You can use Spectra SC-1 as a pickling/storage solution to keep the system growth free for 6 months. Propylene Glycol is used to keep the system growth free for up to one year, as well as offers some extra weather protection. Additional details are below.
Storage: SC-1 prevents biological growth when your system sits idle. It should not be used as a cleaning chemical, nor will it protect your system from freezing. An 8 oz. bag of SC-1 is mixed with three gallons of product or dechlorinated fresh water and circulated through the system. This treatment will protect the system for six months, after which the SC-1 treatment must be repeated. To use SC-1, follow the instructions for Storage Procedure outlined in your owner's manual. Spectra systems should be stored with Propylene Glycol if freezing is likely to occur. Propylene glycol can be used instead of Spectra SC-1 storage chemical for storage in any climate, and treatment is effective for one year. Propylene glycol is a food-grade antifreeze used to winterize RV's, boats, and cabins. Do not use ethylene glycol automotive antifreeze, which is toxic and will damage the system.
The propylene glycol formulations sold in marine and RV stores are usually diluted with water. The water remaining in the watermaker before the storage procedure will further dilute the antifreeze, reducing the microbial protection and increasing the temperature at which the mixture will freeze.
Antifreeze labeled "Minus Fifty" is a 25% solution and will begin to form an icy slush at about +15Degrees F (-10C) and will only provide burst protection to about Zero F (-18C). After a further 50% percent dilution by water remaining in the watermaker, "Minus Fifty" antifreeze will only protect from bursting down to about +25F (-4C). Therefore, if low temperature freezing protection is required a 60% or stronger antifreeze should be used. 60% solutions are labeled "Minus 100" and will provide burst protection to -15F (-27C) even after a fifty percent dilution with residual water. "Minus 200" formulations are pure propylene glycol.
Complete microbial protection requires a 25% solution of propylene glycol, so care must be taken that the solution remaining in the watermaker during long term storage is at least 25%, even if freeze protection is not required. For these reasons Spectra recommends that all pickling be carried out with a 60% or greater concentration.
Note: Propylene glycol can be difficult to flush from a membrane, especially after extended storage periods. This results in high salinity water (high PPM) and residual flavor in the product water. We recommend flushing the system WITH THE PRESSURE RELIEF VALVE OPEN for 4-6 hours after storage with propylene glycol—the longer the better. If, after extended flushing, you still experience low product water quality, cleaning with SC-2 usually removes all traces of propylene glycol and returns the salinity to the level it was before storage with propylene glycol.
See https://katadyngroup.ladesk.com/405233-SC-2--SC-3-AlkalineAcid-Cleaning-Procedure-Automated for SC-2 Procedure for an automated system or https://katadyngroup.ladesk.com/565186-SC-2--SC-3-AlkalineAcid-Cleaning-Procedure-ManualDeluxe for a manual system.