Use/treatment of High TDS (30000 PPM)

"Weak seawater". OK.

Carry out an economic-driven water balance study on the facility and look for ways of re-using the water rather than have multiple streams in parallel.

• Are there cooling towers on site that can take a feed from a first or second stage Reverse Osmosis [RO] facility?
• What about boilers taking a second stage RO?
• What about garden irrigation (unlikely with that TDS, though worth looking at)?
• What about fire-fighting water storage ?
• What about wash-down facilities, where a potable water is not needed?

If clean-up is advocated, then for a 6.3m³/h inlet investigate package pretreatment units and package seawater RO equipment so as to remove most of the salts and make a 3.5-4m³/h pseudo-drinking-water stream from it. Or go one stage further and make a technical water 2.5-3m³/h by adding a second stage RO; the second stage reject is fed back to the inlet of the first stage, making the plant smaller. This water is the one to look at for cooling towers and boilers, which are essentially the same thing.

The economics of single stage RO are unlikely to stack-up against buying-in potable water from the town main at this time unless there is a corresponding volumetric charge on the effluent disposal to be saved; if there is then it might be an attractive possibility given sight of the local money-numbers.

Seawater RO on land is usually uneconomic unless it is done on a large scale either in the absence of river and borehole water sources or close to major centres of population where local rivers and boreholes are inadequate.

Seawater RO at sea or on highly-populated islands surrounded by sea is an attractive proposition and usually wins against evaporation unless there is a large source of low-grade heat available.

As always, local economic factors dictate the way to go and there is no "one size fits all" solution.

It looks like treated acidic effluent with lime.

RO membranes do get chocked with sticky CaSO4. So 2nd stage RO may not be a solution.

If you have disposal problem, you may try to convert the contents to Sodium Salts by Ion exchange and then do 2nd Stage RO , so that you get back part of quantity(around 30/40%) back for usage.

R.O concentrate is high. What % of recovery R. O plant is working? You can consider to install another R.o plant to treat this waste water and use it for required purpose if it is cost effective.

One of the banes of RO system is disposal of high TDS water. If it is left near the source of water, it would contaminate the bores or sources of water around. If it is left into rivers or drains, it would add to the TDS there.

Actually the best RO plants would be run from the sea water, taking salty water, making it as near potable water as possible, and then return the high TDS to Sea, which can tolerate any amount of TDS. Unfortunately, all locations are not sea /coast based.

The next ideal method would be to evaporate the high TDS water, using impermeable pans, exposed to sun, as they do in salt pans, and then bag the crystals and throw them into the sea. If it is economical to separate any specific salt/metal from the material, well and good, otherwise, throwing the solid waste into the sea is the only way out, according to me.

Gadepalli Subrahmanyam

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