The Details of Back Washing for MMDF Tank

After discussing to you the details of regeneration for water softener, a client who read this blog called me up and ask me about the details of back washing for MMDF tank.

So, I again asked him what was the instruction to him by the company that installed his system?, he told me that it’s every other day in order to insure that media tanks are always clean.

Here is a comparative analogy, MMDF tank is the same as the sediment filter (cartridge type), and they have the same function which is to trap particulates or suspended solids from the water. If you use the cartridge type, you will just wait for the cartridge to clog up before you replace it with new one. From the time you place the cartridge until the time it clogs up, it would take so many days (depending on the size of the cartridge, volume of water and level of suspended solids from the water), and once it clogs up then that is the time to replace the cartridge.

If you are using the MMDF (tank type/back washable),once there is a clogging, then that is the time to backwash, considering the length of time before the MMDF tank clogs up would definitely be longer than the cartridge type.

I do not see any reason why we should back wash the MMDF every other day just to insure that filter is clean because it is designed to trap particulates and the accumulation of particulates to the surface of the media elements would even increase the filtration efficiency
of the media tank( Principles of the Brownian Motion).

The reason for back washing is slow flow rate due to clogging and not because it is dirty, you cannot expect a floor map to be clean because its purpose is to wipe out dirt from the floor.

If you use a cartridge type filter and tank type filter, both will function effectively and the only difference is the service life, so what is the logic of frequent back washing?

The purpose of frequent back washing is to prevent ‘channeling’, as water enters the media tank it flows down to the center portion of the media elements, just beside the riser tube, thus creating a deep mark on the layer of the media elements and as the process continues, mark will become deeper and deeper decreasing the bed height of the media elements at the center portion and will result to the uneven performance of the media in terms of efficiency.

During the back washing, media elements will be lifted up wards and as it comes down, media surface becomes even thus improving the filtration efficiency of the media tank.

Hoping that this concept will clarify the back washing issues of MMDF tank so as not to create false belief about the importance of back washing.

Sad to say, not all WRS are advised properly that is why most are operating their systems blindly.

- Orly Shanghai