Managing Short term Water Quality Problems
18 September 2007
ELGA Process Water looks at the challenges in solving the problems of water quality variations
Modern 100bar power station boiler technology has pushed furnace heat fluxes above 500kW/m² and any deposition of scale on the heat transfer surfaces can be enough to cause a tube failure. That is why boiler make-up water quality is typically specified with conductivity 0.1µS/cm and silica less than 20ppb. This means that the make-up water treatment plant is, arguably, the most critical item in the power station.
The cost of producing high purity boiler make-up water is a significant contributor to the overall costs of generation and the water treatment plant is normally designed to minimise chemical and power consumption. To achieve these minimised operating costs, the water treatment plant designer will need to know as much as possible about the quality of the raw water.
For example, alkaline borehole waters are usually most efficiently treated by ion exchange demineralisation using stratified beds of weak and strong cation and anion exchange resins with intermediate degassing to remove the carbon dioxide generated by cation exchange.
Lowland river waters usually have similar levels of dissolved salts but lower alkalinity, favouring reverse osmosis. Waters from moorland sources usually have low levels of dissolved salts. If these waters are treated by ion exchange, the resin beds will be smaller than those required for a similar flow of borehole water, but the high levels of natural organic matter mean that pre-treatment by organic scavenging or activated carbon will be needed to prevent fouling of anion exchange resins or reverse osmosis membranes.
In the past many power stations took their water from municipal supplies – many still do – and this was a guarantee of consistent quality. Reservoir waters have always been affected by seasonal variations in organics, but these changes are predictable and can be easily controlled by techniques like brine washing. Now, however, things are changing. Municipal water supplies are treated to meet drinking water quality standards and the increasing pressure for compliance has resulted in increased costs as water works install new technologies to remove nitrates. As a result some power stations have opted for private water supplies, even using their discharged cooling water.
Even those who still use municipal water are subject to changes in water quality. Considerations of water resources sometimes force water suppliers to change the source of their water supply from river to borehole or vice versa, with a sudden and dramatic change in TDS. This already happens but it will become more frequent as global warming places water resources under increasing stress.
In each case, although the water still meets the drinking water regulations, the effect on the water treatment plant can be devastating: ion exchange plants designed for thin, upland waters may have insufficient capacity to treat thicker borehole waters whilst, those designed for hard waters may suffer from fouling by higher concentrations of natural organic matter in supplies derived from moorland reservoirs.
The most troublesome elements are those like barium that are not normally monitored. So what happens if the quality of the water supply changes so the make-up water treatment plant cannot treat it economically?
To solve these short-term problems, consider ELGA's AQUAMOVE MORO mobile RO and MODI ion exchange systems. The MODI units are installed in standard 40 foot insulated trailers; self-contained all that is required on site is a connection to the water supply. The MORO units are installed in 20 foot containers. Pre-treatment is usually provided by a number of MOFI pressure vessels. And once the raw water quality is back to normal the mobile unit is returned to the supplier, making it a highly cost-effective option.
