Research and development specifically aimed at more cost-effective management of mine water usage has led local water treatment specialists, TAG Water Systems/MC Process, to design a new turnkey water treatment system, the first of which has been commissioned in the Democratic Republic of Congo (DRC).
“Clean potable drinking and process water, to run the plant, was the motivation to develop a dual purpose system.” says TAG managing director Rogan Roulstone. “To meet the criteria we designed a dissolved air flotation (DAF) system with a combined potable drinking water treatment unit that recovers process water unfit for human consumption, but suitable for re-use in the plant,” he adds.
“With this in mind – plus the fact that water-borne diseases are rife in Africa – we developed a skidmounted, modular, potable drinking water treatment plant that draws water from the DAF,” Roulstone continues. “The dual-functional unit is a vital piece of equipment, especially in remote locations, as is the case with so many rural and dry mining areas on the continent.”
The DAF designed for the specific DRC project is a further first for the company, as it is a dissolved air flotation (DAF) process integrated into a conventional clarifier. It removes fine suspended material from an aqueous suspension.
This is achieved by using extremely fine high pressure air bubbles. The attraction between the air bubbles and particles results in a reduction of the density of the particle, and the outcome is increased buoyancy that causes the suspended solids to float.
The distinctiveness of this particular design is that it integrates two technical processes – flotation and clarification – which means that capital investment is lower and the footprint is smaller than it would be in two separate units.
The operating advantage is that the system not only takes care of fine suspended particles, but also removes any particle where the gravitational rate is greater than the flotation rate.
“There are many benefits to having two solid liquid separation principles in one compact and effective unit,” comments Roulstone. “Over and above it being quicker to manufacture, there is an immense saving on energy costs. It eliminates the need for an entire pumping system.”
The primary purpose is the cleaning of bulk water so that it can meet the requirements for re-using the process water on the plant. A key benefit is that the DAF can handle a variety of feed streams, and allows consistent output in terms of volume, as well as the level of contamination. The system has an exclusive static inline mixing system that allows accurate mixing of dosing, and pH control chemicals. This mixing assists in ensuring a consistent output stream.
“There are other similar units, but none that we know of offer the levels of automation that we have designed,” claims Roulstone. “The criterion of designing a unit that has virtually no human interference is vital. Most mining locations are fairly remote and the benefit is that operator training and interface is minimised.”
The system produces three output phases: the top phase is the flotation drawn off by means of a scrapper mechanism; the bottom phase is the settled higher density waste’ and in the middle is the clean and treated water.
According to Roulstone, the potable water that is produced is of international standard ‘bottled water’ quality, making it the purest drinking water that is available. “The drinking water produced exceeds world health standards,” he contends.
The system operates with two high pressure water pumps that draw water from the DAF product overflow box at a rate of 5m3 per hour. After ozone pre-treatment, the feed water is pumped to two sand filters, removing any solids down to a size of 10 microns.
Water from the sand filters reports to a micron cartridge filter where the TSS (Total Suspended Solids) are reduced to less than 0.01% at 1 micron. Flow switches send off a warning when the unit has a dirty cartridge that needs to be cleaned or changed.
In the final stage of water purification, water is passed to a reverse osmosis unit where the final purification takes place. This system can be used in areas where it takes water from any or most sources, however in this instance Tag Water Systems is making use of the water from the DAF system.
“We have designed a reverse osmosis process that uses semi-permeable, spiral-wound membranes that separate and remove dissolved solids, organics, pyrogens, submicron colloidal matter and bacteria from the water,” says Roulstone. “It has an Ozone O3 generator and contact system that kills any residual potential bacteria that may accumulate in the storage tank. An ozone level of 0./4 ppm for four minutes has been shown to kill any bacteria, virus, mould or fungi,” he explains.
The company claims that this is a far more efficient method than a chlorine system, which depends on diffusion into the cell protoplasm and inactivation of enzymes. TAG Water Systems does not limit its service to this particular type of technology – it specialises in designing equipment to suit the application.