Water Purification – Chapter 3 – Filter Life
In our conversations we have regarding mobile water purification, there is one question that will almost always come up… ‘How long do the filters last?’
It’s a simple question, and a fair one, that would likely be asked of any manufacturer or brand of water purifier throughout the world; it is in our minds, the “The Golden Question” … The problem is that it is practically impossible to answer with any real definitive answer.
The performance of any filtration system is bound by the nature of the filtration components, the quality and volume of water passing through it coupled with the level of maintenance carried on the machine. The variables involved that would allow an intelligent answer are numerous and often will interlink and affect one another.
For our explanation as to why, we shall stick with our Séon mobile water purifiers. Séon has been designed to for ease of transportation and to purify water from any available water source. The water quality at these sources will likely be untested for a thorough break down of contaminants; most likely just known to be salty or not, how dirty or turbid it looks to the eye and does it smell a bit funky.
Séon water purification units use mechanical filter cartridges as first line of defence. These cartridges are designed to remove particulate contaminant from the water and some cartridges (activated carbon) can also absorb Volatile Organic Compounds (VOC`s) and some dissolved chemicals and heavy metals.
Whilst having superb performance these cartridges have a finite life and regarded as disposable. The cartridge traps suspended particles preventing them passing through whilst the carbon within absorbs the VOC`s and chemicals. Eventually the cartridge will become saturated with the contaminants and water flow through will drop off.
It is fair to say that: `The life of the cartridge is completely dependent upon the volume and the quality of the water that passes through it`.
Source Water Contamination
Now, without knowing exactly what the contamination is and the levels are in the water source it is almost impossible to predict how long the cartridge will last.
Water contamination can be a near infinite number of types, but typically this can be divided into two categories:
Dissolved Solids
Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular (colloidal sol) suspended form. TDS are often measured in parts per million (ppm). TDS in water can be measured using a digital meter. Salt in seawater is a prime example.
Suspended Solids
Suspended solids are the fine particles of sediment in the water that are not dissolved. Total suspended solids (TSS), turbidity, and water clarity are all methods of measuring the amount of sediment in water. Suspended solids can be organic and inorganic.
Water contamination can come from an infinite number of sources and reasons. A water source can change significantly in quality can by the hour. Seasons alter weather which affects water tables and plant growth, heavy rains can wash down into rivers raising turbidity significantly. Weather changes can cause green algae bloom to water whilst autumnal leaf fall contain tannins which cause water to go brown or tea coloured.
Human activity also plays a big part in source water quality from farming, industrial activity, sewage discharge and even war.
The picture shows a number of water samples taken from a very small section of the Orange River in South Africa.
Measuring Water Turbidity
Water turbidity can be measured. Measurements are commonly taken using the Nephelometric Turbidity Unit (NTU). The turbidity of water is based on the amount of light scattered by particles in the water. The more particles that are present, the more light that will be scattered. The particles are undissolved solids suspended in the water.
It should be noted that any dissolved solids in water actually absorb light rather than deflect it.
Another standard measurement is to weigh the undissolved or Total Suspended Solids (TSS) in the water by trapping them and then drying them; though this is more of a laboratory test. You can commonly see these two measurements in water and filter descriptions displayed as NTU, mg/l or %.
However, as mentioned above, dissolved solids absorb light rather than scatter it which can affect true readings. Salt water will cause suspended solids to clump and sink which is why seawater will regularly look clearer than fresh water bodies.
Removing the Contaminants
Particulate matter has to go somewhere during the filtration process. In Séon UV this is performed by physically trapping the sediment in the 3 filter cartridges. This method has been selected for its simplicity and practicality to fit within the size constraints of a mobile system. Larger systems can utilise self-cleaning/separation methods to remove the particles.
One should also consider what matter and its size is in the water creating turbidity. For example; you could spread out further and cover more densely across the floor a kilo of sand than you could a kilo of pebbles. Organic matter suspended in the water can spread out more finely again. The matter will spread across the surface are of the filter cartridge in the same manner and if fine enough will pass through onto the next cartridge. Organic matter also has the ability to grow once it has been trapped clogging the filter cartridge even more.
Carbon is excellent at absorbing chlorine, (as well as taste, colour and odour) from water. However, being a dissolved solid, will require another form of water testing to understand how much is present in the water and thus how many litres the cartridge would be able to absorb before saturation.
This picture highlights the matter that a filter cartridge can collect from a small beaker of water. This demonstrates the hard work filters have to do and the importance of choosing the clearest water sources.
Settling Tanks
At Wananchi we offer our Transfer Tank Kit to help prolong filter life. The kit actually has a number of important uses; it makes water extraction from source easier so that the water purification system can be left in camp or base away from the source, provide a secure water source within the camp and of course provides a place to help settle the suspended particles in the water. Thus, helping make filter cartridges last longer.
To help with the sediment settling the transfer kit is supplied with 2 x 20ml dosing shots: Dosing Shot A – Coagulant, Dosing Shot B – Flocculent.
The dosing shots help clump together sediment in the water and makes it easier for it to settle to the bottom where the clumps are heavier. It takes about 30 minutes to clump and sink to the bottom of the tank.
When the sediment has settled the water is clearer where we will see significantly increased filter life during water purification.
Conclusion
As you can appreciate, water quality variables are infinite, couple this with an unknown volume of water to pass through the cartridge and you can begin to understand how difficult it is to predict exact life of a filter cartridge.
However, for a rule of thumb; the clearer the water, the longer the life of a filter cartridge. A clear, untrusted water source may allow a cartridge to filter hundreds of thousands of litres of water before saturation, try processing water from a thick, muddy puddle and it is likely to clog in minutes.