Rainwater harvesting is the process of collecting and storing rainwater for reuse. A common use is to divert rainwater from rooftops and store it in rain barrels or cisterns. This provides a sustainable source of water for irrigation, washing, and other non-potable uses. However, stored rainwater can become stagnant, smelly, and host to mosquito larvae if not properly managed. Stagnant water poses health risks when used for washing or entering drinking water supplies. Thankfully, there are several methods to prevent stagnation and keep your rainwater fresh.
Why Can Stored Rainwater Become Stagnant?
Rainwater is naturally pure and devoid of minerals, chlorine, and other constituents of tap water that prevent microbial growth. When freshly collected, rainwater will be oxygenated from contact with the atmosphere. However, stored rainwater can stratify, with oxygen-deprived water at the bottom of containers. Anaerobic bacteria thrive in oxygen-depleted conditions, resulting in decomposition of organic matter and decreased overall water quality. The lack of air circulation, presence of debris, and extended storage times exacerbate stagnation. Signs of stagnant rainwater include:
- Cloudiness and discoloration
- Floating debris and organic film on the surface
- Foul odors
- Protozoa, algae, and mosquito larvae
If left unchecked, stagnation will render rainwater unusable and potentially unsafe if ingested. Proper design, system maintenance, and usage management are key to keeping stored rainwater fresh.
Rain Barrel Design and Placement
Selecting the right rain barrel and situating it appropriately can inhibit stagnation from the start. Look for the following features when purchasing a rain barrel:
- Opaque material. Sunlight promotes algae growth.
- Tight-fitting lid. Screens keep out debris and mosquitoes.
- Spigot near bottom. Avoids stagnant water at bottom.
- Overflow outlet. Drains top water during heavy rain.
- Elevated base. Improves gravity drainage.
The rain barrel should also be placed appropriately:
- On a firm, level surface
- Near gutter downspouts for easy filling
- In partial or full shade to keep cool
- With a sloped landscape to facilitate drainage
- With overflow piping directed away from foundations
Following these guidelines allows for optimal rain barrel function.
Regular Maintenance
To keep rainwater fresh, the system requires periodic maintenance:
- Inspect for leaks and cracks. Seal any openings that allow light, debris, insects, or vermin to enter.
- Check mosquito screens. Clear blockages and patch holes.
- Remove debris and leaves. Cut back overhanging tree branches.
- Clean the roof catchment area. Sweep and hose gutters and downspouts.
- Drain and scrub the rain barrel. Use a bottle brush and non-toxic cleaner.
- Check connections. Tighten downspout/rain barrel fittings.
Twice yearly deep cleaning prevents major buildup. Consult rain barrel instructions for cleaning agents to avoid. Bleach is too harsh for plastic barrels.
Water Usage and Rotation
The key is to not let rainwater sit untouched for extended periods. The more the rainwater is utilized and replenished, the less likely stagnation becomes. Here are tips for maintaining circulation:
- Use rainwater at least weekly to water gardens, wash cars, etc.
- Install a spigot near the base to drain out sediment.
- Have a plan to use excess rainwater if barrel becomes full.
- Manually agitate water to discourage stratification.
- Cover or drain the barrel during rainy seasons if unused.
- Consider investing in multiple barrels for rotation.
By actively managing rainwater, turnover will prevent any one batch from going anaerobic.
Filtration and Purification
You can further improve harvested rainwater and guard against stagnation using filters and purifiers:
- First flush diverters – Catch initial roof runoff containing most debris and pollutants.
- Float filters – Remove particles and organic matter floating at the top.
- Sediment filters – Exclude particles based on pore sizes.
- Activated charcoal filters – Absorb chemicals, odors, and colors.
- Slow sand filters – Use biological processes for purification.
- Chlorine – Disinfects stored rainwater against microbes.
- UV purification – Inactivates microorganisms with ultraviolet light.
Note chlorine and UV are for potable water treatment. Use non-chemical filters if the water will go to gardens.
Testing for Safety
When employing rainwater for expanded uses (drinking, cooking, bathing, laundry), it is vital to test it periodically for safety:
- Check turbidity. Turbid water holds increased microbes.
- Test pH. Water outside 6.5-8.5 may corrode pipes or contain metals.
- Measure total dissolved solids (TDS). High TDS indicates dissolved pollution.
- Detect coliform bacteria. Coliforms signal potential pathogen contamination.
- Watch for mosquito larvae. Larvae indicate stagnant conditions.
Treat or dispose of rainwater if tests reveal hazardous conditions. Never drink rainwater with uncertain purity.
Supplemental Water Treatment
Additional water treatment steps may be necessary depending on rainwater storage conditions and desired end uses:
Aeration
Exposing stored rainwater to air and oxygen discourages anaerobic bacteria. Methods include spraying, fountains, bubblers, or simply stirring before use.
Carbon Dioxide Removal
Adsorption processes (activated carbon filters) or membrane contactors remove excessive carbon dioxide gas that lowers pH.
pH Adjustment
Calcium hydroxide, sodium hydroxide, or carbon dioxide gas can be used to raise pH if rainwater becomes too acidic.
Coagulation and Flocculation
Chemicals like aluminum sulfate cause small suspended particles to clump together and settle out. This removes turbidity.
Sedimentation
Settling tanks allow debris, sediments, and flocs to sink so water can be decanted from the top.
Granular Filtration
Passing water through layers of sand, gravel, and charcoal filters out particles and impurities.
Membrane Filtration
Microfiltration and ultrafiltration membranes have pore sizes small enough to remove bacteria and protozoa.
Disinfection
Chlorine, ozone, UV radiation or other methods inactivate pathogens to produce potable water.
Professional expertise is advisable when applying supplemental water treatments.
Alternative Collection Methods
In addition to rooftop rainwater harvesting, other collection sources include:
Rain Gardens and Man-Made Wetlands
Landscape depressions planted with water-tolerant vegetation filter and absorb rainwater runoff.
Permeable Pavements
Porous concrete, asphalt, pavers, and gravel allow rainwater to infiltrate into the ground rather than pooling stagnantly.
Stormwater Ponds
Retention and detention basins collect and manage stormwater runoff from large surfaces like roads, parking lots, and roofs.
Wells or Rainwater Bladders
Wells and covered bladders store rainwater underground away from light and heat. This intrinsically avoids stagnation.
Fog Harvesting
Mesh nets condense moisture out of fog banks. This functions similarly to rainwater harvesting.
Conclusion
With proper planning and maintenance, collecting rainwater can provide a renewable, sustainable water source without resulting in stagnation, mosquitoes, and health risks. Maximizing water circulation while minimizing debris, organic matter, and pathogens is key. Methods exist to properly design systems, maintain freshness, manage usage, and supplement treatment. Implementing these best practices for harvesting clean rainwater will pay off with improved water security and vibrant plant growth. Remember that rainwater stagnation is preventable with a bit of knowledge and regular upkeep. Now you have the information to store fresh rainwater and keep it from going bad.