Air scouring in membranes is the cleaning of the membrane surface on the feed side of the membrane by rapidly abrading, scraping the membrane surface using pressurised air bubbles moving in a direction parallel to the membrane surface.
Air scouring makes use of specific gravity difference between water and air, limited solubility of air in water and the brownian motion-like movement of macro, micro and nano bubbles of air in water & inherent behaviour of air to rise up in water. In Ultrafiltration or an MBR process, After a round of filtration cycles, there is a build up of contaminants on the feed side of the membrane, which reduces the flux on the membrane. To clean this, Pressurised air is allowed into the membrane module from the bottom on the feed side to rapidly make contact with the suspended solid, biofilm & contaminant layer to slough off this layer from the membrane surface, thereby contributing substantially to the cleaning of the membrane. This also ensures that the resistance to backwash flow is reduced, due to the decrease in the amount of contaminants on the membrane surface.
Common Myths vs Reality
Damages Membranes
Air scouring when done properly, at controlled inlet pressures and controlled flow rates is effective in cleaning the membranes while having little to no effect on the surface of the membrane over long periods of time. Air scouring is generally done at 1-2 bars of pressure for UF membranes. Flow rates during air scouring can be between 0.1 m3/m2 of membrane/hour to 1.5 m3/m2 of membrane/hour. Of course, these are flow rates and not hourly volume requirements of air. This heavily depends on feed water conditions and operating conditions. For even more specific selection, you may contact Theway engineers for a MEPSOR projection that will give you recommended details on the operation of the plant including air scouring.
Uses Extra Energy
After every filtration cycle, it is imperative to clean membranes. Cleaning of membranes are traditionally done with only a backwash - permeate water flowing back from the permeate side to the feed side, loosening and removing the contaminants on the feed side of the membrane. But permeate water is very precious, in the sense, permeate water is the desired product of an UF plant. With extensive backwash, more and more permeate water is used.
a. Air scouring provides you with the valuable opportunity to clean membranes with air instead of precious permeate water.
b. In most cases, the backwash pump has a higher wattage rating than an air scouring compressor.
By using air scouring, backwash duration can be reduced , backwash flux can be reduced, both of which will directly reduce the size, flow rate and wattage of backwash pump. Yes, in a few cases, when the plant size is too small, then there is a possibility for air scouring CAPEX to be prohibitive. But in most plants that cross a threshold of size, air scouring is recommended.
Unnecessary addition
Myth is that air scouring needs an extra compressor and costs more. This is mostly not true. Many sites have existing compressed air lines or have an onsite compressor that is used to supply pressurised air to operate the pneumatic valves present in the UF plant. The air needed for air scouring can be tapped in from these existing lines itself.
Even when there is a situation which requires a separate air compressor, it is mostly worthwhile. While evaluating air scouring, the following parameters need to be considered -
Presence of air scouring - Increase of Recovery, reduction of permeate water loss to drain, lesser drain water to handle in case of ZLD, lesser backwash flux, lesser backwash duration, longer membrane life, reduction in dead spots inside the membrane module, general reduction in OPEX, downtimes with very marginal increase in CAPEX
Absence of Air scouring - Decrease in recovery, increased loss of permeate water to drain, more membranes for required KLD capacity, higher backwash flux, higher fiber breakage, more frequent backwash, lesser membrane life, higher backwash duration and increase in OPEX with marginal decrease in CAPEX.
Of course, there are cases of exception to the above. But in majority of cases, air scouring cannot be taken as an unnecessary addition.
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The points explained in this blog are to be treated as suggestions and are not to be taken as guidelines.
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