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Water Reuse: The System Perspective

Water reuse could be viewed as a system composed by different elements, which work together to achieve a final objective (goal) – the beneficial use of reclaimed water. In this article we consider the reuse of municipal wastewater, which is treated and applied for a secondary beneficial use.

An example of a water reuse system, focusing on the reuse of municipal wastewater, is shown in Fig. 1. Its two major elements are the wastewater subsystem and the reuse subsystem; the last includes the hydraulic structure, which conveys the recycled water to the consumer. The first has two secondary subsystems – the sewerage and the wastewater treatment facility. Each one of the sub-systems could function independently and are entities on its own. In addition, within the level 2 subsystems, each treatment unit or pumping station might be regarded as a system of its own. In this article, the system perspective gives emphasis on the linking and inter-relation of the subsystems and elements for optimum performance and results. From this perspective the water reuse system is part of the larger system of the water engineering structures at population center level and also forms part of the broader system at catchment or higher level, including the available water resources. Fig 1 should be viewed as an example only; in practice a large variety of alternatives might be possible, based on the elements incorporated and the objectives of the system, e.g. the system might include combined used of recycled water and storm water, or the objectives of the reuse might require the incorporation of both – the water supply and the wastewater system.

wrsystem.jpg

Figure 1. The structure of a wastewater reuse system

Content Table

Types/categories of water reuse

The beneficial use of reclaimed water can be grouped broadly into different categories, based on the intended use and the location of the reuse system.

Based on the intended beneficial use, we can differentiate between potable (drinking) and non-potable use. Potable use is a controversial issue and regulating agencies are cautious to recommend it due to limited information regarding public health hazards and emerging constituents [1, 2, 3]. Non-potable reuse is widely applied and recommended, especially in areas where fresh water resources are scarce [4,5]. The most common practically implemented cases include agricultural and landscape irrigation. Other non-potable types of use include ground and surface water recharge (augmentation), toilet flushing, fire fighting, dust washing, etc [1, 2, 3].

Two categories of water reuse can be mentioned based on the system location: indoor and outdoor use. Indoor use is mostly associated with toilet flushing, the rest of the different types of use fall in the outdoor category.

Another important grouping or classification is related to the link between the reclaimed water and a natural water body. Direct reuse is related to types of beneficial use, which consume only reclaimed water, directly after wastewater treatment. The example in Fig 1 shows a direct form of reuse. Indirect reuse is the case, when the reclaimed water is discharged into a natural water body before its intended use. The consumer uses mixed reclaimed and natural water. Both, surface or ground water recharge are examples of indirect reuse in the cases when these natural water bodies are used as sources of different human activities, such as irrigation, recreation or water supply. In many densely populated areas, treated effluents are discharged into surface water bodies, which are used for different purposes downstream of the discharge. Such cases might be considered as indirect form of reuse.

Spatial scale

The spatial scale of the water reuse system is based on the scale of the wastewater system, which can be centralized, decentralized or mixed pattern.

Centralized wastewater systems are the classic solution, where the wastewater is collected in a bulk sewer system, treated and discharged to surface water bodies or reused for different purposes. Historically, centralized systems were built to convey the wastewater far from the source of generation and to dispose it in a convenient way. Water reuse in centralized systems deals with a bulk amount of reclaimed water. In medium size population centers usually one system is developed, but in large population centers several systems might be found.

Decentralized wastewater systems consist of a large number of small systems within a population center [6]. Examples of decentralized wastewater systems are the applications of on-site sanitation, where the wastewater generated is treated and disposed or reused on-site, in close vicinity to the source of generation. In such cases the collection system is eliminated. Decentralization can be applied also at neighborhood level, or suburb/township level, where the required sewer system will cover relatively small area and the wastewater will be treated and disposed or reused in the vicinity.

Mixed pattern wastewater systems are represented by a combination of both – centralized and decentralized systems. Usually, the population center is served by a centralized system, but selected areas (suburbs) are served by smaller treatment plants or by on-site sanitation facilities. This pattern is typical for developing countries, where the centralized sewerage is under development and serves only parts of the population centers. As an example, the City of Harare, which is served by a well-developed wastewater system, has two major centralized wastewater sub-systems with advanced wastewater treatment, several small sub-systems serving selected suburbs, equipped with waste stabilization ponds and a large area of low-density residential suburbs, which are served by on-site facilities (septic tanks with infiltration trenches) [7].

Flow segregation

Classic wastewater systems collect and convey together all types of wastewater from different household appurtenances. A new development in this direction is the flow segregation of gray and black water. Gray water [8] is the water from washing and bathing activities, while black water is the wastewater from toilets. Flow segregation allows for separate treatment of the different flows and corresponding reuse, but requires dual piping [9,10] or dual water supply system [11]. Flow segregation at on-site level is encouraged because of the possibility to reuse gray water close to the source of generation and the opportunity to reduce drinking water for activities, which do not require high water quality standard, such as landscape irrigation, toilet flushing or washing. Also, the control the generated wastewater quality is improved [5, 12].

Gray water - It is collected from all types of domestic appurtenances and machinery used for washing activities. The wastewater generated from kitchen sinks contains oils, fats and food segments, which should not be discharged into the gray water system. For this reason, a preliminary treatment and removal should be provided, or alternatively, the kitchen sink outflow could be directed to the black water system [8]. Gray water comprises 50-80% of the daily household consumption and its quality is of much better standard, compared to mixed domestic wastewater [1,2,13]. Consequently, it requires minimum treatment in order to be reused for purposes, which do not require high water quality status. The most common reuse alternatives are toilet flushing and landscape irrigation [8, 12]. In some cases, treatment might not be required, except a simple screening or filtration, such as direct use for toilet flushing [14] or subsurface land irrigation [15].

Black water – This is the wastewater from toilets, which is collected and treated separately from gray water. It is more concentrated then the mixed wastewater requires higher level of treatment regarding any type of wastewater reuse and produces sludge as a by-product. The higher organic content of this type of wastewater makes the application of anaerobic treatment methods suitable [16].

Source separation – this process includes the use of special urine separation toilets, where urine is separated and conveyed separately from the black water flow. The major advantage of this system is the reduction of the nitrates in the wastewater and the possibility to reuse the urine as a fertilizer. It is acknowledged [17, 18] that this system is still under research and its practical implementation is hindered by technical problems associated with scaling of the urine pipeline system and the safe reuse of the product. Also, the social acceptability of these types of toilets is at low level. This technology is considered appropriate for developing countries at decentralized levels, which lack existing wastewater systems [4,12].

Linking and relation of elements and systems

Matching water quantities and quality requirements of the subsystems

The objective of wastewater reuse systems is to provide recycled water of appropriate quantity and quality to the consumer. From this perspective, the choice of reuse alternative will depend on the wastewater system and its ability to provide reliable water supply.

Matching available water quantities generated by the wastewater subsystem to the requirements of the reuse subsystem is of primary concern. In many cases additional water supply might be needed to satisfy peak demands, or the recycled water may be in excess and a disposal option might be necessary for selected periods of time. Diurnal and seasonal variations of both the wastewater and the reuse subsystem must be accounted for and storage facilities should be provided.

The type of reuse chosen would determine the required quality of the recycled water and the corresponding level of treatment. In this regard, guidelines and regulations play essential role in order to implement a safe and sustainable application and to protect public and environmental health [1, 2, 12, 13].

 The system approach to wastewater reuse allows for analysis of different viable reuse alternatives with corresponding wastewater subsystems design, and the choice of an optimum solution for the whole water reuse system.

The system in the context of urban water management and the hydrological cycle

Wastewater reuse systems are closely interrelated to the water engineering infrastructure and the environmental systems (soil and water). They should be considered as a subsystem of the existing water engineering structures. Their importance and benefits can be evaluated only in this context, e.g. the introduction of landscape irrigation with recycled water will have impact on the existing water supply system by reducing the demand for potable water supply and will also influence the environment by reducing pollution loads and the need for fresh water abstraction. Therefore, the development of water reuse projects should apply the system approach and should analyze their application in this broader context.

In some cases, the implementation of wastewater reuse projects in selected parts o the urban water system can lead to a conflict regarding existing practices, e.g. if an existing wastewater system has the objective of centralized reuse, the implementation of decentralized reuse options might reduce considerably the projected water quantities of the centralized scheme and to jeopardize the existing goal. Similarly, the combined use of recycled water and storm water at large scale might reduce significantly the runoff and replenishment of natural water resources.  

The system analysis process during the planning stage of any water reuse project would help to formulate major goals and objectives, identify the most important variables, and provide optimal solutions for the water reuse system in specific and in the context of the urban water cycle in general. Such an approach would lead to sustainable, reliable and economically viable applications.

References

  1. Metcalf and Eddy, 2003. Wastewater Engineering, treatment and reuse, 4th ed. McGraw – Hill Inc., Int. edition.
  2. Asano T (ed). Wastewater reclamation and reuse Water Quality management Library Vol 10, Technomic Publications, USA
  3. Asano T. Water reuse: issues, technologies, and applications, Metcalf and Eddy, Inc, USA
  4. http://en.wikipedia.org/wiki/Water_management_hierarchy
  5. http://www.melbournewater.com.au/content/water_recycling/what_is_recycled_water/what_is_recycled_water.asp
  6. http://www.iied.org/pubs/pdfs/G00485.pdf
  7. Hranova R. (2005) Characteristics of an urban environment in the context of diffuse pollution control, in Hranova (ed.) Diffuse pollution of water resources – principles and case studies in the Southern African region Taylor and Francis/ Balkema, ISBN 0415 38391 9, pp 47-66.
  8. http://www.graywater.net/
  9. http://www.commonfloor.com/articles/dual-piping-in-bangalore-apartments-378.html
  10. http://www.servinghistory.com/topics/Dual_piping
  11. http://www.iwapublishing.com/template.cfm?name=isbn1843391325
  12. http://www.eawag.ch/medien/bulletin/archiv/2007/20070307/index_EN
  13. http://www.epa.gov/ORD/NRMRL/pubs/625r04108/625r04108.htm
  14. http://www.treehugger.com/files/2006/10/watersaver_tech.php
  15. Hranova, R., 2005. Gray water characteristics and implications on regulations for a sustainable wastewater reuse, Proceedings of the IWA Specialty Conference “Wastewater reclamation & reuse for sustainability”, 8-11 November 2005, Jeju, S. Korea (in CD format)
  16. Treatment of Domestic Wastewater using anaerobic sludge blanket reactor, Banu, Kaliappan, Yeom
  17. van Betuw, W., Mels, A. and Braatbaard, O., 2007. Technology selection and comparative performance of source-separating wastewater management systems in Sweden and the Netherlands, In: Proceedings of the IWA specialized conference “Advanced sanitation”, Aachen, Germany, 2007 (in CD format)  
  18. Lens, P., Zeeman, G., Lettinga, G., 2001 Decentralised Sanitation and Reuse: Concepts, Systems and Implemenation, IWA Publishing

Author

This article was written by Roumiana Hranova:

Dr R.K Hranova specializes in the field of Water Quality Management and Environmental Engineering. She has worked as consultant and as an academic in Bulgaria, Zimbabwe and Botswana and has vast experience in problems related to Water Resources Management in developing countries. Currently, she is a Senior Lecturer at the Civil Engineering Department of the University of Botswana. She has developed a number of courses at B.Sc. and M.Sc. level and has introduced innovative teaching methods, such as the use of web-based distributed learning courses in the "Black-board environment". The on-going research/consultancy of Dr Hranova is in the field of Integrated Water Resources Management, Wastewater Reuse and Environmental Engineering Systems analysis.

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