Total Dissolved Solids (TDS): When to Drain Your Pool

What is Total Dissolved Solids (TDS)?

Total Dissolved Solids (TDS) measures the concentration of all dissolved substances in your pool water, expressed in parts per million (ppm). Think of it as the "stuff" that's been dissolved in your water over time - everything from pool chemicals and minerals from your source water to contaminants brought in by swimmers and the environment.

Unlike other pool chemistry measurements that focus on specific chemicals, TDS gives you the big picture of how "loaded" your water has become with dissolved materials.

What Contributes to TDS?

TDS accumulates from multiple sources over time:

Pool Chemicals

• Chlorine products (sodium hypochlorite, calcium hypochlorite)
• pH adjusters (sodium carbonate, muriatic acid)
• Alkalinity increasers (sodium bicarbonate)
• Calcium chloride for hardness adjustment
• Salt from salt water chlorine generators

Environmental Factors

• Minerals naturally present in fill water
• Dirt, dust, and debris
• Rainwater (TDS varies greatly depending on local air quality and pollution)
• Fertilizer runoff and organic matter

Swimmer-Introduced Contaminants

• Body oils, sweat, and cosmetics
• Sunscreen and lotions
• Hair products and deodorants

Why TDS Matters

High TDS levels create several problems that affect both water quality and swimmer comfort:

Reduced Chemical Effectiveness

As TDS increases, your pool chemicals become less effective. Chlorine works harder to sanitize water with high dissolved solid content, requiring higher doses to maintain proper free chlorine levels.

Water Feel and Clarity

High TDS water often feels "heavy" or "sticky" to swimmers. While the water may test chemically balanced, it doesn't feel fresh or inviting.

Equipment Strain

Salt water chlorine generators and other equipment can struggle with very high TDS levels, leading to reduced efficiency and potential damage.

Difficult Balancing

Water with excessive TDS becomes increasingly difficult to balance. pH and alkalinity may fluctuate more frequently, and shocking becomes less effective.

How to Measure TDS

Testing Process

1. Turn off circulation and let water settle for 30 minutes
2. Submerge TDS meter probe completely
3. Wait for reading to stabilize (usually 10-15 seconds)
4. Record the measurement in ppm
5. Clean and store meter according to manufacturer instructions

Establishing Your Baseline

The key to TDS management is knowing your starting point. Test your source water (tap or well) to establish a baseline TDS level. This baseline varies significantly by location - some areas have naturally high TDS (500+ ppm) while others start very low (under 100 ppm).

When to Take Action

The general rule is to consider action when your pool's TDS exceeds your source water TDS by 2000 ppm. However, you may need to act sooner if you notice:

• Difficulty maintaining proper chlorine levels
• Water feels heavy or uncomfortable
• Frequent pH or alkalinity swings
• Cloudy water despite balanced chemistry
• Ineffective shocking

How to Lower TDS

Unfortunately, there's no chemical way to remove dissolved solids from pool water. The only effective method is dilution through partial drain and refill.

Partial Drain and Refill Process

1. Calculate the amount: Typically, draining 1/3 to 1/2 of your pool volume is sufficient
2. Drain water: Use a submersible pump or your pool's drain system
3. Refill with fresh water: Add source water to restore normal level
4. Rebalance chemistry: Test and adjust pH, alkalinity, and sanitizer levels
5. Test new TDS: Verify that levels have dropped appropriately

TDS Reduction Calculation

A 50% water change will reduce your TDS by approximately 50%. For example:

• Starting TDS: 3000 ppm
• Source water TDS: 300 ppm
• After 50% drain/refill: ~1650 ppm

Special Considerations for Spas

Hot tubs and spas require more frequent attention to TDS due to:

• Higher chemical usage: Hot water requires more sanitizer and frequent pH adjustment
• Increased evaporation: Concentrates dissolved solids more quickly
• Higher bather load: More contaminants per gallon of water
• Smaller volume: Changes accumulate faster

Spa TDS Guidelines

• Test frequency: Weekly during regular use
• Action threshold: 1500 ppm above source water (lower than pools)
• Drain schedule: Every 3-4 months regardless of TDS for heavily used spas

For spas with salt water systems, TDS will naturally be higher due to salt content (typically 3000-4000 ppm just from salt). Monitor the increase above your salt-adjusted baseline.

Maintenance Schedule

Include TDS testing in your regular maintenance routine:

Understanding and managing TDS is crucial for maintaining pristine pool water. While it's not something that requires daily attention, regular monitoring helps you stay ahead of water quality issues and maintain the most comfortable swimming environment possible.