FILTRATION FOR RAINWATER RECOVERY

webmaster59208
Sep 29, 2023
5 min read

Updated: Oct 30, 2023

In doubt about how best to filter rainwater before using it? Look no further! In this info sheet you will find answers to :

Where do I best place a rainwater filter?
What types of rainwater filter exist?
For which application do I choose which type of filter ?

The filtration of your rainwater is an essential step in the rainwater recovery process. There are often different pollutants in rainwater, but with the right filter set(s) you can perfectly reuse your rainwater for various applications both indoors and outdoors. This will save you a lot of money by not using expensive tap water!

There are different types of filters. A first distinction can be made between pre-filters and post-filters. Pre-filters are those that are placed in the system 'before' your cistern and ensure that your cistern is less likely to be soiled by leaves, twigs, and coarse sand. Post-filters are filters where the water from your cistern is pumped through, with much smaller meshes and stronger filtration, depending on the purpose of your rainwater use . The link between filter and pump is very important with these filters. If your pump is not powerful enough to push the water through your filter and there is too much loss of pressure, it can disrupt the proper functioning of your household appliances. See our blog for more information on the right rainwater pump for you.

The Flemish Environmental Society [1] recommends integrating both types of filters into rainwater systems, so your water is of good quality to connect your washing machine, toilet, or outdoor taps . At SmartWaterLoop, we are happy to help you in choosing correctly between all types of filters, and incorporating them into a plan tailored to your project. If you still have questions about filtration after reading this info sheet, or are ready to take action, feel free to contact us on smartwaterloop.com

Pre-filtration

Pre-filters are placed just in front of or on top of the rainwater tank as primary filtration for large dirt particles from your roof. These filters often have larger mesh sizes (180-700 micrometres) and mainly hold back leaves, twigs, moss, or other larger debris so they don't end up in your cistern. There are different types of pre-filters, varying operation and cost . A first important choice is the degree of self-cleaning. Some filters are not self-cleaning and thus need to be cleaned regularly (every 2 to 4 weeks) for optimal operation. The next category is partially self-cleaning through clever use of the water flow (e.g., cyclone filters) and it suffices to check once every few months or in case of heavy rainfall to make sure the filter has not yet clogged. An example of a dirty and clean filter housing is visible in the images below of one of our test setups.

Clean filter housing Dirty filter housing

A final category uses active cleaning mechanisms with water sprayers on rainwater or potable water pressure pipes. They require no additional cleaning but are vulnerable to frost damage if not installed properly.

Underground and in-tank pre-filters

These filters are placed between the rainwater collection point (your roof or terrace) and the cistern, but underground. Underground pre-filters can often also be placed in the tank itself, which can save space and excavation work (see image). They are usually a bit larger, have mesh sizes between 200 and 500 micrometers [3] and already filter the first sediment and other coarse debris from your rainwater quite well. They do have a higher price tag and are slightly more difficult to access for maintenance .

Vlario (knowledge center rain and wastewater Flanders) describes the 4 different types of pre-filters: Cyclone filters, Cascade filters, Inline filters, tank filters and tank filters with a drop-through system [5]. Thus, tailor-made solutions are certainly possible for each project. The table on the following page runs through each of these filtration devices.

Post-filtration

Post-filters are filtration devices placed between the storage in the rainwater tank and the end appliances, through which the water is pushed with a pump. In Belgium, these filters are available in many different sizes and dimensions. The basic versions have one filter step, mid-range models have two filter steps and the most advanced have a third step that, among other things, absorbs organic pollution with activated charcoal and/or disinfects the water biologically with UV. So, depending on the filters, your water becomes increasingly pure for usage in home. Note that in Flanders, even with the very best filters it is not recommended to use rainwater for showers or drinking water [10]. For those purposes, it is best to continue to use water from your drinking water company. With every type of post filters, maintenance is very important step in keeping the system running smoothly. These filters do not self-clean , but their placement in a basement or storage room makes them easier to access. You can do this maintenance yourself or have it done by a professional.

Single and double post filters

In one- or two-stage (single- or double-stage) post-filters, rainwater is propelled through one or two cylindrical filter housings with a filter fleece or filter candles in a plastic casing. Most suppliers offer different options in terms of pore size typically between 200 microns (quite rough and large) to 5 microns (very fine). For double post-filters, a rough filter can be combined with a fine one or the second filter step can also be filled with activated carbon, which filters away odors and colors from the rainwater. The exact workings of such active-carbon filtering is explained in the following article by Buildwise [11].

cintropur NW25 [12] Honeywell FF40 Duplex [13]

Triple Post-filters

Three-step (or triplex) post filters work in a similar fashion as single or double versions, but the third step is always an activated carbon filter. This absorbs the organic pollutants from your rainwater, along with any odors and coloration. They are slightly pricier, mainly because of the extra filtering step and the cost of the activated carbon material. Maintenance is recommended a few times a year, to check whether your filter fleece or candles should be replaced. Checking at least once every 6 months should suffice.

Honeywell-Braukmann Triplex filter with mounting set [14]

UV and Ultrafiltration

For most households, one of the above options (with or without activated carbon) is often sufficient filtration to safely use rainwater for household appliances. However, some top of the range filtration devices provide a few more options to improve the quality of your rainwater even more.

UV systems: These can be added to a double or triple post-filter, further treating the water for general dirt and odor/color cleaning, and almost completely killing all germs and bacteria from your rainwater [15]. The lifespan of such UV-lamps is limited though, and it must be on permanently, bringing along continuous energy consumption.

Ultrafiltration: These filter devices are usually used as filtration in tap water production to remove minerals and bacteria, or in water recirculation at large consumers such as a restaurant kitchen or a car wash. There are small systems that can be incorporated for your rainwater, but they do come at a considerable cost and require painstaking maintenance. The finest category of ultrafiltration is called reverse osmosis [16,17].

There are a few caveats with these systems: some producers argue that their products make your rainwater safe enough to wash with or drink , and in remote areas (e.g. inland Australia) or developing countries (e.g. Africa) this is indeed often the purest source of water. However, it is not so easy to always ensure the correct operation of the system in a small installation. In Flanders, we have a high-quality and accessible drinking water network so the use of rainwater for showers or as actual drinkable water is not recommended.

How can Smartwaterloop help?

The right filter choice clearly requires some research and expertise before an optimal decision can be made. Below is a final summary of the 4 types of filtration devices, along with their price points, mesh sizes and degree of maintenance. With our Smart Water Calculator, we take all these parameters into consideration to determine a solution tailored to your project and your budget! If you want to know how this calculator works, check here . Interested? Be sure to contact us!