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.