Rainwater harvesting/Rainwater post-filtration, treatment and disinfection

Depending on the desired end-use and the local codes, laws and regulations, different combinations of or variations in prefiltration and post-filtration, treatment and disinfection may be required to avoid system malfunctions and human health hazards. Prefiltration is not included in this category, as some systems focus entirely on filtering the debris and organic material from the water before it enters the tank.

Post Tank Filtration, Treatment and Disinfection
Post-filtration is the filtration that occurs afters harvested rainwater leaves the tank. After water is discharged from the primary storage units via gravity or by pump, filters of much fine screens and meshes may be installed to remove finer particulates and sedimentation. Typically these devices and systems require pressurized water to function, and depending on the local codes, laws and regulations, may be required for any kind of indoor water use, potable and non-potable (i.e. flushing toilets, washing, or showering).

Sediment Filtration
Sediment filters remove fine particulate sediment, color, taste and odor that may remain in the collected rainwater even after pre-filtration. Typical mesh sizes for sediment filters include 500 microns, 20 microns, 5 microns and 1 microns. Large mesh sizes are usually made of plastic fabrics, while the smaller sizes can be made out of carbon filters.

UV Disinfection
Ultraviolet (UV) disinfection devices use the principal of ultraviolet germicidal radiation for sterilizing and inactivating microorganisms in the water. In decreasing order of effectiveness, UV lights work to eliminate bacteria and protozoa, viruses, bacterial spores, adenovirus and algae. UV disinfection is always used prior to chlorination of the water to ensure a low microbial count in the water (thus high quality), and to reduce the intensity of the chlorine dosage required.

Pre-filtration at or less than 10 microns is required in order to remove particulates that pathogens could find safe harbor on and avoid the effects of the UV light.

UV devices are important to deactivate chlorine-resistant parasites such as Cryptosporidium and Giardia.

Ozone Injection Disinfection
Ozone injection is the more expensive alternative to chlorination or UV disinfection, however, as of the late 20th century, it is has been gaining prominence. It has an ability to oxidize, not merely deactivate, a wide range of organic and inorganic compounds, which makes it more effective in eliminating the hazards posed by trace organics such as pesticides and chlorine-derived disinfection by-products. Ozone is generally followed by low-level chlorination in order ensure residual disinfection. Ozone is not suitable for residue formation as it decomposes in water relatively quickly, so chlorine disinfection may be preferred for water disinfection that continues downstream.

Chlorine and Chloramine Disinfection
The injection of chlorine or chloramine is used to sterilize water sources. Chlorine is common is system where a distinctive odor and taste are desired to prove the cleanliness of water. It is extremely important to not mix the chlorination process with the UV disinfection process as the UV light can create compounds that are harmful to human health to ingest.