Challenge

A polysilicon manufacturer in the Midwest required a final hydrogen peroxide/hydrofluoric acid wash followed by a 50-gallon per minute rinse. The process was to be continuous 24 hour per day during production runs.  The facility was unable to obtain an industrial wastewater discharge permit to the local POTW and was faced with the alternative of trucking the rinse wastewater 2-3 hours to a treatment facility. A tanker truck would have to be available 24 hours per day during production runs to transport the wastewater.  The waste stream was anticipated to contain up to 90 mg/l of hydrogen peroxide and 120 mg/l of hydrofluoric (HF) acid.  Trace metals were also anticipated to be present. Siemens was asked to provide a solution to the manufacturer’s challenges for this waste stream. 

Solution

Siemens designed a process which first started with pH adjustment utilizing a standard NS-50-1 pH neutralization system.  This is a single stage pH neutralization system capable of maintaining a steady pH with varying flow rates.  pH adjustment was critical to maintain all of the HF in the fluoride anion form.  The pH adjusted water was then treated using activated carbon for the catalytic destruction of the hydrogen peroxide.  Hydrogen peroxide is detrimental to many polymeric membranes and must be maintained at a very low level of concentration. Trace organic components would also be removed by the activated carbon.
 
The next step was to remove the fluoride ion and any other trace metal ions from the solution.  This was accomplished by the utilization of a standard single pass reverse osmosis unit. The permeate from this unit was sent to a permeate collection tank which was then polished using ion exchange resin technology and returned to the process rinse water make-up.  The concentrate was fed to a brine reverse osmosis unit and concentrated further.  Permeate from this unit was also sent to the permeate collection tank and returned to the process.

Results

The initial design allowed for 97.5% recovery of the wastewater.  The system has been in operation for over 4 years.  The robustness of the system design has allowed the manufacturer to permit concentrations as high as 250 mg/l in hydrofluoric acid with no detrimental effect to the recycle system.  The manufacturer has since found a use for the 1.5-gallons per minute reject of the brine reverse osmosis in a scrubber system used onsite, thus making complete reuse of the water possible.