Plate & frame modules made from flat sheet forward osmosis membranes
Plate & frame (stacked) FO module summary
|Packing density||typically below 100 m2/m3|
|Advantage||ease of operation when waste streams contain high amounts of fouling agents and/or solutions entering the module have high viscosities|
|Disadvantage||large footprint increases space requirements – not suitable for high volume applications|
Plate & frame membrane modules – also known as stacked membrane modules – are used in many water treatment applications where the waste streams to be treated contain high amounts of fouling agents and/or have high viscosities. In fact, many commercial membrane bioreactor (MBR) modules belong to the plate & frame configuration. Plate & frame modules typically consist of flat sheet membranes sealed to frames, which provide the overall mechanical integrity and flow distribution needed to stack individual frames together in a modular way. Thus, individual frames function as membrane cassettes where the waste stream to be treated typically flows outside the cassette with the clean water permeating to the inner volume of the cassette for subsequent collection.
Forward osmosis plate & frame modules are – in principle – constructed in a similar manner with the added complexity that the frame/cassette/module designs must accommodate cross flow distribution of feed and draw streams across each individual membrane layer while avoiding direct mixing of said feed and draw streams.
From an engineering point of view, it is difficult to achieve such cross flow distributions, and at the same time avoid unstirred regions, when the distance between individual membrane layers is reduced. As a result, plate and frame forward osmosis membrane modules typically have the lowest packing density / largest footprint of the 4 module design variants considered here (see the table below).
The large footprint of plate & frame forward osmosis membrane modules excludes these modules from being used in high volume applications such as municipal waste water treatment and desalination of seawater. However, in many lower volume applications, where the waste streams to be treated contain high amounts of fouling agents and/or have high viscosities, the low packing density of plate & frame modules represents an operational advantage. The reason being, that a larger distance between membrane sheets results in a lower pressure drop across the module (i.e. lower energy requirement for pumping solutions through the module) as well as a lower propensity towards clogging of flow channels due to accumulation of fouling agents.
|Area of individual sheets (including sealing)||0,25m2|
|Active membrane area per sheet (excluding sealing)||0,23m2|
|Thickness of individual membranes||200μm|
|Distance between membrane sheets||8mm|
|Number of sheets per module||43|
|Internal volume of module||0,09m3|
|Active area per module||9,9m2|
The links below lead to detailed descriptions of other forward osmosis module designs: