Operation Status and Effective Operation Management Model for On-Site Swine Wastewater Treatment Facilities

: This study was conducted to examine the operation statuses of on-site swine wastewater treatment facilities through surveys and field surveys on pig farms and to propose effective operation models. Field analyses and surveys of pig farms indicate that technical and financial support systems are required for both farms and on-site swine wastewater treatment facilities. Public environmental services have been proposed as an effective support system, in which private sewage treatment facilities are entrusted to professional environmental management agencies, and the costs are shared by local governments and facility owners. However, securing a stable budget represents a challenge to implementing public environmental services. Thus, this study proposes a collaborative manure management model composed of individual farms, professional environmental management agencies, and local governments to address the shortcomings of public environmental services. To effectively manage pig manure, the flexible application of discharge standards, diversification of pollutant reduction management techniques (e.g., feed management), and periodic review of permits must be considered. Additionally, the reinforcement of discharge standards for individual purification facilities, control of the livestock density and number, and impacts of livestock manure discharge on riverine ecosystems must be considered.


Introduction
Pigs represent a major livestock species reared worldwide.China produced 454.8 million head of swine and United States produced 74.1 million in 2021 [1].As the number of pigs reared for livestock increases, the generation of pig manure wastewater is also increasing [2,3].
The management of livestock manure, including pig manure, is a challenge not only in Korea but also in advanced foreign countries [1,[4][5][6][7][8].Furthermore, the method of handling and managing pig manure varies by country [9,10].In countries with advanced agricultural practices, such as the Netherlands and Belgium, pig manure is primarily sprayed on farmland to be used as a nutrient for crops [11,12].Pig manure is rich in nutrients such as nitrogen and phosphorus [13] and thus can be utilized as a nutrient source for crops [14] and a soil amendment [9,12].
In countries with ample farmland, pig manure can be utilized to enhance the growth and productivity of crops [14,15].From a resource cycling and recycling perspective, using pig manure as fertilizer represents the most effective method of managing this waste.However, sufficient farmland must be available for the application of pig manure [16].Moreover, the amount of nutrients needed by crops at different growth stages limits the amount of manure that can be applied during the year [17].
In other countries, such as Korea and Japan, pig manure is partially treated using manure treatment facilities [13,18].Specifically, in Korea, approximately 18.6% of the Water 2024, 16, 1794 2 of 20 total manure generated is treated by facilities operated by local governments and other public institutions [19].Additionally, approximately 8.7% of the total manure is treated by individual purification facilities operated directly by pig farms [19]; thus, a total of approximately 27.3% is treated in facilities.The rest is used as organic fertilizers such as liquid manure and compost.
When pig manure is discharged into nearby rivers after being treated in on-site swine wastewater treatment facilities and/or public treatment facilities, the stability of river water quality is maintained, because the manure is treated to meet effluent quality standards before discharge [13].However, if the wastewater from pigs is not appropriately treated in the facilities, then the quality of river water will be negatively affected.Additionally, if pig farms are overly concentrated in a specific area, then the quality of nearby rivers may also be negatively affected, which can lead to various environmental issues, such as eutrophication, aquatic habitat destruction, harmful algae growth, and drinking water source contamination [33][34][35].A study by Kim et al. found that 60.3 to 73.1% of the nitrogen content in rivers near livestock-concentrated farms is contributed by individual purification facilities [13].This means that the density of livestock farms has a significant impact on river water quality.
Facilities operated by local governments or public institutions tend to manage water quality more stably than individual facilities [15], because they have stable budgets for facility operations and are operated by qualified technical personnel.Moreover, public treatment facilities have stricter effluent standards [15] and compliance regulations than individual purification facilities.Moreover, individual purification facilities operated directly by farmers have limitations in terms of specialized knowledge and budgets for operation management.Additionally, the effluent standards for these facilities are higher than those for public treatment facilities [15,36], indicating that individual purification facilities have a larger impact on the quality of nearby river waters.Thus, effective management strategies for individual purification facilities are needed to manage pig manure and maintain the quality of nearby rivers.
To diagnose and improve the management issues that affect individual purification facilities, field investigations and studies on measures applied to manage other pollution sources are necessary.Such work can lead to the development of alternative approaches that can be applied to manage pig manure.
Therefore, we conducted a field survey on the operational statuses of individual purification facilities located in the Gwangcheon Stream watershed in Hongseong County, Chungcheongnam-do, Korea, where pig farms are concentrated and issues with the water quality and pollution of nearby rivers due to pig manure are of great concern.Surveys targeting farm owners on compliance with legal regulations and areas needing improvement were conducted to identify effective operational strategies for individual purification facilities.In addition, management techniques or systems for managing pig manure from integrated environmental management systems in the EU and the National Pollution Discharge Elimination System (NPDES) of the US were researched.Subsequently, management plans suitable for application to pig farms were proposed.
Public environmental systems applied to individually operated sewage treatment facilities in Korea were also investigated.Such systems lead to an increased water quality and provide stable facility management.Based on examples of public environmental systems, an effective management model for individual purification facilities was proposed.

Survey Target Area
We conducted a field survey on the operational statuses of individual purification facilities located in the Gwangcheoncheon watershed in Hongseong County, Chungcheongnamdo, South Korea, which is densely populated with pig farms (Figure 1).This area was designated as a priority management area (Category I, high risk) in a 2016 livestock manure survey [37].Approximately 26% (32 facilities) of the individual purification facilities in the Gwangcheoncheon watershed are located in this area.The density of individual purification facilities per unit area in this region is 0.22 facilities/km 2 , which is relatively high compared to that of other areas, thus raising concerns about the impact of these facilities on the quality of nearby rivers.
facilities in Korea were also investigated.Such systems lead to an increased water quality and provide stable facility management.Based on examples of public environmental systems, an effective management model for individual purification facilities was proposed.

Survey Target Area
We conducted a field survey on the operational statuses of individual purification facilities located in the Gwangcheoncheon watershed in Hongseong County, Chungcheongnam-do, South Korea, which is densely populated with pig farms (Figure 1).This area was designated as a priority management area (Category I, high risk) in a 2016 livestock manure survey [37].Approximately 26% (32 facilities) of the individual purification facilities in the Gwangcheoncheon watershed are located in this area.The density of individual purification facilities per unit area in this region is 0.22 facilities/km 2 , which is relatively high compared to that of other areas, thus raising concerns about the impact of these facilities on the quality of nearby rivers.

River Water Quality Monitoring
The monitoring points for river water quality were set up based on the spatial distribution of individual livestock manure treatment facilities, resulting in a total of three locations (GC1, GC2, and GC3).The densities of individual livestock manure treatment facilities in GC1, GC2, and GC3 were 0.00, 0.99, and 0.78 facilities/km 2 , respectively.Monitoring was performed once a month from June 2016 to January 2020, and the mon-

River Water Quality Monitoring
The monitoring points for river water quality were set up based on the spatial distribution of individual livestock manure treatment facilities, resulting in a total of three locations (GC1, GC2, and GC3).The densities of individual livestock manure treatment facilities in GC1, GC2, and GC3 were 0.00, 0.99, and 0.78 facilities/km 2 , respectively.Monitoring was performed once a month from June 2016 to January 2020, and the monitored parameters included the biochemical oxygen demand (BOD), suspended solids (SS), total nitrogen (TN), and total phosphorus (TP).

Survey and Analysis of Individual Treatment Facilities
To analyze the operational management statuses of individual livestock manure treatment facilities installed in pig farms, surveys were conducted on 74 farms in the Gwangcheon River basin (field survey: 38 farms, questionnaire survey: 74 farms; some farms were surveyed several times).The surveys assessed the compliance with relevant regulations stipulated in the "Livestock Manure Management and Utilization Act" regarding the continuous operation of individual livestock manure treatment facilities and derived points for improvement from the results.

Case Study Analysis for Establishing Management Strategies
To develop effective management strategies for individual livestock manure treatment facilities in pig farms, relevant domestic and international regulations and cases were investigated.In domestic cases, the delegated management system known as the local management system under the former Environmental Public Service Act as well as operational cases of the Integrated Environmental Management System (IEMS) implemented in Korea in 2017 were examined.For foreign countries, the IEMS of the EU, which implements various measures such as permit reviews and pollution reduction management techniques, and the NPDES of the US, which applies manure management based on livestock population and analyzes the impact on surrounding rivers, were also studied.

Field Survey Results for the Operation Statuses of On-Site Swine Wastewater Treatment Facilities
The field survey identified the following types of biological treatment processes applied in pig farms: Modified Ludzack-Ettinger (MLE), extended aeration (EA), postdenitrification (PD), and membrane bioreactor (MBR) processes (Figure 2).These treatment methods are the main wastewater treatment methods used for biological removal of nitrogen and phosphorus based on the traditional activated sludge process.The MLE process is a biological nitrogen removal method that is known for its high nitrogen removal efficiency.It combines anoxic and aerobic zones in a reaction tank where organic matter is removed and nitrified, and the nitrified nitrogen is then internally recycled from the aerobic to anoxic zone for denitrification and nitrogen removal [38].The EA process has a long hydraulic retention time (HRT), resulting in low sludge production and very high BOD removal efficiencies of 90-98%, and it is easy to operate [39].However, it presents high energy costs and requires large aeration tanks.The PD process removes nitrogen that was not denitrified in the denitrification process by using an external carbon source, such as methanol.EA is an activated sludge process with an HRT of approximately 18-36 h, which is longer compared to those of other biological treatment processes [39].The MBR process combines biological treatment with membrane technology for solid-liquid separation [40], thus providing a higher separation efficiency and better effluent quality than traditional activated sludge processes [40].Moreover, this method is also known for its excellent nitrification and denitrification performances [40].
The most problematic water quality parameters in pig farms are nitrogen and phosphorus.Among these, nitrogen removal poses particular challenges, leading to the predominant adoption of advanced treatment processes specifically designed for nitrogen removal, such as MLE and EA.In the surveyed area, the MLE and EA methods are the most commonly used due to their favorable nitrogen removal and BOD reduction capabilities.
Phosphorus can also be removed biologically.In the MLE process, phosphorusaccumulating organisms (PAOs) need to be cultivated under alternating anaerobic and oxic conditions [38].These PAOs uptake phosphorus under aerobic conditions.During sludge wasting, the excess sludge and sorbed phosphorus are removed simultaneously [41].Because the MLE process does not have a strict anaerobic zone, its ability to remove TP is not ideal [41].However, phosphorus is relatively easier to remove.If phosphorus is not sufficiently removed in the biological treatment process, coagulants can be used to adsorb phosphorus onto sludge for removal.Additionally, microbes also settle and are removed during coagulation precipitation, thus removing the phosphorus contained within the biomass.Typically, coagulants are added to the wastewater after it passes through the bioreactor, and the coagulated sludge is separated and removed in the secondary clarifier.and oxic conditions [38].These PAOs uptake phosphorus under aerobic conditions.During sludge wasting, the excess sludge and sorbed phosphorus are removed simultaneously [41].Because the MLE process does not have a strict anaerobic zone, its ability to remove TP is not ideal [41].However, phosphorus is relatively easier to remove.If phosphorus is not sufficiently removed in the biological treatment process, coagulants can be used to adsorb phosphorus onto sludge for removal.Additionally, microbes also settle and are removed during coagulation precipitation, thus removing the phosphorus contained within the biomass.Typically, coagulants are added to the wastewater after it passes through the bioreactor, and the coagulated sludge is separated and removed in the secondary clarifier.Among the four types, the MLE process exhibits a superior performance in terms of its organic matter, nitrogen, and phosphorus removal efficiency and water quality improvements compared with the EA and PD processes (Figure 3).However, the pollutant removal efficiency of the MBR process is not presented here, because almost none of the surveyed farms applied this method.
However, because treatment facilities are mostly operated by non-expert farmers, the measured values are variable.This indicates that water quality values can vary depending on the operational conditions of the treatment facility.
In Figure 3, the number (n) of the removal efficiency population surveyed for each method represents the sum of the total number of times the treatment efficiency of individual purification treatment facilities applied to the method during the study period.This means that certain facilities may have been inspected more than once.This is because we conducted more than two surveys to consider the influence of various factors such as seasonal treatment characteristics for each method.Among the four types, the MLE process exhibits a superior performance in terms of its organic matter, nitrogen, and phosphorus removal efficiency and water quality improvements compared with the EA and PD processes (Figure 3).However, the pollutant removal efficiency of the MBR process is not presented here, because almost none of the surveyed farms applied this method.Furthermore, for effective pollutant removal using the biological and chemical treatment processes of individual livestock manure treatment facilities, the commonly used MLE and EA methods require the proper implementation of important operational factors such as sludge return and internal circulation.However, according to the on-site survey results shown in Figure 4, 37% of facilities implementing the MLE process perform both sludge return and internal circulation, 11% perform only sludge return, 33% perform only internal circulation, and 19% do not perform either of them, indicating inadequate operational practices in 63% of the facilities.For the EA process, 77% of the facilities do not perform sludge return.Therefore, the appropriate implementation of sludge return and internal circulation is insufficient on the surveyed farms.However, because treatment facilities are mostly operated by non-expert farmers, the measured values are variable.This indicates that water quality values can vary depending on the operational conditions of the treatment facility.
In Figure 3, the number (n) of the removal efficiency population surveyed for each method represents the sum of the total number of times the treatment efficiency of individual purification treatment facilities applied to the method during the study period.This means that certain facilities may have been inspected more than once.This is because we conducted more than two surveys to consider the influence of various factors such as seasonal treatment characteristics for each method.
Furthermore, for effective pollutant removal using the biological and chemical treatment processes of individual livestock manure treatment facilities, the commonly used MLE and EA methods require the proper implementation of important operational factors such as sludge return and internal circulation.However, according to the on-site survey results shown in Figure 4, 37% of facilities implementing the MLE process perform both sludge return and internal circulation, 11% perform only sludge return, 33% perform only internal circulation, and 19% do not perform either of them, indicating inadequate operational practices in 63% of the facilities.For the EA process, 77% of the facilities do not perform sludge return.Therefore, the appropriate implementation of sludge return and internal circulation is insufficient on the surveyed farms.Furthermore, for effective pollutant removal using the biological and chemical treatment processes of individual livestock manure treatment facilities, the commonly used MLE and EA methods require the proper implementation of important operational factors such as sludge return and internal circulation.However, according to the on-site survey results shown in Figure 4, 37% of facilities implementing the MLE process perform both sludge return and internal circulation, 11% perform only sludge return, 33% perform only internal circulation, and 19% do not perform either of them, indicating inadequate operational practices in 63% of the facilities.For the EA process, 77% of the facilities do not perform sludge return.Therefore, the appropriate implementation of sludge return and internal circulation is insufficient on the surveyed farms.If key factors such as sludge return and internal circulation (IR) [42] are not properly maintained, then the effluent water quality will deteriorate.IR rates that are too high can lead to lower efficiency due to decreased phosphorous removal and increased operational costs from excessive pumping [43].On the other hand, IR flow that is too low limits the denitrification process, thus accumulating nitrites and nitrates in the settling tank, which is usually related to uncontrolled denitrification in the settler [42,44].This If key factors such as sludge return and internal circulation (IR) [42] are not properly maintained, then the effluent water quality will deteriorate.IR rates that are too high can lead to lower efficiency due to decreased phosphorous removal and increased operational costs from excessive pumping [43].On the other hand, IR flow that is too low limits the denitrification process, thus accumulating nitrites and nitrates in the settling tank, which is usually related to uncontrolled denitrification in the settler [42,44].This restriction leads to an increased anaerobic phase, which can lead to significant modifications in the microeukaryotic communities [44].
The on-site survey results for the MLE process in Figure 5 also indicate that when internal circulation and sludge return are not adequately maintained, the exceedance rates of effluent water quality standards increase significantly.However, maintaining appropriate sludge return and internal circulation rates is challenging even for skilled professionals who possess specialized knowledge.Therefore, to operate individual purification facilities properly and stabilize the treatment of pig manure before discharge, supportive measures for facility operation and management are necessary.
Through a questionnaire survey of farm owners and on-site surveys of individual purification facilities, it was investigated whether internal circulation and sludge return were properly maintained.Whether the discharge water quality standard was exceeded according to the proper internal circulation and sludge return was evaluated based on the results of the discharge water quality monitoring conducted by the farm itself and the discharge water quality test conducted by the authors.purification facilities, it was investigated whether internal circulation and sludge return were properly maintained.Whether the discharge water quality standard was exceeded according to the proper internal circulation and sludge return was evaluated based on the results of the discharge water quality monitoring conducted by the farm itself and the discharge water quality test conducted by the authors.

Survey Results on Compliance with Laws and Regulations
In Korea, the management and utilization of livestock manure is governed by the "Livestock Manure Management and Utilization Act."According to this law, livestock manure must be properly treated through the installation of purification or resource facilities [36,45].Depending on their size, these facilities are only registered once during their initial installation through permits or notifications.The operation of swine wastewater individual treatment facilities on farms must follow the installation and management criteria.
According to these regulations, farms must install auxiliary facilities to handle situations in which the amount of pig manure exceeds the designed inflow of the individual purification facility or when the facility is out of operation due to maintenance.Individual purification facilities are regulated to operate continuously to prevent the illegal discharge of untreated pig manure into rivers.For example, the facilities must be cleaned at least once a year to maintain optimal operational conditions and treatment efficiencies, operational records must be kept continuously to monitor the state of the facility, and these records should be stored for three years.Moreover, farms are required to test the quality of the treated water periodically, with licensed facilities required to test quarterly and reported facilities required to test at least once every six months.Detailed legal provisions are summarized in Table 1.

Survey Results on Compliance with Laws and Regulations
In Korea, the management and utilization of livestock manure is governed by the "Livestock Manure Management and Utilization Act".According to this law, livestock manure must be properly treated through the installation of purification or resource facilities [36,45].Depending on their size, these facilities are only registered once during their initial installation through permits or notifications.The operation of swine wastewater individual treatment facilities on farms must follow the installation and management criteria.
According to these regulations, farms must install auxiliary facilities to handle situations in which the amount of pig manure exceeds the designed inflow of the individual purification facility or when the facility is out of operation due to maintenance.Individual purification facilities are regulated to operate continuously to prevent the illegal discharge of untreated pig manure into rivers.For example, the facilities must be cleaned at least once a year to maintain optimal operational conditions and treatment efficiencies, operational records must be kept continuously to monitor the state of the facility, and these records should be stored for three years.Moreover, farms are required to test the quality of the treated water periodically, with licensed facilities required to test quarterly and reported facilities required to test at least once every six months.Detailed legal provisions are summarized in Table 1.
The survey results show that record keeping and compliance with testing intervals are well observed, with compliance rates of 91% and 93%, respectively.However, compliance rates associated with sufficient storage facilities, the periodic cleaning of reactors, and mandatory training are low.Most farms are small and cannot afford the significant costs required to secure sufficient land for storage facilities.Additionally, stopping continuously operated reactors for periodic cleaning is challenging considering the limited environmental management personnel on the farms.The survey found that most farms (more than 95%) have only one environmental manager.Attendance at required training is also low, with most farms unable to participate due to time constraints.
Although individual purification facilities are supposed to operate continuously, the actual continuous operation rate was found to be 81%.In addition, whether farms deliberately stop the operation of individual purification facilities or perform temporary Water 2024, 16, 1794 8 of 20 shutdowns due to specific reasons was not confirmed; however, since the operation of these facilities directly affects the quality of nearby river water, they must operate continuously.As the survey results indicate, individual purification facilities face several challenges compared to public treatment facilities, including a lack of operational personnel, a lack of expertise, absence of analytical equipment, and limited maintenance and emergency response capabilities.These challenges lead to non-compliance with regulations [18].
Legal effluent quality standards are also not always met.For example, public facilities exceeded BOD standards by 2.5% [18], while individual licensed purification facilities exceeded BOD standards by 35%, TN by 35%, SS by 8.8%, and TP by 3.8% [18].This is because despite applying sophisticated treatment methods, most individual purification facilities are managed and operated by farm owners who lack the necessary expertise to ensure compliance.

Survey Results on the Operational Capabilities of On-Site Swine Wastewater Treatment Facilities and Need for Management by Specialized Organizations
The results of the survey on the awareness of outsourcing management to specialized agencies and the understanding of key operational parameters for on-site swine wastewater treatment facilities are presented in Figure 6.In most agricultural households, the attitude towards outsourcing management of individual livestock manure treatment facilities to specialized agencies or public treatment of livestock manure is positive.This demonstrates the difficulty of operating and managing on-site swine wastewater treatment facilities and the burden felt by individual farm households.The priority item for outsourcing management is the compliance with effluent water quality, which is considered the most necessary field (60%).In addition, there is a demand for outsourcing management in operational areas such as operational diagnosis, maintenance methods, and specialized training (each at 13.3%).
The survey results regarding the awareness of key operational parameters for on-site swine wastewater treatment facilities highlight that these facilities are not aware that technical managers must possess the relevant knowledge at a satisfactory level.Particularly, the level of awareness regarding crucial aspects such as the MLSS concentration, sludge return, and internal circulation in biological advanced treatment facilities is considerably low.This ultimately hinders the stable operation of individual livestock manure treatment facilities, Water 2024, 16, 1794 9 of 20 leading to a deteriorated effluent water quality and significant impacts on surrounding rivers.Farm owners generally operate their facilities using the initial settings established at installation.However, operating factors should always be adjusted to optimal conditions considering the variable nature of incoming pig manure and the operational state of the treatment facility.However, over 99% of the surveyed farm owners do not have the expertise to operate under optimal conditions.Many are also unaware of the key management factors required to operate individual purification facilities, as shown in Figure 6.Therefore, most farm owners feel a significant burden in operating treatment facilities themselves.As indicated by the survey results in Figure 6, there is a significant need for a system that can support the operation of individual purification facilities.Most farm owners agree on entrusting the operation of these facilities to professional organizations.
Therefore, support measures are required for treatment facilities that are operated by technical managers lacking specialized knowledge.
tion, sludge return, and internal circulation in biological advanced treatment facilities considerably low.This ultimately hinders the stable operation of individual livesto manure treatment facilities, leading to a deteriorated effluent water quality and sign cant impacts on surrounding rivers.Farm owners generally operate their facilities usi the initial settings established at installation.However, operating factors should alwa be adjusted to optimal conditions considering the variable nature of incoming pig m nure and the operational state of the treatment facility.However, over 99% of the su veyed farm owners do not have the expertise to operate under optimal conditions.Ma are also unaware of the key management factors required to operate individual purifi tion facilities, as shown in Figure 6.Therefore, most farm owners feel a significant bu den in operating treatment facilities themselves.As indicated by the survey results Figure 6, there is a significant need for a system that can support the operation of in vidual purification facilities.Most farm owners agree on entrusting the operation these facilities to professional organizations.
Therefore, support measures are required for treatment facilities that are operat by technical managers lacking specialized knowledge.

Potential Environmental Management Systems for Effective Livestock Manure Management 3.4.1. Environmental Public Service Systems and Local Management System
Considering the lack of expertise among non-expert livestock farmers operating developed wastewater treatment facilities and the significant financial burden on farmers for facility improvement and maintenance, the option of implementing an environmental public service system (EPSS) to address these issues must be explored.Since 2006, an EPSS has been implemented by Gyeonggi Province to protect the largest water source in South Korea, the upstream area of the Pal-dang Lake, which has been deteriorating due to the proliferation of small-scale pollutant discharge facilities below the regulatory scale.The key features of the EPSS involve outsourcing the operations and management of small-scale individual livestock manure treatment facilities (5-50 tons/d) to specialized environmental management companies and receiving partial financial support provided by local governments for maintenance, management, and facility improvement costs [46].
In the past, operational issues related to individual livestock manure treatment facilities have been attributed to facility management by non-experts, such as owners, who lack management capabilities and a sense of responsibility, as well as to the lack of environmental awareness, financial burden on owners, absence of installation standards based on wastewater flow fluctuations, and difficulties in facility maintenance due to structural characteristics [47].Moreover, operating individual livestock manure treatment facilities with effluent water quality standards consistent with that of public sewage treatment facilities has gained interest.These problems are also observed in the operation of on-site swine wastewater treatment facilities in pig farming.However, since allocating public funds to pollution dischargers is contradictory to the "polluter pays" principle, the system has transitioned to regional management since 2016 to address such concerns [46].
As a result of the implementation of the EPSS, the violation rate of effluent water quality standards has significantly decreased from 52% to 4%, and the BOD concentration in effluent water has reduced from 35 mg/L to 6.3 mg/L (Figure 7) [46].Therefore, the effectiveness of the system has been demonstrated.However, owing to difficulties in securing funds, the project costs and number of supported facilities have steadily decreased, reaching only approximately one-tenth compared with the figures in 2015.
In the past, operational issues related to individual livestock manure treatment facilities have been attributed to facility management by non-experts, such as owners, who lack management capabilities and a sense of responsibility, as well as to the lack of environmental awareness, financial burden on owners, absence of installation standards based on wastewater flow fluctuations, and difficulties in facility maintenance due to structural characteristics [47].Moreover, operating individual livestock manure treatment facilities with effluent water quality standards consistent with that of public sewage treatment facilities has gained interest.These problems are also observed in the operation of on-site swine wastewater treatment facilities in pig farming.However, since allocating public funds to pollution dischargers is contradictory to the "polluter pays" principle, the system has transitioned to regional management since 2016 to address such concerns [46].
As a result of the implementation of the EPSS, the violation rate of effluent water quality standards has significantly decreased from 52% to 4%, and the BOD concentration in effluent water has reduced from 35 mg/L to 6.3 mg/L (Figure 7) [46].Therefore, the effectiveness of the system has been demonstrated.However, owing to difficulties in securing funds, the project costs and number of supported facilities have steadily decreased, reaching only approximately one-tenth compared with the figures in 2015.The local management system divides sensitive water quality areas into regions and assigns dedicated companies to conduct comprehensive inspections and facility diagnostics for individual sewage treatment facilities with a capacity of <50 tons/d.However, The local management system divides sensitive water quality areas into regions and assigns dedicated companies to conduct comprehensive inspections and facility diagnostics for individual sewage treatment facilities with a capacity of <50 tons/d.However, it does not impose obligations or responsibilities for management [46].The regional management system aims to supplement the lack of manpower in local governments by entrusting mapping and inspection tasks to specialized companies.
Key achievements of the regional management system include a 2.4-fold increase in the excellent grade of facility condition from 20% to 48% and a decrease in the number of facilities under intensive management from 28% to 7% (Figure 8).However, achievements in the water quality improvement have not been clearly reported.This suggests that while facility improvement and management are important, the EPSS, which focuses on the operation of sewage treatment facilities for water quality improvements in effluents, is considered more effective.The regional management system also faces challenges in securing stable funding [46].The main details of the EPSS and regional management system are summarized in Table 2.
ments in the water quality improvement have not been clearly reported.This suggests that while facility improvement and management are important, the EPSS, which focuses on the operation of sewage treatment facilities for water quality improvements in effluents, is considered more effective.The regional management system also faces challenges in securing stable funding [46].The main details of the EPSS and regional management system are summarized in Table 2.In conclusion, the background for the introduction of the environmental public operation system lies in the operational issues faced by individual livestock manure treatment facilities.These problems are also observed in individual livestock manure treatment facilities, thus highlighting the need for the involvement of specialized environmental management agencies for effective facility management.By delegating management to professional environmental management organizations, improvements in the water quality of the discharged effluent, stable facility operation, and alleviation of financial burdens on farms are expected.In conclusion, the background for the introduction of the environmental public operation system lies in the operational issues faced by individual livestock manure treatment facilities.These problems are also observed in individual livestock manure treatment facilities, thus highlighting the need for the involvement of specialized environmental management agencies for effective facility management.By delegating management to professional environmental management organizations, improvements in the water quality of the discharged effluent, stable facility operation, and alleviation of financial burdens on farms are expected.

•
Flexible application of discharge permit standards: Prior to the implementation of the IEMS, discharge permit standards in Korea were applied uniformly based on the type of pollution source.However, under the IEMS, even for similar pollution sources, the discharge permit standards are set differently depending on factors such as the level of pollutant emissions and quality of the surrounding environment.Currently, the effluent quality standards for individual livestock manure treatment facilities are applied uniformly across specific areas.However, considering factors such as the density of livestock facilities and results of discharge impact assessments, discharge permit standards must be flexibly applied.In other words, in areas with a high concentration of livestock facilities or where the impact from pollutant emissions is significant, the discharge permit standards must be strengthened to reduce the environmental impact on the surrounding area.
Research by Kim et al. [13] in the Cheongmicheon area, in which pig farming is concentrated, near the Gwangcheoncheon region predicted that reducing the total nitrogen discharge standard of individual purification facilities from the current 250 mg/L to 60 mg/L, which is the standard for public treatment facilities, could decrease the nitrogen levels in Cheongmicheon by approximately 50%.This research demonstrates that the flexible adjustment of discharge standards for individual purification facilities can prevent the deterioration of the water quality of nearby rivers when pig farming is concentrated in an area and causes water quality degradation.

•
Diversification of management techniques to reduce pollutant emissions: The primary approach to reducing pollutant emissions in livestock farms is to strengthen the discharge levels according to water quality standards.However, the IEMS for the pig and poultry sectors in the EU presents various optimal available techniques.This means that in addition to enhancing wastewater treatment in livestock manure facilities, measures for managing nitrogen and phosphorus from the production and supply stages of feed are also required.
To decrease nitrogen emissions from manure, various methods have been implemented, such as reducing the protein content in feed, differentiating feed according to pig growth stages, and using certified additives (enzymes, growth promoters, and microorganisms).These measures have resulted in a reduction in nutrient excretion by 3% [48] and a decrease in the nitrogen content in manure to 7.0-13.0kg N/head/y for fattening pigs (compared with 22.2 kg N/head/year in domestic fattening pigs) and 17.0-30.0kg N/head/y for sows [49].To decrease phosphorus emissions from manure, a number of measures have been implemented, such as differentiating feed according to pig growth stages, using phytase enzymes, and utilizing easily digestible inorganic feed phosphates.These measures have led to a decrease in the phosphorus content in manure to levels of 3.2-5.4kg P 2 O 5 /head/y for fattening pigs (compared with 7.3 kg P 2 O 5 /head/y in domestic fattening pigs) and 9.0-15.0kg P 2 O 5 /head/year for sows [49].Therefore, various methods for reducing pollutant emissions must be explored in all aspects of livestock farming and management in addition to pollution reduction through individual livestock manure treatment facilities.

•
Periodic permit reassessment: In the previous permit system, which was known as the permanent permit system, supplementing permit conditions that did not comply with current requirements was difficult even after the issuance of the permit.However, under the IEMS in Korea, permits are subject to reassessment every 5 years (or 5-8 years for environmentally well-managed facilities).The purpose of permit reassessments is to address whether existing permit conditions are appropriate at the present time or if adjustments are necessary and to examine the compliance with permit conditions and consider changes in the operating conditions of the facility.Additionally, permit reassessment serves as a means to review and manage the operations of permitted facilities, strengthening permit conditions for facilities with inadequate management.Moreover, when pig farming increases or the density of livestock increases, thus leading to the deterioration or non-improvement of the water quality in nearby rivers, it may be necessary to review and strengthen the discharge standards of individual purification facilities to the level of public treatment facilities, as suggested by Kim et al.'s research [13].
Therefore, the operation of the permit reassessment system for livestock farms above a certain scale can lead to the more effective management of individual livestock manure treatment facilities.

National Pollutant Discharge Elimination System
• Analysis of river impacts caused by livestock manure discharge: The IEMS was introduced in 2017 in Korea with the aim of centralizing permits [50].Currently, it covers approximately 1411 facilities across 20 industries, including the livestock sector [51].The system requires predictions of the impact of pollutant emissions from these facilities on the surrounding environment.If the impact on the surrounding environmental quality is significant, strict permit standards are set.
The impact of pollutant emissions from livestock manure treatment facilities on nearby rivers must be assessed through discharge impact analysis.Currently, the monitoring of river pollution levels mainly focuses on the water quality status and trends through the National Water Quality Monitoring Network and national/local R&D projects.Therefore, an impact analysis of livestock manure is mandatory when planning river environmental management.Depending on the impact, additional measures such as site restrictions and strengthened discharge standards should be implemented.
In the US NPDES, stricter limitations are imposed on pollutant discharge from concentrated animal feeding operations if it causes or is likely to cause an impairment to water bodies or if the expected discharge (manure or process wastewater) is likely to exceed water quality standards [52].In other words, by analyzing the impact of livestock manure discharge, the system aims to determine whether water quality standards are violated and more strictly manage pollutant emissions from livestock manure facilities.Therefore, a periodic analysis of the impact of livestock manure on rivers should be conducted in a preventive manner to ensure that the river water quality remains within the acceptable range.

•
Permitting system based on livestock population: The US NPDES differentiates and manages concentrated animal feeding operations based on factors such as the pig weight and number of animals being raised.However, in South Korea, the classification is based on the area of the livestock facility or size of the exercise yard, both of which are categorized as below the reporting threshold, at the reporting target, or at the permit target.However, because the key factors in livestock waste management, such as the amount and load of livestock waste, are directly related to the number of animals being raised, managing permits based on the livestock population rather than the facility area is more reasonable.As evident from the 2018 national pollution source survey data, the number of pigs in facilities below the reporting threshold (0-46,738) was not insignificant when compared with that at the reporting target (0-30,500) and permit target (0-40,134).Therefore, a permit management system based on the livestock population must be considered, because it provides a more rational approach to livestock waste management in South Korea.

•
Pig farming density management: Pig farming density can be categorized into cases where a large number of pigs are housed within a farm and cases where many pig farms are densely concentrated in a specific area.
In the US, the NPDES categorizes livestock facilities based on size into Animal Feeding Operations (AFOs) and Concentrated Animal Feeding Operations (CAFOs).CAFOs are classified into large, medium, and small scales, with large-scale CAFOs housing over 2500 pigs weighing more than 55 pounds, medium-scale CAFOs housing between 750 and 2499 pigs, and small-scale CAFOs housing fewer than 750 pigs.
In the US, livestock facilities above a certain size are required to obtain a permit under the Clean Water Act.CAFOs must comply with the NPDES and thus are required to submit information about the owner, location, size of the livestock facility, and available land area for use and provide a topographical map of the area and a nutrient management plan.This ensures that facilities with large pig populations, which can generate significant pollution from pig manure, are managed stringently by government agencies.
Moreover, Pyo et al. [53] also found that livestock manure discharged from concentrated livestock areas greatly affects the water quality of nearby rivers.Their results showed that the BOD load of Shinryeong Stream increased from 2798 kg/d before livestock manure discharge to 6262 kg/d after; thus, the organic load was more than doubled.Additionally, the discharged livestock manure contributed to 29.5% of the total phosphorus quality in Cheongtong Stream [53].
The pollution levels of nearby rivers were also investigated according to the density of individual livestock manure treatment facilities in pig farms.The BOD, SS, TN, and TP box plot results of rivers near the survey areas are shown in Figure 9. Based on the median values of the BOD concentration, as the density of individual wastewater livestock manure facilities increased, the concentration distributions of GC3 and GC2 were approximately 8 and 18 times higher than that of GC1, respectively.According to Figure 10, the density of individual livestock manure treatment facilities directly affects the pollution levels of surrounding rivers.Therefore, considering the objectives for nearby rivers, the establishment of excessive pig farms must be restricted to facilitate better water quality management.Thus, when setting water quality management or target levels for a river, the environmental capacity of the water body must The SS concentration distributions of GC2 and GC3 were approximately 2-3 times higher, the TN concentrations of GC3 and GC2 were approximately 3-4 times higher, and the TP concentrations of GC3 and GC2 were approximately 6-7 times higher than that of GC1.As evident from the water quality assessment results, the pollution levels of surrounding rivers increased significantly with the density of individual livestock manure treatment facilities, ranging from approximately 2 to 18 times depending on the water quality parameters.Therefore, to protect the water quality of nearby rivers and consider the local environmental capacity, managing the density of individual livestock manure treatment facilities is necessary.
According to Figure 10, the density of individual livestock manure treatment facilities directly affects the pollution levels of surrounding rivers.Therefore, considering the objectives for nearby rivers, the establishment of excessive pig farms must be restricted to facilitate better water quality management.Thus, when setting water quality management or target levels for a river, the environmental capacity of the water body must be considered.In the US and Canada, nutrient (e.g., nitrogen and phosphorus) discharge standards are based on the environmental capacity of the water body.According to Figure 10, the density of individual livestock manure treatment facilities directly affects the pollution levels of surrounding rivers.Therefore, considering the objectives for nearby rivers, the establishment of excessive pig farms must be restricted to facilitate better water quality management.Thus, when setting water quality management or target levels for a river, the environmental capacity of the water body must be considered.In the US and Canada, nutrient (e.g., nitrogen and phosphorus) discharge standards are based on the environmental capacity of the water body.
Many wastewater treatment plants in the central region of North America do not need to remove nutrients.However, for sensitive water bodies, the discharge limits for TN and TP can be less than 3 mg/L and 0.07 mg/L, respectively [54,55].Considering the current number of operational individual wastewater treatment facilities, regulatory authorities need to carefully review whether to approve additional permits.Many wastewater treatment plants in the central region of North America do not need to remove nutrients.However, for sensitive water bodies, the discharge limits for TN and TP can be less than 3 mg/L and 0.07 mg/L, respectively [54,55].Considering the current number of operational individual wastewater treatment facilities, regulatory authorities need to carefully review whether to approve additional permits.Based on the results of on-site surveys on swine wastewater management and a review of potential case studies for effective swine manure management, we propose the introduction of a livestock manure collaborative management model involving individual farms, specialized management companies, and local government authorities.This model incorporates the advantages of environmental public good systems and regional management systems while addressing their limitations.It draws inspiration from the IEMS in the EU and South Korea, as well as operational practices under the US NPDES program when necessary (Table 3) [52,56,57].One notable distinction from environmental public good systems and regional management systems is the adoption of a long-term installment repayment scheme rather than free support to address budget constraints and ensure compliance with the principle of "polluter pays".This approach aims to establish a livestock manure management fund to improve budget stability and prevent budgetary shortfalls.The IEMS incorporates a periodic renewal of permit conditions, considers the contribution of swine manure to the water quality of surrounding streams, and allows for the flexible application of discharge standards based on factors such as the density of livestock farming facilities (including individual treatment facilities).The NPDES cases include impact assessments of swine manure on adjacent streams and permit requirements based on the swine stocking density.

Expected Benefits of the EPSS
The livestock manure collaborative management model can resolve issues caused by the operation of individual wastewater treatment facilities by unskilled individuals and promote timely facility improvements through continuous facility diagnostics.Improvements in the treatment water quality can also be achieved, and the burden of facility upgrades and operating costs for individual farms can be reduced (Figure 11).Local governments can provide stable support to farms by funding and fostering specialized management companies in the region to serve as technical support channels.Such changes will enable financial and technical support for individual livestock manure treatment facilities.The specialized management companies are also expected to generate employment opportunities through the utilization of specialized knowledge (Figure 11).companies can lead to improvements in the wastewater discharge water quality, strengthen the discharge water quality standards, which are currently set higher than those for public treatment facilities, and reduce the impact on surrounding streams.Additionally, the system could be complemented with further improvements, such as the introduction of permit renewal, to realize the proactive management of livestock manure.

Conclusions
Based on the field and survey results for swine farms, the following issues must be considered for the effective management of on-site swine wastewater treatment facilities and the reduction of pollution caused by swine manure: (1) A technical and financial support system must be developed for swine farms to ensure the stable operation of on-site swine wastewater treatment facilities managed by non-experts, such as farm owners.
(2) The feasibility of introducing the EPSS, which has been verified to achieve stable water quality and effective results, must be determined by applying it to small-scale individual sewage treatment facilities.
(3) Various pollution reduction techniques and permitting systems must be introduced based on the livestock numbers to reduce pollution caused by pig manure.This approach aligns with the practices applied in the IEMS and NPDES for the management of livestock wastewater.Furthermore, employing entrusted management from specialized management companies can lead to improvements in the wastewater discharge water quality, strengthen the discharge water quality standards, which are currently set higher than those for public treatment facilities, and reduce the impact on surrounding streams.Additionally, the system could be complemented with further improvements, such as the introduction of permit renewal, to realize the proactive management of livestock manure.

Conclusions
Based on the field and survey results for swine farms, the following issues must be considered for the effective management of on-site swine wastewater treatment facilities and the reduction of pollution caused by swine manure: (1) A technical and financial support system must be developed for swine farms to ensure the stable operation of on-site swine wastewater treatment facilities managed by non-experts, such as farm owners.
(2) The feasibility of introducing the EPSS, which has been verified to achieve stable water quality and effective results, must be determined by applying it to small-scale individual sewage treatment facilities.
(3) Various pollution reduction techniques and permitting systems must be introduced based on the livestock numbers to reduce pollution caused by pig manure.This approach aligns with the practices applied in the IEMS and NPDES for the management of livestock wastewater.
(4) The feasibility of introducing a livestock manure collaborative management model involving individual farms, specialized environmental management agencies, and local authorities that considers the shortcomings of the EPSS and the livestock waste management policies of the IEMS and NPDES must be considered.
(5) The livestock manure collaborative management model must support the efficient operation of on-site swine wastewater treatment facilities but should also offer solutions to resolving budgetary constraints by establishing swine manure management funds and implementing long-term installment repayment schemes instead of providing free support.

Figure 1 .
Figure 1.Distribution and monitoring points of individual wastewater treatment facilities in the Gwangcheon livestock farming watershed.

Figure 1 .
Figure 1.Distribution and monitoring points of individual wastewater treatment facilities in the Gwangcheon livestock farming watershed.

Figure 3 .
Figure 3. Pollutant removal efficiencies using advanced biological treatment methods.

Figure 3 .
Figure 3. Pollutant removal efficiencies using advanced biological treatment methods.

Figure 3 .
Figure 3. Pollutant removal efficiencies using advanced biological treatment methods.

Figure 5 .
Figure 5. Exceedance rates of effluent water quality standards based on internal circulation and sludge return in the MLE process.

Figure 5 .
Figure 5. Exceedance rates of effluent water quality standards based on internal circulation and sludge return in the MLE process.

Figure 6 .
Figure 6.Support rates for outsourced management using specialized agencies (a) and awaren of key operational items for individual livestock manure treatment facilities (b).

Figure 6 .
Figure 6.Support rates for outsourced management using specialized agencies (a) and awareness of key operational items for individual livestock manure treatment facilities (b).

Figure 7 .
Figure 7. Trends in water quality standard violation rates and effluent BOD concentrations resulting from the implementation of the environmental public service system (EPSS) in Gyeonggi Province.

Figure 7 .
Figure 7. Trends in water quality standard violation rates and effluent BOD concentrations resulting from the implementation of the environmental public service system (EPSS) in Gyeonggi Province.

Figure 8 .
Figure 8. Annual trends of excellent and intensive management facilities under the implementation of the regional management system in Gyeonggi Province.

Figure 8 .
Figure 8. Annual trends of excellent and intensive management facilities under the implementation of the regional management system in Gyeonggi Province.

Water 2024 ,
16, 1794 15 of 21 water quality parameters.Therefore, to protect the water quality of nearby rivers and consider the local environmental capacity, managing the density of individual livestock manure treatment facilities is necessary.

Figure 10 .
Figure 10.Impact of individual livestock manure treatment facility density on BOD.

Figure 10 .
Figure 10.Impact of individual livestock manure treatment facility density on BOD.

3. 5 .
Proposal for the Introduction of a Livestock Manure Collaborative Management Model 3.5.1.Key Elements of the Livestock Manure Collaborative Management Model

Figure 11 .
Figure 11.Roles and expected outcomes of stakeholders in the livestock manure collaborative management model.

Figure 11 .
Figure 11.Roles and expected outcomes of stakeholders in the livestock manure collaborative management model.

Table 1 .
Compliance statuses of major legal regulations for individual livestock manure treatment facility operations.

Table 2 .
Key contents of the environmental public operation and regional management systems.

Table 2 .
Key contents of the environmental public operation and regional management systems.

Table 3 .
Key components of the livestock manure collaborative management model.