Optimising Water Quality in Reverse Osmosis Plants with a Rehardener

Rehardening is a crucial process for reverse osmosis (RO) plants, ensuring the treated water is stable, non-corrosive, and palatable. 

The Nature of RO Permeate

RO permeate, the product of the reverse osmosis process, often lacks natural inhibitors like calcium or magnesium hardness. This absence leads to an extremely aggressive nature and fluctuating pH levels due to trace amounts of absorbed carbon dioxide and minimal buffering capacity. Untreated RO permeate not only tastes ‘flat’ but can also be corrosive to materials used in water distribution and storage systems, potentially causing the ‘Red Water’ effect by stripping corrosion deposits from legacy pipe systems. 

Salt Sep Rehardeners

In 1997, Salt Sep were awarded a DTI R&D grant for designing rehardener vessels specifically for RO permeate treatment. This enabled numerous trials to optimise rehardener sizing and CO2 injection rates for the optimal RO permeate treatment suitable for consumption.

Importance of Rehardening

To mitigate the aggressive nature of RO permeate, it is essential to reintroduce hardness, typically raising it to 40-80 mg/ l (as CaCO3). The target pH should be between 6.5 and 8.5, with a slightly positive Langelier Saturation Index (LSI) to encourage calcium carbonate precipitation rather than dissolution. RO permeate generally starts with a hardness of about 10 mg/I (as CaCO3).

How Rehardening Works

Rehardening is achieved by passing the permeate through a rehardener vessel filled with calcium-rich media. This process increases the water’s alkalinity and hardness, achieving a slightly positive LSI around +0.5.

CO2 can be injected into the water before entering the rehardening vessel to aid media absorption. A flowmeter, check valve, and solenoid valve control CO2 flow, adjusted by a needle valve on a flowmeter. 

The juraperle media, with over 99% calcium carbonate purity, gradually dissolves during use and requires periodic replacement. 

Typically, the vessel media is topped up every 1 to 2 months. 

Adjusting and Sampling

The proportion of permeate passing through the media bed is controlled using inlet and by-pass valves, with flow adjustments monitored via a by-pass flowmeter. Adjustments ensure the desired level of pH and hardness at the product water outlet. 

Rehardener Vessel Design

The rehardener vessel is generally designed per PD5500 standards. Typical specifications include:

  • Construction: 316L stainless steel vessel.
  • Media: Filled with 3-6mm gravel and juraperle rehardener media. 
  • Flow System: A lateral system at the bottom allows permeate to pass through the media to the top of the vessel, where another lateral enables the permeate to exit while retaining the media. 
  • Monitoring and Maintenance: Features like a slight glass for media level indication and a filling port for media topping up. A rehardener relief valve set at 6 barg protects the vessel from over-pressurisation. 

Rehardener Top-Up Procedure

Topping up the rehardener involves the following steps: 

  1. Shut Down the RO Plant: As per the specific procedure.
  2. Close Valves: Inlet, outlet isolation and bypass.
  3. Drain the Vessel: Open the drain valve and flushing valve.
  4. Add Media: Open the top filling port, pour in the media, and level it until two-thirds of the sight glass is covered. 
  5. Seal and Restart: Replace the filling port nut, open the inlet valve, close the drain valve, and restart the RO plant.
  6. Flush/Rinse: Open supply valve and flush rehardener before use to rinse out any fines from the new media.
  7. CO2 Injection and By-pass Adjustments: If necessary, the CO2 injection, bypass and inlet valves can be adjusted to regulate the quality of RO permeate (i.e. 6.5 to 8.5 in pH and 40 to 80 mg/l hardness as CaCO3).


By implementing a rehardener in your RO plant, you can ensure efficient and effective water treatment, providing peace of mind and high-quality water to end-users.  At Salt Separation Services, we can provide a full range of spares and consumables for your water treatment plant, including rehardener media such as juraperle. To contact us about our spares and consumables, please email aftersales@saltsep.co.uk.