The global freshwater supply is being put under increasing pressure by sustained development, and in most areas there is no further borehole or surface water. Therefore, the desalination of seawater is viewed as an effective option. With reverse osmosis being one of most accepted ways of desalinating seawater, it is seen that desalination largely depends on the control of waste concentrate, possible negative environmental impacts, and the fact that it is expensive to implement. At the same time, desalination provides much-needed water to arid and water contaminated areas.
This is especially pertinent to the Western Cape, where below average rainfall and poor management of water resources has resulted in water crises for the city of Cape Town (Blersch, 2017). Thus in this essay we argue whether desalination is a viable option as a source of clean drinking water in the Western Cape, from primarily a biological and physical perspective, looking at the impacts that desalination will have on the environment and its associated biota.
The desalination plants that will be installed in and around Cape Town will use the reverse osmosis process (Laird, Wright, Massie & Clark, 2017). This is the most efficient process as it uses less energy and the costs are lower. The seawater passes through a pre-treatment system which is made up of sand filters, micro filters and a mechanism for chemical dosing. The feed pump generates seawater flow at a pressure of 55-80 atmospheres through the membrane system (Einav, Harussi & Perry, 2002).
During the final treatment, the desalinated water undergoes an adjustment of its reactivity ratio, the reduction of its corrosivity and it is also disinfected.
The discharged brine passes through the turbine, which recovers 30-40% of the energy invested by the process pump and is then returned to the sea (Einav et al., 2002).
Desalination as a whole can have many negative impacts on the environment. Issues include the concentrate and chemical discharges to the marine environment, the emissions of air pollutants and the energy demand of the processes. If not properly addressed, processed seawater from desalination plants can have adverse effects on water and sediment quality, impair marine life and the functioning and wellbeing of coastal ecosystems (Latteman & Höpner, 2008). This is mainly due to increased temperature of the discharge, which contains traces of pre-treatment and chemical cleaners that react to have corrosive effects. Furthermore, culling of aquatic organisms may result from open intake impingement, where animals collide with intake screens, or entrainment, as they are drawn into the plant with the source water. However, these impacts to the marine environment can be minimized by locating the intake in a geographical position where productivity is low. Productivity tends to decline from shallow to deep water, particularly below the photic zone (Missimer & Maliva, 2018). Disruption to the seabed via intake installations can cause possibly unwanted sediments, nutrients and pollutants to reenter the water column (Latteman & Höpner, 2008).
These intakes can also act as artificial reefs for marine organisms, providing a spawning ground for many species. Impingement organisms are defined as organisms large enough to be retained by the various meshes of the intake, and this group includes juvenile and adult organisms. Thus, entrainment organisms include smaller organisms (or life stages of organisms) with limited maneuverability, such as ichthyoplankton, which lack the ability to avoid intakes regardless of intake velocity (Hogan, 2015). Ichthyoplankton are the eggs and larvae of fish. They are mostly found in the sunlit zone or photic zone of the water column, less than 200 metres deep.
The construction of the desalination plants will have short term effects on the environment and its associated biota. During the placement of the pipelines, fast swimming mobile fish and elasmobranchs will be able to move to adjacent areas, whereas the majority of slow swimming fish, crabs and benthic infauna should be able to slowly move out of the way of the pipeline (Laird et al., 2017). Since the pipelines are laid in sections, the mortality of the fish in the area is unlikely as this will give the fish time to move out the area. The complete recovery of benthic fauna should occur quickly as a result of the dynamic of soft benthic habitats in depths that are shallower than 30 meters on the exposed coasts (Laird et al., 2017). However, hard substrate environments should be avoided as much as possible as it is more difficult for this type of environment to recover. Beach macrofauna are tolerant to short-term, localised disturbances as result of the nature of sandy beaches (Laird et al., 2017). The majority of species that occur in the intertidal and nearshore areas of a beach are usually opportunistic pioneer species with high reproductive and growth rates (Newell et al. 1998), while populations of long-lived species take more time to re-establish (Kenny & Rees, 1996). This is also true for established species that are found on rocky shore and mixed shore habitats (Branch & Branch,1981). Oceanic birds that are feeding and/or roosting in or around the construction area will be displaced during the construction activities. However, the birds are expected to return permanently once the pipeline placement is finished. Intertidal invertebrate fauna will most likely be crushed during the construction phase (Laird et al., 2017). A net loss of sandy habitat will not occur during the construction since the benthic organisms will still be able to burrow in the sediment above the pipelines once they have been installed. Marine invertebrates will also begin to re-colonise the areas where the construction takes place through recruitment from adjacent rocky and sandy habitats immediately after the construction is finished (Laird et al., 2017). The installation of the pipes could also possibly lead to tidal disturbance of the sediment resulting in erosion and deposition as well as uneven subsidence around the pipelines (Liu, Sheu & Tseng, 2013). Due to the fact that the area that the pipeline rests on at each site is rather small and since algae and invertebrates will colonise the pipeline, there is no need for mitigation, the action of reducing severity, techniques in the construction phase (Laird et al., 2017).
There is also the possibility of increased salinity as a result of the brine discharge which could have long term effects on the environment. This can cause a change in water column structure, stratification, as well as the chemistry of the water such as the dissolved oxygen concentration and turbidity. A change in water stratification could shift the distribution of organisms in the water columns and the sediments. If there is an altered dissolved oxygen level, as a result of a change in temperature or salinity of the sweater, this will affect the osmoregulation in organisms which could have disastrous effects. Hypoxic water, water that has a concentration of less than 2mm of oxygen per litre, can cause mass mortalities of benthos and fish (Diaz & Rosenberg, 1995). Organisms try to maintain oxygen delivery by increasing their respiration rates, the number of red blood cells or by increasing the oxygen binding capacity of haemoglobin. If there is prolonged hypoxia, the organisms will resort to anaerobic respiration (Wu, 2002). Therefore, hypoxia reduces the growth and suppresses feeding which may result in a change of the individual’s fitness. Turbidity could affect the photosynthetic processes of organisms as less sunlight will enter the water (Miri & Chouikhi, 2005). Invertebrates, mainly crustaceans, with long stomachs are more susceptible to a rise in salt concentrations than those that have shorter stomachs. Some of the species, mainly the diatoms, are resistant to high salinity levels, but most of the species will not survive. There are lethal effects for seagrass species such as a 100% mortality rate for Mediterranean seagrass, Posidonia oceania, with salinity levels of 50 PSU (Latorre, 2005).
Noise pollution and vibrations are also products off the desalination process. The noise produced could disturb birds as well as noise-sensitive marine mammals. This could result in the relocation of animals, separation of groups as well as hearing damage and abnormal behaviour (Laird et al., 2017) Foraging and roosting birds will be disturbed during the construction phase, however, other marine life will most likely be unaffected. Fish and mammals will move away temporarily whereas invertebrates are less sensitive to noise and will most likely remain. The impact of this effect can be lessened by subjecting the construction equipment to noise test parameters (Laird et al., 2017).
Elevated water temperatures, as a result on plant discharge, can have effects on the physiology of associated biota such as their growth and metabolism, reproduction timing and the success, mobility and migration patterns as well as production. The functioning of the ecosystem may also change as a result of a change in the oxygen solubility. An elevated temperature level of less than 5 degrees above the normal seawater temperature results in little or no effects on the abundance and distribution patterns of species. (van Ballegooyen et al., 2005). However, some detrimental effects were observed in the development of eggs and larvae (Cook 1978, Sanstrom et al., 1997, Luksiene et al., 2010) with a temperature increase of less than 5 degrees. The duration of larval development decreased (Thiyagarain et al., 2003) and there was suppressed growth in the post larvae of the spiny lobster, Panulirus argus (Lellis & Russsel 1990), as well as an increased photosynthetic and biological community metabolism rates (Parsons et al. 1977). Elevated temperatures can result in a decrease in the rate of production of algae, a replacement of algae with less desirable species and a decrease in the abundance and diversity of phytoplankton (Miri & Chouikhi, 2005). Zooplankton may have a strong mortality rate due to their thermal override tolerance. Due to the fact that benthic invertebrates have rather limited mobility, there can be a reduction in the growth of the clam but a faster growth in oysters as well as a significant mortality rate of adult benthic invertebrates. An increase in temperature affects the swimming speed of fish as well as their behaviour which will therefore alter their distribution (Miri & Chouikhi, 2005). It also affects southern African west coast intertidal (the white mussel, Donax serra) or rocky bottom species (Halitos midae, the kelp Laminaria pallida, mytilid mussels and the rock lobster, Jasus lalandii). Cooler-water species would be the most affected by an increase in water temperature as this will be more favourable for warm-water species (Laird et al., 2017). An example of this would be the possibility of the invasive mussel species, Mytilus galloprovincialis, outcompeting the native mussel, Choromytilus meridionalis, due to the fact that juveniles of Mytilus galloprovincialis grow faster in a temperature range between 17 and 22 degrees. (van Erkom Schurink & Griffiths, 1993) The rock lobster, Jasus lalandii, was fairly tolerant to a temperature change of more than 6 degrees and actually had an increase in their growth rate. However, there was a serious impact on the reproductive cycle of the adult female lobster as the incubation period for the egg was shorter and less larvae survived the various developmental stages (Cook, 1978). There was also a lower respiration rate in the rock lobsters due to increased temperatures. Thermal barriers can be put in place to limit or alter the migration pathways of marine organisms, however, thermoclines will need to be consistent over time and also cover a large cross-sectional area of the water body (Laird et al., 2017). The altered physiological functioning of marine life as a result of the elevated temperature is considered to be insignificant as the effluent temperatures at the desalination plants are expected to only be around 2 degrees warmer than the intake water and will also meet the Water Quality Guidelines almost immediately after discharge (Laird et al., 2017).
Common mitigation practices include using modern surface or subsurface intake designs, restocking the marine system with either fish and invertebrate eggs, larvae, or juvenile or small adult forms, and the creation of marine wetland areas adjacent to the plant for fish and invertebrate spawning (Missimer & Maliva, 2018). The discharge of effluent offshore beyond the surf zone would also be an effective mitigation technique (Laird et al., 2017).
The installation of desalination plants in the Western Cape will be an effective solution to the water crisis in Cape Town as long as the right mitigation techniques are implemented to minimise any environmental impacts. If there are no mitigation techniques put in place during the construction and operation of the desalination plants, then there could be disastrous short-term and long-term impacts on the environment and the associated biota.