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Actual "Backflushing" of RO membranes is not that common nor all that necessary. "Forward flushing" is the preferred method using appropriate chemicals to remove debris, and to restore flow characteristics etc. The choice of prefiltration retention and surface area will diminish the need for 'backflush' of the RO membrane.
To properly backflush an RO membrane, one must remember that the transmembrane pressure to properly 'make water' is 800+ psi, and with the common 'spiral' formed membranes the normal flow is essentially from 'outside' the spiral to 'inside' the spiral and putting 800 psi in the wrong direction across the 'pack' can easily burst or break the membrane. RO membranes are 'tangential flow devices', the fluid travels across the surface of the membrane and at high pressure a portion of the fluid enters and passes through the membrane as product water. The PROPER velocity (amount) of the inlet water flowing across the outer surface of the membrane will or should keep the membrane 'clean', usually this is already in the 'design' of a prepackaged or 'skidded' system.
However, the entrance areas to the 'spirals' do in fact eventually become plugged with particles (mostly deformable 'slimes', etc. and low pressure backflushing in the 'reverse direction' across the 'upstream' side of the membrane with clean 'product water' may be of benefit to dislodge the particles that 'bridge' and plug the 'entry' to the membrane package. A better way is to 'tighten' the retention µM of the prefilters (or/and select a prefilter that has high removal efficiency) and increase their surface area to insure the correct 'delivery flow' to the membrane - prefilters are more easily 'backflushed' (not to exceed ~15 psi when backflushing) than RO membranes, but dont wait until the prefilters become plugged before backwashing!!!!!!. Typical backwashing of prefilters is done with no more than 15-20% of the 'forward' flow at 15 psid MAX. pressure, ... with HOT (160-170° max. temp.) clean product water if you can.
There are specialized chemicals for sanitization and 'cleaning'
1. sodium/potassium metabisulfite will kill/stun and keep the biological growth low inside (both sides - retentate and permeate) the membrane. Biofouling is by very 'sticky' and soft particles that can easily 'plug' an RO membrane. Metabisulfites are entered into the system by 'forward flushing' ... just add the chemical solution to a quanity of previously made input 'product water'. Severe flushing after 'pickling' is required to remove the Mbsulphites as they are not compatible with 'human consumption'.
The following are common to industry and require precise usage by chemical and safety expertise:
2. Sodium Hydroxide mixtures are used to dissolve organic fouling on the surface and partly 'into' the membrane; you essentially 'soak' the membrane in situ, then flush. Precise chemical expertise is needed to insurer the ALL the NaOH is removed before you start to use/store the product water. Sodium Hydroxide is 'lye'. A hydroxide flush is done in the 'forward' direction.
3. Peracetic acid + hydrogen peroxide mixtures. (requires severe chemical knowledge and the usage of 'indicator strips' to insure total removal when finished) Do websearch for "MinnCare". Peracetic+H202 will dissolve 'salts' entrapped in the RO membrane 'pores', plus will kill virtually 'everything' inside the system.
4. inorganic 'hard particles' .... use 'better' prefiltration - tighter and more 'efficient' and 'larger surface area' filters upstream of the RO portion.
Watermaker usage should be limited only to 'visibly' clear/clean input water to avoid premature plugging of the RO membranes. 'Watermakers' are not 'absolute' in retention and should NOT be used in areas of suspected high bacteria or virus, etc. concentrations: harbors, anchorages, other boats, cities, large population areas, sewage treatment plants etc. If a 'single' microorganism passes through the RO membrane it can rapidly divide/multiply in the relatively 'stagnant' water on the 'downstream' of the membrane and then begin to very rapidly produce massive colonies in the piping, tanks, your intestines, etc. RO product water from a 'watermaker' should always be further dosed with a bacteriostat - clorox or other chlorine agent, etc.
Watermakers should be routinely and periodically 'pickled' with at least potassium/sodium meta bisulphite to reduce the 'bio-burden' or 'growth' inside them.
FORWARD FLUSHING after usage is a damn good idea.
Suggestion is made to make a few gallons (~5+) of product water into a separate 'container' and when finished making your water for the day, etc. shut off the raw water inlet and suck in the ~5+ gallons to 'clean out the low and high pressure pump', the prefilters, the lines, the surface and 'some' of the 'pores' of the RO membrane .... this will dilute the bio-burden that was accumulated during water production (reduces 'metabolites'), reduce the 'salt content' of the water to protect any stainless steel, brass, etc. surfaces.
Last edited by RichH; 12-14-2010 at 11:42 AM.