Water Journal : Water Journal April 2013
WATER APRIL 2013 66 Feature Article concentrations should be adopted for risk assessments in the interim. Appropriate elements of an effective stormwater treatment train depend on the intended end-uses as summarised in Table 1 (Wong et al., 2012). It has been recognised that WSUD stormwater systems (such as ponds, swales, wetlands and bio lters) should be used at the very start to capture and pre-treat stormwater. These stormwater treatment elements provide a buffer to the highly variable pollutant concentrations, and bring stormwater quality to a predictable and consistently narrow range. For example, stormwater bio lters are able to reduce Total Suspended Solids (TSS) to around 5mg/L (Bratieres et al., 2008), while reducing metal concentrations below drinking water targets in most cases (Feng et al., 2012). However, to meet current national stormwater harvesting guidelines, the following applies: • If water is used for restricted or drip irrigation, no additional treatment/ disinfection is needed; • For all other uses disinfection is needed and for high level exposure uses (e.g. toilet ushing, cooling etc.) further treatment is also necessary. Novel WSUD systems are under development to target micro-pollutant and pathogen removal. They are mainly based on further enhancement of bio lter design to incorporate novel media for pathogen inactivation (Li, Deletic & McCarthy, 2012) and plant species that have anti-microbial properties (Chandrasena et al., 2012). Storage of stormwater in con ned and highly valued urban space is another issue. Aquifer Storage and Recovery (ASR) should be explored as the rst preference when site conditions are favourable, since such schemes are usually the most cost effective. Schemes involving other forms of storage (e.g., underground and aboveground tanks, ponds, lakes, etc.) should be generally designed to meet a moderate volumetric reliability to limit storage costs; e.g., if the mean annual runoff volume is well in excess of the water demand at a site, it is generally optimal for stormwater harvesting schemes to meet between 50 to 80 per cent of total water demand (Mitchell et al., 2006). The site rainfall variability and demand regime (i.e., seasonal or non-seasonal) are the most important factors in determining the storage size required, and modelling using very ne resolution of rainfall data is necessary for reliable storage design (Mitchell et al., 2008). Sewage treatment plants will be places of recovery of resources including energy, water and nutrients. Localised wastewater recycling will be linked to local productive landscapes as a means of passive nutrient recovery. There has been a clear move across the industry to recognise that there is no such thing as 'wastewater' any more, but there could be 'wasted water' if it is not adequately used and re-used. In this context, the transition in strategic directions and long-term planning at a number of utilities from wastewater treatment to resource recovery is one of the obvious signs of such a shift in thinking. In economic terms, the major bene ts at present can be gained from water recovery, with energy and nutrient recovery gaining increasing relevance with the higher power costs and also rising value of nutrients, such as phosphorus, nitrogen and even potassium. The magnitude of the economic bene ts has already been recognised by many industries such as breweries, food producers, re neries, etc. There have been several water recycling systems installed in the last 10 years in such companies, simply for the direct economic bene t they can generate by reducing the water intake and the trade waste discharge costs, which clearly outweigh the operational and even amortisation costs of water recycling installations. In the process, there are often valuable additional returns generated by the energy recovery (mainly through biogas from anaerobic systems), as well as direct operational bene ts in the processing side, due to better control of the water quality from the water recycling systems. There are also direct bene ts that can be gained in the public sector from such resource recovery initiatives. Water recycling could, for example, off-set water production from surface waters, providing either improved environmental situations through more ows in rivers, but it could also increase the available water supply security from a given dam or catchment area. This can reduce the need for alternative water supplies such as desalination in case of droughts, but it can also increase the ood mitigation capacity of large dams such as Wivenhoe or Warragamba, which may be highly bene cial in ood situations too. At present there are also major efforts underway into improving the energy ef ciency of the public water systems due to the rapidly increasing electricity costs across Australia. Here, the reduction of energy required for wastewater treatment is going hand-in-hand with the increased energy recovery within these plants. This is where substantial new developments will take place over the coming years and the CRC-WSC has projects in this area to assist the partners and the industry with making this shift in an optimal way. In the context of resource recovery there are often also strong views expressed regarding the need to have decentralised processing rather than the current more centralised treatment systems. However, these 'fundamental' debates often ignore some key factors that need to be taken into consideration. Most importantly, public health can never be compromised in any water supply or sewage collection and treatment option. Using a decentralised approach can drastically increase the challenges in achieving this fundamental goal, particularly when dealing with combined sewage or blackwater (from toilets). The same can be true for water supply options from potentially compromised sources. On the other hand, water recycling for non-potable applications is likely best achieved at a more decentralised level, possibly even at neighbourhood or household level. This minimises the need for large distance piping and pumping requirements and can be implemented in a very exible way. When it comes to energy and nutrient recovery, the need for integration of various scale processes is even more evident. While anaerobic processes and biogas utilisation for power generation are clearly more attractive and viable at a reasonably large scale (thousands of properties to whole city catchments), the recovery of heat energy may likely be best done at the household level. Newly emerging technologies, such as recirculating showers, not only save potentially up to 80 per cent of the water use for a shower, but the energy saving is of a similar magnitude and far more valuable from a nancial perspective. These examples demonstrate that future water systems, in order to be more sustainable, will very likely need to optimally integrate various processes across all scales and provide solutions that are suitable for a range of water sources and end uses. CITIES PROVIDING ECOSYSTEM SERVICES Ecosystem services refer to the concept by which humans derive bene t from surrounding ecosystems. Historically, cities have always depended on ecosystems beyond the limits of the urban area; however, with rapid urbanisation the distinction between pristine ecological systems and urban areas has become less de ned (Bolund and Hunhammar, 1999).
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