Integrated concepts for reuse of upgraded wastewater AQUAREC (2003-2006)

Funding body: European Commission (Framework 5)

Wastewater reuse presents a feasible solution to the growing pressure on Europe's water resources. However, wastewater reuse implementation faces obstacles that include insufficient public acceptance, technical, economic and hygienic risks and further uncertainties caused by a lack of awareness, accepted standards, guidelines and uniform European legislation.

So far, there are no European regulations on water reuse and further development is slowed by lack of standards in water quality, treatment and distribution systems. While guidelines for agricultural water reuse have been defined by the World Health Organisation, and by different states such as the USA and Saudi Arabia, a uniform solution for Europe is lacking. European standards have to take a complex water policy and management framework into account and have to balance the protection of water resources, economic and regional interests and consumer-related safety standards.

The Centre's involvement concentrates on WP8 of the project dealing with optimal total system design, using multi-objective Genetic Algorithms with mathematical modelling tools for different components of the reuse system (piping network, wastewater treatment plants, upgrading, etc).

Working as part of the AQUAREC consortium (www.aquarec.org), the Centre for Water Systems, University of Exeter recently developed decision support software (DSS) for Water Treatment for Reuse and Network Distribution (WTRNet). As the name suggests, the software has been developed that addresses both the treatment and distribution aspects of potential water reuse schemes, considered at a planning level. This article describes in brief the process followed in the development of WTRNet software and provides a description of its key features. A PowerPoint presentation can be found here (4MB).

Reclaimed water projects typically include construction of new treatment or upgrades to a municipality’s treatment systems to clean wastewater to the required quality level, and construction of distribution systems which may include pipelines, pumping and storage facilities for reclaimed water. Therefore, the complexity associated with the planning of water reuse systems is very high, as a very large number of design combinations is possible. Also, one of the requirements of the AQUAREC project was that the spatial relation between wastewater treatment sites and potential reuse locations has to be considered. To aid in the planning of water reuse schemes several DSS have been developed in the past. A comprehensive literature review of DSS in the water reuse area revealed that the majority of water reuse research has been focused on the generation, evaluation and optimisation of treatment, despite the fact that “no single factor is likely to influence the cost of water reclamation more than the conveyance and distribution of the reclaimed water from its source to its point of use” (US EPA Guidelines for Water Reuse, EPA/625/R-04/108, 2004). Several researchers considered the distribution system in their evaluation of reuse schemes, with significant simplifications of treatment processes.

The key objective in the development of WTRNet was to develop a DSS that overcomes the limitations of existing tools, by addressing both the treatment and distribution aspects of water reuse schemes in an integrated manner and with sufficient detail. The software development focused on allowing the user to deal with the following questions:

What processes are needed to produce reclaimed water of adequate quality for a specified influent quantity/quality and end-user requirements?
How is the reclaimed water going to be delivered to end-users (i.e., sizes of distribution system components required)?
Who should receive the reclaimed water (i.e. best selection of customers from the identified potential end-users)?
Click here if you want to download a PowerPoint presentation of the work on development of WTRNet.

In order to conduct evaluations required to answer the above questions in an efficient manner, the simulation component was first developed along with a user-friendly interface. The simulation model includes a default knowledge base stored in the project file, as well as separate computational modules for treatment performance and sizing of the reclaimed water distribution system. The model knowledge base contains the following information: the design and costing information on unit processes, water quality requirements for different types of end uses of reclaimed water, suggestions for treatment trains that could be used for influent quality / end use combinations, rules for combining unit processes, and the design and costing information on the distribution system components. Information on treatment processes has been verified by comparing the software outputs with existing reuse schemes and values for cost and performance of treatment options reported in the literature.

The treatment performance has been developed with functionality to perform the evaluation of user-selected combinations of unit processes in a treatment train. The evaluation of treatment train performance and the display of treatment train evaluation results are carried out as changes to the treatment train are made. Since the evaluation results in a large output, the calculated data is displayed through four separate frames on the form: effluent quality, pollutant percent removed, evaluation criteria scores and costs and resources.

The distribution system performance computational module is used to optimally size the distribution system elements. The sizing is carried out based on a pre-determined branched layout and preferences of the user for locating the pumping and storage facilities, entered using a user-friendly interface. The method used is a two-step procedure that first determines the optimal allocation of reclaimed water (along with optimal sizes of seasonal storage), followed by the sizing of pipes and pumping stations.

In addition to allowing the software user to evaluate a large number of design alternatives using the simulation component of WTRNet, the software includes optimisation routines for conducting the least-cost planning of integrated water reuse systems. The verified simulation software was used as basis for the optimisation components, initially to determine the numbers of possible design alternatives involving different end-uses and numbers of customers. This results of this exercise showed that incorporating a single optimisation methodology would not be appropriate, since the number of design alternatives changes by several orders of magnitude, depending on the influent quality an the number of potential end-users considered.

In order to accommodate the wide range of the number of possible design alternatives, three algorithms are incorporated in the optimisation module. If the secondary effluent is to be reclaimed and the number of potential customers is not large, exhaustive enumeration is used to determine the least-cost design alternatives for all combinations of end-users. If the secondary effluent is to be reclaimed for a (potentially) large number of end-users, a simple Genetic Algorithm (GA) is used for optimal user selection. Finally, if the source of water is raw sewage or primary effluent, the optimisation algorithm used is a GA with customised operators. The algorithm conducts a simultaneous search of least-cost design alternatives and the best selection of customers.

The WTRNet decision support tool provides a platform that can be used to conduct the integrated assessment of water reuse options in an efficient manner. The tool has been successfully applied on a case study of water reuse options in the city of Kyjov, Czech Republic, and London, England.

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