Reverse Osmosis Faq's

Reverse Osmosis - Frequently Asked Questions

What is a Reverse Osmosis Plant?

Reverse Osmosis Plants use the principal of "osmosis" to produce high-quality purity water.

Reverse Osmosis is a process which forces a highly concentrated stream of dissolved salts through a semi-permeable membrane into two separate streams:

a) A stream of purified water, known as permeate;
and
b) A stream of the removed highly-concentrated dissolved solids, known as concentrate or reject.

Water pressure at typical pressure of 200 to 400 psi reverses the natural osmotic tendency of the solution, hence the term "reverse osmosis".

What are the typical components of a Derwent Reverse Osmosis Plant?

The typical simplified Derwent Reverse Osmosis Plant consists of:

Pre-filter
Typically of less than 5 micron rating.

Main feed pump
Multi-stage simplex or duplex pumps to generate osmotic pressure.

Membranes and Element housings containing reverse osmosis membranes
membrane housing interconnected with 316 stainless steel pipework.

Control panel
Incorporating pressure gauges, flows, alarms, conductivity measurement and meters.

(Normally all skid-mounted for ease of installation)
What is meant by % recovery and % reject?

Typical % recovery is limited to 70% to 80% of the total feed water volume. This limit is due to osmotic pressure, limited solubility of the ions and dissolved solids, and silica left behind in the reject water.

In the example above, the 100 ltr/hr of feed-water has produced 75 ltr/hr of permeate and 25% of concentrated reject water. Hence, we have 75% recovery and 25% reject.

What impurities can Reverse Osmosis membranes remove from water?

They can remove ionic species, macro-molecules, suspended solids, dissolved solids, organics, bacteria and viruses.

What are Multi-Stage Osmosis Plants?

Multi-stage osmosis plants are designed to produce higher-purity water than from a single stage unit. In the multi-stage process, the permeate from the first stream becomes the feed stream for the 2nd pass. The reject from the 2nd pass is then re-cycled back to mix with the incoming feed-water stream.

Does the feed-water to a Reverse Osmosis Plant require pre-treatment?

Pre-treatment of the water is often required to ensure that the feed stream will not scale or foul the membranes.

Calculations are undertaken by Derwent Water Services' specialists to assess the scaling potential of the water with special regard to calcium sulphate, calcium carbonate and silica scaling. Treatment can then be advised to make the water non-scaling. This can be acid dosing, anti-scalent inhibitor or, more frequently now, base/exchange softening of the feed-water stream prior to the reverse osmosis unit.

With suspended solids present in the feed-water stream, then filtration or ultra-filtration may be required.

What is Reverse Osmosis water used for?

Reverse Osmosis is commonly used in desalination and waste-water treatment. It is finding increasing use in the treatment of brewing liquor and in low-pressure boiler feed water.

Unlike demineralisation, it has the advantage of not using acid or alkali regenerants but it does have the disadvantage of a reject stream which can be typically 25% of the feed water. Unless this water can be re-used, it can add to the cost of the total plant operation.

What types of membranes are used in the typical Derwent Reverse Osmosis Plant?

Derwent Water Services uses thin film composite membranes as a standard but other membranes (such as cellulose acetate) are available and are sometimes used in project systems.

Do the Reverse Osmosis Plant membranes need to be cleaned?

Periodically, the membrane may need to be cleaned to remove foulants and scale. The foulants may be organic and of micro-biological origin, or inorganic of iron and calcium origin. Normally, a two-stage clean is employed:

a) An alkaline clean with surfactants;

and

b) An organic acid clean.

Derwent Water Services can provide on site cleaning of membranes as part of its service package.

Can Derwent Water Services service Reverse Osmosis Plants?

Yes.We offer single-day services or service contracts on all water treatment equipment. All of our water treatment service engineers are fully qualified and experienced on the full spectrum of water treatment equipment available.We have defined service specification sheets (available on request) which define the exact service schedule proposed for a Reverse Osmosis Plant.

Please see our service package facility on the home page.

How do I know what size of Reverse Osmosis Plant I require?

The fail-safe way of sizing any type of Reverse Osmosis Plant is to contact our technical team at Derwent Water Services Limited.