Page 84 - Membranes for Industrial Wastewater Recovery and Re-Use

64 Membranesfor lndustrial Wastewater Recovery and Re-use

RSI = 2pHs - pH (2.39)

An RSI of 7 indicates a water more or less at equilibrium. As the value falls the
water becomes more scaling and as it rises the water becomes more corrosive.
The Langelier and Ryznar saturation indices can be used to predict scaling in
most waters but they become unreliable when the total dissolved solids content
approaches 5000 mg IF1. Above this level the Stiff and Davis index (ASTM
D4582) or a similar method must be adopted.
Not all scale-forming compounds have a pH-dependent deposition. Some
scalants have simple chemistry, analogous to that given in Equation (2.30):
Mzf + YZ-AMY (2.41)

or
Mz+ + ZY-AMYz (2.40)

Scalants having the chemistry represented by Equation (2.39) are mainly the
sulphates of magnesium, barium and strontium (ASTM D4692).
The simplest means of preventing scale formation in RO systems is to operate
at a conversion sufficiently low that the reject stream is not so concentrated that
solubility problems are encountered. This, of course, has economic implications
for the operation of the plant that may be unacceptable, and in such cases some
form of scale prevention must be used. Where calcium carbonate or some other
hydrolysable scalant is the main problem, i.e. those salts containing hydroxide
(OH-) and carbonate (C032-) which therefore have associated pH-dependent
solubility due to hydrolysis reactions (i.e. reaction with acid, H+) of these anions,
it is often possible to adjust the LSI by acid dosing. This converts bicarbonate into
carbon dioxide which can be removed by degassing of either the feed or the
permeate. Depending on whether hydrochloric or sulphuric acid is used there
will be an increase in sulphate or chloride concentration. Increasing the
sulphate may give rise to calcium sulphate precipitation.
Scale-inhibiting chemicals can be used to delay precipitation of some salts by
interfering with the crystallization process forming microcrystals which do not
cause fouling and which will not show significant agglomeration at least until
the concentrate stream has left the RO unit. Most of the commercially available
chemicals work well on calcium carbonate, provided that the LSI in the
concentrate stream is less than +2, but less well on other salts. The most
established scale inhibitors are termed “glassy polyphosphates”, typically
sodium hexametaphosphate (Calgon@). These work by absorbing into the
nanoscopic protonuclei forming during the incipient stages of precipitation and
destabilising the subsequent crystal nuclei. They are not the most effective
chemicals for the application and revert quite quickly in solution to
orthophosphate, producing calcium phosphate sludge which can cause blocking
of separators and small bore pipes. Since the development of Calgon, more