Building resilience in water supply infrastructure in the face of future uncertainties: Insight from Cape Town

ions are perceived to also have a high resilience to flood events (Defra, 2015). 8.3.3.3 Option 3: wastewater reuse treatment plant Wastewater reuse provides a high degree of resilience to droughts and other extreme events due to the nature of the closed-loop system and should be considered as a baseline supply option as opposed to an additional source (Defra, 2015). However, similar to desalination, as the plant capacity is technically locked-in, it is unable to produce more water if demand increases beyond its capacity. 8.3.3.4 Option 4: surface water transfer scheme Surface water transfer schemes from river to reservoir are reliant upon rainfall and as the water resource is exposed, they have medium resilience to temperature extremes (Defra, 2015). However, reservoirs are able to store excess water when supply is plentiful, especially during high rainfall or flooding events, which can be utilised during low rainfall periods. This scheme has medium resilience to short term droughts but has low resilience for longer, multi-term droughts. 8.3.3.5 Summary of resilience characteristics analysis Table 8.2 summarises the results of the resilience assessment of the four options. The sum of the scores of the seven characteristics for each option show that wastewater reuse scores the highest, and hence can be deemed as the most resilient option. Figure 8.6 illustrates these results schematically. 8.3.4 Criteria 4 (C4): environmental impacts The environmental impacts of each of the options is assessed qualitatively in the following sections and scored subjectively based on low (1), moderate (2) and severe (3) negative impacts on the environment, relative to each other, and are summarised in Figure 8.7. 8.3.4.1 Option 1: desalination plant Plants between 100 000–200 000 m/day consume 3.5–4 kWh/m of energy (Zarzo & Prats, 2018), equating to 1.4–1.8 kg CO2 per cubic meter of produced water which makes the carbon footprint of large-scale desalination plants substantial (Elimelech & Phillip, 2011). This is particularly concerning since about 80% of South Africa’s primary energy needs are provided by coal, which is unlikely to change significantly in the next two decades (Energy RSA, 2018). Studies have shown that a major concern with SWRO desalination, as is being considered in Cape Town, is the effect that seawater intake will have on marine organisms – entrainment can kill a large number of fish and small planktonic organisms if open surface intakes are not implemented safely (Elimelech & Building resilience in water supply infrastructure in the face of future 217 Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/911929/9781789060768_0201.pdf by guest on 03 September 2021 Phillip, 2011). Furthermore, the increased salinity of SWRO brines, which is about twice that of seawater, and the chemicals used in the desalination process, also pose environmental risks to the marine ecosystem (Elimelech & Phillip, 2011). 8.3.4.2 Option 2: groundwater augmentation scheme There are several environmental concerns with groundwater augmentation, particularly the impacts it may have on terrestrial ecosystems, biodiversity and water table levels. Currently it appears that there is a lack of widespread knowledge on groundwater data and monitoring in Cape Town, therefore it is unknown what effect drawdown will have on the water table (Parsons, 2018), especially if it is being abstracted faster than it is being recharged (EEA, 2018). Furthermore, there are fears that drilling boreholes in environmentally sensitive areas such as the Table Mountain Group Aquifer will threaten critically endangered species with extinction, degrade ecosystems and wetlands and drastically alter the hydrology of catchments, all while violating environmental management regulations of the National Environmental Management Act (Slingsby, 2018). There is also some concern that large-scale abstraction can affect river flows (DWAF, 2007a). 8.3.4.3 Option 3: wastewater reuse treatment plant There are several benefits of reusing effluent water, such as reducing the volumes of treated sewage effluent and industrial discharges into the environment, and reducing the dependency on surface water thereby enhancing flows through the system (Toze, 2006). However, there are a number of potential risk factors that need to be considered. The physical characteristics of the recycled water, such as the pH, salinity, dissolved oxygen and suspended solid content, have an impact on the soil environment in which it is used, particularly when the water is used for irrigation (Toze, 2006). Furthermore, the presence of enteric pathogens, including Figure 8.7 Summary of environmental impact analysis. Water-Wise Cities and Sustainable Water Systems 218 Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/911929/9781789060768_0201.pdf by guest on 03 September 2021 viruses, bacteria, protozoa and helminths, in recycled water can contaminate the water bodies that are in contact with it and impact the ecosystems dependent on it (Toze, 2006). However, these risks can be mitigated by sufficiently treating the effluent to suit the requirements of its purpose, be it for potable consumption or non-potable activities such as irrigation. 8.3.4.4 Option 4: surface water transfer scheme Transferring water from the Berg River to the Voëlvlei Dam has the potential to provide augmentation of low river flows to mitigate losses in the dam during the drought (Defra, 2015). However, there will be negative changes in the water balance between the donor and receiving catchments and the potential transfer of alien species or diseases (Environment Agency, 2006). This will impact the water quality and ecology of the receiving watercourse and the ecosystems that depend on it. 8.3.5 Criteria 5 (C5): social considerations Consumer perception regarding the price and quality of water reaching households and places of business can affect the ultimate decision on the type of water supply. The perception of the source and quality of water it produced was only significantly different for wastewater reuse when compared to the other options. Wastewater reuse can be used directly for potable supplies, however there is general consensus globally that there is presumption against it, and in order to use it for this purpose there would need to be considerable change in public perception and acceptance of it, together with regulatory changes, which would take time to enact (Defra, 2015). Using treated wastewater for non-potable use such as irrigation is less controversial, although there are not as many examples globally of current practise and, as noted earlier, it would also have to be treated sufficiently to meet environmental regulations (Defra, 2015). With that being said, the quality of water has been disregarded as a sub-criterion for consideration in this research. The issue of pricing water, particularly in the South African context, is also complex. A balance has to be found between how much of the water required needs to be treated and delivered, and how much the consumers can afford or are willing to pay for it. The price of water, which is directly correlated to the level of tariff applied and more deeply connected to the issue of social equity and access to water, varied between the options and hence is a critical social consideration for policymakers. Average tariffs are determined by dividing the total cost of providing the service by the volume of water sold (DWS, 2018), therefore the higher the cost per unit of water produced, the higher the tariff will be. Cape Town’s tariff structure is dependent on large volumes of water being sold at higher levels in order to subsidise water at lower levels (DWS, 2018). If the price of water increases, it is likely to push wealthier households to invest in decentralised Building resilience in water supply infrastructure in the face of future 219 Downloaded from http://iwaponline.com/ebooks/book/chapter-pdf/911929/9781789060768_0201.pdf by guest on 03 September 2021 Ta b le 8 .3 S u m m a ry o fo p tio n ch a ra ct e ris at io n re su lt C ri te ri a C 1 : y ie ld (m 3 ///// d a y ) C 2 : c o s t ($ ///// m 3 ) C 3 : re s ili e n c e