Introduction
Shock treatment is one of the most powerful tools in pool maintenance. When executed correctly, it quickly resolves problems that regular maintenance products cannot eliminate.
In Margem Sul, especially during the intense summer, pools are subject to heavy use, high temperatures and contamination from swimmers. Under these conditions, residual chlorine can be quickly consumed, leaving the water vulnerable to problems.
This guide teaches you when shock treatment is necessary, which product to use, how to calculate dosages and how to execute the procedure safely and effectively.
What is Shock Treatment
Definition
Shock treatment consists of adding a high dose of chlorine (or other oxidiser) to the pool to:
| Objective | Description |
|---|
| Eliminate chloramines | Compounds that cause "chlorine" smell and irritation |
| Destroy algae | Kills visible algae and spores |
| Oxidise contaminants | Removes organic matter, sweat, urine |
| Restore free chlorine | Returns active chlorine to the water |
| Completely sanitise | Eliminates bacteria and pathogens |
Shock vs. Normal Chlorination
| Aspect | Normal Chlorination | Shock Treatment |
|---|
| Chlorine dose | 1-3 ppm | 10-30 ppm |
| Frequency | Daily/continuous | When needed |
| Objective | Maintain residual | Eliminate problems |
| Time to swim | Immediate | 8-24 hours |
| Product type | Stabilised | Non-stabilised |
Breakpoint Chlorination
Shock treatment works through the concept of "breakpoint chlorination":
| Phase | What Happens |
|---|
| Low dose | Chlorine binds to contaminants, forms chloramines |
| Medium dose | More chloramines form, worse quality |
| Breakpoint | Sufficient dose to destroy all chloramines |
| After breakpoint | Free chlorine available, clean water |
💡 Dica Profissional
If you add too little chlorine, you can make the situation worse (more chloramines). It's important to reach the breakpoint - generally 10x the amount of combined chloramines present.
When to Shock Treat
Situations Requiring Shock
| Situation | Urgency | Dose |
|---|
| Green water (algae) | Immediate | Double or triple |
| Cloudy water without apparent cause | High | Normal to double |
| Strong "chlorine" smell | Medium | Normal |
| Irritated eyes after swimming | Medium | Normal |
| After party/intensive use | After event | Normal |
| Combined chlorine > 0.5 ppm | Medium | Normal |
| Free chlorine = 0 | High | Normal to double |
| After heavy rain | Medium | Normal |
| Season opening | Before use | Double |
| Faecal contamination | Immediate | Triple |
Recommended Frequency
| Usage Situation | Shock Frequency |
|---|
| Family use (2-4 people) | Fortnightly |
| Moderate use (4-8 people) | Weekly |
| Intensive use (parties, many swimmers) | 2x per week |
| Commercial pool | Daily or alternate days |
| Rarely used pool | Monthly |
Signs of Chloramines (Combined Chlorine)
| Sign | Indicates |
|---|
| Strong "chlorine" smell | Paradoxically, lack of free chlorine |
| Eye irritation | Irritating chloramines |
| Dry skin after swimming | Chloramines |
| Water looks "dull" | Non-oxidised contaminants |
| Test shows total chlorine > free chlorine | Presence of combined chlorine |
Types of Shock Products
Product Comparison
| Product | Active Chlorine | pH | Advantages | Disadvantages |
|---|
| Calcium hypochlorite | 65-70% | 12 | Strong, economical | Raises pH, leaves residue |
| Sodium dichlor | 56-62% | 6-7 | Neutral pH, dissolves well | Adds stabiliser |
| Sodium hypochlorite (bleach) | 10-15% | 12-13 | Liquid, easy | Weak, raises pH a lot |
| Potassium monopersulfate | 0% (oxidiser) | 2-3 | No chlorine, swim in 15 min | Doesn't sanitise |
Calcium Hypochlorite (Recommended)
Characteristics:
- 65-70% active chlorine
- Granular or tablets
- Doesn't add stabiliser
- Ideal for shock
| Advantage | Description |
|---|
| Powerful | High chlorine concentration |
| No CYA | Doesn't increase cyanuric acid |
| Economical | Good cost-effectiveness ratio |
| Fast acting | Dissolves and acts quickly |
| Disadvantage | Solution |
|---|
| Raises pH | Correct pH afterwards |
| May cloud water | Wait to settle, filter |
| Calcium residue | Don't add directly |
Dichlor (Sodium Dichlor)
Characteristics:
- 56-62% active chlorine
- Granular, dissolves easily
- Neutral pH
- Contains stabiliser
| When to Use | When to Avoid |
|---|
| pH sensitive | CYA already high (>50 ppm) |
| Vinyl pools | Frequent shock use |
| Quick dissolving needed | Pools using bleach |
Monopersulfate (Non-Chlorine Shock)
Characteristics:
- Doesn't add chlorine
- Oxidiser, not sanitiser
- Can swim after 15 minutes
- Ideal for chloramines
| Ideal For | Not Suitable For |
|---|
| Eliminating chloramines | Killing algae |
| Routine shock | Faecal contamination |
| Swimming quickly | Green water |
| Spas and hot tubs | Serious problems |
Calculating Dosages
Standard Dose (Normal Shock)
Calcium hypochlorite (65%) to reach 10 ppm:
| Pool Volume | Quantity |
|---|
| 20 m³ | 300g |
| 30 m³ | 460g |
| 40 m³ | 615g |
| 50 m³ | 770g |
| 60 m³ | 920g |
| 80 m³ | 1230g |
| 100 m³ | 1540g |
Double Dose (Algae, Green Water)
| Pool Volume | Quantity |
|---|
| 20 m³ | 600g |
| 30 m³ | 920g |
| 40 m³ | 1230g |
| 50 m³ | 1540g |
| 60 m³ | 1840g |
| 80 m³ | 2460g |
| 100 m³ | 3080g |
Triple Dose (Severe Contamination)
| Pool Volume | Quantity |
|---|
| 20 m³ | 900g |
| 30 m³ | 1380g |
| 40 m³ | 1845g |
| 50 m³ | 2310g |
| 60 m³ | 2760g |
| 80 m³ | 3690g |
| 100 m³ | 4620g |
Calculation Formula
Quantity (g) = Volume (m³) × Desired dose (ppm) × 1.54
For 65% calcium hypochlorite. Adjust according to product concentration.
💡 Dica Profissional
For green algae, use double dose. For yellow (mustard) algae, use triple dose. For black algae, you may need quadruple dose and localised treatment with chlorine tablets.
Step-by-Step Procedure
Preparation
1. Test the water:
| Parameter | Check | Action |
|---|
| pH | 7.0-7.4 | Adjust before shock |
| Free chlorine | Note | Calculate dose |
| Total chlorine | Note | Calculate chloramines |
| CYA | < 50 ppm | If high, consider dilution |
2. Prepare equipment:
- Large bucket (15-20 litres)
- Rubber gloves
- Safety goggles
- Clothes that can stain
- Stirring rod
3. Choose the time:
| Time | Recommendation |
|---|
| Sunset | Ideal - less UV loss |
| Night | Excellent - chlorine works all night |
| Morning | Acceptable if necessary |
| Midday | Avoid - significant loss |
Preparing the Solution
4. Correct dissolving method:
| Step | Action |
|---|
| 1 | Fill bucket with pool water |
| 2 | Add product slowly |
| 3 | Stir with rod (not hands!) |
| 4 | Wait for complete dissolution |
| 5 | Some residue may remain (normal) |
**WARNING**
NEVER add water to chemical product. ALWAYS add product to water. This prevents violent reactions and splashing.
Application
5. Distribute the solution:
| Method | When to Use |
|---|
| Distribute around perimeter | Recommended |
| Pour in front of return | Good distribution |
| Add via skimmer | Small quantities only |
| Add directly (granular) | Never on vinyl |
6. During application:
- Pump running
- Walk around the pool
- Distribute evenly
- Avoid concentrated areas
Post-Application
7. After adding:
| Action | Duration |
|---|
| Keep pump running | Minimum 8 hours |
| Don't cover the pool | Allows off-gassing |
| Don't swim | Until chlorine < 3 ppm |
| Test in the morning | Check result |
8. Expected result:
| Time | What to Observe |
|---|
| 1 hour | Water may look worse (normal) |
| 4 hours | Starts to clear |
| 8-12 hours | Visibly clearer |
| 24 hours | Crystal clear (if successful) |
How Long to Wait Before Swimming
Safe Levels
| Free Chlorine | Safety |
|---|
| > 10 ppm | Dangerous - don't swim |
| 5-10 ppm | Wait |
| 3-5 ppm | Borderline, sensitive people avoid |
| 1-3 ppm | Safe to swim |
| < 1 ppm | Ideal, but may need more chlorine |
Typical Waiting Time
| Shock Type | Minimum Time | Recommended Time |
|---|
| Normal shock | 8 hours | 12-24 hours |
| Double shock | 12 hours | 24-36 hours |
| Triple shock | 24 hours | 36-48 hours |
| Monopersulfate | 15 minutes | 1 hour |
💡 Dica Profissional
Don't rely on time - always test chlorine before allowing swimming. On hot, sunny days, chlorine degrades faster. On cool, cloudy days, it takes longer.
Treating Specific Situations
Green Water (Algae)
| Step | Action |
|---|
| 1 | Brush walls and floor |
| 2 | Adjust pH to 7.0-7.2 |
| 3 | Apply double or triple dose |
| 4 | Run pump for 24 hours |
| 5 | Brush again after 12 hours |
| 6 | Vacuum dead algae to waste |
| 7 | Backwash filter |
| 8 | Repeat if necessary |
After Party/Intensive Use
| When | Action |
|---|
| During the party | None (let them enjoy) |
| After last swimmer | Normal shock dose |
| Next morning | Test and adjust |
Cloudy Water (No Algae)
| Possible Cause | Treatment |
|---|
| Chloramines | Normal shock |
| Suspended particles | Shock + flocculant |
| Early algae | Double shock |
| New water | Shock + adjust chemistry |
After Heavy Rain
| Situation | Action |
|---|
| Light rain | Test, adjust if needed |
| Moderate rain | Preventive shock (half dose) |
| Heavy rain/storm | Normal shock |
| Flooding | Double shock + check pH |
Faecal Contamination
| Type | CDC Protocol |
|---|
| Solid faeces | Remove, double shock, wait 30 min at 20 ppm |
| Diarrhoea | Remove, triple shock, wait 13 hours at 20 ppm |
**IMPORTANT**
In case of faecal contamination, especially diarrhoea, follow strict CDC protocols. Cryptosporidium is resistant to chlorine and requires very high levels for extended periods.
Common Mistakes to Avoid
Dosing Errors
| Mistake | Consequence | Solution |
|---|
| Insufficient dose | Doesn't reach breakpoint, worse result | Calculate correctly |
| Excessive dose | Waste, long wait | Follow tables |
| Not measuring volume | Wrong doses | Calculate actual volume |
Procedure Errors
| Mistake | Consequence | Solution |
|---|
| Adding during day | 50%+ loss to UV | Do at dusk |
| Not dissolving | Vinyl damage, stains | Always pre-dissolve |
| Mixing products | Dangerous reaction | Never mix |
| pH not adjusted | Chlorine ineffective | Correct pH first |
Safety Errors
| Mistake | Risk | Prevention |
|---|
| No protection | Burns, inhalation | Use PPE |
| Poor storage | Degradation, danger | Cool, dry place |
| Children present | Accidents | Keep away |
Safe Handling
Personal Protective Equipment
| PPE | Required |
|---|
| Rubber gloves | Yes |
| Safety goggles | Yes |
| Mask (if powder) | Recommended |
| Suitable clothing | Yes (can stain) |
Storage
| Rule | Reason |
|---|
| Dry location | Moisture degrades product |
| Cool location | Heat accelerates degradation |
| Away from acids | Dangerous reaction |
| Away from organics | Combustion risk |
| Sealed container | Maintains effectiveness |
| Out of children's reach | Safety |
Incompatibilities
| Never Mix | Result |
|---|
| Chlorine + acid | Toxic chlorine gas |
| Chlorine + ammonia | Toxic gas |
| Different types of chlorine | Violent reaction |
| Chlorine + organic matter | Combustion |
Shock in Special Situations
Saltwater Pools
| Aspect | Consideration |
|---|
| Chlorinator | Turn off during shock |
| Product | Calcium hypochlorite OK |
| Cells | Protect from high concentration |
| Return | Turn on after normalising |
Vinyl Pools
| Precaution | Reason |
|---|
| Always pre-dissolve | Prevents stains |
| Distribute well | Avoids concentration |
| Never granular direct | Damages vinyl |
| Avoid tablet chlorine | Slow dissolution, damage |
Spas and Hot Tubs
| Difference | Adjustment |
|---|
| Small volume | Proportionally small doses |
| High temperature | Chlorine degrades faster |
| Intensive use | More frequent shocking |
| Jets | Don't run during shock |
Alternatives to Chlorine Shock
Active Oxygen Shock
| Aspect | Description |
|---|
| Product | Potassium monopersulfate |
| Dose | According to manufacturer |
| Time to swim | 15 minutes |
| Effectiveness | Good for chloramines, weak for algae |
UV + Reduced Shock
| System | Benefit |
|---|
| UV installed | Reduces shock need |
| Ozone | Continuous oxidation |
| Combination | Fewer chemicals, more pleasant water |
Shock Treatment Checklist
Before
- [ ] Test pH (adjust if necessary)
- [ ] Test chlorine (note values)
- [ ] Calculate pool volume
- [ ] Calculate required dose
- [ ] Prepare protective equipment
- [ ] Check time (prefer dusk)
During
- [ ] Use PPE (gloves, goggles)
- [ ] Dissolve product in bucket
- [ ] Pump running
- [ ] Distribute around perimeter
- [ ] Avoid splashing
After
- [ ] Keep pump running 8+ hours
- [ ] Don't cover pool
- [ ] Don't allow swimming
- [ ] Test chlorine next morning
- [ ] Wait for chlorine < 3 ppm
- [ ] Check pH (may have changed)
Frequently Asked Questions
Can I shock with chlorine tablets?
Not recommended. Tablets (trichlor) are slow-dissolving and contain stabiliser. For shock, you need a fast-dissolving, high-potency product like granular calcium hypochlorite.
Why does my pool smell like "chlorine"?
Paradoxically, a strong chlorine smell indicates lack of free chlorine. What you smell are chloramines - compounds formed when chlorine reacts with sweat, urine and contaminants. Shock treatment eliminates chloramines and the smell.
Can I shock and add algaecide at the same time?
Not ideal. The high chlorine from shock can neutralise some algaecides. Do the shock, wait for chlorine to drop below 5 ppm, then add algaecide.
How often should I shock?
Depends on usage. For normal family use, fortnightly is sufficient. In summers with intensive use or parties, weekly. Whenever you notice chlorine smell, irritated eyes or cloudy water.
Can shock damage the liner/vinyl?
Yes, if poorly applied. NEVER add granular directly onto vinyl. Always dissolve in bucket first and distribute the solution. High localised concentrations can discolour or weaken vinyl.
Conclusion
Shock treatment is essential for keeping the pool healthy and safe. The fundamental points are:
- Do at dusk - avoids chlorine loss to UV
- Adjust pH first - between 7.0 and 7.4 for maximum effectiveness
- Use the correct dose - calculate based on actual volume
- Always dissolve - especially in vinyl pools
- Wait before swimming - always test before allowing bathing
- Use protection - chemicals are dangerous
- Don't mix products - risk of dangerous reactions
A regular shock treatment programme, combined with daily maintenance, keeps water crystal clear and safe for all swimmers.
If you need professional help with your pool's chemical treatment in Margem Sul, ManutençãoPiscinas is here to help. Contact us for a no-obligation quote.
