Recovering a System Severely Scaled with Calcium Sulfate

A blueberry farm in South America was using RO treated water from brackish water wells for irrigation. All local wells had extremely high sulfate concentrations.

The original wells which had been selected were abandoned in favor of different wells. However, the water quality of the new wells was unknown and the antiscalant selection and dosage had been based on the original water quality. Furthermore, there were several incidents of antiscalant dosing interruptions, and operation at a recovery rate beyond the design recovery. The feed pressures started to rise suggesting that scaling was forming, and differential pressure across the system climbed to 68 psi (4.7 bar). Permeate water quality was deteriorating as conductivity quickly climbed.

When an element was removed for membrane autopsy, it was found to weigh 129 lbs (58.5 kg). It was so severely scaled by calcium sulfate (gypsum) that feed channels were nearly completely closed off. The scale had torn into the membrane leaves, resulting in an irreversible loss in salt rejection. Lab testing was performed to determine the most effective cleaning protocol. AWC’s gypsum scale disintegrator, AWC C-238, was found to efficiently break apart the hard crystalline scales to an easily flushable slurry

With the help of one of AWC’s local partners, AWC C-238 was deployed onsite and used to clean the system. During the cleaning, the bag filters were manually cleaned every hour to remove the large masses of gypsum scale that had been released from the membrane elements.

System performance significantly improved post CIP, cutting differential pressure in the system by half without further degrading membrane rejection. Although the final permeate quality was not optimal, it was sufficient for the berry farm to continue using the permeate for irrigation until the membranes could be replaced. AWC C-238 successfully removed all calcium sulfate scale from membranes weighing >100 lbs (>45 kg), an accomplishment unique to AWC within the membrane water treatment industry.