What's Actually in Collected Rainwater
Understanding the contaminants informs the treatment approach. Rainwater collected off a roof is not the same as rain falling in a pristine wilderness — it carries whatever the roof, gutters, and atmosphere have accumulated.
Biological contaminants:
- Bird and rodent feces deposited on the roof — the primary pathogen source. Contains Campylobacter, Salmonella, E. coli, Cryptosporidium
- Accumulated organic debris in gutters — bacterial growth substrate
- Algal growth in storage tanks exposed to light
Chemical contaminants:
- Asphalt shingles: polycyclic aromatic hydrocarbons (PAHs)
- Galvanized gutters: zinc (typically at low levels)
- Lead paint: older roofs may have lead paint residue
- Atmospheric pollution: sulfur compounds (acid rain), nitrogen compounds, particulates
- Pesticides and herbicides (from aerial application or drift)
- In industrial areas: VOCs and heavy metals from air pollution
The good news: A well-designed system with first flush diversion addresses most of these. The biological load is the highest-risk category and also the most treatable.
Treatment for Drinking Water Use
The Sequence
Treatment should proceed in this order — each stage improves efficiency of the next:
1. First flush diversion (not filtration, but essential first)
The first 1 gallon per 100 sq ft of roof should be diverted away from storage before any rain event. This removes the highest-concentration biological and chemical contamination from the initial roof wash.
2. Sediment pre-filter (20-50 micron)
Installed on the tank inlet or outlet. Removes large particles, leaf debris, dust, and visible organic matter. Should be inspected and cleaned regularly.
3. Activated carbon filter
Removes PAHs, organic chemicals, taste and odor compounds, chlorine (if added upstream), and heavy metals (lead, zinc, copper). Critical for asphalt roof systems.
4. Hollow fiber membrane filter (0.1-0.2 micron)
Removes bacteria and protozoa including Cryptosporidium. This is the primary biological barrier. NSF/ANSI 53 or 58 certified filters for drinking water use.
5. UV disinfection or chemical disinfection
UV at 40 mJ/cm² or chlorine dioxide addresses viruses and any biological organisms not removed by the hollow fiber stage. The water entering this stage should already be clear (turbidity < 1 NTU) for UV to work effectively.
Simplified System for Non-Asphalt Roofs
If your roof is metal or tile (not asphalt), the PAH concern is reduced. A simplified drinking water treatment:
- First flush diverter
- Sediment filter
- Hollow fiber filter
- UV treatment
This handles the primary biological concerns effectively without the carbon stage.
Full System for Asphalt Roofs
For composition shingle (asphalt) roofs, particularly newer roofs still off-gassing compounds:
- First flush diverter
- Sediment filter
- Activated carbon block filter
- Hollow fiber filter
- UV treatment
Wait 1-2 years before using water from a brand-new asphalt roof for drinking — the initial off-gassing period is highest.
Equipment Specifications
Sediment filter: A 20-50 micron filter housing with replaceable cartridges. Standard whole-house filter housings work. Replace based on flow rate decline.
Activated carbon: Block carbon filter rated for drinking water (NSF/ANSI 42 and 53). Coconut shell carbon removes PAHs more effectively than coal-based carbon.
Hollow fiber: Sawyer PointONE inline (0.1 micron) or a comparable filter rated for pathogen removal (NSF/ANSI P231). Can be connected inline with standard 1/2-inch or 3/4-inch fittings.
UV unit: A flow-through UV unit sized for your flow rate. Look for NSF/ANSI 55 Class B (40 mJ/cm² dose). Most residential units handle 2-10 gallons per minute.
Testing
After installing any new system, before using the water for drinking, test it:
Minimum: Coliform bacteria test. A positive result means the treatment system has failed or is not properly installed. This test is available as a mail-in kit from IDEXX Laboratories and others, or at some hardware stores.
Comprehensive: Full potable water panel including coliform, nitrates, lead, pH, hardness, and turbidity. A certified laboratory can provide this for $50-100.
Ongoing: Test annually, after any system maintenance that disrupts the filtration sequence, or after any events that may have compromised the system (flood, roof damage, extended power outage affecting UV unit).
Non-Potable vs. Potable Uses
Not all rainwater needs to be treated to drinking quality. A tiered system serves different uses:
Non-potable (collected, lightly filtered): Toilet flushing, outdoor irrigation, washing cars, laundry
Potable (full treatment): Drinking, cooking, food preparation, ice
Many residential rainwater systems are designed primarily for non-potable use with a separate treated drinking water supply. The non-potable system reduces household water consumption significantly without the treatment investment of a full potable system.
Sources
Frequently Asked Questions
Is rainwater from the sky clean enough to drink directly?
Rain in remote areas with low atmospheric pollution is relatively clean, but not reliably safe for direct drinking. It picks up atmospheric particulates, bacteria, and dissolved gases on the way down and whatever is on the catchment surface (roof). Direct rain collection in a clean container (not off a roof) in a pollution-free area is safer than roof runoff, but should still be filtered and disinfected. Rain collected off any roof should always be treated before drinking.
What specific contaminants are in roof-collected rainwater?
The most common: bird and animal feces (bacteria, Campylobacter, Salmonella, Cryptosporidium), dust and particulates, pollen, atmospheric nitrogen compounds, heavy metals from roofing materials and gutters (lead from paint residue, zinc from galvanized gutters, copper from copper flashing), and polycyclic aromatic hydrocarbons (PAHs) from asphalt shingles. First flush diversion removes most biological contaminants. Filtration handles the rest.
How do you test if rainwater is safe after treatment?
Basic water test kits (available online or at hardware stores) check for coliform bacteria, nitrates, pH, hardness, and chlorine residual. A positive coliform test means the treatment has failed or has been compromised. For drinking water confidence, testing annually or after system changes is good practice. You can also use a certified lab — most charge $30-100 for a comprehensive potable water panel.