

The most important aim of this study is to achieve zero-food waste through a centralized automatic biomass recovery system in high-rise apartments. Therefore, a full-scale zero-waste system is another title for this system. This system is to provide a convenient, hygienic, and eco-friendly solution to chronic problems and carbon reduction effects by zero-food waste emissions for the ever-increasing high-rise apartment society. The recovery of high-quality biomass in a ready-to-use state can reduce the burden on local governments by eliminating the need for separate collection, transportation vehicles, and recycling facilities. To further complete the full-scale system, the objectives of the pilot study are as follows:

1. Improvement of hygienic, clean, and convenient system

2. Improvement of technical system with reliability

3. Zero-food waste by biomass recovery in high-rise apartment.

4.2 Improvement of full-scale system using MBT-ICT convergence technology

The pilot study yielded many important findings, including the suitability of the system functionality, potential for zero-food waste emissions from solid resource recovery, minimal impact on the sewage environment, good quality of recovered biomass resources, and minimal impact on utility consumption. However, the prototype system of the pilot study was not suitable for application in a full-scale system of high-rise apartments or for marketing the technology. Therefore, functional, structural, and performance improvements were necessary. The full-scale system also consisted of cutting and transfer, centralized automatic biomass recovery, and piping facilities connecting them. Accordingly, the basic concept of the system was reimagined and redesigned as ergonomic and eco-friendly concepts by adding appropriate functions and rearranging the process of MBT and ICT convergence technologies. A bio-drying process using biological treatment technology was integrated with MT to reduce the moisture content of the recovered biomass.

The moisture content of the biomass recovered in the pilot study was 65–70% in the dehydrated state, and decomposition by microorganisms may cause biomass deterioration and odor. Therefore, drying was necessary to reduce the moisture content as much as possible to a level where microorganisms cannot function. Because a thermal drying method is not suitable for developing this system because of the burden of the external energy supply, a bio-drying process that does not require external energy rather uses the heat of decomposition of the biomass itself to dry it was applied [15, 16].

As a result, the temperature within the process increased to approximately 40–50°C, allowing the heat necessary for drying to be supplied, and the moisture content of recovered biomass could be reduced to less than 15% by supplying excess air. This mechanism could be explained by the fact that evaporation was driven by the difference between the saturated water vapor pressure of the waste surface and the water vapor pressure in the passing air, which increased either due to temperature increases associated with biodegradation or the dryness of the introduced air. A high airflow rate (AFR) can enhance the water removal rate, and water removal associated with air replacement was generally greater than that associated with temperature increases caused by biodegradation. However, an excessive AFR terminated biodegradation owing to the reduced moisture content, even though organics remained [15]. The AFR was confirmed as a key operational parameter significantly affecting both the bio-drying performance and efficiency [16]. Excess air created an aerobic atmosphere inside the module, thereby preventing anaerobic decomposition.

Additionally, the automatic module containing these three processes was redesigned from a horizontal structure in the pilot study to an upright, human-like vertical structure in the full-scale system. Consequently, materials within the system could be moved by gravity, and the top-down upright system structure can save space and energy. For automation and unmanned management of the entire system, interlocking and ICT functions were added, and compact modularization can help with installation and maintenance convenience.

4.3 Installation and operation of full-scale system for high-rise apartment

The zero-food waste system for high-rise apartments using MBT-ICT convergence technology consisted of cutting and transport devices, dual automatic modules, and single manual modules in a dedicated space on a basement floor, and it was installed in each building. This system was installed as a highly complete built-in unit in about 1451 households in nine buildings of 35 floors in Seoul, South Korea, after a three-month construction period starting in July 2021, and it has been operating without any breakdown for 3 years. The layout of the system is shown in Figure 4.

Figure 4.

Layout of a zero-food-waste system for high-rise apartments using MBT-ICT convergence technology.

To prepare for system operational failures and special situations, all buildings were equipped with multiple modules (a pair of automatic modules and one manual module). The two automatic modules can operate simultaneously or independently, and a manual module is implemented to prepare for emergencies. Each automatic module was equipped with a solid-liquid separation process, dewatering process, bio-drying process, and temporary storage box of recovered biomass in a vertical structure (see Figure 5). Each process was connected by an interlocking device such that each process was automatically operated individually or simultaneously. Signals regarding the operating status of the parts were displayed on the manager’s cell phone so that remote managers and related personnel could check them at any location, eliminating the need for a resident manager.

Figure 5.

Flow chart (a) and picture of automatic module inside (b), and bio-drying process (c), and bio-dried biomass in temporary storing box (d).

The food waste placed into the cutting and transfer device of each household is cut and reduced to an appropriate size within 30 s by a multi-stage impeller and transferred to an underground automatic module through the horizontal and vertical kitchen sewage pipes. This system operates automatically and intermittently, when necessary, 24 hours a day, and even if one module breaks down, the other modules start operating. In the case of a disrupted process, the failure of a specific part is displayed on the LCD, and notification is sent concurrently with the manager’s smartphone so that repairs can proceed quickly. An emergency manual module can be operated in the presence of a manager under abnormal conditions, such as when excessive food waste is generated or when the cutting and transfer device in homes is out of order. This means that the system has a dual emergency measure by the monitoring and emergency call line connected to an automatic remote notification to relevant personnel and the integrated management server of the central disaster prevention center.

4.4 Results and discussion

Solid waste management may be defined as the discipline associated with the control of generation, storage, collection, transfer and transport, processing, and disposal of solid wastes in a manner that is in accord with the best principles of public health, economics, engineering, conservation, esthetics, and other environmental considerations, and that is also responsive to public attitudes. In its scope, solid waste management includes all administrative, financial, legal, planning, and engineering functions involved in solutions to all problems of solid waste. The solution may involve complex interdisciplinary relationships among such fields as political science, city and regional planning, geography, economics, public health, sociology, demography, communications, and conservation, as well as engineering and material science [17]. Integrated solid waste management (ISWM) can be defined as the selection and application of suitable techniques, technologies, and management programs to achieve specific waste management objectives and goals [18].

In order to solve the food waste problem, complex relationships in all processes of the food waste management system must be considered, but in order to achieve specific purpose waste management, it is necessary to select and apply integrated and appropriate technologies. This study was conducted to propose a technically integrated solution against chronic problems of the present food waste management system in the manner of best principles and the public attitude.

4.4.1 Improvement of hygienic, clean, and convenient system

The first purpose of the pilot study sponsored by the Seoul Metropolitan Government is “to resolve citizen inconvenience caused by the unhygienic separate collection of food waste and introduce a food waste source treatment and resource recovery system with a treatment method suitable for various residential environments.” By improving the results obtained in the pilot study, the full-scale project of a zero-food waste system for high-rise apartments achieved its goal in accordance with the best principles and public awareness and in accordance with the increasing trend of high-rise apartments.

This system is a convenient system that disposes of food waste at the sink in a household without having to take it out of the home. That is, the path through which food waste passes is a closed sewer pipe rather than a hand-carried elevator, and the centralized automatic biomass recovery system located underground is also packaged in compact style. Aerobic condition in the module is maintained by fresh excess air of the bio-drying process. Therefore, odor generation conditions do not exist inside the module.

As a result of operation and maintenance over 3 years, it was confirmed that the hygienic, no-odor, convenient, and zero-food waste system could be provided to households by the recovery of high-quality biomass. In addition, the recovered biomass can be recycled in fresh condition for a long period, allowing it to be directly recycled as compost and biomass solid fuel without additional processing with long-term storage. Figure 6 shows the recovered biomass that can be stored long without changing the quality at room temperature.

Figure 6.

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

4.4.2 Improvement of MBT-ICT system: Effluent quality

Effluent which is mixed water of water from the solid-liquid separation device and dehydrated water from the dehydrator is discharged into the public sewer. Although there are no effluent water quality standards in Korea, sediment should not deposit inside the sewer and the effluent from the system should not be impacted on the sewage treatment plant.

Figure 7 shows the daily changes in water quality of effluent randomly sampled at intervals of 1–2 h except late at night on November 7, 2023 (Tuesday) and May 19, 2024 (Sunday). The daily average values of the effluent water quality for both days were near the typical medium sanitary wastewater concentration in the United States [11] and were lower except for n-hexane extractable material than the effluent standard of the drainage treatment type of disposer system in Japan, which are 300 mg/L in BOD₅, 300 mg/L in SS, and 30 mg/L in n-hexane extractable material [19]. In other words, it is believed that the development of the full-scale system has increased the reliability of the discharge water quality of the full-scale system compared to the pilot study. In the case of n-hexane extractable material, it is considered to be somewhat high as there is no such standard in Korea. There was no case of shock load to a sewer system, and even in cases where it was somewhat high, the impact on the sewer system is expected to be minimal because the amount of kitchen sewage is less than 1/10 of the total amount of domestic sewage. That is, it is believed that the development of the full-scale system has increased the reliability of the effluent water quality compared to the pilot study.

Figure 7.

Daily variation and average of water quality of effluent from the full-scale system on Tuesday, November 7, 2023, and Sunday, May 19, 2024.

4.4.3 Improvement of MBT-ICT system: Recovered biomass

Table 1 shows the results of the analysis of the compost and feed quality of the biomass recovered from food waste by this zero-food-waste system. The quality of the compost and feed met legal standards. In particular, many heavy metals were undetected or were detected at very low levels, whereas Escherichia coli and Salmonella were not detected. The organic-matter–nitrogen ratio, moisture content, and salt content were also very low, and all met the legal standards. This was possible because the food waste discharged from homes was collected in a very fresh state, washed with water, and dried to a moisture content of less than 15% via the bio-drying process. Therefore, the recovered biomass did not deteriorate biologically, even when stored for a long period.

Compost quality				Feed quality
Item	Unit	Result	Legal Stand.	Item	Unit	Result	Legal Stand.
OM	%	82.75	≧30	Moisture	%	13.70	≦14
Arsenic	mg/kg-dry	N.D.	≦45	Crude protein	%	22.79	—
Cadmium	mg/kg-dry	N.D.	≦5	Crude fat	%	3.84	—
Mercury	mg/kg-dry	N.D.	≦2	Crude fiber	%	24.14	—
Lead	mg/kg-dry	0.68	≦130	Crude ash	%	3.22	—
Chrome	mg/kg-dry	6.03	≦200	Salinity	%	0.29	—
Copper	mg/kg-dry	12.29	≦360	Phosphorus	%	0.20	—
Nickel	mg/kg-dry	2.66	≦45	Lead	ppm	0.01	≦20
Zinc	mg/kg-dry	39.81	≦900	Mercury	ppm	≦0.01	—
E. coli	—	N.D.	N.D.	Cadmium	ppm	N.D.	—
Salmonella	—	N.D.	N.D.	Arsenic	ppm	0.01	—
ON	%-	23.71	≦45	Fluorine	ppm	N.D.	—
Salinity	%	0.13	≦2.0	Copper	ppm	10.52	≦29
Moisture	%	13.98	≦55	Zinc	ppm	20.58	≦87
Maturity (S)	—	4	≧4	Selenium	ppm	0.09	≦2
HCl insol.	%	0.42	≦25	Aflatoxin	ppb	N.D.	≦50
Nitrogen	%	3.49	—	Salmonella	—	N.D.	N.D.
pH	—	5.15	—	Melamine	—	N.D.	—

Table 1.

Qualities and standards for compost and feed of recovered biomass.

OM, organic matter; E. coli, E. coli O157:H7; ON, organic nitrogen; Maturity (S), maturity (Solvita).

Table 2 shows an analysis of the fuel characteristics. The system produces an excellent quality bio-SRF and can be stored for long periods of time. Additionally, the biological methane potential (BMP) was approximately 0.339 Nm³ CH₄/kg VS or higher under the substrate-to-inoculum (S/I) ratio of 0.3, meaning that a considerable amount of biodegradable organic substances remained in the biomass. Meanwhile, when calculating the theoretical amount of methane generated according to the Buswell and Mueller equation, it was found to be 0.477 Nm³ CH₄/kg VS, which is similar to that of food waste, supporting the BMP test results [20, 21]. Considering the above results, zero-food waste emissions can be achieved by producing excellent biomass resources that can be used directly from food waste without a separate treatment system.

Item		Unit	Result
Proximate analysis	Moisture	%	14.06
	Volatile matter	%	67.06
	Fixed carbon	%	16.15
	Ash	%	2.74
Heating value	HHV (dry basis)	kcal/kg	4843
	LHV (wet basis)	kcal/kg	3759
Ultimate analysis	C (carbon)	%	48.85
	H (hydrogen)	%	6.87
	O (oxygen)	%	40.58
	N (nitrogen)	%	4.03
	S (sulfur)	%	0.25

Table 2.

Quality analysis results for bio-SRF of recovered biomass.

To verify the quantitative value, evaluating the amount of resources that could be secured is necessary. However, this is difficult to estimate immediately because there were not enough cases. In this study, a simple comparison between energy usage and existing food waste recycling methods was performed.

4.4.4 Comparison of energy usage with existing methods

Waste is a by-product of economic activities, and sustainable solutions based on market economic principles must be sought for a circular economy. However, because waste management policy in Korea is based on government subsidies rather than market economic principles, decision-making in the market is difficult. For such decision-making, methods such as LCA, life cycle cost, social LCA, and Life Cycle Sustainability Assessment (LCSA) have been proposed; however, these methods have limitations in terms of standardization and application owing to the different situations of the three pillars of economy, society, and the environment. Therefore, instead of evaluating the impact of the entire process, this study only compared the energy consumption of food waste disposal methods.

The comparison target was one building (174 households) among the high-rise apartment complexes where this research system was installed. Based on the same criteria, the centralized automatic biomass recovery system using MBT-ICT convergence technology (hereinafter zero-food-waste system) was compared with feeding and composting system, which are representative food waste treatment technologies that have been used for a long time. Bio-gasification not only integrates the treatment of organic waste such as sewage, food waste, and excrement but also uses the produced biogas as a heat source for facilities, so it was excluded from comparison due to different standards for energy usage. Table 3 shows the operating conditions of the technologies being compared. For simplicity, dry feeding is indicated as (S1), composting as (S2), and zero-food-waste system as (S3).

Category	Basic operating conditions
Persons	174 × 2.41 person/household [22]
Emission unit (apartment)	209.72 g/capita-day [23]
Number of times food waste is discharged (S1, S2)	2.02 times/week [24]
Energy consumption per elevator use (S1, S2)	30 Wh [25]
Amount of kitchen sewage generated (S3)	30 L/capita-day [26]
Water for device (S3)	5 L/capita-day [26]
Power consumption (S3)	15.89 kWh/day (measured value)

Table 3.

System operating conditions.

Table 4 shows the pathway of food waste for this comparative analysis research. Food waste has high moisture and biodegradable contents. Therefore, it is easily perishable at room temperature, and a large amount of leachate is produced physically and biologically during every stage of pathways. The leachate is difficult to treat at every stage and is sent to a nearby sewer or a separate treatment facility, but most of it is sent to a sewage treatment plant. The impact on sewage caused by leachate is the same in all cases, so it can be ignored in comparison. Here, the value of recycled products by the food waste recycling process cannot be judged normal due to chronic problems in the recycling market [14], so it was not included in the calculation.

System	Step
Dry feeding; (S1)	Discharge (by elevators) → Collection → Transportation → Dry feeding; ↘leachate ↘leachate ↘leachate ↘leachate to sewer
Composting; (S2)	Discharge (by elevators) → Collection → Transportation → Composting; ↘leachate ↘leachate ↘leachate ↘leachate to sewer
Zero-food waste (S3)	Cutting and transfer device → Centralized automatic biomass recovery system; ↘Effluent to sewer

Table 4.

Food waste processing flows for this comparative analysis research.

When the total energy consumption required to process 1 ton of food waste in each process was converted into tons of oil equivalent (toe), as shown in Table 5. The dry feeding process (S1) showed the largest energy consumption; the next was the zero-food waste system (S3) of this study, and the last was the composting process (S2). Although the difference between these three processes was not so big, the zero-food waste system (S3) consumed 14.4% less than dry feeding (S1) but 26.7% more than composting (S2). But the dry feeding (S1) consumed 51.1% more energy than composting (S2). Although no meaningful calculations have been made based solely on energy usage by process, the zero-food waste system (S3) provided an approximately 570 times reduction of elevator uses compared to composting (S2) and dry feeding (S1) when processing 1 ton of food waste. That is, these results confirmed that the zero-food waste system (S3) of this study provide not only the biggest advantages of zero-foot waste emission through biomass recovery but also convenience and hygienic environment to residents.

Scenario	Sum	Step
		Separation	Collection-transportation	Disposal
S1	0.04733	0.00392	0.00331 [27]	0.04010 [28]
S2	0.03033			0.02310 [28]
S3	0.04138	0.04138

Table 5.

Energy use by scenario. (Unit: toe/t-FW).

4.4.5 Synergy effect on resident’s voluntary participation in sustainable behaviors of waste management

Northern European countries, such as Finland and Denmark, have high global happiness scores, according to the annual announcement by the United Nations [29]. Interestingly, sustainable behaviors regarding waste management were linked to sustainable happiness because happier people are more likely to engage in waste management and sustainable behaviors, and waste management and sustainable behaviors induce happiness in people [30].

In high-rise apartment buildings, where full-scale centralized automatic biomass recovery systems have been installed, a communal waste separating room has been operated for the voluntary separation of recyclables and special items in waste. It is located between two elevator entrances and connected to an underground communal parking lot. Figure 8 shows the residents’ communal waste separating room being operated cleanly and a centralized automatic biomass recovery system room (usually locked). In the communal separating room, various types of household waste, excluding food waste, were easily collected through the voluntary participation of residents so that the waste could be sorted by item for recycling or appropriate disposal. The contents shown in each photograph are as follows: (a) food waste facility room (usually locked), auxiliary container for volume-rate fee waste, waste home appliances, waste furniture, waste fluorescent lamps, and waste batteries; (b) waste cooking oil, emergency food waste, non-deposited glass bottles, deposited glass bottles, ceramics, disposable ice packs, handbags, shoes, carpets, curtains, blankets, and used clothes; (c) scrap metal, cans, vinyl, transparent plastics, Styrofoam; (d) cardboard, other plastics, and three types of papers; (e) an communal waste separating room between the underground elevators and parking lot (f) a communal waste separating room of next building and food waste facility room (open for inspection). This voluntary participation in waste separating behaviors for 24 items is possible because of no hand sorting for food waste included to residents. Ensuring zero-food waste emission through a centralized automatic biomass recovery system can have a synergic effect on sustainable behaviors, which is participation in waste separation practices in this case. Therefore, a sustainable high-rise apartment may be created by a higher resource circulation rate. An increase of voluntary participation in sustainable behaviors may be linked to sustainable happiness and sustainable high-rise apartments and cities further.

Figure 8.

Photos of residents’ communal separation room (24 items) for recyclable and special items and food waste treatment facility room since July 2021.

5. Conclusions

The supply of apartments and high-rise buildings is increasing dynamically, given economic development and social change in Korea. An integrated system appropriate for high-rise apartments was developed to eliminate insanitation and inconvenience during the separation of food waste and to achieve zero-food waste emissions by recovering biomass as a ready-to-use resource. This system consisted of two parts: a cutting and transfer device of food waste under the sink of each household and a centralized automatic module in the basement. The cutting and transfer device has the shape of a typical disposer but different roles and functions owing to the special design. The centralized automatic module is a system in which three processes, namely solid-liquid separation, dehydration, and bio-drying, are combined with ergonomically and eco-friendly designed MBT-ICT. Therefore, a convenient and sanitary environment is created inside the apartment building. The effluent was in the typical range of medium sanitary wastewater concentrations, which could not cause troubles in a sewer system. And the recovered biomass had good quality and low moisture content, which could be recycled directly and long stored without further treatment. All the processes in the system are operated automatically, interlocked, and connected to the cell phones of remote and control center managers. This technology was developed for high-rise apartments by improving the results of a pilot study supported by the Seoul Metropolitan Government and redesigned for sustainable waste management. A full-scale system has been installed in two high-rise apartment complexes including 1451 homes in nine buildings with 35 floors without troubling residents for over 3 years. A technical solution approach to sustainable waste management for high-rise apartments was proposed by implementing a zero-food-waste system in this paper. In addition, synergistic effects on residents’ participation in sustainable behaviors in waste management are expected with this zero-food waste system.