direct reclamation of potable water at windhoek's goreangab reclamation plant

Presented at the International Conference on Integrated Concepts on Water Recycling, Wollongong, NSW, Australia,14–17 February 2005.

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved

Desalination 188 (2006) 79–88

Direct reclamation of potable water at Windhoek’sGoreangab reclamation plant

Petrus L. du PisaniStrategic Executive Infrastructure, Water and Technical Services, City of Windhoek, Namibia

Tel. +264 (61) 290-2335, Fax: +264 (61) 290-2114, email: [email protected]

Received 14 December 2004; accepted 29 April 2005

Abstract

The Windhoek Goreangab reclamation plant has been a pioneer in direct potable reclamation and is still today, asfar as known to the author, the only commercial-scale operation in existence. From a humble beginning in 1969 untilthe commissioning of a new high technology plant in 2002, Windhoek has managed to do applied research and theoriginal plant had been upgraded on a number of occasions. In any discussion on water reclamation from wastewater,the name Windhoek and its Goreangab reclamation plant enjoy a fair amount of recognition. An article by I.B. Law,published in May 2003 in the publication Water of the Australian Water Association was titled, Advanced Re-use —from Windhoek to Singapore and Beyond. Windhoek in Namibia, was indeed the pioneer in direct potable reuse.

Keywords: Arid climate; Direct potable reclamation

1. Introduction

The City of Windhoek is the capital ofNamibia, which is located in southwest Africa.Namibia has a number of distinguishing featuressuch as natural beauty and wildlife. One otherfeature is that it is the most arid country in sub-Saharan Africa. The country has a surface area of825,000 km2 and has a total population of 1.8million, making it one of the most sparselypopulated countries in Africa. There are only

ephemeral rivers in the interior of the country.The only perennial rivers are on the northern andsouthern borders of the country, respectively, 750and 900 km from Windhoek (see Fig. 1). Thetotal length of the western coastline on the Atlan-tic Ocean is covered by the Namib Desert, one ofthe oldest deserts on earth. In the southeast,Namibia shares the Kalahari Desert with SouthAfrica and Botswana. Although the desert hasmajor advantages in terms of tourism, because of

doi:10.1016/j.desal.2005.04.104

P.L. du Pisani / Desalination 188 (2006) 79–8880

Fig. 1. Location of Namibia and perennial rivers inAfrica.

its rare beauty, it is also indicative of the pre-vailing climatic conditions.

2. City of WindhoekWindhoek is also the seat of government. The

population of Windhoek is approximately250,000, which in the context of the country,makes it a city. It is situated almost in the centreof the country.

The average annual rainfall is 360 mm or14.4". The annual evaporation amounts to3400 mm or 136". The city relies for 70% of itswater on three surface reservoirs. These reser-voirs are built on ephemeral rivers, these beingthe rivers that run only for a few days after heavyrainfall events. The three dams range frombetween 70 and 160 km from the city and areoperated by a parastatal water utility calledNamWater. These dams were built during theperiod from 1978 to 1993 to supply water to thecentral areas of Namibia. Windhoek utilizes

approximately 90% of the water consumed in thecentral areas.

During the last 10 rainy seasons, only onaverage three out of ten seasons yielded aboveaverage inflow into these dams. The efficiency ofthese reservoirs is also such that the main “con-sumer” of water is evaporation, which accounts attimes for a volume double that of the waterutilized by consumers. Security of the watersupply to the central areas of Namibia and theCity of Windhoek is therefore a major challenge,both for the bulk water supplier, NamWater andthe City of Windhoek.

3. Innovations in an arid landThe reason why a settlement originated at

Windhoek was the presence of both hot and coldwater springs. As the settlement grew, so didexploitation of these sources with the addeddigging of wells in the area. The water tablesubsided as a result, and the first municipalborehole was acquired around 1912. Over theperiod from 1912 to today, some 60 municipalboreholes have been developed in an aquifer witha safe assured yield of 1.73 Mm³/y.

Groundwater remained to be the sole source ofwater for Windhoek until 1933 when the AvisDam with a capacity of 2.4 Mm³ was constructed.This dam has a small catchment area andtherefore a very small assured yield; it oftencould not supply any water at all. This dam iscurrently used exclusively for recreationalpurposes.

During 1958, a second small surface reservoir,the Goreangab Dam, with a capacity of 3.6 Mm³,was built and a conventional treatment plant wasconstructed to treat the water from this reservoirto potable standards.

4. Goreangab reclamation plantIn 1969, this plant was converted to treat not

only the water from the Goreangab Dam, but also

P.L. du Pisani / Desalination 188 (2006) 79–88 81

Fig. 2. Old Goreangab process train.

the final effluent from the City’s Gammamswastewater treatment plant. The Goreangab recla-mation plant was born. It had an initial capacityof 4,300 m³/d. This reclaimed water was blendedwith water from the well field.

Due to the fact that the whole city as well asits informal settlements lie within the catchmentof the Goreangab Dam, the quality of the waterfrom this reservoir is often of a worse qualitythan the treated sewage and therefore unfit forreclamation.

From its inception, one of the cornerstones ofreclamation was that the city separated industrialeffluent from domestic effluent and divertedindustrial effluents to a separate treatment plant.This is regarded as a first barrier. The effluentused for reclamation, therefore, originates mostlyfrom domestic and business wastewater.

The Goreangab plant went through a series ofupgrades, the latest being completed during 1997.

The ultimate capacity of the “old” Goreangabplant was 7 500 m³/d of potable water. The pro-cess train in its ultimate configuration is depictedin Fig. 2.

5. New Goreangab water reclamation plant

After independence in 1990, the population ofWindhoek started growing at a more rapid rate,currently accepted as 5% per annum. This, toge-ther with increased investment and developmentin the city, placed ever-increasing pressure on thesupply of water. As the easily accessible naturalresources had to a large extent been fully exploi-ted and demand management measures success-fully implemented, extended reclamation provedto be the logical choice to augment supply. Forthis purpose, the City of Windhoek obtained aloan from European financial institutions to con-struct a new 21,000 m³/d reclamation plant on a


site adjacent to the old plant. This plant can nowprovide 35% of the daily potable requirements ofthe city.

5.1. Multiple barrier system

The design philosophy for the new plant isbased on a multiple barrier system. In this systema certain number of safety barriers are set up,depending on the risk associated with a particularsubstance or contaminant in the water to the user.These barriers can be one of three types: treat-ment, non-treatment or operational. The non-treatment barriers in the Goreangab reclamationplant include:C thorough policing and diversion of industrial

effluents to a separate treatment plant;C complete monitoring at the inlet and outlet of

the sewerage treatment plant, allowing actionto be taken before the water reaches the recla-mation plant;

C extensive monitoring of drinking waterquality;

C blending the water derived from reclamationwith water of different origins so that at most35% of the drinking water is reclaimed water.

Apart from these barriers, operational systemsare in use that take on the role of a further barrier.One example of an operational barrier is thepossibility of dosing powdered activated carbonin case the adsorption capacity of the granularactivated carbon (GAC) in the process train dropstoo low or the organic loading to the plant is toohigh. Treatment barriers are defined as “con-tinually present systems that reduce the undesiredsubstances in the water to an acceptable level”. Itis clear that a complete removal of impurities ispractically impossible without reverse osmosis,so the barriers are designed to reduce concen-trations of substances to fall within drinkingwater guidelines.

A single complete barrier for turbidity is con-sidered to be a combination such as flocculation,

dissolved air flotation and dual-media filtration,even if turbidity is not reduced by 100%. On theother hand, these three steps are not considered tobe a barrier for chemical oxygen demand anddissolved organic carbon, but rather only as a stepin their partial removal.

As the danger associated with specificimpurities varies significantly, different numbersof barriers have been defined:C For aesthetic parameters, such as turbidity and

colour, for which there is no direct correlationbetween them, and detrimental effects onhealth, two complete barriers;

C For microbiological pollutants, three barriers;C For parameters without health risk, such as

calcium carbonate, only one barrier wasimplemented.

The design of the plant is based on the exper-ience gained over 30 years of reclamation, butalso includes new processes such as ozone andmembrane ultrafiltration. The latter two processeswere pilot tested on site over a period of30 months, whereby the performance with thisspecific raw water was thoroughly tested anddesign decisions could be based on actual re-corded results.

During these trials and the design process, theCity of Windhoek sought the advice of recog-nized experts in the fields of ozone, membranesand GAC/BAC, as used in the process. Theoperational protocol of the plant makes provisionfor the multiple barrier system to be operationalat all times.

5.2. Selection of the processes

The new Goreangab reclamation plant is madeup of a series of processes, as represented inFig. 3. Perhaps the most important cornerstone ofpotable reclamation is public acceptance and trustof consumers in the quality of this water. Themost difficult thing for anyone wanting to


Fig. 3. New Goreangab process train.

emulate what is done at Goreangab in Windhoek,would be to break down the psychological barrierto the principle of direct re-use for potablepurposes.

Van Vuuren, a pioneer of water reclamation inSouth Africa during the 1970s, coined a phrase:“Water should be judged not by its history, but byits quality.” To get public acceptance of thisviewpoint is, however, not easy and it places asevere responsibility on the city to exercise therequired level of control over quality. For thispurpose, the city has over the years investedsubstantially in laboratory facilities and man-power. The Gammams Laboratory of the city’sScientific Services Division therefore boastsstate-of-the-art facilities and analytical equipmentto provide customers with a level of comfort thatthey are prepared to accept.

Reclaimed water is widely used for aquiferrecharge, during which time it loses its identity as

sewage water. Water Factory 21 in OrangeCounty, California, is a prime example, wherereclaimed water is injected into the aquifer toprevent saline intrusion into the fresh wateraquifer. This water ultimately re-enters thedrinking water stream in an indirect manner.Often treated sewage is discharged to waterbodies such as rivers and lakes. It is not un-common that downstream users re-use suchreleases which are then treated as natural waterthrough conventional methods. Although it haslost its sewage identity, this could be termedindirect reclamation or re-use. At Goreangab, thehistory of the feed water is recognized as treatedsewage and treatment is designed to cope withjust that.

To retain public confidence, quality monitor-ing and control are of utmost importance. Waterquality is monitored on an on-going basis throughon-line instrumentation as well as composite


samplers after every process step. In the event ofany quality parameter exceeding an absolutevalue, the plant goes into recycle mode and wateris not delivered. The final product water is alsocontinuously sampled by way of online instru-mentation and composite sampling and analyzedfor the full range of parameters, inclusive of thepathogens Giardia and Cryptosporidium. Where-as viruses constitute a real threat in recycledwastewater, these are equally monitored andstudied as an on-going research project. Virusstudies worldwide are not plentiful or are in theresearch phase, and much work needs to bedevoted to this subject.

The public of Windhoek is furthermore wellinformed through the media and a municipalnewsletter of the importance of wise water use.An on-going water demand management cam-paign is aimed at and has been successful inreducing water consumption to the extent that itis currently estimated that a further reduction indemand of 13% would lead to an adverse effecton the economy of the city. Reclamation is wellpublicized and the new plant was launched by thePresident of the Republic of Namibia, with fullTV and media coverage. The plant is often visitedby schools and the scientific community, local aswell as international. No consumer surveys havebeen conducted, but over the last 6 years, noconsumer complaints have been lodged about theuse of or quality of reclaimed water in Windhoek.The citizens of Windhoek have over time becomeused to the idea that potable re-use is included inthe water provision process and they harbour afair amount of pride in the fact that their city inmany respects leads the world in directreclamation.

7. Water quality guidelines

Due to the fact that direct potable reclamationis not widely practiced, specific water qualityguidelines for reclaimed potable water were not

readily available. The city therefore selected fromrelevant drinking water standards such as theNamibian Guideline, USEPA, EU, WHO andRand Water (South Africa), and compiled aspecification for treated water.

In order to ensure optimal performance of theprocess steps, intermediate treated water criteriawere stipulated. These are aimed, for instance, atmaximum organic removal through enhancedcoagulation to extend carbon life and the effectiveremoval of iron and manganese to protect themembranes. These criteria consist of target valuesand absolute values that have to be maintained.Failure to meet target values attracts performancefailure penalties as described hereunder. Failureto meet absolute values precludes the delivery ofwater and causes the plant to go into recyclemode, until such target values are reached. Thetreated water specifications and the intermediatetreated water criteria are given in Tables 1 and 2.

8. Operation and maintenance

From the inception of the project, it had beenenvisaged that the city itself would operate thenew plant in the way it had operated the recla-mation plant from 1969. The loan conditions ofthe European Investment Bank, however, dictatedthat the operation and maintenance of the plantwould include the involvement of internationallyrecognized water sector players. This involve-ment would be for the term of the loan, whichwas 20 years.

As construction was already in progress, theoptions of BOO, BOT or BOOT were not avail-able and the City of Windhoek decided on anoperation and maintenance model. The cityobtained the services of Stallard Burnsbridge,later Katalyst Solutions, to manage the inter-national procurement process for an operator thatwould satisfy the criteria of the loan agreement.

The ultimate winner of this bidding processwas a consortium of Vivendi Water (now Veolia),


Table 1Treated water specifications

Units Target values(maximum UNO)

Absolute values(maximum UNO)

Physical and organoleptic constituents:

Calcium carbonate precipitation potential CaCO3 mg/l N/A Must lie in range 0 to 8Chemical oxygen demand mg/l 10 15Colour mg/l Pt 8 10Dissolved organic carbon mg/l 3 5Total dissolved solids mg/l Greater of 1000 or 200

above raw waterGreater of 1200 or 250above raw water

Turbidity NTU 0.1 0.2UV254 Abs/cm N/A 0.06Macro elements:Aluminium Al mg/l N/A 0.15Ammonia N mg/l N/A 0.10Chloride Cl mg/l Not removed by processIron Fe mg/l 0.05 0.1Manganese Mn mg/l 0.01 0.025Nitrate and nitrite N mg/l Not removed by processNitrite N mg/l Not removed by processSulfate SO4 mg/l Not removed by processMicrobiological indicators: UnitsHeterotrophic plate count Per 1 ml 80 100Total toliforms Per 100 ml N/A 0Faecal coliforms Per 100 ml N/A 0E. Coli Per 100 ml N/A 0Coliphage Per 100 ml N/A 0Enteric viruses Per 10 l N/A Greater of 0 per 10 l or a 4

log removalFaecal streptococci Per 100 ml N/A 0Clostridium spores Per 100 ml N/A 0Clostridium viable cells Per 100 ml N/A 0Disinfection by-products:Total trihalomethanes µg/l 20 40Biological indicators:Chlorophyll a µg/l N/A 1Giardia Per 100 l Greater of 0 per 100 l or a

6 log removalGreater of 0 per 100 l or a5 log removal

Cryptosporidium Per 100 l Greater of 0 per 100 l or a6 log removal

Greater of 0 per 100 l or a5 log removal

Note: Other parameters that are not included in Table 2 will be required to comply with the Rand Water Standards (RSA)for potable water as valid at the effective date. The treated water will not exceed the lower of the RSA limits or thebackground concentration for those parameters as found in the raw water.


Table 2Intermediate treated water criteria

Treatment process/parameters

Units Target values Target values(maximum UNO)a

Absolute values(maximum UNO)

After DAF:Turbidity NTU 1.5 (exceeded by no more

than eight readings in oneday; readings are taken at15-min intervals)

5.0 (exceeded by no morethan four readings in oneday; readings are taken at15-min intervals)

5.0 (exceeded by no morethan four readings in oneday; readings are takenat 15-min intervals)

8.0 (absolute maximumpeak reading; readingsare taken by on-linemeasuring equipmentat preset intervals andcomputed for 15-minintervals as perSection 2.21 ofAnnexure 9 Part 1)

After rapid sand filters:Turbidity NTU 0.2 (exceeded by no more

than four readings in oneday; readings are taken at15-min intervals)

0.35 (exceeded by nomore than four readingsin one day; readings aretaken at 15-min intervals)

0.5 (absolute maximumpeak reading; readingsare taken by on-linemeasuring equipmentat preset intervals andcomputed for 15-minintervals as per Section2.21 of Annexure 9Part 1)

Manganese mg/l 0.03 0.05 N/AIron mg/l 0.05 0.05 N/AAfter ozonation:Ozone concentration mg/l Minimum (absolute

minimum trough asregistered by on-linemeasuring equipment)

COD mg/l 25 25 N/ADOC mg/l 15 15 N/AMicrobiological, disinfection by-products and

According to Treated Water Specification (Table 2)

After GAC filters:DOC mg/l 5 (exceeded by no more

than four readings;readings are taken at 15-min intervals)

5 (exceeded by no morethan four readings in oneday; readings are takenat 15-min intervals)

8 (absolute maximumpeak reading; readingsare taken by on-linemeasuring equipmentat preset intervals andcomputed for 15-min teintervals as per Section2.21 of Annexure 9Part 1)

aRefer clause 5.5.5 of Part 1 of Annexure 11.


Berlinwasser International and VA Tech Wabag.A contract, the Private Management Agreement(PMA) was concluded, with work starting inSeptember 2002, for a period of 20 years. Duringthis period, the manager is responsible for thetotal maintenance of the plant, all scheduledreplacements and specific hand back conditions,which are stipulated.

The PMA was crafted in a way that wouldprovide maximum incentive to the manager toreach quality guidelines and to produce accordingto the requirements of the City. Payment to themanager consists of two parts or tolls, being theaggregate of the availability toll and the volu-metric toll. Both these tolls are subject to per-formance failure factors and availability factors.

Failure to meet the daily treated waterrequirement would attract an availability factor(AF) of less than one, which would mean apercentage reduction in the availability rate. Thisrate could be further reduced through the appli-cation of performance failure factors (PFF),which could vary from 0.96 to 0.98 for everyquality parameter breached. These PFFs areapplicable to breaches of both intermediate andfinal water quality specifications.

In the event that the raw water does not meetspecifications, the manager can opt to treat suchwater, but all quality guidelines/specificationsremain in force. The manager would, however, bepaid a raw water surcharge, based on the actualadditional cost of treatment. The manager wouldfurthermore pay for raw water consumed orwasted, and would pay a charge for the treatmentof effluents emanating from the treatment pro-cess. This is very strong motivation to reducewaste and to optimize the processes for maximumrecovery.

The payment due to the manager would thenbe computed as follows:Treated water toll for the day = (availability toll

for the day) + (volumetric toll for the day)+ (raw water surcharge) ! (raw waterconsumption) ! (wastewater treatment)

During the first year of operation, the plantproduced 5,955,347 m³ of high-quality water atan average treatment cost per m3 of N$2.76 (equalto US $0.46). If one the cost of capital is added,it would add a further N$1.84 to the price ofwater from the plant. The average price wouldthen be N$4.69 or US $0.77/m³. During the sameperiod, the cost of water from the national bulkwater supplier (surface water), was on averageN$4.36 (US $0.73).

In the case of Windhoek, however, the valueof water is not calculated in financial terms only,but in the value of security of supply. If one looksat the cost of alternatives, such as bringing waterfrom the perennial rivers on the border, the costof water increases fourfold. The investment inreclamation is therefore economically, financiallyand environmentally a sound one. What isevident, however, is that the cost of water is verysensitive to the quantity produced, and the Citystrives to have maximum production wheneverpossible.

9. Conclusions

The new plant went into operation in August2002, and is being operated by WINGOC on a20-year operation and maintenance contract. Theplant is currently shut down due to a failure of theoxygen generation system but has delivered waterof excellent quality over its first year of com-mercial operation.

From the Windhoek experience it is evidentthat treated domestic sewage can be successfullyre-used for potable purposes. In the case ofWindhoek, a combination of factors, with the lackof alternatives probably the most notable, makesdirect potable reclamation a viable option, even infinancial terms. It is furthermore evident that thetechnology exists to produce water which meetsall drinking water guidelines and standards and toprovide the user with an acceptable level ofconfidence as to the risk of potable re-use.


The main obstacle to newly introducingpotable reclamation will, however, be public per-ception. It is the perception of the author that onlyin cases where no viable alternatives exist will itbe possible to consider the introduction of directpotable reclamation. The use of reclaimed waterto replace the use of potable water in non-drinking water uses such as industry, agricultureor artificial recharge of aquifers, should howeverbe rigorously pursued.

The old Goreangab plant, on a reduced pro-cess train, provides reclaimed water for the irri-gation of all sports fields and public parks in thecity. In the very near future, all excess reclaimedwater will be used to artificially recharge theWindhoek aquifer, albeit under very strong qual-ity constraints.

Goreangab is an excellent example of one ofthe innovations practiced in a country with fewresources, both natural and financial. Directpotable reclamation has proven in Windhoek that,

if it is possible to overcome public prejudice, it isa viable option for arid countries and fits in verywell into the concept of integrated water re-sources management.

Bibliography

[1] J. Haarhof, C.J. van der Walt and B.F. van derMerwe, Process design considerations for the Wind-hoek water reclamation plant, unpublished report,1999.

[2] FMG–Joint Venture, Goreangab water reclamationplant, Design report to the City of Windhoek, 1998,unpublished.

[3] K Wolz, Reclamation of waste water at the newGoreangab water reclamation plant, Presented at the4th Biennial Congress of the African Division of theInternational Association of Hydraulic Research,Windhoek, Namibia.

[4] I.B. Law, Advanced Reuse — From Windhoek toSingapore and Beyond, Water, May 2003, p. 44.

direct reclamation of potable water at windhoek's goreangab reclamation plant

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