1. Introduction: saving water with cooling tower blowdown reuse
Cooling towers are open recirculating system designed to dissipate the heat removed from any commercial or industrial process by means of water evaporation.
As a portion of the water evaporates in the cooling tower, all dissolved ions (Calcium, Magnesium, Silica, etc) remain in the recirculating water stream. As more water evaporates, the concentration of dissolved ions in the water stream also increases and if this situation persists without restriction, the solubility limit is reached and salts precipitate as scale.
Scale is problematic in many ways:
- Blocks the heat transfer in heat exchangers
- Reduces the effective pipe cross-section
- Leads to corrosion problems
- Is difficult and expensive to remove
The concentration of dissolved ions is reduced by ‘bleeding’ a portion of the recirculating water and adding make-up water to replace it. To keep the water balance, the ‘bleed’ (or blowdown) plus the evaporated water must be replaced by make-up water.
Depending on the industry, the cooling tower blowdown stream can represent a significant portion of the total wastewater and recycling such source can provide significant water savings.
2. Cooling tower blowdown chemistry
The chemistry of the cooling tower blowdown depends mostly on the chemistry of the make-up water and the chemicals used for the cooling tower water treatment.
From a composition point of view, cooling tower blowdown is quite different to the wastewater generated by other industries such as
textile or food and beverages.
Blowdown contains a high concentration of inorganic salts (hardness, alkalinity, sulphates, phosphates, silica) and a few organic compounds used as part of the chemical cooling tower water treatment (antiscalants and corrosion inhibitors). However the the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of the cooling tower blowdown stream are usually relatively low.
In these circumstances, the primary treatment required for recycling the blowdown stream is reverse osmosis desalination. The main challenge is reducing the high concentration of dissolved ions coming from the concentration effect of the stream evaporation in cooling tower.
Apart from the high salinity or concentration of dissolved ions, other typical blowdown water parameters that may be of concern require pretreatment before the RO desalination are:
Cooling tower blowdown turbidity can sometimes exceed the threshold of 1 NTU, which represents the top limit for any RO treatment. If that is the case and based on the actual turbidity value, sediment filtration for turbidity removal or ultrafiltration are good alternatives to reduce the turbidity and bring it to acceptable levels.
2.2 Total Organic Carbon
Total Organic Carbon (TOC) and Total Dissolved Carbon (TDC) in the blowdown stream can indicate the presence of algae in the cooling tower water or can be originated by biodospersants dosed as part of the chemicals treatment.
Depending on the quantities, origin and dissolved or suspended status, TOC can be reduced to less than 2ppm by activated carbon filtration or ultrafiltration.
Iron in oxidised form can be present in the cooling tower water due to two main reasons: its presence in the make-up water or as a result of corrosion in the system. Dissolved (reduced Iron) is very uncommon in cooling tower water as aeration will oxidise any trace of Iron very quickly.
Iron levels as low as 0.05 ppm can be extremely damaging to reverse osmosis membranes and need to be removed before the RO desalination. Sediment filtration , standard Iron removal filters or Ultrafiltration are different alternatives to reduce the Iron to acceptable levels.
3. Reverse Osmosis for cooling tower blowdown treatment and reuse
Management of the cooling towers blowdown stream is one of the key activities within the operation and maintenance tasks.
The two main options for the blowdown disposal are:
- Discharge to sewage
- Treatment and reuse
Installing a Reverse Osmosis treatment on the cooling tower blowdown is an option to reduce the facility’s water needs. Reverse Osmosis membranes are used to separate the dissolved ions from the blowdown stream resulting in a high quality water output that can be reused as make-up water or for the same cooling tower or as a high quality water feed for any other process.
Depending on the blowdown quality, a number of pre-treatment options may be needed to make it suitable for the RO process: filtration, PH adjustment by acid injection and/or softening.
The main benefits of applying a reverse osmosis desalination treatment to the cooling tower blowdown are:
- Lower make-up water demand
- Easier regulation compliance
- Higher number of concentration cycles
- Reduction in chemical treatment
- Consistent water quality
4. Other opportunities to reduce cooling tower water demand
- When Calcium or Magnesium Carbonates and Sulphates are main factor limiting the cycles of concentration, softening the make-up water can allow the increase of recirculation cycles and reduce the blowdown and with it, the facility’s overall water consumption.
- Side-stream filtration systems can improve the quality of the recirculating water removing silt and suspended solids.
5. Conclusion: cooling tower water reuse to reduce operational and environmental costs
Water and wastewater reuse in industrial applications has become a growing trend due to higher water demand, constant water scarcity and more stringent regulations.
Most cooling towers use tap water as main make-up water supply, what adds pressure to the utility demand. Instead of discharging the blowdown and replacing it with fresh water, many industries are relaying on reusing the cooling tower blowdown to reduce their operational and environmental costs.
By removing a high portion of the total dissolved solids in the blowdown, reverse osmosis simplifies the reuse if the blowdown, allows higher concentration cycles and ultimately reduces the amount of blowdown and fresh make-up water required for the cooling tower operation.