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A MODEL FOR DETERMINING THE FEASABILITY OF BRACKISH WATER DESALINATION
USING REVERSE OSMOSIS TO SUPPLY IRRIGATION WATER TO FARMERS
Cornelius J Coomans and Christian D Swartz
Chris Swartz Water Utilisation Engineers
The salinity levels of groundwater in the Western Cape varies from low (having an Electrical Conductivity (EC)
of less than 70 mS/m) to very high (with EC in excess of 520 mS/m). Due to the high salinity of many
groundwater sources, farmers are mostly unable to use the water for crop irrigating or for livestock watering.
New and existing farming enterprises around the Cape Metro area (but also in other areas of the Western Cape)
have requested the installation of desalination systems on their farms to desalinate the water that they utilise for
irrigation and livestock watering.
Although some salts, such as calcium, magnesium and certain heavy metals, can be removed by chemical
precipitation, most of the salts dissolved in water can only be removed by distillation or by highly sophisticated
physical-chemical separation technologies. All these technologies are characterised by their high cost and/or
their high energy requirements. Of all the available technology for desalination, reverse osmosis (RO) is still the
most suited for brackish water desalination on remote rural sites.
A desktop study was performed by the Department of Agriculture, Western Cape in which enterprise budgets of
different crop and animal enterprises were used and the costs of water and irrigation schemes determined. There
was a need to extent this study by developing a costing model for brackish water desalination for irrigation
purposes.
A sensitivity model was developed that indicates the effect of the additional cost of desalination on the gross
margin of the different crops/animal enterprises. The model consists of a spreadsheet-based user interface from
which the cost of desalination using RO membranes is calculated. The model has the option of either using RO
water only, or blends of RO water with raw water. In both cases the different volumes of raw water required,
clean water produced and brine produced are provided by the model, as well as the EC levels of these streams.
The user can select multiple crop and/or animal species as well as expected water volumes that would be
required for irrigation and livestock watering. The user supplies the model with the quality and flow rate of the
raw water. With these inputs, the model calculates the maximum salinity that can be tolerated based on the
sensitivity of the most sensitive species. With the final product water salinity fixed, and the salinity of the raw
water known, the model determines whether blending is possible or not, and calculates all the applicable costs
relating to the use of the technology.
With this model it is now possible to determine for which crops/animal enterprise it would be economical and
sustainable to purify and utilize water sources of different quality.
Keywords: Groundwater, Desalination, Reverse Osmosis, Crop Irrigation, Feasibility model.
Topic: Modelling/Ground Water Remediation