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Using Recycled STP Water as Cooling Tower Feed Water: A Sustainable Solution that Requires Special Treatment

Using treated water from a Sewage Treatment Plant (STP) or Wastewater Treatment Plant (WWTP) as cooling tower makeup water is a brilliant move for water conservation and operational cost efficiency. However, this step is not without challenges. STP recycled water has characteristics that are very different from regular raw water and requires much more rigorous pretreatment.

Treating it as ‘ordinary water’ and relying solely on sand filters and carbon filters is a major risk that can quickly damage your cooling tower.

Why Are Sand & Carbon Filters Not Enough?

Sand and carbon filters are excellent for removing suspended solids (TSS) and some colour, odour, and organic compounds.

However, STP recycle water contains pollutants that cannot be removed by either filter:

  1. High Total Dissolved Solids (TDS): STP water contains high levels of TDS, TSS, turbidity, etc.

  2. High Organic Nutrients: Nutrient residues are easy food for microorganisms, triggering rapid growth of biofilm and algae within the system.

  3. Ammonia & Other Nitrogen Compounds: Can trigger microbial growth and increase the demand for biocide.

  4. Reactive Silica: Has the potential to cause very hard silica scaling that is difficult to clean.

  5. Trace Contaminant Residues: Other complex organic and inorganic contents that require special treatment.

Relying solely on sand and carbon filters will allow all of these dissolved contaminants to enter the cooling tower, which will cause:

  • Extremely Rapid Corrosion due to chloride ions and high TDS.

  • Severe Scaling due to hardness and silica.

  • Uncontrolled Biofouling that clogs the system and increases the risk of Legionella.

  • Decreased Efficiency and Frequent Downtime.

Mandatory Pretreatment Steps and Systems

To safely use STP recycle water, you need a comprehensive, specially designed pretreatment system. Here is a common and effective scheme:

1. Primary Filtration (Initial Step):

  • Disc Filter or Multi-Media Filter: Filters large particles that may have escaped the STP process. This is the first line of defence to protect equipment in subsequent stages.

2. Advanced Pretreatment (Core of the System):

  • Ultrafiltration (UF) or Microfiltration (MF): This is a critical step that replaces/complements conventional filters. UF/MF membranes have very small pores (0.01 microns) that can effectively filter not only sludge but also bacteria, viruses, and colloids. This greatly reduces the organic and microbiological load.

  • Reverse Osmosis (RO): This is the heart of the solution. RO systems can remove up to 99% of dissolved contaminants, including:

    • Salts (high TDS)

    • Hardness ions that cause scaling

    • Silica

    • Most remaining organic matter

    • And almost all other ions

    • The output from RO is very clean water that is suitable for use as cooling tower feed.

3. Polishing Treatment (Optional for Best Results):

  • Electrodeionisation (EDI): For applications requiring water of very high purity.

What to Do?

  1. Comprehensive Water Analysis: The first and most important step is to analyse the composition of your STP recycle water in a laboratory. Key parameters to test: TDS, Chloride, Hardness, Silica, COD/BOD, Ammonia, and TSS.

  2. Proper System Design: Based on the lab results, the BHL engineering team will design the most effective and economical pretreatment scheme, usually centred on Ultrafiltration and Reverse Osmosis.

  3. Integration with Chemical Treatment Programme: Even with excellent pretreatment, the water treatment programme (corrosion inhibitor, dispersant, biocide) must be optimised to handle the specific characteristics of RO water.

  4. Regular Monitoring: Conduct regular monitoring of water quality before and after pretreatment to ensure the system is running optimally.

Conclusion: Investment for Sustainability and Reliability

Using recycled STP water is an investment in sustainability that also brings significant water savings. However, a half-hearted approach will only result in damage and costly repairs.

Investing in the right pretreatment system, such as UF + RO, is not an expense but a guarantee that your water recycling strategy will succeed and your cooling tower will remain protected, reliable, and efficient in the long term.

Make Your Water-Saving Strategy a Safe Success

Consult your plans with our experts. BHL is ready to assist from water analysis, system design, to implementing the right pretreatment solutions to ensure smooth operation of STP recycled water usage.