Reverse Osmosis Energy Efficiency

How to drink sea water

Make sea water drinkable
Make sea water drinkable

The world is running out of drinking water. Where is it going, how are we using it and what are engineers doing to combat this. That’s what we’ll be discussing in this article. A key topic we’ll cover is how to turn sea water into freshwater efficiently, and we can do that using the isave by Danfoss.
Watch the YouTube video at the end of the page.

Learn how the iSave makes sea water reverse osmosis even more efficient and helps deliver fresh water to people all over the world. Click here to learn more about iSave

Access free case stories, data sheets, and the iSave calculation and selection tool.
🏆 Visit the iSave homepage –

We all need to consume around 2 litres per day just to survive. Every living cell in our body needs water to keep functioning. Without water our bodies will simply shutdown and we will perish within somewhere between 3 and 7 days, typically.

Daily water consumption
Daily water consumption

But that’s not the only water we use, there’s all the other daily routines which consume water, such as

Showering, flushing toilets, washing hands, food preparation, cleaning dishes, watering plants etc

Then there’s the water used for agriculture and manufacturing.

We’re using so much water that major cities have started to run out. In February 2018, the city of Cape town in south Africa began drastic water restrictions limiting citizens to just 50 litres a day per person, in an attempt to avoid what they called, day zero, the day they would run out of water.

Over consumption and drought led to the depletion of water reserves and cape town was set to become the first major, modern, city in the world to run out of water. Engineers predicted that by April 21st 2018, the water levels in the dams would be so low that they would have to turn the pumps off and water would be handed out at communal collection points throughout the city. Luckily the city took drastic action and avoided this, this action alone isn’t enough though and day zero is predicted to return in 2019.

Water access world
Water access world

When you look at our planet, around 70% of it’s surface it covered by water. The problem is, around 67% of this is sea water, only 3% is fresh and even then only 0.4% is actually accessible. The other 2.6% is locked in places like ice caps, glaciers or is in the atmosphere.   So, we’re only left with that ~0.4% which is both accessible, usable and drinkable.

That has to be shared with the estimated 7.7 billion humans on earth, and this population is growing by approximately ~80 million additional people per year.

So, what are we doing to combat this? This is where the engineers come in.

There are many technologies out there, one of the most popular and most obvious is to use sea water.

Of course, you can’t drink sea water. So, we need a way to purify it. A technique we use to do this is called reverse osmosis.

The process works like this. You take sea water and remove as much dissolved salts, organics and pyrogens as possible.

The problem is sea water doesn’t naturally want to give up this stuff, so we have to persuade it.

Basic reverse osmosis
Basic reverse osmosis

First, we need a semi-permeable membrane.  Membrane refers to the boundary and semi-permeable means to permit some.  What that means is you pass sea water through a special boundary, kind of like a filter, and that will allow certain substances to pass through it whereas other substances like dissolved salts will be held back. So, you put sea water in and get fresh water out. It still needs a little more work done to it, but it’s usable.

This is a slow process though so to clean a lot of sea water we need some big sets of semi-permeable membranes and then we need some very high-pressure pumps to force the water through the membrane. Not all of the sea water will be turned into fresh water, some of it is used to flush away all the dissolved salts etc.

Simplified reverse osmosis schematic
Simplified reverse osmosis schematic

As you can imagine, this is an energy intensive process. the pumps are large and consume a lot of energy to create the high pressures required to squeeze the water through the membrane and remove the unwanted substances.

To reduce the energy consumption, we need a way to recover some of the energy that’s wasted and reuse this. So, engineers looked at the high pressure concentrate waste water coming out of the membrane and decided to find a way to reuse it. This is the water that didn’t make it to becoming fresh water and flushes the dissolved salts etc away.

isave reverse osmosis
isave reverse osmosis

What the engineers came up with is this, the isave energy recovery device with an isobaric pressure exchanger and a high-pressure positive displacement pump, all driven by a single induction motor.

We connect this into the system by taking the high pressure concentrate waste water and passing this into the pressure exchanger. We then take another line of low-pressure sea water and also run this into the pressure exchanger. The two streams briefly come into contact with each other. We’ll see how a little later in the video, but the brief contact time is enough for the high-pressure waste water to ram the low-pressure sea water and in doing so it transfers its pressure, so we have recovered this. The concentrate waste water leaves the pressure exchanger as a low pressure concentrate and the sea water leaves as a high-pressure sea water.

Energy efficient reverse osmosis
Energy efficient reverse osmosis

The positive displacement pump at the end of the unit is then used to force the water into the membrane along with the other sea water from the high-pressure pump, and the process continues as before and we get fresh water out of the system, but we can reduce our energy consumption by up to 60%

So what’s happening inside the isave and how does it work?
For detailed working animations, watch our YouTube video at the end of the article.

We have the three main components to the isave. The induction motor which drives the unit, the pressure exchanger which recovers the pressure and then the pump which pushes the recovered pressurised water to the membrane.

Inside the pressure exchanger we have a number of empty chambers. These will be filled with water and is where the pressure is transferred. There are some plates at each end to control the flow of water.

The low-pressure sea water enters the unit and begins to fill up some of the chambers. The induction motor then rotates the entire pressure exchanger and so the chamber of low-pressure sea water then rotates around to the high-pressure side. The high pressure concentrate waste water then enters the unit and rams into this low-pressure sea water inside the chamber

How the isave energy efficient reverse osmosis works
How the isave energy efficient reverse osmosis works

As the two waters collide the pressure is transferred from the high-pressure side and into the low-pressure side. This forces the sea water out of the chamber and into the pump, As the sea water leaves the chamber, the chamber then fills with the concentrate waste water.

The sea water is now pressurised as it took some of the pressure from the concentrate during the collision. The now high-pressure sea water then enters the pump and is forced into the membrane for the reverse osmosis to take place. The concentrate water has lost some of its pressure so is now a low pressure concentrate waste water. This is still inside the chamber so as that rotates it comes back around to where the low-pressure sea water is entering, the sea water pushes the concentrate out and the chamber then fills with sea water where the whole process will repeat.

By doing this we have recovered the pressure from the waste water so the work load by the main high-pressure pump has been reduced, making the entire operation more efficient and cheaper to produce.

The two streams do come into direct contact with each other so there is some mixing of the fluids, but the whole thing happens very fast so the amount of mixing is small. The sea water still has to pass through the membrane to be cleaned so this mixing is not a major issue.



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