For example, a common compromise is to use the same flow control orifice on a multimedia filter discharge line to control both the filter backwash flow rate and the rinse flow rate performed after a backwashing. This method results in roughly the same flow rate being used for both steps. But where a backwash flow rate based on 12 gpm/ft2 of cross-sectional area is appropriate for obtaining 40 percent expansion of the media granules ( at 54 degrees Fahrenheit), this same rinse flow rate will compact the media granules under a pressure drop exceeding 10 psid (pounds per square inch differential). This will tend to push any suspended particles still in the upper section of the media filters deeply into the media bed. Acceptable performance will only begin to be achieved after the flow rate has been reduced to the normal service flow rate. If this only occurs while the filter is in service, much of the solids shed by the filters will end up in the RO cartridge pre-filters and in the RO membrane elements.
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Another common mistake with media filters is not installing individual flow meters on each of multiple filters in parallel. Without these flow readings there is no way to know if flow rates are balanced between the filters. If any particular filter starts to plug up with solids, more flow will divert to the other filters. If the media filter is not capable of providing water with a maximum silt density index (SDI) of five, as noted as a requirement on some membrane manufacturers’ element specification sheets, a fatal mistake is to inject a polymeric filtration aid directly prior to the media filters. This mistake is particularly devious in how it appears to dramatically improve the effluent quality of the filters. What does not show up in the effluent turbidity or SDI analysis is the residual polymer breaking through the filter.
Because of the polymer’s charge characteristics, it will permanently bond with the RO membrane. Any suspended solids will now attach to the polymer rather than migrate along the membrane surface. The rate of RO fouling will increase and cleanings will no longer restore original performance because it will not be possible to get the polymer off the membrane. The membrane elements will need to be replaced.
If media filters are not providing water of a sufficient quality, there are ways to improve their performance. A common misconception of pressurized filters is that they provide the best filtration at a flow rate of 5 gpm/ft2. Actually, filter performance will keep improving as the flow velocity is reduced until reaching the limits of the ability of the distribution laterals to prevent channeling.
It may be necessary to coagulate fine colloids upstream using a coagulant. If so, an inorganic coagulant should be employed, such as an aluminum product or ferric chloride. If these materials break through the media filter, they will also foul the downstream RO, but they can be cleaned. They should be used in a reaction tank of sufficient size to allow the reaction time necessary for the suspended solids to bind with the coagulant before getting to the media filters.
Chlorine Elimination
The polyamide thin-film membrane commonly used in most RO systems cannot handle chlorine. Some membrane manufacturers have promoted that their membrane could tolerate free-chlorine equivalent to the exposure of 1 ppm over a period of 1000 hours before a doubling of salt passage would occur. This guideline has often been misinterpreted as meaning that it is acceptable to allow chlorine to occasionally contact the RO membrane as a means of reducing biological fouling. But membrane damage will soon occur if it is exposed to any amount of chlorine and will be cumulative. The damage will be worse if iron or other transition metals have fouled out on the membrane.
Sodium bisulfite is often used to reduce the chlorine concentration going into the system. But sodium bisulfite will also react with dissolved oxygen in the water and any excess bisulfite will tend to reduce the oxygen concentration, increasing the potential for increased anaerobic biological growth. These are the species responsible for heavy slime formations that can rapidly foul the systems. A definitive symptom of this is the sulfur dioxide, rotten-egg smell noted when membrane vessels are opened.
The optimum concentration of sodium bisulfite can be difficult to maintain. Sodium bisulfite present in the injection day tank or in chemical totes will degrade over time as it reacts with oxygen from the atmosphere. If sodium hypochlorite (bleach) is injected upstream, its concentration will also change depending on its age.
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