Many coastal towns in the Cape Province feel the pressure with water demands exceeding supply, especially during peak holiday seasons. An example is Lamberts Bay, a town about 280 kilometres from Cape Town - along the Cape West Coast, in Cederberg which has only six boreholes to supply water to the entire region.
To resolve this dilemma, the Department of Water Affairs (DWA) and the Cederberg Municipality first considered two possibilities: provide additional boreholes; or install a 61-kilometre pipeline feeding from the Clanwilliam Dam. Investigative studies, however, revealed the pipeline would not guarantee a sustainable water supply and test boreholes exposed overly excessive iron and manganese content in the water. The DWA and the town's municipality therefore decided to commission a new desalination plant adjacent to the town's existing water purification plant.
Seawater desalination reduces water dependency on other regions, eliminating the need to import and transport water over long distances. It reduces logistical costs and pressures associated with transporting water.
The Lamberts Bay desalination plant, which came into full operation during the second quarter of 2014 at an approximate cost of R 17 million, uses reverse osmosis(RO) membrane desalination to separate salt from seawater. RO is realised by forcing seawater through a membrane using high external pressure to separate it from the salt, and is one stage in the process enabling the plant to provide 1.7 mega litres of potable water per day. The plant's infrastructure has been designed to accommodate future expansion of up to 5 mega litres per day, providing the capacity to supplement the region's water usage by approximately 50%.
The recovery rate of clean, drinkable water across these RO membranes is 42%. The remaining solution, called "brine", contains the extracted salts in up to double the original concentration of the water. The recovery rate and concentration factors vary, depending on the quality of the seawater feed source.
As part of the solution, Veolia installed an innovative energy recovery system which recycles the excess pressure energy from the brine back into the system in order to compensate for high energy required to power the high-pressure pumps. This plant uses approximately 3.75 kW of electricity per cubic metre of potable water produced.
Veolia's conducts research into technology and process improvement for ways to minimise energy consumption
Veolia supplies Hydrex chemicals to the plant and services and maintains plant equipment.
Four stages of treatment enable desalination at the plant: pre-treatment and preparation, seawater reverse osmosis (SWRO), treatment up to potable standard, and backwash effluent treatment, all controlled by the plant management system - a Supervisory Control and Data Acquisition (SCADA) computer system.
SCADA systems are used to remotely monitor and control industrial plant equipment. Inconsistencies and problems show quickly, alerting the operator to the need for maintenance. A testing lab for the desalination plant was built adjacent to the operator's room, where samples of the water are tested regularly for pH, turbidity and chlorination levels. Results are compared with the required values in the SCADA system.
Pre-treatment and preparation
Seawater is extracted through four boreholes located on the beach 3 kilometres south-west of the plant and pump station. The sand acts as a'pre-filter and the water, through an underground feed, is then pre-treated with a shock dose of chlorine to kill any biological growth in the pipeline. Ferric chloride is added as a coagulant to bind dirt and suspended particles, 95% of which are removed as the water filters through two layers of silica and anthracite in Dual Media Filter (DMF) tanks. An antiscalant is added to the filtered water before it moves into the RO system, which inhibits build-up of minerals ensuring the membranes remain clean for longer periods.
The plant contains two RO'trains' (the infrastructure provides for six) consisting of a series of long pressure vessels pumping the water at high pressure. The concentrated brine solution produced is returned to sea via a specifically-designed brine outfall system to minimise the impact of brine on the surrounding seawater, and the remaining water is now sterile.
Treating to potable standard
To stabilise the clean water further treatment is needed. Carbon dioxide is injected into the water, adding carbonic species before it is fed into a limestone filter. The calcium from the limestone dissolves in the water before it is fed to a large permeate tank, where soda ash is added to meet the SANS 241 2006 Class 1 specifications. These are stringent standards which prescribe the amounts of minerals the water must contain, as well as the ideal pH, for balancing to a high quality potable standard. Before the water is fed to the municipal water reservoir, chlorine is dosed to prevent biological growth in the product water pipeline.
Backwash effluent treatment
The DMF filters are backwashed regularly to remove the build-up of organic matter. The resulting effluent is fed into a buffer sump, where it is treated with a coagulant and a flocculant, which binds the particles to form larger-sized clusters. This effluent passes upwards through a lamella clarifier that leaves the solid particles behind. The clarified backwash water is then discharged into the sea, along with the RO brine.
Environmental Impact Assessment
The Environmental Impact Assessment (EIA) determined that the solids determined that the solids retained must be removed before dispersing the effluent. The resulting "sludge" is transferred to a drying bed where the remaining water evaporates and the dry deposit is disposed of.
The plant is designed according to client specifications to minimise impact on the environment, which included removing impurities from backwash effluent before it returns to sea. The EIA involved the deployment of divers out to sea to explore the best location for the outflow pipe in order to have the least impact. The outflow pipe extends 150 m into the ocean, which improves saline dispersion, avoiding potential negative impacts on the shoreline.
The water quality and salinity levels are frequently tested to ensure that there is no harmful effect. Rehabilitation of biodiversity was encouraged where the pipelines were installed.
For efficiency and cost-effectiveness, the plant will supplement Cederberg's water supply mainly during peak seasons.
Veolia has constructed seven desalination plants along the Cape coastline since 2009.