Advanced processed wastewater for different uses: constellations favouring future implementation of a multimodal water reuse concept

Advanced wastewater treatment is often used to produce one single water quality. In recent years, technologies have been developed that allow the production of different qualities that are fit for their purpose. These technology bundles are still not being marketed, and market requirements are unclear. Two constellations in West Basin, California and in Oman were analysed to shed light on the different constellations of actors, resource situations and institutions. The first led to the industrial reuse of several water qualities, while the second produced an application in holiday resorts, leisure and food production. A hypothetical solution was contrasted with an historical case. The analysis of the constellations showed that multi-sectoral investments and dependencies require strong cooperation arrangements and long-term agreements. Governmental institutions were revealed to be suitable for coordinating the process, especially during the initial phase, but also in view of security of supply. A (comparative) examination allows an initial, still provisional systematic overview of other constellations that favour systems with recycled water of different qualities. Further research is required to understand the welfare and distribution effects of multimodal water pricing policies and the feasibility of co-financing of agricultural irrigation and opportunities for more sustainable water reuse.


INTRODUCTION
Water reuse can support 9 of the 17 Sustainable Development Goals: zero hunger, good health and well-being, clean water and sanitation, decent work and economic growth, industrial innovation and infrastructure, sustainable cities and communities, responsible consumption and production, life below water and partnerships for the goals.
Advanced treatment was initially developed for a small number of high-value water reuse applications, in particular groundwater recharge and drinking water production (Rizzo et al. ). For most applications, simpler treatment processes were initially considered sufficient. In the past, treatment was very simple, especially for agricultural applications; in many cases, secondary wastewater treatment with additional hygienisation is still considered sufficient (see European Union ). However, examples from many countries now show that such simple wastewater treatment can lead to environmental damage such as groundwater pollution, for example through the emission of environmental chemicals and anthropogenic micropollutants, especially contaminants of emerging concern (CEC, e.g. endocrine disruptors; Seis et al. ). Such long-term consequences are considered unacceptable in sustainability assessments (Kennedy & Tsuchihashi ). Accordingly, efforts are being made, such as advanced water treatment that includes further process stages (Kennedy & Tsuchiha-shi ; Drewes et al. ). These innovations potentially increase the range of sustainable applications for water reuse. In addition to agricultural irrigation, it can also be used for livestock breeding, for irrigating tourist facilities and public green areas, and for a variety of industrial, commercial and domestic purposes (Drewes et al. ).
In many cases, advanced treatment has so far only been applied to produce a single water quality. In view of the numerous possible applications, however, it is also conceivable that the wastewater treatment plant (WWTP) effluent is treated to produce different qualities for various uses. The provision and supply of different water qualities, which are used for different purposes, are abbreviated here as multimodal. Multimodal supply is the (usually simultaneous) supply of water of varying quality, which is differentiated and usually carried out using different transport carriers (usually pipelines). Accordingly, multimodal water use refers to the use of several separately transported water qualities (usually) for different purposes. • What are the (existing) constellations for water reuse of different water qualities where advanced treatment is appropriate?
• Which aspects promote their realisation and sustainable operation?
The identified cases were examined using CA to assess whether they lead to a sustainable solution.

METHODS AND MATERIALS
The CA concept The method used in this paper is based on CA, as explained by Schön et al. (). CA was developed to analyse and solve complex problems involving social, technical and natural aspects that are closely interlinked. Furthermore, CA claims to bridge prevailing problems in interdisciplinary settings and to integrate practical and scientific knowledge of various disciplines and societal groups (Ohlhorst & Schön ). Elements of the constellation are heterogeneousindividual or collective actors as well as technical norms and laws, technical objects and natural resources. Each of these element types is treated as equally important in the CA; the elements can either be the components of a specific problem situation or parts of a solution. It is possible to identify promoting and hindering factors of innovations, and a special feature of CA is the visualisation of the constellation, which helps to identify networks, dynamics and the relationships between the elements. Conventional water reuse is analysed in Nölting & Mann (). The process can be summarised in the following steps: • identification of the elements of the constellation and description of their relations, • analysis of the functional principles and characteristics of the constellation under consideration, including the identification of destabilising and stabilising elements, • examination of the dynamics in the constellation and its development and • evaluation of the constellation and strategy making (optional).
The CA was applied in an interdisciplinary research process in which competencies from ecology, engineering (from water management, agriculture and chemical engineering, landscape planning) and social sciences were used. Thus, the knowledge considering the elements and their relationships could be drawn from various scientific fields and combined with practical knowledge.
Broad expertise and the graphically based design procedure of CA allow a quasi-experimental optimisation and stabilisation of constellations in its development process.
Thus, it is possible to simplify or improve the constellation, e.g. by omitting or replacing actors or by changing relationships and incorporating technical innovations. Inhibiting factors (e.g. acceptance problems of traditional farmers) can also be changed by focusing on new products and sales markets.

Concept application
In this paper, constellations of two different cases are considered and evaluated. The cases describe examples of water reuse in different sectors, which are applied in practice in the fields of industry, agriculture, tourism and landscape irrigation. The first case has its starting point in the constellation that led to the reuse of different water qualities by industry in California's West Basin district in 1994.
The second case deals with the water reuse in the Arab peninsula and analyses so far hypothetical applications in aquaculture, tourism and landscape.
Based on an initial market analysis for the MULTI-ReUse methods (Becker et al. ), typical problem situations were identified and described. In an interdisciplinary group process, several cases were discussed with the aim of identifying the benefits of providing different water qualities and highlighting bottlenecks for sustainable development. For different cases (an industrial application and an agricultural and domestic application), a situation was selected that is as conducive as possible in order to present possibilities that favour advanced water reuse and where the financial feasibility of the supply of different water qualities can be assumed.

THE MULTI-REUSE CONCEPT
The MULTI-ReUse process produces three different, precisely defined water qualities from a base product of municipal wastewater already being conventionally treated.
The water is processed further either at the same treatment plant or at its site of use (cf. Nahrstedt et al. ). Thus, no raw wastewater is used, but rather the effluent from the WWTP. Due to its orientation towards user requirements, it can be seen as a fit-for-purpose approach. In the production process, which is quality-assured using specific monitoring procedures, certain modules of an advanced water treatment system are used according to the defined quality. The MULTI-ReUse process also includes observation of the microbiological quality of the treated water, which is close to real-time control. The process has been tested since 2018 in a trial and demonstration plant (pilot plant) at the Nordenham WWTP by the MULTI-ReUse consortium. There, the following qualities were produced, including side by side if required (cf. Nahrstedt et al. ).
• Process water 1 is practically free of undissolved substances and pathogenic bacteria, but still contains nutrient salts and CEC. The water can be used industrially for washing processes (e.g. for street cleaning) or for cooling processes with low requirements (e.g. for the concentration of dissolved salts). It can also be used for domestic applications (e.g. toilet flushing and washing machine) and for irrigation of urban greenery and energy and industrial plants. However, the nutrient content may affect water storage and distribution (which needs to be considered during the operation of the plants).
• Process water 2 is practically free of undissolved ingredients and pathogenic bacteria and viruses. The concentration of nutrients is considerably reduced compared with process water 1 (this increases the microbiological stability of the water, i.e. less microbial regrowth during storage and distribution). It also contains significantly fewer CEC of anthropogenic origin. The water can be used for industrial washing or cooling processes with higher quality requirements, as well as for high-quality agricultural applications (e.g. underground irrigation of fruit and vegetables for human consumption and supply of livestock) or for groundwater recharge.
• Reverse osmosis (RO) water (process water 3) is free of particles and pathogenic germs, and ions and macromolecules (e.g. CEC) dissolved in the water are removed as far as possible by RO in further process steps. This highly treated water can, therefore, be used in a wide range of applications as process water, e.g. as boiler feed water, for the production of ultrapure water or as mixed water for dilution purposes. However, the low ion concentration and free carbonic acid cause the water to have a corrosive effect on certain metallic materials, which should be compensated for by suitable materials during water storage and distribution (alternatively, the water can be adjusted by buffering and pH correction).
In comparison with the other two water qualities, a maximum of microbiological stability is also achieved (cf. Nocker et al. ). RO water is used in industrial processes and agriculture, for example, especially in greenhouse horticulture.

Preconditions and framework analysis
The United States, with an area of approximately 9.06 million km², is the third largest country in the world. Due to the enormous size of the country, the population density is relatively low at 33 inhabitants per km² (BMWi ), despite a population of more than 321 million (as of 2016). In some states, the increased salinity in aquifers and surface waters poses a serious water quality problem.
The steady increase in salinity is a result of excessive irrigation and increasing urban water use. In California, the biggest salinity problems are in the aquifers in the southern Central Valley and the Salton Sea. There is currently no sign of a slowdown in the trend (BMWi ).
Further saline inputs to groundwater are via saltwater intrusions in coastal areas.

Economy and legal framework
The US water sector generates annual revenues of more than US$ 157 billion (as of 2015) and, with average growth of about 4%, is one of the promising future markets for global investors and new water-related technologies. The market is influenced by various external factors such as regulations or the financing instruments offered, resulting in a state of constant change in the water sector (Water Technology ). Water abstraction law in the USA is regulated very heterogeneously due to the size of the country. In the east and mid-west of the country, most states apply the Riparian Law, which links water abstraction to land ownership. The person who owns the land with the water source may use the water within reasonable limits. What is meant by reasonable use, however, remains a matter of interpretation and, in case of doubt, will be decided by the competent courts.
In some countries, water abstraction also requires a permit from the competent environmental authority. In the western states, on the other hand, the Prior Appropriation Doctrine is predominantly represented.
According to this legal text, the use of water for the benefit of the general public (Beneficial Use) enjoys priority. Water abstraction is, therefore, independent of land ownership. In California and the other remaining states, aspects of both systems are combined into a hybrid system (BMWi ). At the federal level, there are also numerous laws and executive orders that specify the practical implementation of environmental laws. The

Water reuse as a challenge
The West Basin Municipal Water District is a water agency that provides imported drinking water to 17 cities (and unincorporated areas) in Los Angeles County. The agency was created by a vote of the people in 1947 to reduce local groundwater over-exploitation and to make local water supplies more reliable (e.g. by groundwater recharge). In

Description of the constellation
In the major drought of the late 1980s and early 1990s, West Basin's Board of Directors led the agency in developing alternative local water supplies that included water recycling for industrial use and landscape management.
Industrial applications were seen as the main applications; in terms of volume, applications with seawater intrusion barriers and landscape irrigation (which were actually the agency's objective) were secondary (Lazarova et al. ). An overview of the constellations of actors, resources, legal constraints and technical solutions in the California West Basin is given in Figure 1. • tertiary water (Title 22) for a wide variety of industrial and irrigation uses, • nitrified water for industrial cooling towers, • pure RO water for refinery low-pressure boiler feed water, • ultrapure RO water for refinery high-pressure boiler feed water and • barrier water or stabilised RO water.
Regarding the use of the recycled water from ECLWRF, the majority (53%) goes to industrial supply, while the seawater intrusion barrier uses 36%, landscape irrigation 10% and mixed use 1%.
Key milestones of the constellation are as follows • West Basin launched its water recycling programme in the mid-1990s.
• Contractual obligations with the industry to be supplied and the operation of the recycling facilities (by SUEZ) led to a public/private partnership.
• West Basin has invested more than $500 million dollars in this programme.
• Daily production capacity is 150,000 m 3 /d (SUEZ group ) and in 25 years has produced 750 million m 3 of recycled waterevery cubic metre recycled is a water volume not imported from Northern California or the Colorado River.

CA: MULTIMODEL SUPPLY OF TOURIST AREAS, AGRICULTURE/AQUACULTURE AND LANDSCAPE
Oman was selected as a potential country for multimodal purpose water reuse for several reasons, explained below.
Beneficial for the analysis was good data availability and the possible transferability of the results to other countries in the Arabian Peninsula.

Preconditions and framework analysis
The Oman is a high-income country and its economy is heavily depending on oil production; other important sectors in terms of GDP are tourism, shipping, mining, manufacturing and gas-based industries (The Heritage Foundation ).
Agriculture, forestry and especially fishery are traditional activities, but account for only 1.3% of the GDP (FAO ).
Economic growth in Oman is estimated to be slowing down, and diversification investments to reduce dependencies will lead to growth in the medium term (World Bank  Just a few years ago, Oman was a country for adventurous individual travellers, but in the meantime, statesponsored tourism has developed that meets international standards. In the last ten years, some government plans have been postponed to develop large areas for high-quality tourism, in particular to build extensive hotel complexes in the northern coastal region (e.g. Al Madina A'Zarqa).
Nevertheless, demand in the area of superior individual tourism (diving, snorkelling and fishing) is growing strongly.
It can, therefore, be assumed that there will be an increasing number of private initiatives. These initiatives offer windows of opportunity to construct sewerage and wastewater treatment plants at the same time, and to protect the sensitive coral reefs and fishing grounds. An overview of a hypothetical favouring constellation of actors, resources, legal constraints and technical solutions is given in Figure 3.
If the water is treated according to the MULTI-ReUse process (cf. Figure 4), it can be reused for various purposes.
In hotels, this water can be applied for garden irrigation and toilet flushing (PW 1), but also to produce fruit, vegetables and fish for tourism and local needs (PW 2). PW 3 is the highest quality, as after ultrafiltration, RO is also applied.
This water is suitable for hydroponic greenhouses, where Wastewater must be treated and discharged in such a way that the sea is not polluted. After all, this is the main attraction for scuba divers and snorkelers in the coral reefs and for fishing tourists. At the same time, the hotel sector also requires high-quality fruit, vegetables and meat, preferably from locally controlled production. Given the scarcity of water, it is also necessary to irrigate oases.

DISCUSSION
Multimodality means that several (e.g. 'dual') supply networks must be set up and operated on a permanent basis. Crucial to the cost of water is the length of the pipes. To save costs and reduce problems due to reinfection of the water, it is important that the advanced treatment takes place close to the water user. The WWTP, however, could be at some distance since one pipe is sufficient to supply treated wastewater for advanced treatment. The Californian example highlights this: the central facility is located near the WWTP, but has satellites in direct proximity to the customers. If only small quantities of one quality are required (e.g. RO water for a hotel's dishwasher), transport, e.g. by trucks, can be considered instead of a cost-intensive network of pipes. In order to prevent water shortages, it is necessary to involve intermediaries for the water management of a region with water stress. Intermediaries can facilitate processes and carry out a comprehensive problem analysis.
They need to be independent of the 'free play of forces' otherwise a short-term analysis of the initial constellation alone may lead to the adoption of the wrong strategy. The Californian example shows that a high level of regional suf- In the two constellations examined, state (or intermunicipal) intermediaries play a decisive role. A utility or an intermediary has a professional perspective on the users' water quality requirements. Therefore, it is possible to find a suitable typology of process water, pooling the different user requirements in order to increase cost efficiency. These classes are also related to the WWTP effluent and the available technology.
In one of the two cases, a constellation in which the national wastewater company is the key actor coordinating the important customers has proven to be stable. As a counterweight in this constellation, it will be necessary for the tourist resort and other users to participate responsibly on a board. However, state (or inter-municipal) independence is not required for the implementation and actual operating phase of the multimodal water reuse. Once the planning phase is finalised, the intermediary can also assign an operator from the private sector (e.g. SUEZ).

CONCLUSIONS
Reclaimed wastewater can be used for many different purposes. In practice, when developing a market for advanced water reuse, it is problematic to limit this to a single use and a single user group since an increase in water stress will affect all user groups. In this respect, a fit-for-purpose approach can be pursued. Based on further constellation analyses and supplementary investigations, it will be possible to introduce multimodal water reuse systematically.
Multimodal water reuse is a technically complex and cost-intensive process. There are constellations in which advanced water reuse is favoured. The CA makes it plausible that advanced treatment for reuse will be implemented more frequently in future due to a reduction in the associated risk to human health and the environment. Furthermore, if constellations can be established by forward-looking governmental agencies, private actors and civil society, such cases will increasingly replace simpler forms of water reuse. The target visions linked to water reuse in the United Nations' Sustainable Development Goals can then be realised sustainably.
The CA revealed a greater complexity regarding cooperation between actors compared with traditional water reuse. Hence, the venture needs to be supported by reliable (perhaps even strong) institutions as essential network nodes, supported by steering committees and participative planning processes for example.
Another drawback of multimodal water reuse is the cost of advanced treatment. In countries where the price of (irrigation) water is not subsidised, the supply of sufficiently treated water will only be available to target groups with sufficient capital. If water treatment can be covered by cross-financing, it might be possible to provide farmers with irrigation water at prices that are adapted to their income and the market prices of their products (e.g. by cross-subsidising the water tariff of irrigation water with the tariff of RO water). In this way, local food production, small-scale agriculture and landscape quality will be preserved.
Based on specific local preconditions, other opportunities for sustainable water supply always need to be considered. Moreover, further research is required to understand the welfare and distribution effects of multimodal water pricing policies and the feasibility of co-financing agricultural irrigation.
The following favourable constellations can be noted: 1. Multimodal water reuse can be established either when sewage transport and (semi-) centralised wastewater treatment is already in place, or in the case of new development.

A multimodal approach is possible in areas in which
water users carry out activities that require a wide range of different water qualities.
3. Multimodal water reuse is appropriate in constellations in which cooperation between heterogeneous actors (from operators of WWTPs to monitoring institutions and customers) can be initiated and maintained in the long term.
4. In principle, multimodal water reuse is suitable for countries that: • offer a high degree of legal security and contractual reliability (customers have to take the water for an agreed period of time), • are comparatively wealthy and have the possibility to engage qualified personnel (advanced wastewater treatment depends on energy and material input as well as on knowledge and skills) and • have different water users capable and willing to pay the cost (in a way that ultimately covers costs).
The stated conditions allow countries to be identified in which constellations can be formed beneficially for multimodal water reuse. Industrialised countries and countries in transition are eligible. These include BRIICS, as well as Thailand, South Korea, OPEC countries and possibly some other MENA countries as well.