Introduction
Woke up after a stormy night and found your pool completely green? You're not alone – this is one of the most common problems we face in Margem Sul, especially during spring and autumn when rains are more frequent and intense.
Green water after rain doesn't happen by chance. Rain brings nutrients, dust, pollen and algae spores, while diluting your pool's chemicals. The result? Perfect conditions for an algae explosion within hours.
In this article, we explain why rain causes green water, how to recover your pool quickly, and most importantly – how to prevent this from happening again.
Why Rain Causes Green Water
What Rain Brings to the Pool
| Element | Origin | Impact |
|---|---|---|
| Nitrates | Atmosphere, pollution | Feeds algae |
| Phosphates | Dust, pollen, leaves | Nutrient for algae |
| Algae spores | Air, vegetation | Direct inoculation |
| Soil particles | Erosion, splashing | Clouds water |
| Organic matter | Leaves, insects | Consumes chlorine |
| Acidic water | Typical rain pH 5-6 | Unbalances pH |
The Dilution Effect
Rain is pure water (or nearly), which dilutes all chemicals:
| Rainfall | In 8x4m Pool (50m³) | Chemical Dilution |
|---|---|---|
| 10 mm | 320 litres | ~0.6% |
| 25 mm | 800 litres | ~1.6% |
| 50 mm | 1,600 litres | ~3.2% |
| 100 mm | 3,200 litres | ~6.4% |
💡 Dica Profissional
In Margem Sul, autumn storms can easily bring 50-100 mm of rain in 24 hours. This means your pool can receive more than 3,000 litres of untreated water in a single night – water loaded with nutrients and with acidic pH.
The Perfect Combination for Algae
After heavy rain, your pool has:
- Reduced chlorine (diluted + consumed)
- Altered pH (usually lower)
- Abundant nutrients (nitrates, phosphates)
- Fresh algae spores
- Decomposing organic matter
- Possibly interrupted circulation
Result: algae can multiply exponentially in 6-12 hours.
Severity Levels
Light Green (Transparent but Green)
Characteristics:
- Water still transparent
- Greenish tint
- Bottom visible
- Walls slightly slippery
Likely cause: Beginning of algae growth, low chlorine
Recovery time: 24-48 hours
Moderate Green (Cloudy and Green)
Characteristics:
- Cloudy water
- Obvious green colour
- Bottom partially visible
- Slippery walls
Likely cause: Active algae growth, unbalanced system
Recovery time: 2-4 days
Severe Green (Swamp)
Characteristics:
- Opaque water
- Dark green or yellow-green
- Bottom invisible
- Possible smell
Likely cause: Massive algae infestation, post-rain neglect
Recovery time: 4-7 days
Step-by-Step Recovery
Step 1: Initial Physical Cleaning
Before any chemical treatment:
- Remove large debris with surface net
- Brush walls and floor vigorously
- Check and clean skimmer basket
- Check and clean pump basket
- Check water level (adjust if necessary)
Important: Don't vacuum to drain if water is very green – can clog piping.
Step 2: Test the Water
After physical cleaning, test:
| Parameter | Expected Post-Rain | Target |
|---|---|---|
| pH | Probably 6.5-7.0 | 7.2-7.6 |
| Free chlorine | Probably 0-0.5 ppm | 1-3 ppm |
| Alkalinity | Possibly reduced | 80-120 ppm |
| Cyanuric acid | Diluted | 30-50 ppm |
Step 3: Adjust pH First
Why pH first?
- Chlorine only works well at correct pH
- At pH 8.0, chlorine is 75% less effective
- Adjusting pH after chlorinating wastes product
How to adjust:
- If pH low (<7.2): Add sodium carbonate
- If pH high (>7.6): Add muriatic acid or sodium bisulphate
- Wait 2-4 hours before proceeding
Step 4: Shock Treatment
Dosage for green water:
| Severity | Shock Chlorine | Per 10,000 L |
|---|---|---|
| Light green | 10 ppm | 170g (hypochlorite) |
| Moderate green | 15-20 ppm | 250-340g |
| Severe green | 20-30 ppm | 340-500g |
Procedure:
- Do treatment in late afternoon/evening
- Dissolve product in bucket with water
- Distribute around entire pool
- Run pump continuously (24 hours)
- Don't use pool until chlorine drops to <3 ppm
💡 Dica Profissional
Use granular calcium hypochlorite for shock treatment on green water. Stabilised chlorine (trichlor, dichlor) is less effective against algae and can over-stabilise the water if you already have cyanuric acid.
Step 5: Supporting Algaecide
After chlorine shock:
| Type of Algaecide | When to Use | Typical Dosage |
|---|---|---|
| Quaternary ammonium | Light green | 5 ml/m³ |
| Copper algaecide | Moderate green | As per label |
| Polymeric algaecide | Severe/prevention | 3 ml/m³ |
Note: Wait 4-6 hours after shock to add algaecide.
Step 6: Intensive Filtration
During recovery:
- Pump 24 hours a day
- Backwash when pressure rises 0.5 bar
- Possibly multiple backwashes per day
- Clean pump basket frequently
Sand filter:
- Sand becomes loaded with dead algae
- Long backwash (3-5 minutes)
- May need chemical clean afterwards
Cartridge filter:
- Clean cartridge every 6-12 hours
- Having spare cartridge helps
- May need replacement if very clogged
Step 7: Clarifier (If Needed)
If after 24-48 hours water is still cloudy:
- pH should be 7.2-7.4
- Add clarifier/flocculant
- Let circulate 2-4 hours
- Turn off pump 8-12 hours
- Vacuum sediment to waste
- Adjust water level
Specific Treatment by Algae Type
Green Algae (Most Common)
Characteristics: Green water, slippery surfaces Treatment: Shock + brushing + filtration Difficulty: Easy
Yellow/Mustard Algae
Characteristics: Yellowish powder in shaded areas Treatment: Triple shock + specific algaecide Difficulty: Moderate
Black Algae
Characteristics: Dark spots, rooted Treatment: Wire brushing + direct chlorine + copper algaecide Difficulty: Difficult
Post-Rain Prevention
Before Rain
If you know a storm is coming:
| Action | Why |
|---|---|
| Raise chlorine to 3-4 ppm | Reserve of sanitisation |
| Check pH (7.2-7.4) | Maximise chlorine effectiveness |
| Put cover on (if possible) | Reduces water volume and debris |
| Remove loose objects | Avoid damage and contamination |
Immediately After Rain
In the first 2-4 hours:
- Remove surface debris
- Test pH and chlorine
- Adjust pH if necessary
- Add chlorine to restore levels
- Turn pump on if it was off
- Brush surfaces
This immediate action can completely prevent green water!
Post-Rain Routine
| Day | Actions |
|---|---|
| Day 0 (after rain) | Cleaning + test + adjustments |
| Day 1 | Retest, adjust if necessary |
| Day 2-3 | Check water appearance |
| Day 7 | Complete water test |
Useful Equipment
For Prevention
| Equipment | Benefit | Cost |
|---|---|---|
| Pool cover | Reduces dilution and debris | €100-500 |
| Automatic chlorinator | Maintains constant chlorine | €80-200 |
| Variable speed pump | Can run 24h economically | €800-2,000 |
For Recovery
| Equipment | Function | Cost |
|---|---|---|
| Complete test kit | Accurate diagnosis | €30-80 |
| Wall brush | Dislodge algae | €15-30 |
| Deep leaf net | Collect debris | €20-40 |
| Manual vacuum | Remove sediments | €30-100 |
Typical Recovery Costs
DIY
| Item | Light Green | Moderate Green | Severe Green |
|---|---|---|---|
| Shock chlorine | €15-25 | €25-40 | €40-60 |
| Algaecide | €10-15 | €15-25 | €25-40 |
| Clarifier | €0-10 | €10-15 | €15-20 |
| pH adjustments | €5-10 | €5-10 | €5-10 |
| **Total** | **€30-60** | **€55-90** | **€85-130** |
Professional
| Service | Cost |
|---|---|
| Basic treatment (light green) | €80-120 |
| Complete treatment (moderate green) | €120-180 |
| Full recovery (severe green) | €180-300 |
| Including filter clean | +€50-80 |
Frequently Asked Questions
Can rain really cause green water in just a few hours?
Yes, especially if:
- The pool already had low chlorine before rain
- The storm was intense (>30 mm)
- Temperature is above 25°C
- There's lots of vegetation around
Can I use the pool during recovery?
Not recommended:
- Chlorine is very high (can cause irritation)
- Dead algae contain toxins
- Water may have bacteria
Do I need to drain the pool if it's very green?
Rarely necessary. Even "swamp" pools can be recovered with proper treatment. Draining:
- Wastes lots of water
- Can damage structure (especially liner)
- Doesn't solve the cause
How long does full recovery take?
| Severity | Average Time |
|---|---|
| Light green | 24-48 hours |
| Moderate green | 2-4 days |
| Severe green | 4-7 days |
Factors that speed up: quick action, powerful pump, good filter.
How do I prevent it happening again?
The three golden rules:
- Keep chlorine always >1 ppm – especially before expected rain
- Act immediately after rain – don't wait until next day
- Use cover when possible – reduces 80% of the problem
Conclusion
Green water after rain is a common but completely preventable and treatable problem. The main points to remember are:
- Rain dilutes chemicals and brings nutrients – perfect environment for algae
- Quick action is crucial – treating in first hours prevents the problem
- Follow steps in correct order – clean, test, adjust pH, shock
- Intensive filtration – pump should work continuously
- Prevention is easier than cure – keep chlorine high before rain
If you need professional help to recover your green pool in Margem Sul, ManutençãoPiscinas is here to help. Contact us for quick intervention.
