SHARON JONES Desalination may be the most sustainable source of water in the absence of natural recharge of surface and groundwater resources
With increased debate around water security in South Africa, as a result of growing surface-water scarcity and future uncertainty, the prospect of implementing large-scale desalination of seawater is also increasing.
Engineering News spoke to SRK Consulting, a firm of specialist consulting engineers and scientists, about the perceptions around licensing large-scale desalination plants, which would ideally be based at the coast to process seawater.
SRK Consulting principal environmental scientist and associate partner Sharon Jones says, that, currently, and especially during times of reduced availability of potable water resources, desalination seems to be perceived as an emergency solution to supplement available potable water. This is especially prevalent in coastal provinces, owing to the fact that such regions are bordered by a seemingly endless supply of water, which is otherwise unusable as a result of it being too salty.
However, assuming that the impacts associated with the discharge or disposal of brine (salt-rich effluent produced by the desalination process) can be adequately managed to minimise environmental impacts, desalination “may be the most sustainable source of water in the absence of natural recharge of surface and groundwater resources”.
Brine can be disposed of using several methods – including the current preferred method of discharging brine into the sea – as well as to evaporation ponds and salt reclamation plants. The option of blending the brine with discharges from other sources, such as wastewater treatment plants, before discharge to the sea, could also be a viable option where facilities are situated close together. This allows for the dilution of both effluent sources and may reduce the need for the construction of additional marine infrastructure.
Desalination technology comprises two types of filtration systems: thermal and reverse osmosis (RO). According to water treatment company Veolia Water Technologies business development manager Chris Braybrooke, the only feasible form of desalination technology to be used in South Africa is RO.
Getting Approval
Building and commissioning a desalination plant requires various permits and approvals from several State departments.
Jones says the two key authorisations locally are an environmental authorisation in terms of the National Environmental Management (Nem) Act, No 107 of 1998, and associated environmental-impact assessment (EIA) regulations, and a Coastal Waters Discharge Permit (CWDP), which is issued in terms of the Nem: Integrated Coastal Management Act, No 24 of 2008, for the discharge of brine generated by a desalination plant into the sea.
An EIA process needs to be performed, in which feasible alternatives to brine disposal must be assessed. “The process of identifying and screening various locations, technology and process alternatives should be carefully documented.”
Authorisations may also be required in terms of the National Heritage Resource Act, No 25 of 1999; the National Water Act, No 108 of 1997; and the NEM: Waste Act, No 59 of 2008, which depends on the scale, nature and location of the project, she adds.
Jones says, in most cases, separate reports and application processes are required because the permits and licences are issued by various government departments or organs of State. In turn, this will necessitate a number of detailed, specialist studies, which can be time-consuming. “On average, it would take one to two years – potentially more – for all the relevant processes to be completed and permits to be issued.”
Jones tells Engineering News that the most significant challenge in terms of licensing a desalination plant is the method used to dispose of brine. “The proposed method of discharging brine plays a big role in determining a suitable location for the site and the size of the area required.”
A CWDP is issued by the Department of Environmental Affairs’ Oceans and Coasts (O&C) division in terms of the Integrated Coastal Management Act, No 24 of 2008. “This is a relatively new requirement and permitting process (discharges to the sea previously required a water-use licence) in terms of the National Water Act, and the O&C is still revising the National Policy of Coastal Effluent Disposal from Land-based Sources and the assessment criteria for applications for CWDPs, as well as the South African Water Quality Guidelines for Coastal Marine Water,” she says.
Mixing Zone
The ultimate goal of discharging effluent, such as brine, into the sea is the effluent’s rapid dilution into the receiving environment. The current method of discharging brine into the sea is applied in areas of high energy and dissipation, such as particularly rocky patches of coast or further out to sea, whereby the action of waves assists in mixing and rapid dilution.
This reduces the size of the area affected by the effluent, also known as the mixing zone, says Jones. To manage this, she says effluent would need to comply with predetermined effluent quality standards.
Further, the mixing zone is the area in which the discharge of brine will have the most significant impact on marine ecology, the significance of which would need to be understood prior to a CWDP being issued. “This will often require the modelling of discharge plumes through numerical marine modelling by marine engineers or specialists.”
The impacts of marine infrastructure and reduced, or poor, marine water quality on the mixing zone may be more significant if there are sensitive receptors and other water users, such as aquaculture facilities, fishing communities and recreational bathers.
Jones highlights that environmental authorities are “eager” to see industry develop any alternative methods of effluent disposal, especially those that might produce a beneficial impact on the environment.