Reducing silica scale on membranes at desalination plants

CHALLENGE: FLUX DECLINE, INCREASE IN PRESSURE DROP AND SALT PASSAGE
Silica scaling on membranes used in the desalination process is a major problem if water that passes through a desalination plant has high silica concentration. Scaling causes a flux decline and increases pressure difference and salt passage.

Silica scaling happens when the concentration of dissolved silica exceeds the solubility limit at the temperature and pH of solution. This is a major problem at Reverse Osmosis plants where silica concentration in feed water is at elevated range (e.g. 25 ppm) and recovery is high (e.g. more than 80%). At these levels, the silica in concentrate stream precipitates out of the aqueous phase.

SOLUTION: KEMIRA’S NEW ANTISCALANT TO CONTROL SILICA SCALING
Kemira KemGuard™ PB20412 antiscalant prevents scale formation by controlling the scaling environment in the Reverse Osmosis process, promoting conditions where scale will not form.
We ran a pilot trial at a desalination plant with a capacity of 110 000 m3/day. The feed water for the plant is mainly surface water and hot spring water.
The plant has a high concentration of silica in the feed (20–25 ppm) and high recovery (85-90%). Before Reverse Osmosis, water goes through coagulation, flocculation, sand filter, and cartridge filter. The Reverse Osmosis unit itself has three stages.

The pilot plant simulated the last of these stages. Feed water flow rate to the pilot
was 1.2 m3/h, inlet pressure 23 bar and recovery for a single 8-inch membrane
was about 90%. The performance of the antiscalant was monitored for a month.
Permeate flow, pressure difference, silica concentration, and conductivity of
permeate and concentrate were monitored and collected for further analysis.
After performance testing, the membrane autopsy was closely studied.

RESULTS: SILICA SCALING OF MEMBRANES SUCCESSFULLY INHIBITED
Based on the performance data (flux, pressure difference, and salt passage),
Kemira’s KemGuard™ PB20412 antiscalant prevented silica scale formation
efficiently. A close study of the membrane shows absence of silica both in
amorphous and crystalline form. Silica scale formation on the membrane was
inhibited as scale was dispersed in the aqueous phase. The pressure difference
over the membrane was not changed during the pilot trial.

Silica concentration in the pilot feed and concentrate during pilot trial

REFERENCES:
Gill, J.S., Inhibition of silica-silicate deposit in industrial waters, Colloids and surfaces A: physicochemical and engineering Aspects, 74 (1993) 101-106
Mickley, Michael C.; Straley, Martin; Coury and Associates, Glenn, An experimental project to determine the parameters of silica scaling in reverse osmosis systems: technical completion report, United States, Office of Water Research and Technology, National government publication, 1982.

For more information contact water@kemira.com and visit www.kemira.com/membrane-program