Water stressed cities are importing water and investing in desalination plants. Could treating sewage plant wastewater offer a local, energy-efficient way of securing water supply?
Stuart Khan, The Guardian, 10/13/14
Of all the clean water that our cities consume, roughly half of it flows down our sewers to sewage treatment plants where it is treated and released back to the environment. Conventional sewage treatment plants are designed to clean this water to a degree that can be discharged to rivers or the ocean without major environmental or public health impacts.
In many parts of the word, sufficient fresh water supplies are increasingly difficult to source. Water stressed cities now import water, pumped over large distances at a considerable energy cost. Los Angeles, for example, imports 8.9bn litres of water a day to meet the city’s needs.
Other cities, such as Ashkelon in Israel, are investing in seawater desalination to produce drinkable water. But this process is also highly energy intensive and its application limited to coastal locations. An alternative opportunity is to reclaim the water that we discharge from sewage treatment plants and treat that to a quality suitable for safe human consumption.
Reusing highly treated municipal sewage effluent is not a new idea. It has traditionally been achieved by a process known as indirect potable reuse (IPR). Examples of unplanned IPR exist throughout the world, such as in Adelaide. In such cases, conventional sewage treatment plants discharge effluents to rivers (in Adelaide’s case into the Murray-Darling Rivers), which are then used as drinking water sources for cities downstream. Alternatively, planned IPR usually involves treating the sewage effluents to a very high degree by advanced water treatment processes before releasing the purified water to a lake or groundwater system used for drinking water supply.
While planned IPR has been an important water supply strategy for a number of decades, an alternative approach, known as direct potable reuse (DPR) is now rapidly gaining favour in countries including the US, South Africa and Australia. This process refers to taking treated municipal wastewater from a sewage treatment plant and, after further treating it to a level suitable for drinking, re-depositing it directly back into a drinking water distribution system. It differs from IPR by not discharging the water back to an environmental system, such as a river, lake or aquifer, prior to re-extracting and reusing it for drinking water supply.
Until very recently, we used to point to the only one DPR scheme in the world, which has been operating in Namibia since 1968. But since 2011, new schemes have come online in the US at Cloudcroft (New Mexico), Big Spring (Texas) and Wichita Falls (Texas).
More significantly, a number of very large Californian cities such as San Diego, Los Angeles and Sacramento are now all actively considering the development of DPR schemes as a major contributor to future water supplies. Major changes to regulation (such as the California Water Code) have been implemented to facilitate these potential projects. This has been accompanied by significant research efforts on the part of the US water industry to address a number of key issues including enhanced treatment process reliability, regulatory requirements and issues related to public perception and acceptance.
Among the key advantages of DPR is that the water tends to be available much closer to the location at which it can be used, compared to water which must be imported over long distances. This advantage is often not as effectively realised for IPR schemes since the water must often be transported large distances (and usually up-hill) to a suitable lake storage or aquifer recharge site. Pumping water long distances and up-hill is a highly energy intensive process. Depending on the mix of energy sources available, this normally implies significant production of greenhouse gasses and associated climate change impact.
As an example, Big Spring, Texas is located in a highly arid environment and pumps drinking water approximately 100km from a small shallow reservoir, which the city must share with a number of other cities. The energy requirements of the DPR water treatment system are entirely offset by energy savings from the avoided raw water pumping. Furthermore, the overall drinking water quality has been improved by the reduced salinity afforded by the blending of reclaimed water after reverse osmosis treatment.
There is now considerable interest in the DPR concept in Australia. The Australian Academy of Technological Sciences and Engineering (ATSE) recently published a report titled Drinking Water through Recycling. In this report, ATSE noted that DPR offered numerous potential advantages as a component of future Australian water supplies. Consequently, ATSE argued that DPR should be accepted as a viable option which should be given equal consideration to other water supply improvement options in the country.
Dr Stuart Khan is associate professor at the School of Civil & Environmental Engineering, University of New South Wales