How does a UV water filter system work?

How does a UV water filter system work?

When talking to customers in the shop, one topic that constantly comes up is how do water filters work and what is a UV water filter. We find commonly is people think that rainwater is safe and pure and fine to drink straight out of the tank. This complacency probably comes from being used to mains scheme water in urban areas where the water is filtered and chlorinated to make it safe to use and drink.

When water is caught off a house or shed roof and is then stored in a water tank, there is a high likelihood that there is pollutants (dust, organic material etc) and microbes that get washed into the tank with the water. The primary sources of these microbes are generally the surrounding environment and also bird droppings off the roof surfaces. These contaminants are managed properly if your keep your roof, gutters and downpipes clean and your water tank properly maintained.

Maintain your water quality When water is stored in a water tank there are a couple of things you need to ensure:

  1. The water going into the tank is clean as possible. Use flush diverters and keep your roof clean makes the water going into your tank as good as possible.
  2. Keep the water cool and fresh. If the water is going to be used over a 6-12 month period, then make sure the tank is well ventilated and can maintain the water as cool as possible. Most good tanks (like our Steelfab tanks) have a whirlybird installed. This keeps water from going stagnant and also provides a cooling effect. If your tank is covered and reflective as possible (silver, cream or white in colour) then this will assist in keeping it cool. This minimises the growth of microbes and things like mosquito larvae.
  3. Clean out your tank occasionally. It is recommended that every couple of years your tank is cleaned out to remove the built up sludge and debris. If you have a major problem with contamination (ie something dead in the water) then the only realistic option is to empty the tank, sterilise it with chlorine or similar and then fill it back up with fresh water. For all other times we recommend our system of vacuuming the sludge out from under the clean water.
  4. You can also add products like Davey Aquasafe to the water. This peroxide based chemical is designed to kill off bacteria in the water and provide a small amount of residual protection against reinfection.

Can I treat the water?

Acquasafe 1 Litre

Davey Aquasafe for keeping your tank water sanitised.

Yes you can treat the water (see above) but that only kills off the majority of microbes in the tank for a while. There will always be residual bacterial populations left in the water and only a handful have to get through into the pipes and into your house to make your water unsafe for your family.

People tell me to put some filters in.

In addition to some basic maintenance, the most effective way to make your water safe to drink is to filter it. Filtering water involves having a fabric or paper membrane or sheet in a housing. The filter cartridge is manufactured to have pores in it that have a consistent maximum size, usually measures in microns (or micrometers), common sizes are 50, 20,10, 5 and 1 micron for drinking water. All the water flow from a pump or tank has to pass through the filter and depending on its size, then the filter cartridge will block certain particles from passing through and into the water supply. The aim with a water filter is to have totally clean and safe water come out from the filter. If the supply water is particularly dirty or has difficult to manage particles like clay, then the filters might block up quickly. If the filters are blocking quickly then this makes things hard and expensive to manage and you will have variable and generally poor water pressure.

A UV filter board

To overcome this problem, you either need to pre-treat the water before it goes the filter or add a couple of different sizes of filter in line. This method called stepped filtration, works on the principle that if you use a series from large pore to small, large particles get caught in the first filter and smaller ones in the latter. This stops the filters blocking up quickly and allows you to dramatically increase the life of your filters. A normal stepped filter setup for a house is a 50 or 20 micron cartridge first and then a 5 or 1 micron cartridge second. If there are still problems with fast blockages, then you either have to settle the water out first with a flocculent (like Alum or PAC) or you can increase the surface area of the filters by running in parallel or with bigger cartridges.

There are 4 standard sizes to domestic water filters,  2 ½” and 4 ½” wide and 10 or 20 inches long. There are other sizes outside of this but the majority of applications fit into these sizes. Housings are standard across most major brands so if you need a filter, most reputable filter retailers will have an option for your on their shelf. Some less reputable filter manufactuers design their own “special” housings or cartridges, sometimes with fairly outrageous claims on what they do. They generally also come with a fairly outrageous price tag too and when you can’t swap in a standard housing, then you are locked into buying their product, BUYER BEWARE!

The 2 sizes of 10 inch filter housing.

The 2 sizes of 20 inch filter housing.

Another feature that is quite common in water filters is carbon. In most smaller micron (10, 5 and 1) cartridges there is the option for them to contain carbon. This is a form or activated charcoal that is designed to remove most colour and off flavours and odours from the water. This lets you make your water taste and look a lot better if there is too much tannin, chlorine, sulphides etc. so it not only has an application for rainwater but a carbon filter can also find itself very useful for town water when people don’t want to feel like they are drinking out of a swimming pool . The carbon in the filter is eventually filled completely with the different tastes and odours and has a capacity. At this point it just stops working as a taste and odour filter but will still physically filter the water.

Changing your filters regularly and sanitising your housings and pipes is a very good practice. This should be done approximately every 6 months for the best result. You should sanitise the empty housings and pipework with chlorine before inserting new filters. This is because the old filters caught all the pollutants and most of the bacteria in the water. As this all sits there for 6 months things can start to grow on the outside of the filter cartridge. When you change the filters this may not all be removed and it will contaminate the new filter and possibly also the downstream pipework.

What does UV light have to do with water filters

Now one of the last things that is always recommended for treating rain water to potable is a UV steriliser. This is a stainless steel housing in which there is a ultraviolet fluorescent light globe

A UV globe and thimble

mounted. This globe is designed to shine ultraviolet light through a quartz glass thimble into the housing and so when the filtered water passes through the housing, any remaining bacteria are sterilised. This means that the small handful of bacteria will still be alive but they will not reproduce and so in 6 hours or so their lives will end and the population goes nowhere. This is a very effective and simple way of chemically free treating of your drinking water. It does rely though on filtering your water first. The basic principle is that if there are any particles left in the water, then they create a shadow which the bacteria cells can hide behind and the UV light will not sterilise them. Then they can simply continue on their way and cause you and your family harm. We always recommend that your filter the water down to a maximum of 5 micron and if possible 1 micron ideally. This means at least a 50 or 20 micron filter in front of all of this to catch the large particles and all of this system will fairly effectively treat your house water.

A Davey UV steriliser

A Davey UV steriliser

How much does it cost? And when should I replace filters.

As with all stand alone water systems they do cost money. Filters and chemicals, along with tanks and pumps are fairly expensive. At the end of the day though, the water has cost your absolutely nothing, there are not rates payable on water that falls from the sky so it is a very wise investment to spend at least some of that saving on a quality water treatment system. There are things that need to be changed:

  1. Filters. Need to be changed every 6 months or sooner if they block up. These can cost anywhere from $15 to $120 each depending on size and type. If you want to get a little more out of a filter go for a quality pleated paper cartridge as you can sometimes wash them out and extend their life. Just remember to clean the housing and pipework with chlorine before installing a new filter.
  2. The UV. This needs to be serviced every 6 months and then a globe changed out every 12 to 18 months. The servicing of a UV sterilizer simply involves checking to make sure the globe is working and then cleaning dirt and algae off the outside of the glass thimble. This allows the light to get though properly and make it work. Be very careful when servicing the thimble as they are brittle and delicate. The globes generally have a working life or 12-18 months, after this period they might look like they work but the UV they are emitting is not intense enough to treat the water. Usually a globe will simply blow and so you will get an alarm on your unit and need to change it. Be very careful of UV globes as they are a tricky piece of electrical circuitry. With any appliance like this, make sure it is turned off at the power point and disconnected before you do any work on it. In addition to the electrical side of the globe, they are also filled with mercury vapour and so should be handled and disposed of very carefully. A globe will cost you somewhere around $100 to $200 and a thimble the same. Whenever you service the thimble also replace the O Ring seals as they are degraded by the UV light. If you don’t know what you are doing or a bit unsure, make sure you get a professional out to do the service as they are very familiar with the equipment and are trained to repair it.

If you want to discuss options and information on water filters further feel free to call our friendly staff today on (08) 9721 3577 or email them .