Water Purification – Chapter 2 – Seawater
In this Chapter we will examine the methods for purifying seawater and how we do this at Wananchi.
What Is Seawater?
The largest water source on the planet is without doubt, the sea. About 97% of the words water is found in oceans whilst oceans cover more than 70% of our planets surface. Seawater abundance makes it a logical go-to source for humans.
Seawater has a high concentration of salt in it, this level is described as saline water.
However, purifying seawater is tricky. The concentration of salt in seawater (its salinity) is about 35 parts per thousand; in other words, about 3.5% of the weight of seawater is made of dissolved salts. We typically describe Seawater salinity as 35,000 ppm (Parts Per Million). Saline water is in the range of 10,000 to 35,000 ppm.
Unlike suspended particles in the water such as dirt and bacteria etc, the dissolved salt cannot be filtered out using conventional particle filters; the salt needs separating somehow from the water. We call this desalination.
Desalination Methods
There are a variety of different methods to desalinate water, here follows a brief overview of some common ones.
Evaporation or Distillation
This happens in nature and is where most of our drinking water originates from. Water when heated by the sun or warm air, turns into a gas and mixes with the air, it then condenses and falls back to earth as rain, snow or ice. Evaporation has been used for at least 1,500 years. It requires significant heating energy and condensing equipment to capture significant quantities of water.
Electrodialysis
This where salt is transported from source water through an ion-exchange membrane into a brine solution. The transportation is made by an applied electric potential difference. Essentially, the salts are charged and pass through the membrane leaving fresh water behind. Electrodialysis is an extremely energy hungry process.
Gas hydrate Formation
Gas hydrate is an ice-like crystalline form of water and low molecular weight gas such as methane. This occurs naturally in marine sediments at depths of approximately 200m in permafrost regions of the world. During the process, all the salts and impurities present in the water disappear and as the temperature increases the gas can be recovered leaving fresh water. High pressures and low cooling requirements demand high energy to achieve water production.
Reverse Osmosis (RO)
RO is essentially the process of pressurising a solute (in our case this is Seawater) at high pressure against a membrane (membrane pore size is around 0.0001 micron), where the salts and over dissolved solids in the water remain on the high-pressure side whilst the solvent (pure water) passes through to the low-pressure side. What is left on the high-pressure side is a concentrate which is rejected to waste, the low-pressure side is the product water used for drinking.
Nano Filtration
Nanofiltration (NF) is a membrane liquid-separation technology sharing many characteristics with reverse osmosis (RO). NF membranes have pore sizes of about 1–10 nanometers, smaller than those used in microfiltration and ultrafiltration, but are slightly bigger than those in RO. Unlike RO, which has high rejection of virtually all dissolved solutes, NF provides high rejection of multivalent ions, such as calcium, and low rejection of monovalent ions, such as chloride. NF uses less energy that RO but salt rejection is 50-90% lower than RO, thus it requires multiple stages to achieve desired results.
Advancing Technology
Technology is forever moving forwards with scientists around the world working hard to find more efficient ways to desalinate seawater. Exciting research in graphene technology for RO membranes is accelerating that could provide more durable and lower pressure membranes significantly increasing system efficiency.
All of the above desalination processes are energy hungry with varying degrees of physical size, efficiency for the production rates and quality of the output water. Today, RO remains the most efficient, reliable and compact answer for most applications.
Wananchi Seawolf Water Reverse Osmosis (RO) Desalination Systems
Our Seawolf series of water purifiers use RO, selected for the reasons above. We use single pass (water is passed once through one set of membranes) to achieve drinking water quality in line with World Health Organisation (WHO) guidelines.
The Seawolf systems do require an array of other components to make a fully functioning system.
Stage 1
Water is drawn from the source and pressurised to push pup it through pre-treatment stages. The pressure setting level is also important to ensure later stages work correctly. This water is now known as the ‘Feed’ water.
Stage 2
The seawater is pre-filtered to remove suspended particles in the water down to 5.0 micron in size or less. This is extremely important to minimise damage to the RO membranes later in the system.
Stage 3
Carbon filtration to remove chlorine from the water. Chlorine, over time, can damage RO membranes. This stage is not necessarily essential, depending on the water source. It can also be combined into stage 2 with a carbon sediment filter.
Stage 4
The treated feed water is now introduced to a High Pressure (HP) recovery pump. The HP pump takes the feed water and boosts the pressure of the water up to approximately 850 psi (58 Bar). This is the ideal pressure range for RO membranes to operate. The HP pumps selected by Wananchi for our Seawolf water makers are the latest technology chosen for highest efficiency and reliability.
Stage 5
The pressurised feed water is directed to the RO membranes which are held within pressure vessels, essentially long tubes. The Product drinking water is directed out of one pipe and the concentrated brine water is directed out of another pipe. The brine is no longer of any use and sent to waste.
From source water we can typically expect 20-30% pure water extracted whilst the rest becomes the concentrate. many factors control this % including the level of contaminants and temperature of the source water. This is known as the recovery rate.
Stage 6
The product water is now passed through UVC germicidal lamps to ensure the water is safe from harmful pathogens. Whilst the RO systems will remove them all, there is the potential for micro-organisms to grow on the product water side. The UVC lamps protect against this.
Stage 7
If installed, the product water is lastly dosed with disinfectant (Chlorine) to provide additional, 3rd layer microbial water treatment and protect the drinking water against cross contamination post-delivery.
Voila! Safe, great tasting drinking water…
Nuclear Biological Chemical (NBC) Water Treatment
Whilst of course Seawolf can desalinate seawater, it also has the capability to remove or reduce harmful levels of many other contaminants from water; even offering NBC (Nuclear Biological Chemical) water purification protection. Some NBC levels may require purifying the water twice; or double pass. This is documented in US Marine Corp document: ARMY TM 10-4610-309-14
Is RO water harmful?
Some claim that RO water is harmful because essential minerals needed for the human body have been removed from the water. This claim is a bit of myth or conspiracy story. While RO is very pure, it is not truly demineralised, and some smaller mineral ions may remain. This is especially true for a single pass RO system.
It should also be remembered that we eat food where we will digest all of the essential nutrients and minerals that we need along with the water consumed; therefore, it is not necessary to re-mineralise RO water.
RO Life and Maintenance
RO Membranes typically have a working life of 5-7 years if they are maintained correctly. As they function differently to conventional fresh water filter cartridges, they need a little bit of extra love to keep performing.
Seawater is very hard water due to such high mineral content. This can cause scale build up on the RO membranes, just like in your kettle at home. This requires periodic descaling using an acid flush. The acid flush used at Wananchi is completely environmentally friendly and safe to use and dispose of.
There is of course lots of biological matter in the source water which can grow on the membranes such as mould and fungus. A environmentally friendly biological cleaner is periodically flushed through the system to remove fouling of the membranes.
Last, but not least, the RO membranes need to stay wet to prevent them drying out and going brittle which can cause them to fail. So, if the RO system is to be unused for a whilst and placed into storage, a storage solution needs to be flushed through the system to prevent any issues.
None of these flushes are complicated, we simply pump the fluid from a bucket by the water purifiers own system and let it circulate for the suggested time.