Water Journal : Water Journal March 2011
6 MARCH 2011 water regular features my point of view The last decade or so has seen a revolution in thinking and action in the urban water cycle, probably a once-in-a-lifetime event. The driving pressures have been extended drought periods in all our capital cities, acceptance of the threat from climate change, concern with greenhouse gas footprints, and overwhelming concern with general environmental sustainability. The new water paradigm has not yet fully emerged but, following WSAA's recent discussion document, Cities of the Future (see page 62), certainly encompasses a combination of climate-independent water services, decentralised supply and sanitation systems, water recycling, urban areas acting as their own water supply catchments, a carbon-neutral water industry, efficient technologies reducing per capita consumption, and reduced impact of urbanisation on the ecosystem health of creeks and waterways. These innovations will be overlaid onto the current traditional centralised water and sewerage systems which are too big, important and expensive to replace, at least over a few decades. Identifying the Game Changers How this mix of options will play out is uncertain. But one thing is for sure -- seawater desalination powered by renewable energy is a game changer. By 2012, it could supply over a third of the water demand of our capital cities, increasing to almost 50 per cent at full build-out (WSAA Report Card 2009-10). Despite its slow adoption and lower social acceptance, indirect potable reuse of highly treated sewage effluent will also be a game changer as it uses existing infrastructure to capture 100 per cent of the urban water market, but with a substantially lower specific energy use (kwh/kL). We need to ask ourselves how other technologies and systems fit into the emerging new-age urban water cycle. Decentralised water and sewerage solutions are widely believed to be potentially superior to centralised systems because long pumping distances and associated high energy costs between supply and demand are removed. However, good intentions can be notoriously difficult to turn into good outcomes, especially in complex biophysical/ social systems where unexpected outcomes often emerge. A good example is biological nutrient removal tertiary sewage treatment plants, which have excelled at producing a high-quality effluent. In South-East Queensland (SEQ), over $700 million invested in this technology over the last eight years has reduced point source nitrogen export to Moreton Bay from 1800 tonnes a year to a little over 400 tonnes. However, energy consumption could be up to three times higher than for secondary treatment plants and the greenhouse gas (GHG) footprint even larger again due to fugitive emissions of the highly potent GHG -- nitrous oxide. Adapting sewage treatment plant operation to recover energy from methane production, and ensuring complete denitrification, could go a long way to offsetting the perverse outcomes of an otherwise desirable technology. These insights were made possible due to the system understanding and detailed monitoring by process engineers, especially from the University of Queensland. Clearly, monitoring is at its most useful when it confirms a process is operating to design specifications, provides insights into how the process might be improved, and identifies otherwise unexpected outcomes -- both perverse and beneficial. Similar measurements and process understanding need to be applied to other industry innovations, including water recycling, decentralised water supply and sanitation, and stormwater management. Recycling Wastewater Water re-use for irrigation is perhaps the most straightforward and best understood in recycling waste products, with the big success stories occurring in the peri-urban horticultural areas of major towns and cities -- especially Adelaide and Melbourne. At the other end of the spectrum is using treated sewage effluent for indirect potable reuse (IPR). The depth and range of monitoring and process understanding of trace organic chemicals, and their removal by IPR technologies, that has built up in Australia and overseas over the last few years is astounding. Continual monitoring over the next few years will be vital to demonstrate to a wary public that not only can IPR plants be continuously and consistently operated to produce potable standard water, but also that operators can quickly adapt treatment technologies to efficiently remove emerging trace contaminants of concern. In between agricultural and drinking water end uses is recycled water for potable substitution, such as toilet flushing, laundry and garden irrigation. This requires dual reticulation in new urban developments. The jury is still out as to whether this is a desirable long-term outcome due to managing cross-connections, the extra cost of infrastructure and asset management, and the possibility of stranded assets if IPR were to become widespread. Dual reticulation has become fashionable in major urban areas of southern Australian, and these schemes provide an excellent opportunity to monitor their economic and environmental sustainability. Utilising Stormwater It is generally agreed that stormwater is the last major untapped source of urban water supply. However, it is likely that two separate paths will emerge. In southern Australia, with its hot, dry summers, public open space irrigation is likely to predominate, while in subtropical SEQ potable substitution via a dual reticulation system to individual dwellings is more likely. In both cases, monitoring to understand types and concentrations of pathogens and the reasons for their presence (for example, sewer overflows) will be The Urban Water Cycle Revolution: Why Monitoring is Vital Ted Gardner has recently retired as a Principal Research Scientist from the Queensland Department of Environment & Resource Management (DERM) and CSIRO, where he led research into the urban water cycle focusing especially on decentralised systems.
Water Journal April 2011