What is Nano Alumina?
It is an oxide of aluminium. There are several oxides that come under the general banner of Alumina but the one used for water filtraton is Aluminium Oxide Hydroxide or boehmite.
How is it used in water filters?
It is made into nanofibres which are about 2 nanometers in diameter and 200 – 300 nanometers in length. To illustrate how small this is, the average sheet of paper is about 100,000 nanometers thick or, a nano alumina fibre is 1/25th to 1/50th the size of a human hair!
These nano alumina fibres are bonded onto submicron glass fibres which are then dispersed throughout a filter medium.
Why use Nano Alumina?
Nano alumina has a special property when it gets wet – it acquires a positive electrical charge. This is important because bacteria, viruses and protozoa (all the nasties that make us really sick) have a negative charge and, as we all know, opposites attract and these little nasties are attracted to the nano alumina fibres dispersed throughout the filter medium and stick to them. (For those that really want to know, this is called adsorption – the adhesion of atoms, ions or molecules from (in our case) a liquid to a surface which occurs through electrostatic attraction.) So, the nasties get stuck onto the filter medium surface and don’t penetrate through it.
Also, the pore size (like pores in our skin this is an opening that lets stuff through) of a nano alumina filter is 2 microns – much smaller than biologicals (bacteria, virus etc) and also much smaller than contaminants such as heavy metals (lead, aluminium, iron, copper, tin, silver etc.) which also make us sick if we ingest too much of them. So, those contaminants that don’t have a negative charge and therefore don’t get stuck can’t get through anyway because the pore size is too small.
So why use carbon as well?
Carbon takes away taste, smell, chlorine and some heavy metals. It provides another level of filtering but, because the pore size of a typical carbon filter is about 40 microns, it can’t stop the contaminants that a nano alumina filter can.
So we use both – one to stop contaminants in their tracks and another to take away any nasty smells or taste. This is one way that you know that you need to replace the filter – if the water from the bottle starts to smell or taste then you know that the carbon in the filter has reached its limit of filtration capability. That doesn’t mean that the nano alumina filtration capacity has stopped – it just means that the water smells and tastes not nice. It’s still safe to drink – but not pleasant!
Water To Go filters have three filtering mechanisms
- Mechanical – the very small pore size won’t let anything larger than 2 nanometres through;
- Electrostatic adsorption – protozoa, bacteria, viruses, cysts all get attracted to and then adhere to the filter and can’t then go anywhere else;
- Carbon – in the form of non-glued carbon particles dispersed throughout the filter which removes taste, smell and some heavy metals.
Only reverse osmosis provides better filtration
Research has shown that nano alumina filters have a higher filtration efficiency and greater dirt holding capacity than the ultraporous membranes that have, until, now been used to produce pure and ultra pure water. Only reverse osmosis provides a more complete filter for drinking water – and that is a very expensive process!