Understanding Media Filtration
Media filtration plays a crucial role in removing relatively large, suspended solids that may be present in the raw water feeding reverse osmosis (RO) plants. This essential process of media filtration ensures the longevity and efficiency of the RO system by preventing fouling of the RO membrane elements due to suspended solids, particulates and colloidal matter.
How does Media Filtration work?
A media filter vessel is partially filled with a combination of varying grades of filter media (typically sand and gravel but other medias are used depending on the application). This setup leaves space above the media to allow for bed expansion during filter backwashing.
- There is a header fitted to the top of the vessel, it distributes feed water during normal operation and allows wastewater to exit during backwashing.
- A lateral system is located at the bottom, it collects filtered water and retains the media, while also allowing wash water to enter during backwashing.
Media Filter design and performance
The media filter vessel is designed to provide sufficient filtered water for the RO plant’s correct operation. It has a design filtration rate which is set to ensure optimal performance even when the plant is operating in poor quality feed water.
Keeping your Media Filter in top shape
During operation, the media filter may become fouled, indicated by an increase in differential pressure. This is monitored by the filter’s inlet and outlet pressure instruments and usually a differential pressure switch (or transmitter). Regular monitoring and maintenance of media filters is essential to avoid operational issues.
The Backwashing Process for Media Filtration
Backwashing is necessary to remove accumulated solid particulates from the media bed. This process involves reversing the normal flow and using a higher flow rate to fluidise the media filter bed, allowing particles to be released and discharged to waste. In some applications, filter backwashing is achieved by operating manual valves on the filter pipework arrangement. The following description relates to an automated backwash sequence:
- Initiation: Backwashing for media filtration starts automatically when the differential pressure reaches 1 barg from the clean condition or every 24 accumulated running hours, whichever comes first. It can also be manually initiated via a panel pushbutton.
- Flowrate Adjustment: The flowrate for backwashing depends on the feedwater temperature. Warmer water has lower viscosity, making it easier to fluidise the media bed with a lower flowrate, while colder water is more viscous and requires a higher flowrate to achieve the same fluidisation. Proper flowrate is crucial for effective backwashing without losing media.
Backwash flow rates based on temperature
As an example, see below typical rates for a 54m³/day RO plant:
| RAW WATER TEMPERATURE | FLOWRATE REQUIRED |
| 5 ºc | 13.2 m³/h |
| 10 ºc | 15.0 m³/h |
| 15 ºc | 16.6 m³/h |
| 20 ºc | 18.8 m³/h |
| 25 ºc | 20.4 m³/h |
| 30 ºc | 22.4 m³/h |
| 35 ºc | 24.2 m³/h |
The flowrate is adjusted manually using the backwash control valve with the flow rate indicated on the media filter feed/backwash flowmeter.
Step-by-Step guide to the Backwashing Process
In normal operation, the media filter will accumulate solid particles from the raw feedwater. To remove these deposits, the filter undergoes a backwashing process, which involves:
- Shutdown: Stop the RO plant before starting the backwashing process.
- Initiation: Begin the backwash sequence automatically or manually to clean the media.
- Backwashing: Reverse the water flow through the media to clean the filter.
- Extended Backwash: For new media or after maintenance, an extended backwash may be necessary until the effluent is clear, viewed through the backwash sightglass.
Media filtration is vital for maintaining the efficiency and longevity of reverse osmosis plants. By removing the majority of suspended solids, media filters protect the RO system from fouling and ensure the production of high-quality water. Proper maintenance, including regular backwashing, is essential to keep the filtration system in optimal condition.
For spares and consumables related to media filtration, please contact our after sales team at aftersales@saltsep.co.uk.
(Image – Aquasana)