Scarcity of water in urban areas and continued pressure on water resources has made rainwater harvesting an attractive solution for reusing rainwater which would have otherwise been lost. These systems provide an alternative supply solution for reducing dependence on mains water supply. Rainwater harvesting systems collect and store rainwater which can later be used for potable or non-potable purposes depending on treatment. Typical uses include washing, toilet flushing and gardening. According to Environment agency data average per person daily water usage in England and Wales is around 150 Litres, out of which 26% is used for toilet flushing and 7% for outdoor use and car washing. 1mm of rainwater collected over 1m2 area equals 1L of water.

In the absence of mains water supply, hard ground conditions and polluted surface water sources, rainwater harvesting can provide an alternate source of water.  Environmental benefits also make rainwater harvesting systems ideal for both domestic and commercial users. This water is not potable and a series of treatment and testing steps are required before it can be used for drinking. Drinking Water Inspectorate provide guidelines for drinking water. Further information on rainwater harvesting is available in BSI 8515.


Water is collected from roof tops through downpipes is discharged into a storage tank. There is normally a filter prior to storage tank to catch larger dirt e.g. leaves, pieces of eroded roof tiles etc. The storage tank can be installed in the roof making it a gravity fed system. The only drawback is limited storage capacity as compared to pumped systems which are usually installed underground and store larger volumes of water.

Water is pumped on demand and the pump is controlled by a controller. A second filter can be installed at pump inlet or outlet which removes smaller debris. A backup water supply might be needed during dry periods and this is usually provided by connecting the system to mains water. Water supply regulations require that there is sufficient backflow to prevent contamination of mains water which is usually achieved by air gap.

Sources of contamination in rainwater

  1. Overhanging trees: leaves can block down pipes, provide nutrients for microorganisms and facilitate contamination by birds, rodents and carcasses.
  2. Asbestos cement sheeting can pose a health risk and block valves and pipes.
  3. Bird and animal faeces can cause contamination by pathogens.
  4. Leptospirosis, giardiasis, and cryptosporidiosis are some of the diseases that can be caused by organisms carried in the gastrointestinal or urinary tracts of mammals; birds and reptiles may also carry some pathogens, such as those causing campylobacteriosis and salmonellosis.
  5. Atmospheric pollutants (industrial, vehicle emissions)

Filtration for rainwater harvesting systems

Siga Filtration can supply filtration products for rainwater harvesting systems. ALSI & BFOS Series Bag Filter housings can provide excellent prefiltration when used with acuraBag range of filter bags in Polypropylene. Our bag filtration systems are capable of providing very high flow rates at low pressure drops.

Water can be further purified using acuraPromelt & acuraMultiflow spun bonded filter cartridge. These cartridges can be used in single or multi cartridge configuration depending on flow rates and provide excellent removal ratings down to 1 micron.

These versatile systems can be used separately or in series to suit a wide range of applications. Filtration combined with UV disinfection systems has been successfully used in rainwater harvesting systems.

1FU Group



1FU Series Single Cartridge Filter Housings


BFOS Series Bag Housing

BFOS Series Bag Filter Housings