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Water Recycling: It’s Time to Go “Direct”

By Jurg Keller

Water supply and wastewater systems are kept separate, but a report into water recycling finds this is no longer the best option.

Water is undoubtedly one of our most important resources, but we take it for granted. We expect it to run, nice and clear, from the tap and then “disappear” again from the sink or shower. But this major achievement of modern life is not without its limitations and challenges.

Through population growth, urbanisation and the growing variability of global climates, we are increasingly recognising that supply will not keep up with demand for much longer – at least not if we only use it once.

Therefore water recycling has to be a key consideration in the diversity of supplies into the future, together with alternative water sources such as seawater or stormwater (urban runoff).

Many cities have recognised this in recent years. Water recycling systems have helped to address the supply shortages and built up valuable experience and public confidence.

This is by no means an obvious or “easy to swallow” proposition, challenging the public and water professionals across the world. The very foundation of our remarkable improvements in public health and life expectancies over the past century or more has been the strict separation of clean drinking water from polluted wastewater. Now we are starting to connect the two systems deliberately, and calling this “progress”.

It has been happening for quite a long time. There are numerous examples where the water supplies of downstream cities along a river contain significant fractions of treated wastewater discharges from upstream populations. Despite its somewhat ad hoc nature, this unintentional water recycling has not caused serious water quality or human health incidences despite its widespread occurrence, particularly in highly populated areas such as central/northern Europe and coastal areas in the US.

The key to this success is the fact that we have developed an excellent understanding of important water quality criteria and how to maintain them, which has translated into well-performing treatment operations at both wastewater discharge and water intake.

The development of intentional potable water recycling in recent years has been enabled by the application of “Advanced Water Treatment” (AWT) technologies – mainly an extension of current water treatment approaches to include a multi-barrier train of different treatment processes such as membranes, activated carbon filtration and advanced oxidation processes.

Together with modern biological wastewater treatment and existing coagulation/filtration/disinfection processes in the water treatment plants, the AWT concept enables extensive purification of the water, yielding a product that is by all accounts at least as good as current drinking water supplies.

Despite these extraordinary advances in the treatment technologies, most potable reuse systems still include an “environmental buffer”, such as an aquifer, dam or a river/wetland. These buffers in the Indirect Potable Reuse (IPR) schemes provide time to react to potential incidents, may achieve some further treatment and potentially create a psychologically important separation of wastewater and water systems.

But are environmental buffers that important? The recycled water going into these buffers is usually far better quality than the “natural” water there already. And with the extensive online monitoring and control systems in modern AWT plants, there is no real need for further reaction time. The control systems simply shut down a process as soon as certain “critical control point” parameters are not met, therefore stopping the entire recycled water production.

It is time to consider “Direct Potable Reuse” (DPR) – without the IPR buffers – to secure diverse water supply options for our growing populations, both in urban and regional areas. This is the conclusion of a recent Australian Academy of Technological Sciences and Engineering report that considered all aspects of DPR, including cost and energy/material requirements relative to alternative water supply options. DPR was often one of the report’s best-ranked options, even compared with non-potable (dual pipe) recycling systems.

DPR requires us to break the age-old tradition of not connecting wastewater and water systems, but with a careful and conscientious implementation strategy it will provide at least the same (and possibly even better) public health protection as current drinking water systems do.

We may still have to deal with the psychological impacts, but humans have successfully adapted to far greater changes in the past.

Professor Jurg Keller FTSE is IWA Fellow, Director of the Advanced Water Management Centre at The University of Queensland and Professor in the School of Chemical Engineering. He is a member of the Executive of the CRC for Water Sensitive Cities.