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? Water flows through the Southern California desert from the Colorado River to the Los Angeles area.
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.
