TECHNICALLY SPEAKING
Figure 2 (a) Unplanned reuse
2(b) Planned indirect potable reuse (IPR) of wastewater
same sort of constituents (sparingly soluble inorganic salts or ‘scalants’) as RO. Of all the various alternative desalination technologies developed, perhaps one of the most ingenious is the forward osmosis process. This process is based on the natural, and almost zero-energy, process of osmosis to draw water into an inert draw solution. The draw solution must necessarily be at a much higher concentration than the feed water, which can be challenging for a seawater feed, and must be recoverable to leave only harmless residual levels of it in the product water.
Figure 3 IPR plants in the world
fouling propensity changes correspondingly. However, notwithstanding the stated extensive and unplanned reuse of water evident in urban conurbations, a far greater impediment to widespread wastewater reuse is public perception. Indeed, there is only one place globally where there is direct, ‘toilet- to-tap’ water reuse and this is Windhoek, Namibia, where no aquifer exists.
Low-energy desalination An alternative to water reuse is the development of novel, low-energy desalination technologies. Desalination is limited by two different natural phenomena. Evaporative processes must overcome the very large latent heat of evaporation of water to produce steam, only part of which is recovered when the steam is condensed to produce the product condensate water. Membrane desalination must overcome the osmotic pressure of salt solutions, which increases with the salt concentration, for
wwtonline.co.uk Fig 4 Forward osmosis principle
water to be forced through the RO membrane. A number of alternative desalting technologies are under development, some of which have reached commercialisation and many of which involve membrane technology. Membrane distillation, for example, uses a membrane to permit evaporation of water at a lower temperature than the boiling point, thereby reducing the energy demand. However, the process still demands some energy and is subject to fouling by the
An example (figure 4) is ammonium carbonate (NH4CO3), which decomposes at temperatures of around 60ºC into the constituent gases of ammonia and carbon dioxide, allowing them to be recovered and reused.
Waste brine
It remains to be seen whether the novel low- energy desalination processes, many of which have at least reached proof-of-concept stage, are ultimately viable at full-scale. Regardless of whether such processes are ultimately commercialised and implemented, the energy demand has no bearing on the problematic waste brine stream generated.
In coastal areas this stream is returned to the sea, but such a practice has seen an alarming increase in seawater salinity is some regions of the world (such as the Arabian Gulf). Continued pressures on freshwater supplies may thus force a reluctant public into accepting much more widespread implementation of wastewater reuse. Given that the technology already exists, such implementation becomes an exercise in the management of expectation and risk. nnn
August 2013 Water & Wastewater Treatment 35
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