AUGMENTATION OF POTABLE WATER SUPPLY TO THE NORTHERN AREAS OF PRETORIA
L. Fouché
Bigen Africa Consulting Engineers (Pty) Ltd, Tel: (012) 8428794. Fax: (012) 8038006.
ABSTRACT
The City of Tshwane Metropolitan Municipality (CTMM) approved the implementation of the Roodeplaat Water Supply Scheme (RWSS) as a local water resource to augment potable water supply to the northern areas of Pretoria. The RWSS comprises of an upgraded outlet works at the Roodeplaat dam, raw water pump station, raw water pipeline, water treatment works, treated and bulk water distribution pipelines for 60 Ml/d in phase 1 and a further 30 Ml/d in phase 2. Multiple outlets are provided at the dam wall, and careful consideration was given to process selection so that the processes would act as efficient barriers against any potentially harmful constituents in the water before being distributed as potable water. Consumption trends were modelled and control systems will be modified to ensure that 60 Ml/d can be delivered into the target area of CTMM continuously. The total project cost amounts to R330 million and will be financed by means of a structured finance mechanism which does not rely on any financial guarantees from CTMM.
INTRODUCTION
A lack of sustainable local water resources available to the City of Tshwane Metropolitan Municipality (CTMM) makes the city dependant on imported water from river systems in Kwa-Zulu Natal, Mpumalanga and Lesotho via Rand Water’s distribution systems.
The distance and elevation of CTMM from these remote resources is such that the imported water is introduced into the supply area of CTMM at a significant premium, and is largely lost as return flows to the Pienaars, Apies and Crocodile River Systems, and eventually into the Limpopo River for international release.
The projected future water demands of CTMM shown in Table 1 indicate that an increasing quantity of CTMM’s demand will be fed from Rand Water supply systems should CTMM not be in a position to develop local resources of their own.
Table 1. CTMM projected water demand (ML/D). Ground Year Roodeplaat Other Vlakfontein Klipfontein Water 2000 - 66 (13%) 73 (14%) 120 (24%) 250 (49%) 2015 (excl RWSS) - 66 (6%) 173 (16%) 300 (28%) 538 (50%) 2015 (incl RWSS) 90 (8%) 66 (6%) 173 (16%) 258 (24%) 490 (46%)
In 1999 the Greater Pretoria Metropolitan Council (GPMC), disestablished in 2000, initiated a feasibility study to identify possible sustainable water resources to the north of Pretoria for potable use. Such local resources would have the advantage of significantly lower capital and operating costs than those associated with importing water from afar.
It would furthermore postpone upgrading of Rand Water’s supply system from the south through built-up areas of Pretoria.
Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 –6 May 2004 ISBN: 1-920-01728-3 Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics RAW WATER SOURCE
At the time that the pre-feasibility study was conducted, previous studies by the Department of Water Affairs and Forestry (DWAF) had indicated that there existed only two possible water resources for CTMM, viz the Crocodile and the Apies/Pienaars River Systems. The Crocodile River System was already over-committed, leaving only the Apies/Pienaars River Systems as possibilities, both having a significant firm yield caused by return flows from CTMM waste water treatment plants. The Pienaars River, feeding into the Roodeplaat Dam was identified as the preferred raw water source, primarily because it was the closest to the identified consumers to the north of the Magaliesberg, and also because the long retention time in the dam will improve the raw water quality entering the proposed Roodeplaat Water Treatment Works.
Previous studies by DWAF indicated that the Roodeplaat Dam sustained yield increases steadily as development and the associated stormwater and return flows increase.
An abstraction licence was subsequently obtained by CTMM from DWAF for an initial abstraction of 60 Ml/d, growing to 90 Ml/d as the yield of the Roodeplaat Dam increases.
The Roodeplaat Water Supply Scheme was hence initiated to supply 60 Ml/d of potable water under phase 1 and a further 30 Ml/d under phase 2 to CTMM.
The scheme comprises of the following main components: • improving the existing outlet works at Roodeplaat Dam. • raw water pump station. • raw water pipelines (one 650NB for phase 1 and a second 650NB for phase 2). • 750NB clear water pipeline to Montana Reservoir. • 700NB pipeline from Montana to Wonderboom. • booster pump station at Montana to transfer water to the south of the Magaliesberg.
RAW WATER QUALITY
The quality of raw water abstracted from any deep impoundment is dependent on the level of abstraction below the surface of the water.
At present water is largely released for irrigation and environmental purposes, with Magalies Water being the only provider of potable water from the Roodeplaat Dam. Water can currently only be discharged from the bottom outlets, some 28 m below the full supply level of the dam, these being the only outlets in operation.
In order to assess the raw water quality which could be expected, several sources of information were considered: • Magalies Water analyses of Wallmannsthal raw water after being conveyed in either the Pienaars River or the irrigation canal both as representing the bottom outlet quality. Refer to Table 2. (1) • Water quality profiles of Inanda Dam (Fig 1) . • Actual analyses conducted at the Roodeplaat Dam wall by DWAF Resource Quality Services (Fig 2).
By considering all the available water quality records, it was decided that the optimum level of abstraction should be approximately 10 m below the water surface where the DO is still sufficient to reduce concentrations of soluble metals, such as Fe and Mn, but where chlorophyll-a and microcystis is less than at higher levels. Table 2. Water quality at Wallmannsthal water treatment works.
Raw Water Potable Water
Analysis Meas. Unit Min Avg Max Min Avg Max
MICROBIOLOGICAL Faecal Coliform Per 100 ml 6 49 >100 0 0 0 AND BIOLOGICAL QUALITY Total Coliform Per 100 ml 12 58 >100 0 0 0
Std. Plate Count Per 1 ml 425 2190 >3000 12 48 154
Chlorofil-a µg/l 3.5 22 87 <1 <1 >1
PHYSICAL QUALITY pH 7.6 8.1 8.4 7.9 8.1 8.2 AND STABILITY Conductivity mS/m 62 68 80 67 72 78
Total Dissolved Solids mg/l 389 431 500 428 454 488
Turbidity NTU 19.0 38.0 52.0 0.4 0.8 1.6
Colour mg/Pt - - - 2.5 5.0 15.0
Total Hardness mg/lCaCO3 146 173 245 151 171 250
Total Alkalinity mg/lCaCO3 163 186 212 161 188 210
Temperature ºC 14.0 23.0 28 14.0 23.0 28
Prec. Potential mg/lCaCO3 -3 9 14.7 4 6 11.0
CHEMICAL QUALITY Calcium mg/lCa 38 44 56 43 45 47
Macro Elements Magnesium mg/lMg 19.0 21 25 20.0 22 22
Sodium mg/lNa 32 66 86 58 75 86
Potassium mg/lK 7.1 11.0 14.0 11.0 12.0 13.6
Ammonium mg/lN <0.1 <0.1 <0.1 <0.1 <0.1 <0.1
Iron mg/lFe 0.1 0.1 0.6 <0.05 0.2 0.6
Copper mg/lCu <0.025 <0.025 <0.025 <0.025 <0.025 <0.025
Manganese mg/lMn <0.05 <0.05 <0.118 <0.05 <0.05 <0.06
Chloride mg/lCl 43.0 58 72 65.0 74 78
Nitrite mg/lN <0.1 0.1 0.8 <0.1 <0.1 <0.1
Nitrate mg/lN 0.9 2.3 4.1 0.2 0.3 3.5
Sulphate mg/lSO4 50 62 74 59 66 74
Ortho-phosphate mg/lP <0.1 0.7 3.9 <0.1 0.7 3.9
Fluoride mg/lF 0.30 0.32 0.38 0.31 0.33 0.35
CHEMICAL QUALITY Lead mg/lPb <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
Micro Elements Cadmium mg/lCd <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
Silver mg/lAg <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
The temperature and DO profiles at the Roodeplaat Dam wall furthermore indicated clear stratification in summer and a mixed water body in winter. It was subsequently concluded that several draw-offs will be necessary to provide flexibility throughout the year to abstract the best quality of raw water.
Dissolved organic carbon (DOC) analyses were not available, but was commenced on the canal water directly downstream of the outlet works. Weekly analyses over the past 3 months indicate DOC levels below 9 mg/l all the time.
Figure 1. Inanda Dam water quality profile(1).
Figure 2. Roodeplaat Dam water quality profile. IMPROVED OUTLET WORKS TO ROODEPLAAT DAM (REFER FIG 3)
The existing outlet works comprise two parallel stacks of 3 x 762 mm diameter steel outlet pipes spaced at vertical intervals of about 6 m. The two lowest level outlets are at present the only ones in use at 28 m below the water surface. The four (2 x 2) higher outlets have been blanked off.
In order to create the requisite flexibility for abstraction at higher levels, each of the two existing stacks will be extended. Two additional 800 mm dia outlets will be installed above the existing outlets at 5,0 m depth intervals. The existing outlets will be connected to the new abstraction pipe system and isolating valves added to each existing outlet.
New galvanised mild steel working platforms will be installed outside the dam shaft at the various outlet levels for purposes of maintenance and operation of the valves.
As a suitable site for the new Roodeplaat Raw Water Pumpstation was not available immediately downstream of the dam wall, the pump station was positioned downstream of the existing outlet tunnel at the closest possible position. As part of the upgrading works, one new 1200 mm diameter outlet pipe will be installed in the outlet tunnel, connecting the upgraded abstraction works in the dam wall to the new raw water pumpstation. The purpose of the pipes is to make the potential energy created by a normally full dam available to the raw water pumps. An estimated annual energy cost saving of around R500 000 is achieved with this design.
Figure 3. Upgraded abstraction works to Roodeplaat Dam.
RAW WATER PUMP STATION
A new raw water pump station will be constructed immediately downstream of the abstraction works tunnel section. Three pumps (1 stand-by) will be installed in phase 1, each with a capacity of 33 Mℓ/d to match the treatment works modules and to cater for 10% losses.
The pumps will be driven with variable speed motors to satisfy the following requirements: • varying dam water levels. • pump system curves which will change when adding the second 650NB raw water pipe in phase 2 • flexibility in raw water feed to match varying water losses at treatment works.
RAW WATER PIPELINE
A single 650NB steel raw water pipe with epoxy lining and medium density polyethylene coating will be implemented over a distance of 3160 m for phase 1. A similar parallel pipe will be supplied during phase 2 of the scheme.
THE ROODEPLAAT WATER TREATMENT WORKS
The proposed process train for Roodeplaat raw water is shown in Fig 4.
Figure 4. Proposed process train for Roodeplaat WTW.
Pre-Treatment Provision is made to pre-treat the raw water with potassium permanganate in cases when severe problems are experienced with iron and manganese complexes. Pre-chlorination is included as a possible process, but is not recommended for normal operation due to the undesirable reactions between chlorine and organics.
Aeration/Inlet Structure The preliminary design provides for raw water to cascade down a widening ramp with splitter vanes.
The purpose of the aeration chamber is to allow aeration and some oxidation to take place during periods that raw water with low oxygen levels is pumped from the Roodeplaat Dam. The chamber also allows for efficient mixing of returned PAC/sludge extracted from the vertical flow sedimentation tanks and fed into the head of works.
The first part of the aeration chamber provides 78 sec reaction time for KMnO4 oxidation, followed by aeration and intense mixing before further reaction time of 180 sec. After chemical dosing, the flow will be split into two 30 Ml/d modules for phase 1 allowing for a third 30 Ml/d module in phase 2.
Coagulation The chemical dosing building has been positioned at the northern end of the aeration/inlet structure.
The full incoming raw water stream of up to 99 Ml/d can be dosed with lime, and after flow splitting, each 33 Ml/d stream with ferric chloride and/or polymer. Secondary ferric chloride or poly-electrolyte will be dosed in the manifold feeding into the sedimentation tanks after flotation. A third ferric chloride dosing option will be provided in the settled water manifold that feeds into the filters, to provide for periods when PAC removal in the sedimentation tanks may be problematic.
The chemical dosing system will be flexible, to enable operators to dose lime either at the head of the works i.e. before ferric chloride is dosed or later in the process to affect stabilisation of the treated water. The later point of application may enhance removal of organic matter.
Flocculation For Phase 1 the two separate streams of 33 Ml/d each will be mixed in separate baffled channels. Flocculation of 10 minutes at a velocity gradient of 40 - 60 sec-1 has been provided.
Dissolved Air Flotation (DAF) System Six separate flotation tanks are provided for Phase 1. The maximum daily flow rate per tank will be 11 000 Ml/d.
The shape of the flotation tanks will be rectangular, 10 m long by 6,5 m wide plus a 10 m long x 1,5 m wide reaction zone. The reaction zone is provided in the middle of the tank, to enhance float layer removal.
The minimum reaction time is 140 sec in the reaction zone, at a hydraulic loading of 42 m/hr.
The sludge float layer removal will be hydraulically actuated by closing two inlet sluices on the adjacent tanks. This will provide an increased vertical flow through the bottom inlet, and a surface flow via the top inlet. The top inlet will create a sweeping action across the tank when the water level in the tank reaches the level of the scum overflow weir. This will temporary increase the vertical rise rate to 23,25 m/hr in the tank being desludged, forcing the sludge layer over the washout weir.
The side depth of the flotation tank is 3 m and the draw off will take place via 4 x 300 mm outlet pipes with orifice inlets on both sides of the settled sludge hopper. The vertical flow rate in the separation zone is 7,75 m/hr.
Allowance was made to produce 10% saturated water (of the mainstream) for the flotation units. Two separate saturators, one for each set of tanks will operate and will be fed from the treated water sump by a single pump. The pump will operate at a head of 450 kPa. A compressor plus air tank will produce the air at a head of 500 kPa for an air concentration of 6 mg/l. A diffuser system with nozzles will distribute the air-saturated water evenly throughout the reactor.
Powdered Activated Carbon (PAC) Provision is made to dose PAC immediately downstream of the flotation tanks as the water enters the PAC reactor tank. Mixing energy of 300 mm head is created in the reactor with a retention period of 25 sec.
It is envisaged that the flotation process will remove the majority of the suspended solids. Dosing the PAC after flotation will therefore increase the efficiency of the PAC. PAC will be dosed as a slurry for taste and odour control and to reduce dissolved organics in the treated water. The new PAC dosing system will handle large bags of 1 m³ capacity each. The bags will be transported from the PAC store with a fork lift to the loading structure.
The method that will be adopted for bulk PAC handling is direct induction of the powder from the bags into a specially designed, high dispersing energy, in-line wetting pump. This pump will withdraw water from one 20 m³ make-up tank and return the mixed PAC slurry to the same tank until the required quantity of PAC has been mixed (5% solution). Two mixers will be installed, one in each of the makeup tanks to keep the PAC in suspension.
Peristaltic pumps will transfer the PAC slurry to the dosing point at a rate of 150 – 1 500 l/hr.
Sedimentation 12 Pyramidal vertical flow sedimentation tanks are proposed.
The inlet velocity at the bottom of the tanks is 2 m/s which will produce hydraulic mixing and flocculation. The hydraulic headloss for flocculation will be 300 mm. The upflow velocity is 2,3 m/hr, and lamella inclined tubes are provided 600 mm deep in the vertical wall section of the clarifiers to provide effective separation.
Sludge collection is by means of suspended sludge hoppers below the lamella packings and the overflow lip of the hopper is in the top portion of the pyramid, arranged to create a deceleration from 2,68 m/hr to 2,29 m/hr as the sludge layer flows vertically past the hopper.
Sludge will collect in the hopper and a variable timer will be installed to desludge the hopper on a selected time cycle. The suspended hoppers will contain mainly PAC sludge, which could be recycled by pumping it into the raw water for optimal utilisation of the PAC.
Rapid Gravity Filtration 8 Filters will be provided with a combined filter area of 600 m². This arrangement will produce a filtering rate of 4,58 m/hr with 8 filters in operation, and 5,23 m/hr when one filter is being backwashed.
The incoming flow will be equally divided on the inlet side to the filters by fixed level weirs and will allow a slow start operation. The outlet valves will allow filter water to waste on a time cycle.
Each filter will have two filter bays. Each filter bay will be backwashed independently, first with air at a rate of 7,5 mm/sec, then with water also at a rate of 7,5 mm/sec.
The minimum water level will be maintained by a fixed outlet weir.
Disinfection A three stage disinfection system will be provided.
UV Disinfection UV disinfection equipment will be installed on the pipeline between the filterblock and the clearwater tank. The complete system will consist of a UV chamber complete with automatic wiper mechanism, UV lamps with quartz sleeves, power control equipment, UV transmittance monitor and UV dose measurement, all fitted in an online system.
The minimum UV dose shall exceed 40 mJ/m² at all times.
Chlorination Provision will be made to feed up to 5 mg/l of chlorine gas at full production capacity as pre- chlorination into the raw water or alternatively at the intermediate dosing point. A dosage of 5 mg/l is allowed for disinfection into the final treated water in the clearwater tank.
Flexibility will be built into the system to apply either pre- or intermediate chlorination, should problems be experienced with removal of manganese complexes. Chlorination points for this application will be provided ahead of sedimentation and filtration. Chloramination Chloramination up to a maximum dosage of 1 mg/l will be provided to make the potable water from Roodeplaat WTW compatible with water supplied to CTMM by Rand Water. The two waters will be mixed in the Montana and Wonderboom Reservoirs during most times of the year. Chloramination will prevent unwanted reactions between the two water types from taking place.
Clearwater Storage A new clearwater tank with a full supply capacity of 4 000 m³ will provide the following functionality: • balancing tank for clearwater pumps. • baffled chlorine contact tank with 42 minutes contact time (at 50% water depth). • pump sump for recycle pumps to DAF. • fixed level pump sump in first section of tank to provide a constant head to the filter washwater pumps.
Sludge Disposal/Filter Washwater Recycling Three sludge dams have been designed at the lower end of the treatment works site. All sludge from the flotation tanks, sedimentation tanks and filter washwater will gravitate to the sludge dams.
Sludge from the sedimentation tanks will be recycled to the raw water before finally being discharged from the flotation tanks to the sludge dams.
The normal operation (series flow) will allow good settling and a recycle pumpstation will feed supernatant to the head of the works.
PIPELINE FROM ROODEPLAAT TO MONTANA
Potable water will be pumped from the Roodeplaat WTW to CTMM’s Montana Reservoir through a 12 km 750ND CML Sinterkote steel pipeline. This pipeline will be constructed generally parallel to road 1386 between Pretoria and Moloto. The pipeline crosses existing and future rail, provincial and national roads along its route. Wayleaves have been granted to CTMM by the authorities responsible for the pipeline. Crossings are generally made inside jacked culverts.
PIPELINE FROM MONTANA TO WONDERBOOM
A significant portion of the demand on the new scheme will be exercised from CTMM’s Wonderboom Reservoir. A new 7,85 km 700ND CML Sinterkote pipeline has been planned to convey this demand under gravity between CTMM’s Montana and Wonderboom Reservoirs. This pipeline passes through well established and gardened residential areas; the pipeline will be installed below the travelled way of residential streets. Traffic and earthworks management plans were prepared and approved by the municipality for this purpose.
DISTRIBUTION OF POTABLE WATER
The objective of the Roodeplaat Scheme is to distribute the full capacity of the Roodeplaat Water Treatment Works (60Ml/d Phase 1, 90 Ml/day during Phase 2) within the target supply areas at all times.
The expected commissioning date for Phase 1 of the Scheme is the first quarter of F2006. Accordingly, demand projections are reported for the period from end 2005 to end 2025. (Refer Fig 5).
Figure 5 shows the monthly distribution and accumulation of demands for the CTMM target areas that will be supplied through the Roodeplaat to Montana Pipeline, based on commissioning of a direct supply to Doornpoort in 2011. Upon commissioning, those areas within the Doornpoort supply at that stage via Montana and Wonderboom will be supplied directly from Roodeplaat via the proposed Doornpoort Reservoirs.
Figure 5. Potable water distribution from Roodeplaat WTW.
As can be seen from Fig 5, the average water demands for the area north of the crest exceed the supply capacity of the Roodeplaat to Montana Pipeline at all times. If all target areas are to be supplied, it can be seen that the lowest LDD is some 67 Ml/d in 2012, which represents a spare capacity of 10% over the capacity of Phase 1 of the Scheme.
Accordingly, water would only be provided from Roodeplaat to the south of the Magaliesberg for limited periods of the year. Potential deliveries to the south of the crest have been analysed and it has been found that the likely supply to Queenswood is only 0,8% of the volume delivered from the Roodeplaat WTW over the analysis period (2005 to 2025). The balance of the supply district to the south of the crest is expected to total around 5,2% of the total water supplied over the analysis period. This also represents the amount of water which will be recirculated into the Roodeplaat dam drainage basin, which can be considered negligible for purposes of TDS accumulation.
The implementation of the Roodeplaat Scheme dictates the revised control philosophy in the CTMM bulk distribution system. Preference is always given to Roodeplaat water. This will entail reversing flows at times to feed the areas south of the Magaliesberg crest during low demand periods.
IMPLEMENTATION MECHANISM
A unique implementation and financing mechanism has been developed for the project. Some brief characteristics of this mechanism include: • A ring-fenced municipal entity (setup in terms of the Municipal Systems Act) is utilised as the implementation vehicle. This vehicle is in the legal form of a trust. • The City of Tshwane Metropolitan Municipality (CTMM) is the only beneficiary of the Trust and enjoys ownership control thereof. • CTMM has the right to designate the majority of the board of trustees, who are responsible for the management and administration of the affairs of the Trust. The municipality also chairs the management committee of the Trust. • The development of the project from initiation to the point where tenders for the construction were awarded was funded by the project sponsors with no risk or liability to CTMM. • Bigen Africa and Absa Bank acted as project sponsors. • All major cost items such as construction, financing and engineering consulting services were procured in terms of the PPPFA. • The long term financing of the project was 100% over-subscribed which enabled the Trust to select the cheapest sources of funding. • The long term financing facility has no recourse to CTMM nor does it rely on any financial guarantees from CTMM. • Under this mechanism, the Trust entered into a bulk water supply agreement with CTMM. In terms of this agreement, CTMM is obliged to purchase the potable water produced by the Trust at a price equal to the ruling Rand Water price. • As the only beneficiary, any profits in the Trust revert back to CTMM – no third party shares in the profits or benefits of the Project.
Through this mechanism, CTMM enjoys all the typical benefits of a private concession or Public Private Partnership without relinquishing its ownership, control or any of the benefits.
CONCLUSIONS
The Roodeplaat Water Supply Scheme of R330 million will augment potable water supply to the northern areas of CTMM at a sustained rate of 60 Ml/d in phase 1 and 90 Ml/d in phase 2.
The development of this local resource will not only benefit CTMM but is in national interest as it reduces the quantity of water imported from remote regions at significant cost.
Provision is made to produce potable water of the highest possible quality at all times which would blend into Rand Water supply comfortably. This required complex modifications to the Roodeplaat Dam outlet works, and sufficient process flexibility at the water treatment works.
The integration of Roodeplaat Water into the CTMM distribution system required extensive modelling to ensure that control systems will be capable of distributing the full quantity of Roodeplaat water at all times.
The implementation of the Roodeplaat Water Supply Scheme was facilitated by a unique financing and implementation mechanism which not only eliminated balance sheet constraints to financing by CTMM but also will unlock capex programmes over the next 20 years to supply water to poorer communities within CTMM.
REFERENCE
1. W.R. Harding, BR Paxton, Cyanobacteria in South Africa: A Reviews, WRC Report TT 153/01, July 2001.