Intuitive Answer –
When it comes to Out-In fibers, the dirt and impurities are on the outer perimeter of the fiber, the inside of the fiber is clean. In IN-OUT, the dirt and impurities are on the inside of the fiber, which increases the probability of a clogged fiber. An IN-OUT fiber is similar to a blockage in an artery. Eventually, such a block can result in the burst of the fiber or the immensely slow movement of the block (clot) along the fiber inside wall (artery)
Suspended Solid Layer –
Let us assume that there is deposition of equal amounts of suspended solids on both IN-OUT and OUT-IN fibers, both of equal outer and inner diameters. This fixed amount of suspended solids is spread over a larger area in an OUT-IN fiber, since it meets the outer surface area of the fiber, as compared to the smaller area when it comes to the smaller inner diameter controlled surface area of the IN-OUT fiber. Therefore for a given TSS level, the thickness of the cake layer of suspended solids deposited in thinner on an OUT-IN fiber as compared to an IN-OUT fiber. This is reflected in higher filtration times when it comes to an OUT-IN fiber, which translates to higher TSS levels of acceptance in an OUT-IN fiber.
Backwash Cycle –
During the backwash cycle, the clogged impurities in an IN-OUT fiber need to travel inside the small inner diameter all the way to the end of the fiber, pushing its way through other possible clogs inside the fiber. Also, once there is a clog in any part of the IN-OUT fiber along its length, that fiber is rendered useless in the current filtration cycle. The fiber becomes a working membrane only after the backwash. In contrary, in an OUT-IN fiber, the clogged impurity sits on top of the outer diameter wall of the fiber, rendering only the area this clog occupies useless until the rest of the filtration cycle, leaving the rest of the fiber in working condition. This is seen by faster dips in flux in an IN-OUT membrane along its filtration cycle as compared to an OUT-IN fiber. Also, during backwash in an OUT-IN fiber, the impurity is effaced from the outer diameter wall and has a lot more free space inside the membrane module to demonstrate free Brownian motion before exiting the UF module.
Above are reasons why backwashing in OUT-IN fiber modules require 10-30% lesser water for cleaning of membranes in similar conditions as compared to IN-OUT fibers.
Air scouring
While both designs allow air scouring, it is very effective only in one. In an OUT-IN fiber, air scouring enables marginal fiber movement with turbulent motion of air in the free space inside a module, enabling effective effacement of suspended solids cake layers and other impurities on the surface of the fibers. In an IN-OUT fiber, the air is scoured in the internal lumen of the fibers. The air flow here is not turbulent enough to cause any fiber movement. Rather, the air encourages pressure buildup inside the fiber and the effectiveness of cleaning is less. At each part of the fiber along its length, the integrity of the fiber depends on the successful dislodging of the clog due to local instantaneous pressure being higher than the resistance offered by clog to move along the fiber’s length, while this instantaneous local pressure is smaller than the fiber break strength. This is not good for the fiber strength and integrity in the long term. This is also the reason why air scouring pressures are higher in IN-OUT fiber systems.
Fibers
Due to above stated reasons of increased pressure on the ID of the fiber, the IN-OUT fibers require higher fiber wall thickness compared to OUT-IN fibers for a given flux rate and similar conditions. This is the reason why multibore membranes were developed, to even out the increased local pressures with a thicker and common wall.
There are two main reasons why we find lesser membrane active surface area in an IN-OUT membrane module vs an OUT-IN membrane module for the same overall volume
1. The internal diameter wall’s surface area is considered as the membrane active surface area in an IN-OUT membrane as compared to the outer diameter’s wall in an OUT-IN membrane.
2. Thicker walls in the case of IN-OUT membranes give rise to lesser fiber packing in a given cross section area of the membrane resulting in a lesser active area compared to an OUT-IN membrane.
Convention
Majority of the manufacturers of UF membranes have chosen to produce OUT-IN membranes. Even manufacturers who had been making IN-OUT membranes in large quantities have moved away to OUT-IN configuration.
There is a strong advantage of using OUT-IN membranes over IN-OUT membranes for any given application. The only major advantage IN-OUT membranes have is reduced dead volume of liquid retention inside the membrane.
Theway continues to manufacture membranes from both configurations, even though Theway prefers OUT-IN membranes. IN-OUT manufacture is to help users who have installed large quantities of IN-OUT modules from other companies. Please see (www.thewaymembranes.com/retrofit)