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Article

Research on a Novel Terminal Water Supply System Based on the Diversion Process

by
Wanghu Sun
1,*,
Yuning Sun
2,*,
Yuan Zhang
1 and
Xiaochun Hong
1
1
College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China
2
Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230027, China
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(24), 16744; https://doi.org/10.3390/su152416744
Submission received: 1 November 2023 / Revised: 3 December 2023 / Accepted: 9 December 2023 / Published: 12 December 2023
(This article belongs to the Section Sustainable Water Management)
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Abstract

:
The pipeline direct drinking water system (PDDWS) has emerged to ensure the quality of direct drinking water. Nevertheless, the existing literature suggests that the PDDWS suffers from inherent structural technological deficiencies, and various internal and external factors hinder its reliability, which raise concerns about the scientific and rational basis of the PDDWS. To address these issues, a new-type terminal differentiated water system (TDWS) is proposed to establish an efficient and health-oriented household water supply system. A water purifier is directly installed at the user’s terminal, and, due to its diversion process, differentiated water is provided, including direct drinking water, clean water and flushing water. Direct drinking water can be produced on demand, without secondary contamination. Clean water is also not stored, thus preventing microbial growth and ensuring superior water quality compared to tap water, suitable for kitchen usage. Flushing water is mixed with tap water for laundry, bathing and toilet flushing. Engineering verification has demonstrated that the quality of the direct drinking water and the clean water exceeds national standards. With the diversion process, the TDWS exhibits benefits related to health, the economy, applicability and environmental friendliness, and it can serve as a supplement and innovation for the PDDWS.

1. Introduction

Water scarcity poses a global and paramount challenge, with particularly heightened prominence in China [1,2]. China is one of the countries with the lowest per capita water resources. Approximately half of the over 600 cities are facing water shortages. Moreover, pollution exacerbates the water quality deterioration, affecting industrial production and residents’ lives [3,4]. Due to issues like water source and supply system pollution [5,6,7], research has shown that only 11% of the drinking water in China meets the requirements specified by the Chinese Standard for Drinking Water Quality (GB 5749-2022) [8,9,10]. As affirmed in the Universal Declaration of Human Rights, the right to an adequate standard of living includes ensuring water safety and quality. In order to meet the demand for high-quality drinking water, the pipeline direct drinking water system (PDDWS) has emerged as an alternative water source [7]. The PDDWS takes tap water and other suitable drinking water sources as raw water, conducts treatment through technical and technological means and transports direct drinking water separately to achieve water quality and diversion goals. The adoption rate of PDDWS is relatively high in the United States, with a penetration rate of 60%, as well as in Europe with 56% and Japan with 38% [11]. However, in most Chinese cities, the adoption rate of PDDWS is less than 1%. The PDDWS aims to achieve a differentiated water supply by separating direct drinking water from non-drinking water to achieve optimal water use.
Currently, there are three main approaches for its implementation. The first approach involves urban water treatment plants, which produce direct drinking water and non-potable water, and the non-potable water supply is provided with dedicated pipelines for industrial use and firefighting. Examples of this approach include the dual-pipeline binary water supply system in the United States, the three-tiered water supply systems in Japan and the two sets of water supply systems in France. The second approach is a community-based differentiated water supply, which utilizes municipal tap water as the source and establishes purification stations within residential areas to convert tap water into direct drinking water, and then the purified water is distributed to households through dedicated pipelines. Currently, this approach is the main method used for the differentiated water supply in China. The third approach is a citywide direct drinking water supply, which involves the production of high-quality drinking water by the city’s water treatment plants and the construction of an urban water distribution network. However, this approach is neither cost-effective nor necessary, and it leads to huge water wastage.
The primary objective of establishing a PDDWS is to provide on-demand water. After comprehensive treatment and transportation through dedicated high-quality water pipelines, direct drinking water is the final water type available for instant consumption [12]. To ensure that the water is suitable for direct consumption, three key requirements must be fulfilled. Firstly, to prevent a prolonged water residence time in the pipeline network and minimize secondary contamination, a pipeline network should be installed to circulate water back to the source tank or purified water tank [13], as depicted in Figure 1. Secondly, to avoid potential secondary contamination from pipe materials and equipment, the materials used should meet certain requirements, such as being stainless, non-corrosive, insoluble in water and resistant to high pressure [14,15]. Lastly, the output water from the purification equipment should meet the relevant standards.
In China, significant progress in the development of PDDWS has been achieved in recent years. For instance, Baotou City has already achieved full coverage of PDDWS. However, the operational outcomes of numerous PDDWS have consistently demonstrated inadequate compliance with water quality standards [16]. Baotou City stands out due to its early and extensive construction of PDDWS and standardized management, and its geographical location falls within a frigid climatic region. During 2016 to 2020, the overall compliance rate for PDDWS water samples in Baotou City was 96.61%, and the highest non-compliance rate was found in the total bacterial count [17]. In regions with warmer climates, such as Southern Chinese areas, the climate is more conducive to the proliferation of most bacteria, further exacerbating the deterioration of the direct drinking water quality. For instance, in Guangzhou, 17.15% of 1959 collected water samples exhibited results that exceeded the limits, and the exceedance rates for oxygen consumption, turbidity, manganese, iron and total bacterial count reached 7.66%, 6.89%, 4.29%, 1.84% and 17.15%, respectively [18].
From the above literature, it can be seen that even with the complex PDDWS structure, the quality of direct drinking water is still inadequate and lacks compliance with water quality standards [19,20]. Hence, this study aims to reveal the theoretical and technological deficiencies related to PDDWS comprehensively and proposes a more scientifically rational differentiated water system, which is a terminal differentiated water system (TDWS). Its innovative structure is designed to achieve the water diversion process, which yields three types of water, namely direct drinking water, clean water and flushing water, to fully meet various residential water requirements. Moreover, due to its structure, the TDWS employs filter automatic flushing, and it prevents secondary pollution caused by prolonged filter usage. The TDWS compensates for the technical deficiencies of the PDDWS and achieves the dual objectives of conserving water resources and ensuring water quality, establishing an efficient and health-oriented household water supply system. Therefore, due to its notable benefits, including energy efficiency, health benefits and environmental friendliness, it holds the potential to serve as a complement and alternative to PDDWS.

2. Materials and Methods

In this section, the scientific optimization of the PDDWS is presented, and then the water quality and water quantity in regard to household water consumption are firstly analyzed for the structural design of the TDWS. Therefore, a TDWS with technological innovation is proposed.

2.1. The Scientific Optimization of PDDWS

Based on the aforementioned analysis, it can be seen that the structure of the PDDWS is complex, resulting in unstable system performance. Therefore, this study aims to optimize this system in the following aspects.
Firstly, the irrational structure of the pipeline network is an internal factor leading to the above issues in water quality. Moreover, the pipeline network, particularly pressurization and depressurization devices in high-rise buildings, is prone to causing secondary contamination. Thus, the optimal solution is to eliminate the pipeline network and devices and directly install water purification equipment at the user’s terminal.
Secondly, directly installing water purification equipment at the user’s terminal is also the most effective approach to reduce the residence time of direct drinking water. This on-demand production prevents secondary contamination.
Thirdly, due to the low water flow rate at the user’s terminal, water can effectively pass the filter. Additionally, considering the extremely small pore size, the accurate detection of filter membrane damage is difficult and its breakage is unavoidable. Therefore, by decentralizing membrane filtration to the user’s terminal, the impact can be greatly controlled. With a damaged filter membrane in the PDDWS of a residential area, the entire community’s users will be affected. However, in a TDWS, filtration is decentralized to the user’s terminal, and the damage to a specific filter membrane will only impact the relevant user and not affect other households. Therefore, the TDWS is proposed to optimize the PDDWS to enhance the assurance of drinking water safety and reduce structural instability.

2.2. Analyzing the Water Quality and Water Quantity in Household Water Consumption

In terms of water quantity, based on the Chinese standard for Urban Residents’ Domestic Water Consumption (GB/T50331-2002) [21], the typical household daily water consumption is as follows: 40 L/d (day) for toilet flushing, 40 L/d for bathing, 9 L/d for laundry, 30 L/d for kitchen use, 3 L/d for drinking, 8 L/d for watering plants and 8 L/d for sanitation, totaling 138 L/d. Surveys have also revealed that the designed direct drinking water consumption is 5 L/d in Shenzhen and 3 L/d in Shanghai.
Therefore, in terms of water quality, domestic water for different usages can be divided into three categories: direct drinking water, clean water and flushing water. Direct drinking water encompasses water used for drinking, cooking, brushing teeth, etc., with the highest requirements for health and taste. Its per capita demand is approximately 10 L/d, representing 7% of the daily water consumption. Clean water has slightly higher microbiological indicators than direct drinking water, and it is needed at 26 L/d per capita, comprising 19% of the daily water consumption. Flushing water is employed in toilets, washing machines, showers, etc., with a per capita requirement of 102 L/d, representing 74% of the daily water consumption. The water quality specifications for flushing water can be fulfilled by municipal tap water. The proportions of direct drinking water, clean water and flushing water consumption are illustrated in Figure 2.

2.3. The Water Purification Process of TDWS

Based on the aforementioned analysis of water quality, the diversion process of the TDWS is designed to fully meet various residential water requirements. Compared to the conventional PDDWS (as depicted in Figure 1), the TDWS eliminates multiple components, including regulating water tanks, centralized water purification equipment, purified water tanks, pressurization and depressurization devices, supply and return pipeline networks, disinfection equipment and equipment rooms. It retains only the tap water supply pipeline network, and a water purifier is directly installed at the user’s terminal, as depicted in Figure 3.
As tap water enters through the inlet of the TDWS, it undergoes different purification paths, yielding three types of water: direct drinking water, clean water and flushing water. These water types vary in terms of quality and pressure. Direct drinking water exhibits the highest quality but a lower flow rate, rendering it suitable for drinking. Moreover, direct drinking water can be produced on demand for oral contact, thus preventing microbial growth. Clean water has good quality and moderate pressure, making it ideal for vegetable washing and cooking in the kitchen. It is also not stored and has superior water quality compared to tap water, suitable for kitchen usage. Flushing water has high pressure and a high flow rate and its quality is similar to that of tap water, meeting the needs of washing machines and toilets and suitable for bathroom activities, flushing and laundry.

3. Results

3.1. The Water Purification Capability of TDWS

It can be seen that the structure and diversion process of the TDWS is appropriate for residents’ scientific water use. The on-demand supply ensures that the water quality, water pressure and water volume are in line with the various water needs of households, and it achieves optimal water use. The third-party testing results of the water quality are listed in Table 1, and the water purifier underwent sanitary safety tests for 14 parameters: chromaticity, turbidity, odor, visible particles, aerobic bacteria, lead, cadmium, mercury, chromium, arsenic, volatile phenols, total bacterial count, total coliform bacteria and heat-resistant coliform bacteria. All 14 parameters complied with the Sanitary Standard for Hygienic Safety and Function Evaluation on Treatment Devices of Drinking Water in China, confirming the water purification capability of the TDWS.

3.2. The Innovative Structure and Technologies of TDWS

As the core component of the TDWS, the appearance and structure of the water purifier are shown in Figure 4, and its structure and functional parameters can be seen in Table 2. It can be seen that one inlet and three outlets are designed to achieve the diversion process. The TDWS integrates multiple filtration treatment components, including a polypropylene (PP) cotton filter, ceramic filter, activated carbon filter and ultrafiltration/nanofiltration membrane filter. The PP cotton filter serves as the primary filter and intercepts large-sized pollutants such as sediment and colloids. The ceramic filter serves as an intermediate filter and captures smaller-sized contaminants, like rust and colloids. The activated carbon filter adsorbs particles, like pigments, odors and colloids. The ultrafiltration/nanofiltration membrane filter intercepts contaminants with micro-sized particles, including large molecular clusters and viruses. The direct drinking water passes through the PP cotton filter, ceramic filter, activated carbon filter and ultrafiltration/nanofiltration membrane filter. Consequently, it attains the highest water quality and is suitable for direct consumption. The clean water passes through the PP cotton filter, ceramic filter and activated carbon filter, and its water quality is suitable for kitchen usage. The flushing water is blocked outside the PP cotton filter, and it has almost the same quality as tap water, being entirely safe for bathroom activities, flushing and laundry.
Moreover, due to its innovative structure, the system employs filter automatic flushing and prevents secondary contamination caused by prolonged filter usage. As depicted in Figure 4, within the sedimentation chamber, due to the low flow rate, sediment carried by tap water settles, and large-sized contaminants are trapped outside the PP cotton filter. When the flushing water is discharged, it removes these pollutants and cleans the filter, extending the filter’s lifespan. Similarly, large-sized contaminants trapped outside the ultrafiltration/nanofiltration membrane are expelled along with the clean water, which avoids the accumulation of pollutants in membrane pores and membrane fouling, which can lead to an increase in water output. By experimental testing, it has been shown that the membrane lifespan can be extended by three to four times. Additionally, this device is compatible with both ultrafiltration and nanofiltration, meeting diverse user requirements while minimizing redundant device investments. Several related Chinese patents can be seen in the Patents section.

3.3. The Applications of TDWS

A demonstration project of a TDWS has been implemented in the Xianzhuang Community in Anhui Province in China. During the project’s initial stage, publicity and necessary technical support were provided. During the construction phase, the tap water supply pipes were only installed in the kitchen, where valves for tap water, direct drinking water and flushing water were placed. The valve for the direct drinking water was connected to the bathroom sink, while the valve for the flushing water was connected to water-consuming devices such as toilets, washing machines and mop sinks. At the decoration stage, a water purifier was installed inside the kitchen cabinets. As depicted in Figure 5, the inlet of this purifier, the outlet for the flushing water and the outlet for the clean water were connected, respectively, to the tap water valve, the flushing water valve and the kitchen sink. The outlet for the direct drinking water was connected to both the direct drinking water valves in the kitchen/bathroom sink.
Moreover, the water purifier has obtained sanitary approval from Chinese government health authorities and received the Third Prize in the 2020 Science and Technology Award from the Jiangsu Provincial Science and Technology Association and the Third Prize in the 2022 Science and Technology Award from the China General Chamber of Commerce. It offers significant advantages in terms of water conservation and land savings, as well as improvements in health, economy, applicability and environmental friendliness.

4. Discussion

In this section, a comprehensive comparison among the TDWS, PDDWS, self-service water purifier and bottled water in terms of water quality, energy efficiency, environmental sustainability and economy is carried out to illustrate the technical sophistication of the TDWS. The inherent structural technological deficiencies, along with various internal and external factors that hinder PDDWS’ reliability, are thoroughly discussed and addressed by the TDWS.
As depicted in Figure 6, the comprehensive comparison analysis of the TDWS, PDDWS, bottled water and self-service water purifier is summarized in terms of water quality, energy efficiency, environmental sustainability and economy.

4.1. Water Quality Analysis

The complex structure of the PDDWS and the long-distance transport of water through pipelines lead to some issues regarding water quality. Among them, secondary contamination is the most prominent, which can be induced by the operation process, equipment materials and complex hydraulic conditions. The PDDWS is required to be independently installed and undergo regular circulation. It is stipulated that direct drinking water should not remain in the supply system for more than 12 h [22]. However, it is difficult to achieve the absolute and complete circulation of the direct drinking water within the whole pipeline. This limitation stems from the fact that the return water network is positioned before the water meter, while numerous shorter pipelines are located after the meter. These abundant peripheral pipelines are excluded from the circulation system, creating stagnant areas for microorganism proliferation, such as nitrifying bacteria and heterotrophic bacteria, which results in the storage of significant volumes of contaminated water [23]. Moreover, during the optimization design of the pipeline network, cost considerations may result in a reduction in network topology, favoring tree-like network configurations [13]. Thus, many peripheral and even branch pipelines may not display the effective circulation of the direct drinking water, leading to prolonged residence times and subsequent microorganism growth [24,25]. Therefore, the prompt discharge of peripheral water would be desirable to minimize contamination within the system. However, in China, there is a high vacancy rate in residential properties, rendering it impractical to expect the timely discharge of contaminated peripheral water. Moreover, the high cost of direct drinking water negatively affects both the adoption rate and usage, and it hinders the timely discharge of peripheral water. Research conducted on a PDDWS in Shanghai revealed that the actual usage percentage ranged from only 30% to 50% [26]. Additionally, direct drinking water often undergoes disinfection processes like ultraviolet (UV) and ozone treatment, without effective filtration measures. This allows the retention of substances such as proteins from the killed bacteria, which provides a material basis for microorganism growth [27].
Moreover, pressure boosters and reducers are installed to meet the water pressure requirements of PDDWS, which may result in secondary contamination. Regarding materials, whether equipment materials, pipeline materials or pipe fittings materials, the stability of their physical and chemical properties is not guaranteed. For example, the ubiquitous presence of chlorine ions [28] can lead to corrosion and then secondary contamination in the direct drinking water.
High-rise buildings’ water supply systems also result in complex hydraulic conditions [29]. The actual flow velocity in the pipeline network is influenced by numerous factors, leading to substantial deviations from the design flow velocity [25]. Regarding the water flow direction, a water contamination incident revealed blue water flowing from the tap due to the addition of a blue toilet detergent to the toilet tank. When the inlet valve of the toilet fails to close properly and experiences a drop in water pressure in the water supply pipeline, it results in siphon backflow, which is subsequently drawn into the water supply system [30]. These alterations in flow direction allow contaminants, such as bacteria present in peripheral pipelines, to be drawn into the primary distribution network. Additionally, it should be noted that secondary contamination also exists in self-service water purifiers. The contaminants outside the filter are difficult to discharge, which leads to a water quality decline and frequent filter replacement. Hence, it is challenging to completely eradicate secondary contamination within PDDWS and self-service water purifiers.
However, the TDWS achieves the on-demand allocation of high-quality water due to its diversion process, which has a significant impact in terms of addressing secondary contamination. As depicted in Figure 4, contaminants will settle in the sedimentation chamber. When the flushing water is discharged, it removes these pollutants and cleans the filter, thus preventing microorganisms’ growth within the contaminants inside the TDWS. The TDWS also addresses the issue of systemic safety risks (e.g., contamination in a PDDWS affecting an entire residential area), because the TDWS is installed at the users’ terminal.

4.2. Energy Efficiency Analysis

At present, the purification process used in PDDWS is a combination of pre-treatment, membrane treatment and post-treatment [31]. Pre-treatment primarily employs adsorption and filtration techniques to eliminate suspended solids, colloids and large particulate pollutants from water. It aims to meet the requirements of membrane filtration and reduce membrane fouling and contamination to prolong the lifespan of the membranes. Membrane treatment involves the utilization of ultrafiltration membranes, nanofiltration membranes or reverse osmosis membranes [32]. Post-treatment mainly employs disinfection and sterilization methods such as ozone, ultraviolet light, sodium hypochlorite and chlorine dioxide.
In pre-treatment, adsorption depends on the porous structure and high surface energy of the filter to adsorb lightweight pollutants. However, the adsorption capacity significantly diminishes when the adsorption sites become saturated, which requires the timely replacement of adsorbent media and presents operational challenges. Moreover, adsorbing pollutants at a high flow rate is difficult, and it requires the control of the water flow rate to reduce the kinetic energy of pollutants. In high-flow-rate water purification systems, the large volume and uneven internal pressure of the filter can result in variations in water molecule movement. In this case, passage through the filter without being adsorbed will lead to a decrease in adsorption efficiency, which is a common issue encountered in PDDWS. Additionally, filtration serves as an interception process to effectively capture larger-sized pollutants. A multi-layered system of coarse, medium and fine filters is typically employed. However, when the filter pores become clogged, the water output decreases significantly; thus, the timely replacement of filters is necessary.
In membrane treatment, the extensive surface area of filtration membranes for PDDWS poses challenges in maintaining their integrity [33,34,35,36]. Once a membrane is damaged, contaminants can bypass the filter and enter the circulation system. Moreover, it should be noted that in large-scale water treatment systems, the replacement and flushing of all filtration membranes is not convenient. The long-term neglect of replacement results in the accumulation of pollutants in membrane pores, leading to membrane fouling.
In post-treatment, the use of sodium hypochlorite and chlorine dioxide affects the taste of water and generates byproducts, and it will result in excessive chlorite levels beyond national standards [37]. The use of ozone and ultraviolet light can effectively kill bacteria but they are unable to eliminate residual substances.
In a TDWS, filter automatic flushing is employed to prevent residual contamination and extend the filter’s lifespan. Within the sedimentation chamber, due to the low flow rate, the filtered contaminants will settle. Once the valve of the flushing water is opened, the water pressure forces contaminants to discharge with the flushing water and cleans the filter, extending the filter’s lifespan. Similarly, large-sized contaminants accumulated outside the ultrafiltration/nanofiltration membrane are expelled along with the clean water, which avoids the accumulation of pollutants in membrane pores and membrane fouling, leading to an increase in water output. By experimental testing, it has been shown that the membrane lifespan can be extended by three to four times.

4.3. Environmental Sustainability Analysis

Currently, the drinking water production within PDDWS is associated with significant water wastage. This issue is especially common in residential areas, where water treatment stations use high-quality tap water as the source, and the generated wastewater is often discharged directly, leading to substantial tap water wastage. According to research [38], the water production rates for ultrafiltration membranes, nanofiltration membranes and reverse osmosis membranes are 75%, 65% and 55%, respectively.
Moreover, the PDDWS requires massive energy consumption for production and operation. The PDDWS involves extensive infrastructure development, where the energy consumption associated with production, operation and maintenance is difficult to measure. In contrast, due to the absence of centralized purification facilities and large-scale equipment, the TDWS eliminates the need for pipeline construction or modifications. Instead, the TDWS only requires the installation of a small-scale water purifier at the user’s terminal.
As for reverse osmosis water purifiers, a typical product of self-service water purifiers, the waste-to-product ratio of reverse osmosis water purifiers is up to 40%, and it results in significant water resource wastage and water expenses. In water-stressed cities like Beijing, over 80,000 tons of treated water are wasted daily. An article titled “Staggering Water Waste of Approximately One Billion Tons Annually in China”, published by the People’s Daily, warned of the treated water wastage by water purifiers. The diversion process of the TDWS achieves the on-demand allocation of water with suitable quality, pressure and quantities, with zero discharge of wastewater, which greatly conserves water resources. It can even operate without electricity, thereby significantly reducing the energy consumption. Additionally, due to regular transportation and distribution and huge solid generation, bottled water is neither cost-effective nor suitable for long-term consumption.
Thus, the TDWS emerges as a suitable water supply method to ensure water safety and health. The environmental friendliness of the TDWS is evident in aspects such as the equipment casings and internal filters. Firstly, the water purifier is compatible with both ultrafiltration and nanofiltration. It meets diverse user needs and reduces repeated investments in equipment, aligning with the “Reuse” principle of the 3Rs (Reduce, Reuse, Recycle) concept in the green circular economy. Secondly, the water purifier is constructed from sanitary-grade stainless steel, which provides good durability. The automatic flushing function reduces the frequency of filter replacement. It also minimizes solid waste generation, thereby promoting environmentally friendly practices and aligning with the “Reduce” principle of the 3Rs concept. Thirdly, the activated carbon filter features user-participatory technology solutions and facilitates filter replacement and regeneration, in line with the “Recycle” principle of the 3Rs concept.

4.4. Economic Analysis

The implementation of a PDDWS requires the establishment of dedicated pipeline network systems, with intricate pipeline installation and high initial investments, which makes it economically viable only under large-scale conditions [39]. Particularly in China, rapid urban development has necessitated extensive renovations of the existing road infrastructure to accommodate water pipelines and the retrofitting of massive existing buildings with water pipelines. This has resulted in immense construction and high engineering costs. In terms of urban PDDWS construction, a study was conducted on the PDDWS in Jinan and a novel model was proposed for the urban water supply. Assuming a population of 4.3 million, the projected investment was 1.365 billion yuan, which excluded the construction of internal and external building pipelines [40]. Another comprehensive analysis was conducted in China regarding the construction of a PDDWS in residential areas. It was concluded that promoting PDDWS in newly developed residential areas has potential to enhance the living quality of residents. The economic analysis indicated a total estimated investment of 7.7 million yuan, with an investment payback period of 6.88 years [41]. The construction of a PDDWS in newly developed residential areas requires substantial manpower and financial investments. The rationale and necessity of incorporating a PDDWS into existing urban areas are under debate. Hence, from both feasibility and long-term operational perspectives, providing PDDWS for residential kitchens is not an ideal solution.
As a crucial infrastructure project, the urban water supply is under government leadership to ensure water security. However, the PDDWS is typically managed by specialized companies motivated by profit, and high operation and maintenance costs are an objective reality in company operation. Currently, in China, the water fees for PDDWS range from 300 to 450 yuan per ton, which is lower than the price of bottled purified water at 600 to 800 yuan per ton.
Compared with the PDDWS, the TDWS only requires the installation of a small-scale water purifier at the user’s terminal, making the cost of using a TDWS extremely low. The longer service life of the equipment and filters and the lack of wastewater discharge also contribute to its benefits. According to calculations, the comprehensive cost of a TDWS is only 3% of that of a PDDWS and 27% of that of a self-service water purifier.
In conclusion, with higher expectations for water quality, bottled water, self-service water purifiers and pipeline direct drinking water have emerged as alternative water sources. Among these options, bottled water is commonly used in situations where water access is inconvenient or water consumption is low, but the usage of bottled water has gradually declined due to high costs and concerns about water quality and nutrition. Self-service water purifiers have become essential devices to improve water quality in Chinese households. Among them, the reverse osmosis water purifier exhibits the best water purification performance, but its water wastage has become increasingly prominent. As for the PDDWS, it complex structure induces some challenges in maintaining water quality. Furthermore, it involves high engineering costs and water fees. These intrinsic and extrinsic factors clearly demonstrate that the current PDDWS is not scientifically reasonable and call for technological innovation. Therefore, the TDWS fills a technological gap and promotes the development of water quality supply systems. The TDWS achieves the on-demand allocation of high-quality water and offers a healthy and convenient water usage method.

5. Conclusions

  • The inherent structural technological deficiencies in PDDWS contribute to unresolved issues of secondary contamination in water quality. External factors, such as poor management and non-standard construction practices, also contribute to secondary contamination in PDDWS.
  • The PDDWS incurs not only substantial construction and operational costs but also introduces additional charges for direct drinking water consumption. Currently, the majority of PDDWS in China fail to achieve optimal water utilization, leading to the wastage of water resources.
  • In comparison to the PDDWS, the TDWS proposed in this study exhibits a scientifically rational structure. The structural simplification saves equipment space and reduces construction and maintenance expenses. The TDWS has been validated through third-party assessments to exhibit reliable water quality and address secondary contamination. Moreover, wastewater discharge is eliminated, and the goals of maximizing water utilization and prioritizing high-quality usage and on-demand production are achieved.
  • The TDWS offers notable technological benefits, including improvements in health, economy, applicability and environmental friendliness. The TDWS establishes an efficient and health-oriented household water supply system. It addresses the technical deficiencies of PDDWS and facilitates improvements in water supply quality. As a result, it has the potential to serve as a complement and alternative to PDDWS.

6. Patents

  • A Stainless Steel Integral Water Divider, Chinese invention patent, ZL202021038581.5;
  • A Multi-Functional Grasping Device, Chinese invention patent, ZL202021038177.8;
  • A Stainless Steel Columnar Water Purifier Shell, Chinese invention patent, ZL202021049433.3;
  • An Automatic Edge Detection System, Chinese invention patent, ZL201821402590.0;
  • A Multi-Functional Water Purifier, Chinese invention patent, ZL201830703622.X;
  • A Multi-Function Controller (Compact-sized), Chinese invention patent, ZL201830704267.8;
  • A Water Purification Process and its Water Purification Device, Chinese invention patent, ZL201410376716.1;
  • A Water Purification Equipment, Chinese invention patent, ZL201420432677.8;
  • A Hydrogen and Oxygen Production System, Chinese invention patent, ZL201310326688.8.

Author Contributions

Conceptualization, methodology and funding acquisition, W.S.; writing and data analysis, W.S. and Y.S.; writing—review and editing, Y.S.; investigation and data acquisition, X.H. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant. number 51878588.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of PDDWS.
Figure 1. Schematic diagram of PDDWS.
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Figure 2. The proportions of direct drinking water, clean water and flushing water in residential daily consumption.
Figure 2. The proportions of direct drinking water, clean water and flushing water in residential daily consumption.
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Figure 3. The schematic diagram of TDWS.
Figure 3. The schematic diagram of TDWS.
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Figure 4. The appearance and structure of the water purification device.
Figure 4. The appearance and structure of the water purification device.
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Figure 5. Installation diagram of TDWS.
Figure 5. Installation diagram of TDWS.
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Figure 6. The current issues existing in PDDWS, bottled water and self-service water purifier.
Figure 6. The current issues existing in PDDWS, bottled water and self-service water purifier.
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Table 1. The third-party testing results of TDWS regarding water quality.
Table 1. The third-party testing results of TDWS regarding water quality.
ParametersUnitResultsHygiene Regulatory RequirementsDetermination
Sample 1Sample 2
ChromaticityCU≤5≤5≤5Compliant
TurbidityNTU0.170.18≤0.5Compliant
Odor/NoneNoneNoneCompliant
Visible particles/NoneNoneNoneCompliant
Aerobic bacteriamg/L0.370.39≤2Compliant
Leadmg/L<0.001<0.001≤0.001Compliant
Cadmiummg/L<0.0005<0.0005≤0.0005Compliant
Mercurymg/L<0.0001<0.0001≤0.0002Compliant
Chromiummg/L<0.004<0.004≤0.005Compliant
Arsenicmg/L<0.001<0.001≤0.001Compliant
Volatile phenolsmg/L<0.002<0.002≤0.002Compliant
Total bacterial countCFU/mLNoneNone≤100Compliant
Total coliform bacteriaMPN/100 mLNoneNoneNoneCompliant
Heat-resistant coliform bacteriaMPN/100 mLNoneNoneNoneCompliant
Table 2. The parameters of the water purifier in TDWS.
Table 2. The parameters of the water purifier in TDWS.
ParametersUnitValue
Heightmm420
Diametermm104
Flow ratesDirect drinking water L/min6
Clean waterL/min12
Flushing waterL/min20
Filter accuracyPP cotton filter μ m5
Ceramic filter μ m0.1
Activated carbon filter μ m0.5
Ultrafiltration membrane filter μ m0.01
Nanofiltration membrane filternm1
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Sun, W.; Sun, Y.; Zhang, Y.; Hong, X. Research on a Novel Terminal Water Supply System Based on the Diversion Process. Sustainability 2023, 15, 16744. https://doi.org/10.3390/su152416744

AMA Style

Sun W, Sun Y, Zhang Y, Hong X. Research on a Novel Terminal Water Supply System Based on the Diversion Process. Sustainability. 2023; 15(24):16744. https://doi.org/10.3390/su152416744

Chicago/Turabian Style

Sun, Wanghu, Yuning Sun, Yuan Zhang, and Xiaochun Hong. 2023. "Research on a Novel Terminal Water Supply System Based on the Diversion Process" Sustainability 15, no. 24: 16744. https://doi.org/10.3390/su152416744

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