Sustainability requirements of implementing water-reuse concepts for new industrial park developments in water-stressed regions

Requirements for wastewater management and water-reuse concepts concerning sustainability are gaining greater importance, especially in times of climate change. Industrial park developments are often hindered due to water scarcity. Thus, nowadays, the reuse of wastewater is becoming more and more important to increase the availability of water and to enable new developments. The sustainable Industrial WasteWater Management Concept with the focus on Reuse (IWMC→ R) provides a solution strategy to produce reuse water from industrial wastewater within production plants. To achieve sustainability, certain requirements are essential since the reuse water can be provided directly via an optimized wastewater treatment process for subsequent use. Hence, it is then ‘fit for purpose’. This enables a more efficient application of reuse water. Furthermore, due to environmental issues, it is important to construct space-saving water resource recovery facilities for reducing land consumption within industrial parks.


GRAPHICAL ABSTRACT INTRODUCTION
In water-scarce regions, for instance, the northwestern parts of China, huge areas have high levels of water stress due to an uneven distribution of natural water resources and high levels of pollution of water bodies (WRI -World Resource Institute ). The development of industrial parks is generally related to the availability of water. Due to increasing water shortages, industrial park developments are often hindered; production plants may have high water requirements depending on their production processes. For instance, food production processes require less water than chemical production processes (Bauer et al. ). However, water is also needed for infrastructural purposes, such as the irrigation of green spaces or street cleaning in the park.
Especially in water-stressed regions, the water demand for production plants increasingly competes with other water requirements such as municipal and agricultural water demand. This is resulting in the limitation of industrial expansions (Bauer et al. a).
To enable new industrial park locations or their expansion in water-scarce regions, a sustainable water supply has to be ensured and water consumption from natural resources has to be reduced. In this context, and due to worsening climate change, new sustainable water-reuse concepts are achieving greater importance. Another important element is that industrial park developments drive urban developments (Zhao et al. ), which usually occur in densely populated urban areas. Thus, in industrial parks, the space for wastewater treatment plants is often limited due to the restricted availability of land as it is mostly provided for production plants. Hence, the reduction of land consumption and a high water-reuse potential is important to implementing water-reuse concepts.
Previous studies in the context of water reuse focus mostly on municipal wastewater treatment and reuse in an urban context. A well-known concept is integrated urban water management (IUWM), which includes all aspects of the water cycle: wastewater, drinking water, stormwater, and evaporation (Sharma et al. ). IUWM is an approach for urban water utilities to minimize their impact on the environment, to maximize their contribution to social and economic issues, and to generate well-being (Maheepala et al. ). Another important approach is 'urban metabolism,' which can be holistically understood according to Musango et al. () as a 'collection of complex socio-technical and socio-ecological processes by which flows of materials, energy, people, and information shape the city, service the needs of its populace, and impact the surrounding hinterland.' A further approach is the innovative SEMIZENTRAL approach (Tolksdorf et al. ; Tolksdorf & Cornel ). It was developed for a semi-centralized supply and disposal system for fast-growing urban areas.
The municipal wastewater is treated in a semi-centralized Resource and Recovery Center (RRC). Instead of only one municipal central wastewater treatment plant in a large urban area, the urban area is divided into smaller units, each of which has an RRC. The RRC provides water 'fit for purpose' via an in-house solution. It integrates the sectors of wastewater treatment, reuse water supply, and waste treatment. The concentration of pollution in industrial wastewater varies greatly and usually depends on the production process. Therefore, studies dealing with the treatment of industrial wastewater are very different and refer to specific case studies. As an example, a project by Whynman et al. () dealing with industrial wastewater reuse at San Francisco airport can be mentioned here.
The particular aspect of the IW 2 MC → R concept presented here is that wastewater flows are clustered according to certain concentrations and can therefore be specifically treated. The wastewater treated according to the 'fit for purpose' principle is reused for infrastructural purposes. This approach is therefore not purely projectspecific or production plant-specific and can therefore be transferred to other projects.

MATERIAL AND METHODS
To enable sustainable industrial park developments, an innovative Industrial WasteWater Management Concept with the focus on Reuse (brand name: IW 2 MC → R) is developed. The concept includes the efficient treatment of industrial wastewater in a central wastewater treatment plant (CWWTP) and an additional water-reuse plant (WRP) which provides reuse water 'fit for purpose' (Bauer et al. a). The treated wastewater can directly be reused for respective applications, for instance, for the irrigation of green spaces. To achieve further aspects of sustainability, the IW 2 MC → R aims to reduce land consumption via an area-saving construction type of the CWWTP and the WRP. This aspect is an especially important issue in densely populated areas. To reduce land consumption, the wastewater treatment processes have to be optimized. Furthermore, the concept enables an optimization of water-reuse opportunities by reaching a high industrial park reuse factor (IPRF). The IPRF describes the relation between the wastewater inflow to the CWWTP and all possible reuse water flows. The IPRF includes the infrastructure reuse factor (IRF), which relates to infrastructural reuse applications, e.g., irrigation, street cleaning, and toilet flushing. Additionally, the IPRF includes the production plant reuse factor (PPRF), whereby it is possible to calculate the reuse water requirements for the production plants, such as process water (Bauer et al. a). The IW 2 MC → R is not focusing on treatment processes for internal water reuse for production plants within the industrial park. This is due to the fact that water reuse for infrastructural purposes is a further water-reuse opportunity as the internal reused water is discharged into the water resource recovery facilities at the end (Bauer et al. b). Furthermore, production processes require a higher quality water than, for example, irrigation does. Consequently, the cost of treatment processes is higher as well. The concept focuses on the outflow of the WRP which is treated 'fit for purpose' for infrastructural purposes. Remaining reuse water can be discharged to the adjacent urban area since the water demand for irrigation and street cleaning cannot be covered by the municipal water resource recovery facility (Bauer et al. b). Hence, it is not necessary to implement further cost-intensive treatment tracks for the treatment of the industrial wastewater for an internal production plant water reuse.
To calculate and evaluate area requirements for the CWWTP and WRP and to calculate the IRF, the creation of a model industrial park (MIP) is essential. Therefore, the case study of China was chosen due to its dynamic in developing greenfield areas as new industrial park locations (Bauer et al. a). Furthermore, due to the uneven distribution of natural water resources, and the relatively high levels of pollution of water bodies, water shortage is often a challenge (WRI -World Resource Institute ). Another reason for choosing this case study area is that besides the United States, China is the country with the highest worldwide water requirements for industrial purposes (FAO ).
For the first IRF calculation, six exemplary production plants were taken into account. Accordingly, the IRF results in ∼25% when including three infrastructural reuse applications: irrigation of green spaces, street cleaning, and toilet flushing (Bauer et al. a). To get more viable results, an extended calculation that considered 19 production plants instead of six production plants was conducted.
Using a MIP as methodology allows the adjustment parameters concerning wastewater quantity and quality. Even if the IRF varies due to the variety of production plants with different water demands, the MIP shows a methodology which calculates the reuse potential of a specific industrial site. If this is applied on real industrial parks, all parameters can be adjusted. The IRF shows a specific reuse potential for further applications as well as the volume of remaining treated wastewater flows that can be discharged into the water body or can be applied for further purposes such as for the irrigation of city parks in an adjacent urban area.
Besides the MIP of six production plants, the reuse applications for cooling water and firefighting water were further considered. Concerning the land consumption of the water resource recovery facilities, the needed treatment technology for the MIP is identified and average sizes of CWWTPs and WRPs are analyzed. However, it should be noted in this context that a MBR plant is not necessarily needed for processing reuse water for infrastructural purposes, even if it takes up little space. If the use of reuse water has higher requirements, it would be possible to include the MBR, even if the costs are higher.

Characteristics of the MIP with 19 production plants
For the MIP, the 19 production plants included several types of industries, such as chemical, paper, food, and beverage production facilities, as they are statistically the most water-intensive industries and various production types are often localized in industrial parks. Additionally, the wastewater of a canteen and generally sanitary wastewater are considered.
Wastewater flows were divided into categories (A to C) according to their carbon, nitrogen, and phosphorus concentration (see Table 1). These parameters were chosen because they are common in the characterization of wastewater flows and are often used to determine the qualities to be maintained for the treatment of wastewater. It was assumed that the wastewater would be of constant quality throughout the entire year. This is due to the fact that in industrial CWWTPs, the equalization tank homogenizes the loads, concentrations, and wastewater flow. In this standard, requirements relating to reuse water qualities for cooling water, washing water, boiler feedwater, and process/product water are prescribed (for extracts see Table 2).
Due to the many food industries in the MIP, which produce highly loaded organic wastewater (Ranade & Bhandari ), in categories A to C the amount of COD (2,488,010 kg/d) is relatively high compared to the amounts of nitrogen (124,987 kg/d) or phosphorus (10,528 kg/d). It is, therefore, appropriate that the CWWTP provides at least two treatment tracks for wastewater treatment: anaerobic treatment to reduce carbon concentration and aerobic treatment to eliminate nutrients and further reduce COD (see Figure 1). The anaerobic treatment makes it possible to obtain biogas and thus achieve the most energy-efficient treatment of wastewater. A detailed consideration of the wastewater flows shows that ethanol (see No. 10, Table 1) must be pre-treated to reduce high concentrations of solids. In this case, a simple sedimentation stage would be the best solution. The anaerobically pre-treated wastewater has to be discharged further into the aerobic process line to achieve further elimination of COD and nutrients. After the aerobic biological treatment, low concentrations of COD and nutrients are reached. To meet the requirements for the reuse standards, the treated wastewater is discharged to the WRP, filtered, disinfected by means of UV, and mixed with chlorine. It can then be reused for different applications. As the Chinese quality requirements of reuse water for irrigation, toilet flushing, firefighting, street   cleaning, and cooling are very similar, the effluent of the CWWTP/WRP is expected to remain within the limits.
Hence, to reach the required water quality, only one WRP treatment technology is necessary. The number of employees in the different parks divided by the respective park size was the second essential average value (employees/ha) to be gathered from the investigated parks. Consequently, the total MIP has a size of 823 ha, the green spaces take up 165 ha (20%), and the area of roads amounts to 74 ha (9%) (see Table 3).
For this enlarged calculation, further applications such as cooling and firefighting water are taken into account.
For the implementation of the IW 2 MC → R, it is premised that with respect to the water shortage in several regions, the industrial park enables only recirculating cooling instead of once-through cooling systems. The latter needs high water flows which cannot be provided in water-stressed regions.
Therefore, the calculation of the IRF only includes the make-up water for cooling towers. Due to the German directives for cooling water (VGB R ; VGB R ), the make-up water is estimated as 2% of the total cooling water demand. It is assumed that this value can also be transferred to other countries, such as China, due to similar technical conditions. Minor deviations according to Chinese conditions would not have a noticeable effect on the result.
For four of the 19 production plants, specific cooling water demand is given (see Table 4). For the first approximation of the others, it is calculated with the average cooling water demand of two existing industrial parks. As in water-stressed regions where the provision of firefighting water could be a challenge, (underground) fire water storage tanks are considered. To maintain the water quality, a partial water exchange in these tanks is expected, which results in permanent water demand of 60 m 3 /d (see Table 4) even if there is no fire. This shows the example of the industrial park InfraServ Gendorf (). For toilet flushing, street cleaning, and irrigation of green spaces, the calculation of the water demand is based on Bauer et al.
(a) (see Table 4). For the MIP with 19 production plants in China, the result is an IRF of ∼14.3% (wastewater flows: 94,685 m 3 /d; reuse water demand: 13,539 m 3 /d), which is less than the IRF of six production plants. The water flows of the MIP are shown in Figure 2. Since there is remaining reuse water, the water can be discharged to the urban area to meet the water requirements that cannot be covered by municipal treated wastewater (Bauer et al. b).
An essential reason for this lower factor is the considerably higher wastewater effluents of the chosen production plants. It should be noted that the IRF depends on the water requirements of the production type and the quantities produced which can be controlled via industrial park management (Bauer et al. ). Furthermore, a low IRF can equally be a potential for the adjacent urban area, as it allows remaining water to be reused for urban purposes such as the irrigation of city parks or street cleaning (Bauer et al. b).
Reaching sustainability by an area saving construction type of the water reuse plant Due to the restricted availability of land, as it is mostly provided for production plants, the size of the CWWTP and     Hence, via an optimized treatment technology providing reuse water 'fit for purpose,' the concept tackles these challenges.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.