Reverse osmosis is becoming an increasingly attractive option for steam system users, argues Mike Griffin.
A combination of recent technical advances and increasing pressure to save energy means that today’s reverse osmosis (RO) systems could be an ideal solution for many steam plant users.
Moreover, new legislation to minimise the presence of lead in drinking water will soon add a further incentive to deploy RO to remove the phosphate that utility companies use to prevent lead from dissolving in mains water.
RO uses semi-permeable membranes to deliver purified water to boilers and clean steam generators. It’s already widely used in industries such as water and wastewater treatment and food production.
But it is only recently that many steam-system users are starting to wise up to the potential benefits.
The biggest driver for most steam users to invest in RO has been the need to save energy and reduce costs. The Chemical Industries Association has estimated that climate-change schemes will double energy costs for many industries by 2020.
No single technology can entirely mitigate the coming price hikes, but RO can slice as much as 3% off the fuel bills of typical steam users.
The technology is commonly used in conjunction with water softening, chemical dosing or filtration to deliver purified water to boilers and clean steam generators.
By stripping out almost 99% of the dissolved solids, RO can reduce boiler blowdown by an order of magnitude, resulting in significant savings in energy, water and treatment chemicals.
Meanwhile, under tight, new EU controls lead levels in drinking water must be below 10 microgrammes per litre by December 2013 - prompting water companies to add phosphate corrosion inhibitors to supplies to prevent lead in old pipes from dissolving.
While water companies and householders have already removed many old lead pipes under existing EU regulations, water to some 8 million UK households is still travelling through lead pipes.
It seems doubtful that lead will be totally eliminated in the foreseeable future, so phosphate dosing looks like it is here to stay.
Left untreated, phosphate can precipitate and cause scale when it reacts with calcium or magnesium in the boiler water. Typically this happens when the boiler water becomes too hard.
Avoiding this situation involves constant management of the alkalinity of the water in the boiler, adding chemical dispersants and ensuring that the level of dissolved solids is kept in check.
By contrast, RO uses a pump to force a stream of water through a semi-permeable membrane, separating it into two streams.
One is the reject stream or concentrate stream and the other is the purified water or permeate stream. The permeate typically flows to a holding tank before being sent to the boiler or clean steam generator.
RO membranes do not need to be regenerated using the acid or caustic chemicals used in conventional dealkalisation or demineralisation plants.
This makes RO generally safer and easier to manage.Used in combination with the right treatments, RO helps cut down on plant outages caused by scale and reduces corrosion in the condensate circuit.