Rainwater Filtration and Disinfection

Summary

Rainwater treatment matches filtration and disinfection to the end use, from none for ornamental watering to multi-stage treatment for potable supply. Treat in a chain, with sediment filtration first so downstream UV and chlorine work, and size equipment to the end use and local rules.

Cols

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Turbidity (NTU): cloudiness that shields microorganisms from disinfection
Sediment filter: removes particles by pore size
Activated carbon: adsorbs organic chemicals, taste, odor, and chlorine
Hollow fiber membrane: membrane filter; 0.1 micron removes bacteria and protozoa and 0.02 micron also removes viruses
UV disinfection: lamp light that disrupts microorganism DNA
Chlorination: chlorine added to kill pathogens
Treatment chain: filters and disinfection in series

Standards

Purity by use: potable use always needs purification; ornamental watering needs none; edible plants depend on context; flushing, washing, and watering have lower requirements
Potable turbidity: filtration to under 1 NTU before disinfection, depending on other treatment
Filtration for flushing: the plumbing code requires a 100 micron or finer filter for toilet and urinal flushing water; see the plumbing code reference
Chlorine residual: limits and testing in the plumbing code reference
Pre-tank filter: a 30 micron filter upstream of the cistern is one design practice
System standards: ARCSA/ASPE/ANSI 63 and CSA B805/ICC 805; NSF/ANSI standards for potable treatment devices
Quality verification: test water at the point of use; ASTM E2727 identifies site conditions that affect raw rainwater quality

Components

Sediment cartridge or bag filters: remove particles upstream of disinfection
Activated carbon block: taste, odor, and chlorine removal; not effective against viruses or bacteria
Ceramic filters: brushable; slow flow, clog easily, usually ineffective against viruses
UV reactor: very effective against viruses and bacteria only when water is clear
Chlorine feed or electrochlorinator: pathogen kill with a residual; chlorine is more effective against viruses and UV is more effective against parasites
Pressure gauges and shutoff valves: across each filter for monitoring and service
Treatment chain example: a filter that reduces turbidity ahead of UV lets a smaller UV lamp work

Detailing

Order: filters first, then disinfection, then storage or point of use
Access: removable access, pressure gauge, and shutoff valves for each filter
Sizing: match flow rate and pressure to the end use
Sediment: protect filters from heavy sediment; membrane filters clog in high-sediment water
Maintenance: replace cartridges and UV lamps on schedule; keep spare parts listed in the O&M manual
UV limits: does not remove non-biological contaminants
Chlorine: byproducts can be toxic, so dose carefully

Uses

Specific uses

Toilet and urinal flushing
Cold water laundry
Irrigation with drip emitters that need filtered water
Potable supply with full treatment

Pros and cons

Pros

Treatment chains can be sized to the end use
Filter and UV equipment is compact
Chlorine residual protects water in distribution

Cons

Cartridges and lamps need scheduled replacement
UV needs power and clear water
Potable use adds testing, approvals, and cost

Manufacturing and cost

Construction

Factory-built filter housings, carbon blocks, and UV reactors mounted on a skid or wall with a gauge and shutoff valves at each filter

Main manufacturers and suppliers

Trade association: ARCSA, American Rainwater Catchment Systems Association.

Sources

Grafman, L.: To Catch the Rain, section 2.6 water purification, https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/02%3A_Components/2.06%3A_Water_Purification
Grafman, L.: To Catch the Rain, section 4.4 first-flush sizing (potable turbidity), https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/04%3A_Calculations/4.04%3A_First-Flush_Sizing
Texas A&M AgriLife: rainwater harvesting, https://rainwaterharvesting.tamu.edu/
American Society of Plumbing Engineers: ARCSA/ASPE/ANSI 63-2020 rainwater catchment systems, https://aspe.org/?p=6144