With ever-increasing demands made upon the world’s water supplies and the upward spiralling cost of water, it is becoming more viable to recycle water. The recent developments in ultrafiltration technology permit high quality water to be produced from a municipal sewage effluent. Where necessary this water can then be treated by reverse osmosis to produce water of sufficiently low salinity for use in industrial water applications such as cooling water or boiler feed water.

Biwater built a 30,000 m³/d plant in Jeddah, Saudi Arabia, treating tertiary effluent by reverse osmosis to produce high quality irrigation water as long ago as 1981. Since this plant was built the improvements in pre-treatment that can be made with ultrafiltration and the improvements in the reverse osmosis membrane have been huge. More recently, Biwater have installed a reverse osmosis plant to recover over 63,000 m³/d at Scottsdale, Arizona.

An alternative to using an ultrafiltration membrane on the outlet of an existing works would be to design the sewage treatment plant as a Membrane Bioreactor or MBR, which would also produce high quality feedwater to the reverse osmosis plant.

There can be a move to desalinate seawater when the water resources are scarce. Whilst seawater desalination is attractive for potable water production it can be expensive to treat all the water in this manner irrespective of the use, use the water once and treat the waste in a sewage works before returning the water to the sea. It may be more appropriate to consider the water supply system as a whole and build a seawater desalination plant for potable production but reuse the water from the sewage works for industrial applications.

Where there is a requirement for irrigation or industrial use it is significantly more cost effective to treat the wastewater with a Membrane BioReactor (MBR) or polish the treated waste water with Ultrafiltration (UF) and then remove the remaining salts and impurities with Reverse Osmosis (RO).

Often, industrial users will further treat the drinking water that is supplied to them to either soften the water or to demineralise the water for boilers. Consequently, desalinated wastewater is of much greater value to some industries than drinking water as the chemical usage in ion exchange systems is reduced significantly.

The capital cost of installing a UF plant to treat secondary effluent followed by a low pressure RO plant is around 70% of an equivalent capacity seawater RO plant. The operating cost of the UF/RO combination to re-use the effluent is around 35% of that of seawater desalination.

Re-use of treated effluent is a cost effective solution to the development of alternative water sources.

Biwater are unique in having a wealth of experience in not just all forms of membrane treatment but decades of experience in “traditional” or “conventional” water and sewage treatment.