A systems approach to urban water services in the context of integrated energy and water planning: a City of Cape Town case study.

Transcription

1 A systems approach to urban water services in the context of integrated energy and water planning: a City of Cape Town case study. An exploration of the energy consumption for water and sanitation services for the City of Cape Town (CCT) with an emphasis on water supply augmentation options for the near future ( ). Fadiel Ahjum and Theodor J. Stewart Energy Systems Analysis and Planning Group Energy Research Centre University of Cape Town

2 Research Focus The CCT municipality estimated that the provision of water and sanitation services for the period 2007/8 accounted for ca. 46% of its electricity consumption with 31% attributed to waste water treatment. The DWA initiated the Western Cape Water Supply System Reconciliation Strategy Study (2007) to examine the future water demands on the regional supply system up to the year 2030 and the options available to prevent a possible supply shortfall as it was anticipated that unconstrained demand would exceed the system yield by Supply- and demand-side options such as water conservation and demand management, sea water desalination, waste water recycling were investigated for their potential yield and their economic, environmental and social cost. This research examined the impact on municipal electricity consumption for the CCT and the impact on future water availability for interventions comprising the identified options.

3 South Africa: Provincial and Water Management Area Boundaries Cape Town

4 Water services for Cape Town 331 Mm 3 of water produced (2009/10) 29.3 Mm 3 supplied to neighbouring municipalities Mm3 supplied to distribution network Mm3 delivered to customers Hydro-power at larger WTWs offset electricity demand from grid. Distribution network predominately gravity based although growth in the northern suburb increasing pumping requirements. Effluent treated: Mm 3 (2009/10). 82% of volume underwent secondary treatment with the Activated Sludge process

5 Water Supply Infrastructure 98.5% of supply from surface water. 6 major dams (905 Mm 3 ) largely comprising the Western Cape Water Supply System (WCWSS). Dams operated as a system. Runoff constantly monitored and supply from individual dams may vary annually to minimise spillage. Theewaterskloof dam, in the Breede WMA, provides bulk storage (53%) and is linked to two other dams via a pipeline and bidirectional tunnel system, thereby connecting the dam to three large WTW and a potential fourth WTW in future. Ground water abstraction (~5 Mm 3 /a) for potable use occurs in the northern area (green square). Aquifer recharge by percolation of secondary treated domestic waste water is also practised. CCT Bulk Water Network Interbasin water transfers

6 Water Supply Infrastructure 98.5% of supply from surface water. 6 major dams (905 Mm 3 ) largely comprising the Western Cape Water Supply System (WCWSS). Dams operated as a system. Runoff constantly monitored and supply from individual dams may vary annually to minimise spillage. Theewaterskloof dam, in the Breede WMA, provides bulk storage (53%) and is linked to two other dams via a pipeline and bidirectional tunnel system, thereby connecting the dam to three large WTW and a potential fourth WTW in future. Ground water abstraction (~5 Mm 3 /a) for potable use occurs in the northern area (green square). Aquifer recharge by percolation of secondary treated domestic waste water is also practised. CCT Bulk Water Network Interbasin water transfers

7 Municipal Operations in Context of Energy Consumption Traffic Signals, 4% Street Lighting, 34% Waste Water Treatment, 31% Electricity Consumption(2007/8) Buildings, 15.50% Bulk Water Supply, 15.50%

8 The Urban Water-Energy Relationship

9 Conceptual CCT Water Services Model WEAP Emphasises the direct energy consumption as studies have indicated that this phase is the most energy intensive over the life span of water and sanitation infrastructure 1. WEAP simulation software utilised to implement the urban water services model as depicted incorporating a sufficiently representative model of the Western Cape Water Supply System (WCWSS) to approximate the inflows to the major dams and to conduct scenario analyses.

10 Conceptual CCT Water Services Model WEAP Emphasises the direct energy consumption as studies have indicated that this phase is the most energy intensive over the life span of water and sanitation infrastructure 1. WEAP simulation software utilised to implement the urban water services model as depicted incorporating a sufficiently representative model of the Western Cape Water Supply System (WCWSS) to approximate the inflows to the major dams and to conduct scenario analyses. Adapted from the California Sustainability Alliance (2008)

11 Modelling Process Energy data: WEAP financial module (kwh/m 3 ) Data Shape files (catchments, etc.) WEAP objects (rivers, dams, etc. Stream inflows Pipeline capacities/losses Dam volume elevation curves Operating rules or preferences Water consumption profiles Energy factors WEAP Water Energy Results

12 Modelling Process Energy data: WEAP financial module (kwh/m 3 ) Data Shape files (catchments, etc.) WEAP objects (rivers, dams, etc. Stream inflows Pipeline capacities/losses Dam volume elevation curves Operating rules or preferences Water consumption profiles Energy factors WEAP Water Energy Results

13 Modelling Process Energy factors attributed to each stage of the urban cycle and summed by water transmission path: n E kwh = e k s k k=1 Energy data collated and categorised by stage (e.g waste water collection and treatment) Energy factors largely obtained from literature where topography and application were similar, empirical data and calculations (e.g. water conveyance pumping energy)

14 Sample Modelling Results: Timeline of Water Supply Portfolio and Associated Energy Consumption The modelling process allows for: the examination of a mix of water supply options; and the associated energy consumption for the different stages for the urban water cycle. For example, the reference scenario is shown with an intervention orientated towards effluent reuse and recycling. In this case, by 2030, 20% of water supply is obtained from the advanced recycling of effluent with a further ca. 10% from the reuse of secondary treated effluent. Ground water also contributes 10% of supply. Reference scenario with intervention (d) Volumes indicate total supply (i.e. excludes conveyance and distribution losses). Energy consumption by stage for the urban water cycle.

15 Scenario Summary Scenario Water Demand Surface Inflow Water Quality Dam Waters Comments 1 High Historic Low degradation Regulated Reference Case 2 High Historic Low degradation Unrestricted 3 High Reduced Inflow Higher degradation Regulated Worst case 4 High Reduced Inflow Higher degradation Unrestricted 5 Low Historic Low degradation Regulated 6 Low Reduced Inflow Higher degradation Unrestricted

16 Future Supply Options 1. Water Conservation / Water Demand Management (WC/WDM): a) partial implementation (17 Mm 3 ) or; b) full implementation (78 Mm 3 ) 2. Additional surface water schemes (e.g augmentation of existing dams with water from secondary rivers) 3. Sea water desalination (SWD) 4. Secondary treated effluent for agriculture/landscape irrigation or wet industry 5. Further processing of secondary effluent for indirect potable supply at WTWs or direct to the distribution network for non potable use or for dilution in service reservoirs. 6. Ground water augmentation of Theewaterskloof dam from TMG aquifers to exploit existing water supply infrastructure

17 Intervention Themes (portfolio of options) Low WC/WDM with SWD Limited WC/WDM Ground water Reuse of secondary treated effluent Sea water desalination (SWD) Low WC/WDM with Surface Water options Limited WC/WDM Surface water scheme expansion Reuse of secondary treated effluent Ground water Tertiary recycling of effluent (potable) High WC/WDM with SWD WC/WDM extension Sea water desalination Reuse of secondary treated effluent High WC/WDM and Waste Water Recycling WC/WDM extension Reuse of secondary treated effluent Tertiary recycling of effluent (potable) Ground water

18 Intervention Timelines and Yields

19 Comparative performance of alternatives (interventions) Three dimensions in the modelling process: Model Parameters System Storage Index (SSI) Reliability (R) (WEAP variable) Basic performance given by: System energy intensity (E i ) α = 1 E i * R* SSI Expressed in logarithmic terms it can be viewed as a simple additive value function which forms the basis of decision analysis in Multi-Attribute Value Theory : Α = ln( α) = ln( 1 E i ) + ln(r) + ln( Reliability) + ln( SSI)

20 Sample Modelling Results: Water Availability vs Unmet Demand and the Energy Intensity of Urban Water and Sanitation

21 Comparative performance of alternatives (interventions) A reference performance index of α = 164.

22 Sample Modelling Results: Municipal Energy Consumption for Water and Sanitation

23 Sample Modelling Results: Municipal Energy Consumption for Water and Sanitation

24 Modelling Implications High growth in demand for water of ca. 3% per annum would result in an increase in energy intensity to maintain security of supply. Sea water desalination (SWD) augmentation is potentially the most intensive (~70 % greater over the period than the reference do nothing case) with effluent recycling and additional surface water schemes the least (~20% & 30% greater). The intervention that emphasises effluent recycling is comparable to the SWD schemes with regard to reliability and dam storage but exhibits a lower water services energy intensity. The energy intensity is 66% of that for the SWD interventions for the period Additional surface water schemes increases the vulnerability of the supply system to the impact climate on inflows to the dams while effluent recycling is sensitive to the volume of return flows. Effluent recycling is a form of water conservation and therefore has greater applicability for the high growth scenario. Low growth in water demand of ca. 1% per annum with a concerted WC/WDM programme would require no additional supply interventions for the near future (2030).

25 Conclusions WEAP is designed for regional water supply modelling therefore the distribution network is aggregated and does allow for explicit representation of dual networks for the case of effluent recycling. Recent release of WEAP allows for energy analysis via integration with LEAP. Possibly an easier more robust and standardised implementation via this approach? Empirical energy data for CCT bulk water and distribution systems are not readily available. Bulk water operations have assessed their vulnerability to electricity outages but overall the City s Water Services Directorate do not incorporate annual energy consumption within their monitoring and reporting programmes. A refinement of model parameters would improve the uncertainty and accuracy of the results: local empirical derivations of energy factors. (e.g. distribution and waste collection, WTWs and conveyance); Improved WEAP hydrological model with (a) linking climate to runoff and (b) linking ground water resources to runoff; Additional work required to further explore the energy-water nexus of municipal ground water usage and aquifer recharge potential along with storm water reuse.

26 Land-use modelling component Initial focus on urban agriculture within Cape Town: The Philippi Horticultural Area (PHA) Examining CLEW tradeoffs in the context of urban food security vs low cost housing in the City of Cape Town. The rapid growth of the City is associated with the urbanisation of poverty PHA

27 Other Water-Energy Projects WORLD BANK Thirsty Energy initiative: South Africa case study. Based on the ERC s South African TIMES (SATIM) model, phase 1 is underway modelling the impact of water pricing on future investment in the energy sector.

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