Can Sulfuric acid or HCl be used with the addition of Citric acid for the control of iron and other scales in a reverse osmosis plant?

First and foremost, citric acid is a highly assimilable organic acid – this means that it would act as a carbon source for growth of microorganisms.  In the few cases we’ve come across where citric acid was being dosed continuously into the RO feed, membrane autopsies found severe fungal growths that lead to extreme declines in permeability.

If the water source contains oxygen (surface water, wastewater), has been exposed to oxidizers, or ferric based coagulants have been used, the iron will be in the ferric state and can typically be controlled by dosing antiscalant to a certain extent.

If the water does not contain any dissolved oxygen, most of the iron will be in the Ferrous state.  Ferrous ions are extremely soluble and easily controlled by most antiscalants without acid.

However, in many cases, some soluble oxygen will be present. It only takes 0.1 ppm dissolved oxygen to oxidize 0.7 ppm of ferrous ions to the ferric state.

$${Fe{^2}{^+} + \frac{1}{4}O_2 + H^+ == Fe{^3}{^+} + \frac{1}{2} H_2O}$$

When the water source is from a deep aquifer, we assume anaerobic conditions where all iron will be in the ferrous state (as long as the water goes directly to the RO with no holding tanks and no dosing of chlorine or other oxidizers).  When the water source is from a surficial aquifer, oxygen may be present and iron can be assumed to be in the ferric state.

Ferric iron can be controlled to a certain extent using specialty antiscalants like AWC® A-119 or Megaflux® AF, but the antiscalant demand becomes significant.  Antiscalants have a higher affinity to trivalent metal hydroxides than other surfaces.  They will therefore preferentially adsorb to colloidal ferric hydroxide while allowing calcium carbonate and other sparingly soluble salts to precipitate and form scale on the membrane surface.  In such cases, very high dosages of antiscalant would be required to control the iron while an excess would be needed to control other scales.  In some cases, the required dosage would be so high that antiscalant overdosing may occur.  This can usually be addressed by using a lower antiscalant dosage in combination with pH reduction.

Some smaller plants use greensand filters for iron removal and they are very effective so long as they are correctly sized and well maintained;  iron concentrations are typically reduced to below 0.1 ppm. But they are a significant capital expense, require a large footprint, and because they require continuous dosing with a strong oxidizer, run a risk of damaging the RO membranes.