Study of Water Resource Allocation and Optimization Considering Reclaimed Water in a Typical Chinese City

Abstract

Reclaimed water is considered to be an important alternative to freshwater to solve the imbalance between the supply and demand of regional water resources; it is also recognized as an effective tool for alleviating ecological problems caused by insufficient water flow. Yiwu City is a typical area experiencing a water shortage in southeastern China because the regional water resources are limited. In this study, the multiple water resource allocations in Yiwu City are optimized, the complex coupling model of multiple water resource allocation is established, and both the economic and ecological effects of multiple water resource allocation in Yiwu City are simulated and analyzed. The simulation results of optimizing the multiple water resource allocations show an efficient way of reclaimed water utilization in this typical Chinese city. In order to ensure the future economic and social development of Yiwu City, it is necessary to introduce reclaimed water into different fields, such as residential water, industrial water, agricultural water, and environmental water. Reclaimed water has also proven to have a high capability for pollutant control and reduction, which is also important to the ecology and environmental protection.

1. Introduction

Recently, China has made great progress in the development and utilization of unconventional water sources, especially the recycling of tailwater from wastewater treatment plants [1,2,3]. The amount of unconventional water utilization in China has increased remarkably from 2000 to 2020, which plays an important role in alleviating the contradiction between water supply and demand, optimizing regional water resource allocation, and protecting the water environment [3,4,5,6,7]. However, the development and utilization of unconventional water is still critical in most Chinese cities due to the serious water shortage problem [5,8,9]. The proportion of reclaimed water utilization in the dual water supply system in China is less than 1.5%, which is relatively low compared to other countries, such as Japan, Israel, and Singapore [10,11,12]. In this study, the tailwaters of wastewater treatment plants are included in a complex system composed of water resource allocation, social and economic development, and the regional water environment. The optimal allocation of reclaimed water into different fields, such as residential water, industrial water, agricultural water, and environmental water are all considered in this study, while the nexus model is established by introducing both the ecological and social factors, as well as the financial factor. As an example, a multidimensional, synergetic, multisource allocation model is developed and utilized in a typical Chinese city.

Reclaimed water has been utilized in urban water resource allocation for decades; many countries have introduced reclaimed water in multisource water configuration systems, especially the United States, Japan, Israel, and Singapore. It is widely acknowledged that early in 1920, the government of Arizona (USA) developed urban wastewater recycling from the experimental research stage to the application stage [13,14,15,16]. In the 1960s, the United States built numbers of largescale wastewater treatment plants to treat and recycle domestic wastewater [17,18]. In 1992, the United States Environmental Protection Agency published guidance on dual water supply systems, including wastewater reuse, reclaimed water quality standards, dual water supplies, etc. [16,17,18,19]. The amount of wastewater recycling in California in 2000 reached 8.64 × 10⁸ m³. The City of Los Angeles plans to use 69% of the total wastewater by the end of 2050, which is approximately 42% of the total water demand of the city [18,20]. As a legal and effective alternative to freshwater, reclaimed water has become an important part of the urban water supply system in the United States [19,20,21,22].

The difference in population densities in different areas of Japan results in serious water shortage problems in some cities with high populations. Therefore, Japan started researching wastewater recycling and reclaimed water utilization in 1955 [21,22]. During the 1970s, two-thirds of the total water used in the Seto Inland Sea was reclaimed water. In the 1980s, approximately 6.3 × 10⁷ m³/d of Japan’s urban wastewater was used as a recycled resource. In 2020, Japan had built more than 2789 reclaimed water utilization facilities, which are mainly used for the treatment and reuse of urban wastewater and a dual water supply after treatment [21,22,23,24].

Israel also suffers from the problems of a water shortage since 70% of Israel’s mainland is desert. The extremely poor water resource in Israel has led to a distinctive reclaimed water usage system in the country [22,23,24,25,26]. During the 1960s, the management and utilization of reclaimed water in Israel became highly efficient. In most Israeli cities, the sewage discharge and water supply systems have been constructed completely; both engineering and non-engineering measures are utilized in order to save water resources [23,24,27]. In the late 1980s, there were 210 municipal wastewater recycling projects in Israel. Currently, almost 100% of domestic wastewater and 72% of municipal wastewater is treated and reused as a reclaimed water resource [27,28,29,30].

As a widely acknowledged alternative to freshwater to solve the imbalance problem between the supply and demand of regional water resources, reclaimed water is the key consideration of this study. A typical city experiencing a water shortage in China has been selected for this study. The allocation and optimization of water resources, taking into consideration reclaimed water, in this city has been executed and simulated in order to show the advantages of the newly derived multidimensional, synergetic, multisource allocation model. The social, economic, and environmental factors of water resource allocation and optimization are also investigated in detail.

2. Study Site

2.1. Characteristics of Yiwu City, China

Although the average annual precipitation of Yiwu City is 1418 mm, the high population density and economic factors of this city are still embarrassing, accounting for 1683 people per square kilometer and a total GDP of CNY 135 million, or USD 22.5 million, for the city. Hence, Yiwu City is widely known as a typical water shortage area in southeastern China; the local water resource per capita is only 20% of the Chinese national average. There are three types of water resources available in Yiwu City. First, Yiwu City has six medium-sized reservoirs and a project that diverts water from Dongyang City to Yiwu City. Together, the six medium-sized reservoirs have a total water storage capacity of 1.56 × 10⁸ m³. As a high-quality water resource, it is mainly used for the domestic urban water supply, agricultural irrigation, etc. Indeed, the regional utilization rate of the water resource in Yiwu City is more than 40%, which is relatively high, while the amount of water diverted from Dongyang City to Yiwu City is more than 0.9 × 10⁸ m³ per year, which is mostly used for the domestic urban water supply. Second, the average annual water resource of the Yiwu River and its tributaries is 1.27 × 10⁹ m³, which is relatively high compared to the water resource of the six medium-sized reservoirs. When considering the water environment, the utilization of reclaimed water in Yiwu City shows advantages in the reduction of COD (chemical oxygen demand) and NH₃-N (ammonia nitrogen) in the water of the Yiwu River, which is usually unsatisfactory due to the pollutants discharged from the urban area [1,3,10,29].

Figure 1 shows the dual water supplement and reclaimed water utilization in Yiwu City. The high-quality water resources in Yiwu City are lacking, and they are usually used for the drinking water system. As a good alternative, the tailwater of the wastewater treatment plant in Yiwu City is considered as another water resource, which equaled approximately 570,000 t/d at the end of 2018, while the water quality indicator of the tailwater is given as COD < 50 mg/L, NH₃-N < 1.0 mg/L. Due to the low water quality of the tailwater from the wastewater treatment plant, the present utilization rate of the tailwater is only 11.24%, which leads to a low level of reclaimed water usage in industry and the agricultural fields of Yiwu City. Although the water quality of the tailwater is unsatisfactory, an additional treatment method is developed and executed in this study in order to improve the water quality and ensure water supply security. Nowadays, reclaimed water is a stable and reliable water resource in Yiwu City, and it can be used for the industrial and environmental water supplies. In the future, the increase in the population of Yiwu City will result in a huge amount of wastewater production as well as reclaimed water resources. Thus, reclaimed water will become a convincing water resource in the future in Yiwu City.

2.2. Dual Water Supply System and Reclaimed Water Utilization in Yiwu City

Due to the fact that the water supply system and multiple water resources are complex in Yiwu City, the water–economy–social nexus model established in this study considers the dual water supply system, the economic effects, and the ecological benefits for Yiwu City.

First, the nexus model is set up by generalizing the water supply projects and wastewater treatment plants. Generally, the social water cycle is considered as a main function in the present model. Second, the relationship between water suppliers and water consumers is fully considered in the present model. Third, the utilization of reclaimed water and the dual water supply system are classified and generalized by considering water quality demand. Finally, according to the demand for water quality, the multiple sources of the water resources in Yiwu City are classified as high-quality water, general water, and reclaimed water [10,25,29,30,31].

Among them, “high-quality water resource” refers to the water storage of the six reservoirs and the water diverted from Dongyang City, outside the local region of Yiwu City, which is mainly allocated for the domestic residents and agriculture, while the general water source refers to the Yiwu River and its tributaries, which is mostly allocated to industrial water, agricultural water, and ecological water. The reclaimed water source refers to the tailwaters of nine wastewater treatment plants in Yiwu City, and it is mostly allocated to the industrial and ecological water systems, depending on the different reclaimed water qualities from different wastewater treatment plants. Figure 2 shows the dual water supply system and the utilization of reclaimed water in Yiwu City.

3. Materials and Methods

3.1. Basic Equations

Based on the above analysis of the internal coupling mechanism and multidimensional, dynamic, synergetic relationship of water resource, economy, ecology, and environment, the utilization of reclaimed water is integrated into the dual water supply system and is fully considered as an independent water resource for the consumers [25,26,28]. The multidimensional, synergetic, multisource allocation model was then derived in this study by combining the effects in the field for society, the economy, and ecology. The basic equations of the present model are given as follow [21,22,23,24,25,26].

The basic objective function includes both the expected benefits and penalties of the water supply capacity, which are differentiated to obtain expected, comprehensive benefits under different allocation scenarios of the multiple water resources in Yiwu City. The penalties include the penalty of the low water assurance rate and unqualified water quality in specific sections of the river. The objective function of the model is given as [31,32,33]:

$$\max F_{Q\mathrm{q}}={\displaystyle \sum _{i=1}^{NI}\left[Q_i^{\max }-Q_i\right]}\times B_i-{\displaystyle \sum _{i=1}^{NI}{\displaystyle \sum _{j=1}^{NJ}\left[Q{E}_{ij}\times C_{ij}+f_1\left(q_{ij}\right)\right]}}$$

(1)

where F_Qq is the expected comprehensive benefit of regional multisource water allocation (CNY or USD); Q_i^max is the maximum water supply capacity of the specific water source (m³); Q_i is the configured water supply volume of the specific water source (m³), B_i is the net benefit coefficient of the specific water source (CNY/m³ or USD/m³); QE_ij is the water shortage cause by the predicted water resource allocation (m³), C_ij is the penalty coefficient of the water shortage (CNY/m³ or USD/m³), Q_ij is the amount of water allocated to local water consumers (m³), and f₁(q_ij) is the penalty coefficient considering the environmental capacity of the local river network (CNY/m³ or USD/m³).

The water resource assurance rate is defined as [6,33]:

$${\rho }_j\ge {\rho }_j^{*}$$

(2)

while the environmental capacity of the local river network, which is also considered regarding the water quality at a specific section, is given as [6,9,28,29,30,31,32,33]:

$${\displaystyle \sum _{i=1}^{NI}{\displaystyle \sum _{j=1}^{NJ}{f}_1\left(q_{ij}\right)=\{\begin{array}{l}D\times \Delta W\begin{array}{cc},& W_{ij}>W_0\end{array}\\ 0\begin{array}{cc},& W_{ij}<W_0\end{array}\end{array}}}$$

(3)

$$W_{ij}=A\times {\gamma }_2+W{F}_{ij}\times {\gamma }_3$$

(4)

where ρ_j is the water supply assurance rate, D is the penalty coefficient for pollutant discharge beyond the capacity to accept pollutants (CNY/t or USD/t); ΔW is the discharged amount of pollutants beyond the environmental capacity (t), ΔW = W_ij − W₀, A is the rainwater collection area of the local river network (km²), WF_ij is the amount of wastewater discharge considering different consumers (t), γ₂ is the discharge coefficient of the non-point source (t/m³), and γ₃ is the discharge coefficient of the point source (t/m³).

The total amount of the local water resource, including reclaimed water, is also presented in the model, given as [33,34,35]:

$$Q_i\le Q_i^{\max }$$

(5)

Additionally, the constrained reservoir dispatching ability, the water supply capacity, and the non-negative constraint are also included in this model.

3.2. Simulation Parameters

According to the hydrological conditions in Yiwu City, 90% of the rainfall frequency is selected in this study for the model simulation and analysis. The simulated results for the environmental capacity of the Yiwu River in 2030 is 9427 T/a of COD and 283 T/a of NH₃-N.

Table 1. Emission of non-point and point sources in Yiwu City.

Indices	Non-Point Source		Point Source
Indicator	COD	NH3-N	COD	NH3-N
Unit	t/km2	t/km2	mg/L	mg/L
Total emission	5.199	0.295	16.57	0.28

shows the emission indicators of the point source and the non-point source.

Table 2. Benefits of different water resources in Yiwu City.

Water Resources	Benefits (CNY/m3), (USD/m3)
	High-Quality Water	General Water	Reclaimed Water
Value	2280, (380)	1140, (190)	570, (95)

and

Table 3. Punishment for different water resource users in Yiwu City.

Water Usage	Punishment (CNY/m3), (USD/m3)
	Residents	Agriculture	Industry	Environment	Others
Value	22,800, (3800)	11,400, (1900)	11,400, (1900)	5700, (950)	5700, (950)

show the value of the net benefit coefficient per cubic meter of water for various water resources and the value of the penalty coefficient for water shortages per cubic meter of water for different water consumers. It should be noted that the penalty for pollutants entering the river that exceed the environmental capacity is relatively high, namely CNY 22,800 /m³, or USD 3800/m³.

Table 4. Parameters of the nexus model.

Category	Indicator	Value
Reclaimed water in dual water supply system	Industrial water replace rate	57%
	Urban public water replace rate	40%
IPSO (immune particle swarm optimization algorithm)	Irrigation	100
	Examples N	100
	Inertia weight	Initial (1.2), Final (0.8)
	Learning factor	2
	New particles M	100
	Randomized particles	2
	Threshold $\mathsf{\xi }$	0.6

shows the parameters of the nexus model during the multiple water resources configuration.

4. Results and Discussion

4.1. Model Testing and Calibration

The model testing was executed by calculating the past water demand and forecasting the future water demand of Yiwu City.

Table 5. The quantity of water resources in Yiwu City considering different sources.

Month	Multisource Water Resources (108 m3)
	Six Reservoirs	Water Diversion Project	Yiwu River and Its Tributaries	Reclaimed Water
1	0.031	0.075	0.427	0.135
2	0.053	0.075	0.722	0.118
3	0.105	0.075	1.445	0.149
4	0.109	0.075	1.489	0.141
5	0.105	0.075	1.445	0.143
6	0.174	0.075	2.388	0.142
7	0.074	0.075	1.017	0.148
8	0.038	0.075	0.516	0.142
9	0.037	0.075	0.501	0.135
10	0.023	0.075	0.310	0.131
11	0.023	0.075	0.310	0.123
12	0.023	0.075	0.310	0.134
In total	0.793	0.900	10.88	1.641

shows the monthly distribution of water resources from all of the water sources, such as the six reservoirs, the water diversion project, the Yiwu River and its tributaries, and the reclaimed water from Yiwu City, with a consideration of 90% rainfall frequency.

Table 6. Past and future water demand in Yiwu City.

Month	Past Water Demand (2020) (108 m3/a)				Future Water Demand (2030) (108 m3/a)
	Irrigation	Industry	Residents	Environment	Irrigation	Industry	Residents	Environment
1	0.038	0.039	0.095	0.038	0.046	0.061	0.158	0.038
2	0.038	0.039	0.067	0	0.046	0.061	0.111	0
3	0.038	0.039	0.090	0	0.046	0.061	0.150	0
4	0.038	0.039	0.103	0	0.046	0.061	0.171	0
5	0.038	0.039	0.108	0	0.046	0.061	0.181	0
6	0.045	0.039	0.117	0	0.054	0.061	0.195	0
7	0.123	0.039	0.121	0	0.157	0.061	0.202	0
8	0.178	0.039	0.141	0.222	0.228	0.061	0.236	0.222
9	0.141	0.039	0.137	0.170	0.179	0.061	0.229	0.170
10	0.045	0.039	0.123	0.134	0.054	0.061	0.204	0.134
11	0.038	0.039	0.127	0.110	0.046	0.061	0.212	0.110
12	0.038	0.039	0.117	0.117	0.046	0.061	0.196	0.117
In total	0.797	0.466	1.346	0.791	0.991	0.734	2.246	0.791

shows the simulation results of water consumption for different fields of Yiwu City in both 2020 and 2030.

Figure 3 shows the results of annual water consumption and predicted water demand for both the past year of 2020 and the future year of 2030. The past data was collected from The Regional Statistical Yearbook and the prediction data was calculated by using the multidimensional, synergetic, multisource allocation model developed in this study.

4.2. Simulation Results

The immune particle swarm optimization algorithm (IPSO) is introduced in this study to solve the optimal allocation model of multiple water sources in Yiwu City based on the prediction for the water demand in 2030. However, as shown in

Table 7. Basic prediction of water resources considering reclaimed water in 2030 (10⁸ m³).

Category	Six Reservoirs	Water Diversion Project	Yiwu River	Reclaimed Water	In Total	Water Shortage
Residential	0.460	0.900	0	0.604	2.246	−0.282
Industrial	0	0	0	0.732	0.732	0
Irrigation	0	0	0.994	0	0.994	0
Environmental	0	0	0.210	0.536	0.746	−0.042
In total	0.460	0.900	1.205	1.872	4.974	−0.324
Water resources remaining	0	0	4.595	0.827

, the simulation results of the basic allocation scheme of multiple water sources, considering a 90% insurance rate, have been proven to be unacceptable and unachievable due to the relatively high amount of total water demand. Therefore, both management and control methods are required in order to reduce the water demand in 2030. Hence, the reduction of water consumption in Yiwu City is anticipated in the future; the updated allocation of the water resources in 2030, considering the limitation of the insurance rate and environmental capacity is shown in

Table 8. IPSO prediction of water resources considering reclaimed water in 2030 (10⁸ m³).

Category	Six Reservoirs	Water Diversion Project	Yiwu River	Reclaimed Water	In Total	Water Shortage
Residential	0.460	0.900	0	0.458	1.818	0
Industrial	0	0	0	0.732	0.732	0
Irrigation	0	0	0.994	0	0.994	0
Environmental	0	0	0.292	0.499	0.791	0
In total	0.460	0.900	1.286	1.689	4.335	0
Water resources remaining	0	0	5.024	0.620

, while the reclaimed water is also proven to be a necessary and precious resource for the water consumers in Yiwu City in 2030.

Figure 4 and Figure 5 show the optimal water resource allocation results for 2030. First, it should be noted that the restriction of the water supply in Yiwu City will be approximately 1.818 × 10⁸ m³/a by the end of 2030 to ensure the water supply guarantee rate. Second, the utilization of reclaimed water will become versatile in the future; thus, the requirement of reclaimed water used in ecological flow will be reduce from 1.005 × 10⁸ m³/a to 7.91 × 10⁷ m³/a; the discrepancy of the reclaimed water used in ecological flow between the past year of 2020 and the future year of 2030 also confirms that reclaimed water will be used in the industrial and agricultural fields and will become an important part of the dual water supply system.

4.3. Analysis and Discussion

It can be concluded from the simulation results of the present multidimensional, synergetic, multisource allocation model that reclaimed water is a reliable and important water source for Yiwu City as an alternative to freshwater and gray water in the industrial, agricultural, and urban water supply, especially in the future year of 2030.

The discrepancies in water resource allocations between traditional predictions and the newly derived model for the year of 2030 are shown in Table 6 and Table 7, while the classification of different water resource comparisons between the traditional and novel models are shown in Figure 4 and Figure 5. The results predicted by traditional method have been obtained from the Statistical Yearbook of Yiwu City and the Regional Water Source Planning Report of Yiwu City. The social, economic, and environmental factors of water resource allocation and optimization are also investigated in detail.

The traditional method predicts that, together, the residential and environmental water resource will not meet the requirements in the year 2030, with a water shortage of 0.282 × 10⁸ m³/a and 0.042 × 10⁸ m³/a, respectively, eventually leading to a total water shortage of 0.324 × 10⁸ m³/a in the year 2030 in Yiwu city, and it becomes embarrassing due to the fact that Yiwu city does not have enough freshwater resource to deal with the large amount of the water demand. However, when considering the reclaimed water as an effective alternative of the freshwater resource, with the new derived model in this study, the forecasting of the water allocation in all the fields of water consumers such as residents, industry, environment except irrigation. The new derived model indeed optimized the water resource in all the fields and results in a remaining water resource of 5.024 × 10⁸ m³/a of Yiwu River and 0.620 × 10⁸ m³/a of reclaimed water. Although the remaining water resource is slightly more than the prediction results from the traditional method, it is critical for Yiwu city to protect, sustain and preserve its society, economy and environment in the future.

Therefore, reclaimed water is proved an effective way to ensure the future economic and social development of Yiwu City. The incorporate of the reclaimed water into the allocation of multiple water resources is proved important and effective. The water supply capacity of the existing six reservoirs, which are recognized as supplying high-quality water, with a water supply capacity of 1.35 × 10⁸ m³/a, while the water demand of Yiwu City was 1.346 × 10⁸ m³/a in 2020; hence, a balance is achieved between the water demand and supply. However, the prediction result of high-quality water is only 2.246 × 10⁸ m³/a in 2030, but the total demand of water resources in Yiwu City will reach 5.061 × 10⁸ m³/a by the end of 2030, which is almost 2.25 times of the high-quality freshwater supply capacity. Due to the fact that Yiwu City is lacking water resources, the most effective way to improve the water supply insurance rate of this city is to incorporate reclaimed water into the multiple water resources allocation system.

Moreover, it is also indicated from the simulation results that if the reclaimed water is not included in the multiple water resources allocation system, the quantity of point source pollutants generated by water consumers in 2030 of Yiwu City will increase by 60% compared to the past year of 2020, leading to a serious water environment problem. When considering the reclaimed water utilization in Yiwu City, the amount of major pollutant discharges will be reduced by 50% compared to the past year of 2020, and the impact on the water environment will also be relieved. Therefore, both water security and water environmental protection can be achieved by introducing reclaimed water into the water supply system in Yiwu City.

5. Conclusions

A multidimensional, synergetic, multisource allocation model has been established for the water supply system in Yiwu City; the internal coupling mechanism and multidimensional, dynamic, synergetic relationship between water resources, economy, ecology and environment have been investigated. The effects of multiple water resource allocations on economics, society, and ecology are represented in the nexus model in both current and future stages.

Neither the high-quality water nor the normal water supply capacity in Yiwu City in 2030 is sufficient to meet the requirements of the calculated water demand. Therefore, reclaimed water has been introduced as a convincing alternative for minimizing the water shortage. The utilization of reclaimed water in both the urban and rural water supply systems are encouraged in the study area. The dual water price for the dual water supply system, including freshwater and reclaimed water, has been anticipated for the future to further accelerate the utilization of reclaimed water in Yiwu City.