Reverse Osmosis: The ins and outs of living on bore water
Saturday, 21st February, 2015
By Craig Brealey
If the Menindee Lakes go dry in April next year, as was predicted this week, then the plan is that the city will switch to bore water.
The drills are working in and around the dry Menindee Lake and the quality of the water they find will be known soon.
Approval is also being sought to sink bores in the Talyawalka floodplain, 20 kilometres to the south of Menindee.
Water that is closer to the surface is not as salty as that which is deeper but to make it drinkable, bore water must be treated by a reverse osmosis plant.
The city’s supplier, Essential Water, has a plant on standby but it is an expensive and, necessarily, a wasteful process.
If the water treated by reverse osmosis is not too saline then up to 80 percent can be used and the remainder is discharged as brine, according to the company. The higher the salt count, the greater the loss.
The plant, which will be set up at the Mica Street works, can produce 6 megalitres a day (MLD) if the water is not very salty, Essential Water told the BDT.
This can then be added to water from the lakes and reservoirs that has been treated at the Mica Street works to produce a palatable “shandy”.
As an example, the company said, if the water treated at Mica St had an EC reading of 2000 (which is salty) then about 24ML of good water could be produced each day.
(EC stand for “electrical conductivity”, a test that shows the level of dissolved salts in water by measuring its ability to carry an electrical current; the higher the reading, the more salt in the water).
With a reading of 4000 ECs, about 9.5MLD could be produced.
A reverse osmosis plant works like this: Salty water comes in, is treated to remove contaminants that might damage the filtering membranes (such iron or manganese) and then it is forced at high pressure through the microscopic membranes to catch the brine.
After that the water has fluoride and chlorine added and the brine is pumped to a site for disposal.
In the last big drought 10 years ago, a waste pipeline was run from Mica St to the garbage depot. The reverse osmosis plant was not used then, and that site can no longer be used either.
Essential Water told a public meeting this week that, wherever the brine went, it would be held in dams lined with plastic to stop it leaching into the soil.
But, after all of that, the salinity of the bore water is not yet known, and it could be that water from the shallow bores does not need to pass through reverse osmosis, the company said.
However, the plant might still be needed to treat poor quality water from Lake Wetherell if Copi Hollow dries out, it said.
The problem is, it seems, what happens if there is no water from the lakes and reservoirs to supplement the bore water? The reverse osmosis plant can produce 6 megalitres a day. Now, under the least stringent of restrictions, the city is using 16 megalitres a day.
Everyone at the public meeting at the Musicians’ Club on Wednesday night, including representatives of the NSW water authorities, murmurmed in agreement when someone said they just hoped for big rain in the Darling River’s catchment.
Treated bore water is palatable - it is what you get in your bottled “spring water” - but it is not a permanent solution. That will come only when the river is allowed to run and the lakes to fill again.