Water efficiency I: Cities
by Petrus L. Du Pisani
"With the new plant, Windhoek is able to incorporate the allowed maximum 35% reclaimed water into its potable water mix at all times. (...) The City of Windhoek is indeed proud of what it has achieved against the odds of natural adversity."
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Located in the southwestern part of Africa, Namibia has a number of distinguishing features, such as natural beauty and wildlife. With a surface area of 825,000 km2 (318,534 mi2) and a total population of 1.8 million, it is one of the most sparsely populated countries in Africa. It is also the most arid country in sub-Saharan Africa.
The total length of Namibia's western coastline, on the Atlantic Ocean, is covered by the Namib Desert, one of the oldest deserts on Earth. In the southeast, Namibia shares the Kalahari Desert with South Africa and Botswana. Although the desert has major advantages in terms of tourism, because of its rare beauty, it is also indicative of the prevailing climatic conditions.
The only perennial rivers to be found in Namibia are on the northern and southern borders of the country, respectively 750 and 900 km (~466 and 560 miles) from Windhoek. In the interior of the country, there are only ephemeral rivers--that is, rivers that run for only a few days at a time, after heavy rainfall events.
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The average annual rainfall is 360 mm (~14.4"). The annual evaporation averages 3,400 mm (~136"). The city currently relies for 70% of its water on three surface reservoirs built on ephemeral rivers. The Omatako, Von Bach, and Swakopoort Dams that create these reservoirs are between 70 and 160 km (~43.5 and 99 miles) distant from Windhoek and are operated by a para-statal water utility called Namwater. They were built from 1978 to 1993 to supply water to central Namibia.
Only three of the last 10 rainy seasons yielded above-average inflow into these reservoirs. The efficiency of these reservoirs is also such that the main "consumer" of water is evaporation, which accounts at times for twice as much water as that utilized by consumers. Security of water supply to central Namibia and the City of Windhoek is therefore a major challenge, both for the bulk water supplier, Namwater, and the City of Windhoek.
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Groundwater remained the sole source of water for Windhoek until 1933 when the Avis Dam, with a capacity of 2.4 Mm3 (~634 MG), was constructed. This dam has a small catchment area and therefore a very small assured yield. From 1962 to 1973 its maximum contribution amounted to only 2.4% of Windhoek's consumption; often, it could not supply any water at all. Thus, Avis Dam is currently used exclusively for recreational purposes.
During 1958, a second small surface reservoir, the Goreangab Dam, with a capacity of 3.6 Mm3 (~951 MG), was built downstream from Windhoek, and a conventional treatment plant was constructed to treat the surface water from this reservoir to potable standards. During 1960, the City commissioned its new 6,800 m3 (~1.8 MG) per day Gammams Sewage Purification Plant on a site adjacent to the Goreangab Dam, to deal with the city's domestic and industrial effluents. Until 1963, these two waste streams were treated together; in 1963, a series of anaerobic and aerobic oxidation ponds were added to Gammams, and the bulk of the industrial effluent load was diverted to these ponds for treatment. Throughout the 1960s, these waste streams, following treatment, were mingled and discharged to a dry river bed; the reclaimed water was not used for potable or other purposes. By 1967, however, the City was seriously considering potable reuse of reclaimed water, and the Gammams Works were extended to provide 14 days retention time in the maturation ponds, thereby improving the quality of final effluent for eventual use as influent to a water reclamation plant.
In 1969, the conventional Goreangab treatment plant was converted to treat not only the surface water from the Goreangab Dam, but also the final effluent from what was by this time called the Gammams Wastewater Treatment Plant (WWTP) in two separate treatment trains (the bulk of the city's industrial effluents were still being diverted from domestic effluents and treated separately). Thus, the Goreangab Reclamation Plant was born. It had an initial capacity of 4,300 m3 (~1.1 MG) per day. This reclaimed water was blended with water from the city's well field and was delivered as drinking water to the city's residents. At this initial stage, reclamation could account for up to 25% of the City's water consumption.
After Independence in 1990, however, the population of Windhoek started growing at a more rapid rate, currently accepted as 5% per annum. This, together with increased investment and development in the City, placed ever increasing pressure on the supply of water. As one way to cope with this pressure, the City instituted progressive water pricing structures and educational programs aimed at reducing average consumption; these met with considerable success. On the technical side, the Goreangab Reclamation Plant went through a series of upgrades, the latest being completed during 1997. The ultimate capacity of this "Old" Goreangab Plant was 7,500 m3 (~2 MG) of potable water per day. The process train in its ultimate configuration comprised the following steps:
However, these innovations were more than offset by the city's population growth. Furthermore, the whole City, including its informal settlements, lies upstream from and within the catchment of the Goreangab Dam, and its growth has created pollution that has seriously compromised the water quality in this reservoir. In fact, the surface water runoff captured by Goreangab Dam is often of a worse quality than the treated sewage from the Gammams WWTP and is therefore at times unfit for reclamation by the process outlined above.
Clearly, something had to be done. As the easily accessible natural resources had to a large extent been fully exploited, and demand management measures successfully implemented, extended reclamation proved to be the logical choice to augment supply. However, it was determined that attempting to upgrade the existing reclamation plant yet again would not be cost-effective and prevailing drought at the time precluded the interim loss of production from the old plant; thus the City of Windhoek obtained loan finance from European Financial Institutions to construct a new 21,000 m3 (~5.5 MG) a day reclamation plant, on a site adjacent to the old plant. Completed in 2002, this plant can now provide 35% of the daily potable requirements of the City.
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Non-treatment barriers are procedures that occur outside the actual water treatment process but have an effect on the final quality of the treated water. Non-treatment barriers in the Goreangab Reclamation Plant include:
Operational barriers are back-up processes that can be implemented if needed to boost the efficiency of the treatment barriers. For example, if the organic load in the water/effluent mix reaching the reclamation plant is exceptionally high, it could decrease the effectiveness of the activated carbon filtration steps to unacceptably low levels. In this case, additional powdered activated carbon is added in order to maintain the desired levels of organics removal.
Treatment barriers are "continually present systems that reduce the undesired substances in the water to an acceptable level." That is, these are the physical processes that remove actual contaminants from the water being treated.
Because the danger to health associated with specific contaminants varies significantly, different numbers of treatment barriers have been specified for different classes of contaminant:
Complete barriers can include more than one treatment process. A single complete barrier for turbidity, for example, is considered to be a combination such as flocculation, dissolved air flotation (DAF) and dual media filtration, even if turbidity is not reduced by one hundred percent. On the other hand, these three steps are not considered to be a complete barrier for chemical oxygen demand (COD) and dissolved organic carbon (DOC), but rather as only a step in their partial removal.
The design of the plant incorporated both the experience gained in Windhoek over 30 years of reclamation and newly developed processes such as ozonation and membrane ultra-filtration. The latter two processes were pilot-tested on-site over a period of 30 months, so that the system's performance with Windhoek's specific raw water could be thoroughly tested and design decisions could be based on actual recorded results. During these trials and the design process, the City of Windhoek sought the specialist advice of recognized experts in the fields of ozone, membranes and granular activated carbon/biological activated carbon (GAC/BAC), as used in the process. The operational protocol of the plant provides for the multiple barrier system to be operational at all times.
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The new processes incorporated in the plant can be briefly described as follows:
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Dr Lucas van Vuuren, a pioneer of water reclamation in South Africa during the 1970's, coined a phrase: "Water should be judged not by its history, but by its quality." To get public acceptance of this viewpoint, however, is not easy; it places a severe responsibility onto the City to exercise the required level of ongoing control over water quality. For this purpose, the City has over the years invested substantially in laboratory facilities and manpower. The Gammams Laboratory of the City's Scientific Services Division boasts state-of-the-art facilities and analytical equipment to ensure customers' continuing comfort with using the treated water.
The fact is that reclaimed water is widely used for aquifer recharge, but often in such a way that it loses its identity as sewage water. Water Factory 21 in Orange County, California, is a prime example, where reclaimed water is injected into the coastal freshwater aquifer to prevent intrusion of saline seawater into the aquifer. This reclaimed water ultimately re-enters the drinking water stream. Also, treated sewage is often discharged to rivers and lakes. It is not uncommon that downstream users reuse such releases which are then treated as natural water through conventional treatment methods. These are in fact instances of indirect reclamation or reuse, even though the water in question has lost its "sewage identity." At Goreangab, the history of the feed water is specifically recognized as treated sewage and treatment is designed to cope with just that. In that sense, the very aspect of this water that initially made people reluctant to use it, is in fact the aspect that leads to its being adequately treated to ensure its quality.
To retain public confidence, water quality at the Goreangab Treatment Plant is monitored on an ongoing basis through on-line instrumentation as well as through collection of composite water samples after every process step. In the event of any quality problems (specified target values that are not met), the plant goes into recycle mode and water is not delivered. The final product water is also continuously sampled by way of online instrumentation and composite sampling and is analyzed for the full range of currently recognized contamination parameters, including pathogens such as viruses, Giardia and Cryptosporidium. (The City recognizes that viruses also constitute a real threat, and these are equally monitored and studied as an ongoing research project; however, virus studies worldwide are mostly in the research phase and much work needs to be devoted to this subject before more specific treatment barriers can be designed. Currently ozonation and ultrafiltration and chlorination combined are deemed to form an effective barrier for viruses.)
With the new plant, Windhoek is able to incorporate the allowed maximum 35% reclaimed water into its potable water mix at all times. Whenever possible, the rest of the City's potable water is taken from surface water. The boreholes that tap the underlying aquifer are the city's savings account, used only when insufficient surface water is available. A normal scenario would then be 25 to 35% reclaimed plus 65 to 75% surface. Under stress, borehole water could account for up to 23% of total consumption, with another 35% from reclaimed water and the rest from surface water resources. During times of water stress, of course, Windhoek also steps up water demand management measures and consumption is reduced substantially.
The citizens of Windhoek have over time become used to the idea that potable reuse is included in their water provision process. In fact, they have grown to harbor a fair amount of pride in the fact that their city in many respects leads the world in direct reclamation.
The new plant went into operation in August 2002, and the City has procured a private sector partner for a 20 year Operation and Maintenance Contract for this plant. The plant is currently shut down due to a failure of the oxygen generation system (PSA) but delivered water of excellent quality over its first year of commercial operation.
Goreangab is an excellent example of one of the innovations practiced in a country with few resources, either natural or financial. The City of Windhoek is indeed proud of what it has achieved against the odds of natural adversity.
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Mr. Petrus L. du Pisani, Professional Engineer, is the City of Windhoek's Strategic Executive for Infrastructure, Water and Technical Services. You can reach him for comment by email at: email@example.com
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City of Windhoek
Windhoek's official web site contains extensive information about all aspects of the city, including the Goreangab Reclamation Plant.
Orange County Water District Projects: Water Factory 21
Details about the Water Factory 21 project that is mentioned in Mr. Du Pisani's article.
This Wild World Ecoregional Profile from the National Geographic and the World Wildlife Fund provides a good description of the Namib Desert, along with links to more extensive information.
About the Arid Lands Newsletter