Initial Hardness Removal from High-TDS Waters with or Without Silica Removal
Authors: Bowman Ronald W. , Craycraft Robert R. , Massaro James C. 
Published: December 2007

Recycle of high hardness, high TDS(total dissolved solids) waters (hardness&gt;1000 parts per million (ppm) as CaCO3 and TDS&gt;10,000 ppm) water for steam generation or other reuse such as irrigation or drinking water is very expensive. Silica content is usually above 250 ppm in such waters which can cause problems in steam generation and with desalination. Many of the these high hardness waters are oil field produced waters but there are other processes which generate waters which require additional treatment. For those high hardness waters, hot lime or hot caustic, followed with strong acid/weak acid resin or just weak acid resin softeners are used in conjunction with oil field steam generation. Treatment of these waters for irrigation or drinking waters involves thermal desalination or reverse osmotic(RO) treatment with biological control.

Silica removal is not required for normal wet steam genera tion. However for desalination operations, whenever the feed water concentrate is to be used for the steam generator feed and the fresh water sold, silica removal is required to aid in scale formation in desalination plus silica level in the concen trate water. The known limits on the concentrate feed water to a wet steam generator are 500 ppm silica and 25,000 - 30,000 ppm TDS of total water ions, based on soluble salts solubility. Silica removal is also required in other waters such as 210,000 ppm TDS water containing sodium carbonate where silica removal is required prior to crystallization of sodium carbonate crystals.

In the softening test work, the high solids addition and disposal associated with a hot lime system was not desired so alternatives were investigated. In addition, better silica removal than silica absorption on magnesium hydroxide or alumina or aluminum was required. Steam stripping of the high pH water and removal of the precipitates using a ceramic crossflow filter for which a special crossflow filter back pulse unit for cleaning was developed. Temperature and pH were increased prior to the steam stripping to decrease steam condensation and drive the reaction. When silica removal was required, a bed of alumina or aluminum was used at the high pH and temperature to put aluminum into solution so aluminum silicates were removed with the hardness precipitates. The solids often contain some oil when using oil field waters so an odor chemical was also developed for the microbiological soil remediation site.

The steam stripping was tested first in a countercurrent mode in a stainless steel column clad with a Hastelloy C 22, packed with stainless steel packing. The second test was by injecting the steam on the outside circumference of a crossflow ceramic microfilter in a cocurrent mode with flashing in a exit vessel. In the countercurrent tower operation, the control of the equili brium of the carbon dioxide, the carbonate and bicarbonate at the top of the tower was more difficult than when contacting with the microfilter. With the microfilter, the equilibrium approach was not a large concern as fresh steam was contacting the water and was then flashed. However the exact control of the steam to water ratio was more difficult in the second case.

Both thermal desalination and RO were pilot tested with waters from 10,000 to 36,000 ppm TDS to produce potable water with a quartz ultraviolet light for biological (disinfection) control. For wet steam generation, the field produced waters(10,000-24,000 TDS) were tested using strong acid/weak acid resin softening with no silica removal in a 1 MM BTU/Hr wet steam generator.1

The overall operational costs were less than normal sequence of processes mentioned in the literature while the capital costs were in the same range. Patents were obtained on the(1) steam stripping softening, (2)silica removal,(3) back pulse on the micro filter and(4) the odor chemical. A patent on the sour gas treatment is pending.