Water Journal : Water Journal May 2011
6 MAY 2011 water regular features my point of view Professor Gray is responsible for the Institute's water research program and is actively involved in water treatment and membrane research. Soaking rains over the east coast of Australia this spring and summer, and the construction and completion of many seawater desalination plants, has led the water industry to focus on improving bottom lines, coping with floods and creating "sustainable water systems". Indeed, the last few contributions of "My Point of View" have identified drivers for change towards more sustainable systems. These challenges include: • A 40% increase in population over the next 20-30 years; • Climate change resulting in sustained drier periods and extreme rainfall events; • The need to reduce energy requirements to lower greenhouse gas emissions; and • How to integrate more sustainable water systems into existing infrastructure, so as to reduce capital expenditure. Rainwater Tank Energy Consumption Survey Decentralised systems are often considered the panacea to these problems, by offering lower transport requirements for water, new non-traditional sources of water, and flexibility in how they integrate into existing infrastructure. However, a recent survey of the energy used by rainwater tanks by the University of Technology Sydney (UTS) (http:// utsescholarship.lib.uts.edu.au/iresearch/scholarly-works/ bitstream/handle/2100/888/retamaletal2009waterenergynexus. pdf?sequence=4) has shown that the actual energy use varied from 0.9--4.9kWh/kL, with a typical energy use of 1.5kWh/kL. This compares to specific energy consumptions of <1kWh/kL for delivery of drinking water in most Australian capital cities (the exception being Adelaide at 1.84kWh/kL) and around 1kWh for brackish water desalination. Clearly, while the theoretical energy required for transport of water from a rainwater tank to household end uses should be lower than sourcing it from distant supplies, this is not currently realised. There is a need to improve the design of rainwater tank systems, and the technologies used in the widespread roll out of water tanks, if lower energy water systems are to be achieved. The issue can be addressed by the use of more efficient pumps and gravity systems in low pressure applications. These approaches need to be promoted through planning policies, so that more of the potential benefits that rainwater tanks can afford may be realised. It also underscores the need for monitoring the performance of alternative water systems, as called for by Ted Gardner in his March 2011 "My Point of View", as there can be large differences between actual and theoretical performances. Small-Scale, High-Cost Solutions Small-scale wastewater treatment plants, often in the basements of large high-rise buildings, are also seen by many as a more sustainable solution for suppling recycled water. Indeed, star rating systems for buildings, such as BASIX, give credit for the inclusion of such systems. While they are capable of supplying high quality water, the cost is usually considerably higher than potable water supplies, with production costs of $20/kL having been claimed in some instances. The high production costs of these systems are associated with the need for compliance monitoring and regular site visits for maintenance. In order for these systems to be financially competitive with centralised reuse systems, improved on-line monitoring and process diagnostics are required. There has been a significant research effort for development of new sensors nationally via the Environmental Biotechnology CRC and CSIRO, as well as international efforts. As yet this has not led to significant outcomes in the reduction of operating costs for small-scale wastewater treatment systems. Perhaps it is time for a targeted research program to identify specific sensors and control strategies to reduce the operational costs of distributed small- scale reuse systems so as to lower their operating costs. In locations where expensive augmentation of distribution and/or collection systems is required, then the use of small- scale reuse systems may be cost competitive. However, in these situations the cost of these systems should be compared against demand management and peak levelling approaches, as these concepts are also suitable under such circumstances. Another approach to decentralised systems is to use stormwater, but to date there are very few working examples available to demonstrate the implementation issues of these systems. As the increasing interest in these systems leads to demonstration sites, monitoring programs along the lines of Is Sustainability Scalable? Professor Stephen Gray, Director of the Institute for Sustainability and Innovation, Victoria University.
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