One of the numerous challenges in
designing a successful inland desalination plant is management of the concentrate
produced by the reverse osmosis (RO) process. One possible option is re-injection
into deep wells, but chemical incompatibility between the concentrate and receiving
formations could cause an unacceptable reduction in aquifer permeability. Assessing
this potential problem would require not only site-specific hydrologic and geochemical
data but also an interpretive analytic tool that could be used to predict performance.
In this case, the goal is not only to maximize recovery of product water but
also to minimize problems with concentrate injection. Here, we report our progress
on building such a tool using PHREEQC (Parkhurst and Appelo, 1999) to evaluate
the geochemical processes expected to occur during re-injection.
In the arid southwestern U.S., the brackish aquifers that are potential feed
waters for desalination exhibit remarkable spatial variation in chemistry. Of
waters with total dissolved solids (TDS) within an acceptable range for desalination
(< 10,000 mg/l), most are Ca-SO4 type waters; a smaller number are Na-Cl
or Na- SO4 type waters. PHREEQC calculations show that these waters are typically
saturated or supersaturated with respect to carbonate minerals and many iron
oxides. Using simple assumptions about the operating efficiencies of the desalination
process, and typical pre-treatment methods such as acidification, addition of
anti-scalants, and degassing excess CO2, we simulate the geochemical character
of RO concentrates derived from the feed waters in our database. Many of these
concentrates are supersaturated with respect to gypsum and are not suitable
for injection. Of the waters that are suitable for injection, many are predicted
to precipitate significant amounts of gypsum upon mixing with certain receiving
formations. We show that the impact of mineral precipitation on formation permeability,
however, will be strongly influenced by hydrologic factors and reaction kinetics,
which are not addressed here. Detailed study of these factors is worthy of future
analysis.