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The Water Wheel July/August 2003

WATER RECLAMATION

Leading edge process techno-logy was integrated into thedesign of the new 21 Ml/d

Goreangab Water ReclamationPlant, which was completed in 2002.It is internationally renowned as thefirst plant in the world to reclaimdomestic sewage for drinking waterpurposes. After 32 years it contin-ues to be the only plant in theworld to do so.

SCARCE RAW WATERRESOURCES

Namibia is the most arid country inSouthern Africa and continuouslyfaces serious water challenges.The initial Goreangab Water Recla-mation Plant was built by the Cityof Windhoek in 1968 to reclaimwater directly from domestic sew-age effluent as a supplement to

Windhoek’s very scarce raw waterresources. The plant can be fedfrom two sources, these being theGammams Sewage Treatment Plantand the Goreangab Dam. The rawwaters are blended and treated as asingle stream. In general, the objec-tive is to utilise a larger portion ofthe effluent from the sewage treat-ment plant than water from theGoreangab Dam.

The Goreangab Water Reclamation Plant in Windhoek is internationally renowned as the firstand only plant in the world to reclaim domestic sewage for potable use as a supplement to

Windhoek’s very scarce raw water resources.

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WATER RECLAMATION

HEALTH CONCERNS

Due to pollution in the catchmentarea of the dam, the quality of thewater had deteriorated over theyears to such an extent that con-ventional treatment methods couldno longer be applied. The twosources had to be combined andtreated in a single extended andupgraded new Goreangab WaterReclamation Plant.

During the early nineties, the needarose to augment the plant signifi-cantly and to upgrade its treatmenttrain with the latest technology toaddress the most recent healthconcerns.

GFJ, South African consulting engi-neers, formed a consortium withFichtner in Germany and Multi-Consult in Namibia and the FMGGoreangab Joint Venture was awarded

the technical responsibility for thischallenging project.

NEW TECHNOLOGY

Since the commissioning of the firstreclamation plant in 1968 a hugeamount of new information onvarious aspects of water treatmentbecame known. Experience ob-tained from the old plant, informa-tion from pilot plant studies and

The original water reclamation plant in Windhoek was developed by the National Institute for Water Re-search (NIWR) of the Council for Scientific and Industrial Research (CSIR) during the 1960s. The design

and operation was based on the results of a pilot plant operated by the NIWR in Windhoek over the period1964 to 1968.

After the Water Research Commission (WRC) was established in 1971 it immediately embarked on an ambi-tious research programme to expedite the development of the technology of water reclamation and to studythe health effects of such water.

A research facility was constructed by the CSIR at Daspoort water treatment works in Pretoria and theWindhoek plant was modified from time to time to incorporate advantages gained from the latest researchresults with financial assistance from the Water Research Commission.

In 1976 the Windhoek plant was upgraded and extended, again with financial support by the WRC. At the inau-guration ceremony Dr GJ Stander, the then chairman of the WRC, stressed the significance and the value of theground-breaking research at Windhoek with a quotation from an American scientist who (at a conference in1969) said: “Windhoek will become famous as the first city in the world to reclaim wastewater on a regular basis fordirect domestic reuse. This plant has been in operation since spring 1969 and the peoples of the world have taken littlenotice of its existence.

“Yet, in this same year that saw science and engineering land a human on the face of the moon, science and engineeringalso brought direct water reuse into reality. The question is, which event will have more intimate impact on our lives andthose of our children?”

LOOKING BACK AT THE BEGINNING…

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WATER RECLAMATION

knowledge of current technologieshad to be integrated into a processdesign which eventually lead to areclamation Plant producing waterof a final quality which is sustainablyfit for human consumption.

RESEARCH ANDDEVELOPMENT

The new treatment plant was de-signed using data from 400 days ofpilot testing and a comprehensivereview of international practices.

This research and development partof the project played an importantrole in eventually determining theprocess train and establishing im-portant design parameters. In thisregard, research was focused on theoptimal removal of organics andharmful pathogens from the water.This was achieved by operating anozone/activated carbon pilot plant, amembrane pilot plant and perform-ing some full-scale studies on theold plant with regard to the precipi-tation of organics at various pHlevels.

Following the research part of theproject, water quality concernswere categorised into the following:

Physical and OrganolepticMacro ElementsMicrobiologicalDisinfection By-products

BARRIERS

The process design was one of a‘multiple barrier’ process whereindividual barriers were establishedfor each of the groups mentionedabove. With a major emphasis be-ing placed on a safe final product,three barriers were set for patho-gens like Cryptosporidium and Giardia.The reduction in organic contentbefore chlorination to reduce theformation of trihalomethanes (po-tentially carcinogenic) also necessi-tated three barriers. Employing thisapproach for each concern wouldensure a final water complying withthe stringent standards required.

Following the establishment of adesign philosophy, the processesrequired to create the multiplebarriers had to be identified andsequenced into a final process de-sign. The old plant was taken as astarting point for processes cur-rently employed and a detailedevaluation of the existing plant indi-cated that the old treatment traincomfortably provided one treat-ment barrier against physical andorganoleptic parameters throughDissolved Air Flotation (DAF), set-tling, and filtration and two barriersagainst most microbiological andbiological parameters (two chlorina-tion steps).

To enhance the multiple barrierconcept further, the following spe-cific adaptations and additions tothe original process were imple-mented:

Powdered Activated Carbon(PAC) dosing, should one of the

The High Lift Pumping Station consists of two pumpsets. The pumping stationfurther houses the raw waterpumps with the raw water inlet works to the

right of the pumps, just below the clear water reservoir. Several on-lineinstruments are situated in the building for quality control purposes.

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The Water Wheel July/August 2003

WATER RECLAMATION

key processes such as ozone,membrane or Granular Acti-vated Carbon (GAC) filtrationfail. This process will only be anoperational barrier.The settling step was shown toadd very little benefit to thephase separation alreadyachieved by DAF and could beexcluded without any compro-mise to the overall plant per-formance.Ozone was added as an addi-tional step before GAC for itsability to destroy cysts, particu-larly Giardia and Cryptosporidium;further to oxidise organics, ironand manganese.Biological Activated Carbon (BAC)to enhance dissolved organics

removal without the regular highcost of regeneration of GAC.Membrane filtration provided anadditional barrier for both thebiological parameters and thephysical parameters.The removal of iron and manga-nese was specifically addressed,especially in view of its foulingeffect on the membrane proc-ess.

This philosophy resulted in theprocess train as shown in the dia-gram and described hereafter.

PROCESS TRAIN

The major features of the plant are:

PAC dosing as a standby barrierfor dissolved organics removal.The provision of pre-ozonationfor oxidation of iron and manga-nese. To achieve this, some ofthe ozonation capacity is avail-able at the inlet box.An acid dosing system (experi-mental) for flexibility when thepH needs to be adjusted forbetter organic precipitation.A ferric chloride dosing systemas well as a polymer dosing sys-tem for flocculation.

Two flocculation units with flex-ibility to vary the mixing intensity.A DAF system for removal ofalgae and other suspended parti-cles.Sand filtration with upstreamdosing of potassium permanga-nate and caustic soda to facili-tate manganese oxidation andremoval in the filters.An ozone facility, including theozone injection units, the ozonecontact tank, and a PressureSwitch Adsorption (PSA) plantfor oxygen production. This is abarrier to Cryptosporidium andGiardia and also oxidises organ-ics and makes it more suscepti-ble for adsorption onto theactivated carbon.A granular activated carbonfacility following ozone consist-ing of three separate steps – thefirst step to be used as a biologi-cal carbon facility and two GACunits to be used as an adsorp-tion facility. The lowering oforganic content is a priority asthis has further implicationsduring final chlorination.An ultrafiltration unit process asa final barrier against cysts andthe further lowering of organiccontent and turbidity.

PAC (Standby Process)

Pre-ozonation

Flocculation

Dissolved Air Flotation

Rapid Sand Filtration(Filter to Waste Facility)

Ozonation

Biological Activated Carbon

Granular Activated Carbon

Membrane Filtration

Chlorine Disinfection

Stabilisation

Existing Train Additions

Note: Sedimentation and secondary chlorinationwere omitted from the existing train

The ultrafiltration unit process, which acts as a final barrier againstharmful pathogens

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A chlorine dosing step with acontact time of one hour, and aresidual of at least 1 mg/l freechlorine when leaving the plant.Final stabilisation to obtain acalcium carbonate precipitationpotential of 4 mg/l is achievedusing caustic soda.

AUTOMATION

Automated processes, which re-quire limited operator interventionand a monitoring Supervisory Con-trol and Data Acquisition (SCADA)system, which stores a number ofcritical parameters for control pur-poses are important features of theplant.

QUALITY CONTROL

Due to the sensitive and multi-disciplinary nature of this TurnkeyProject, the Consultant employedsite personnel on a full time basis.Personnel from FMG supervisedconstruction and ensured that allquality control documents werecompleted. Specialist personnelwere utilised for each disciplinenamely civil, mechanical, electricaland instrumentation/control. In-spections were done on the mem-branes in Holland and on the PSAPlant in Italy. The Client and Con-sultant in Johannesburg witnessed a

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factory acceptance test on theSCADA.

The Contractor had to prove allintermediate and final guaranteevalues during a four week trial pe-riod. These guarantee values dealtwith all aspects of the project, fromprocess related values such as wa-ter quality constituents to engineer-ing issues such as the correct back-wash rates and final water produc-tion rates. During this period theConsultant’s personnel checkedthat all guarantee values were met.The reclamation plant was acceptedas fully functional towards the endof 2002 and the Plant was officiallyinaugurated on 2 December 2002.

WATER QUALITY

The water quality produced meetsthe quality guidelines of the WorldHealth Organisation, Rand Water(South Africa) and the Water Qual-ity Guidelines for Namibia. Watersamples are taken on a four hourlybasis after each treatment processfor analysis to ensure compliancewith the stringent standards re-quired. Health standards inWindhoek have been monitoredclosely since the inception of directreclamation in 1968. No knownwater related outbreak of diseasehas been experienced.

COSTS

The cost of providing water fromthe Goreangab Plant is less than thealternative of developing furtherdams, conveyance and treatmentsystems in distant catchments. Theenvironmental impact of dam con-struction and inter catchmenttransfer systems are also avoided.

The project was awarded at a valueof N$92 million (R92 million) andthe client made provision for con-tingencies of just over N$5 million.The project was completed forN$95 million resulting in a saving ofmore than N$2 million (N$1 = R1).The project was financed by theKreditanstalt für Wiederaufbau(KfW) in Germany, the EuropeanInvestment Bank (EIB) and the Cityof Windhoek.

Potable reuse is an indispensableelement of the Windhoek watersystem and has proven to be a reli-able and sustainable option.Water of exceptional quality canconsistently be produced fromtreated domestic sewage.

For more information, please con-tact Charl van der Walt at GFJConsulting Engineers, Pretoria.Tel: (012) 342 1234E-mail: charl.vdwalt@gfj.co.za