State of California requires homeowner’s only install salt based softeners that meet California salt efficiency standards i.e., ones installed after 2000 and 2002 (has higher efficiency rating than 2000). The reason for this is reduce the impact of the salt (sodium) which produces a lower quality of effluent and its impact on the environment.
Water Quality Association (an independent, highly recognized & respected nonprofit) foundation has published a research study detailing the environment impact of ion-exchange water softeners (salt-based softeners).
Bottom line summary of the study: Addition of regeneration wastes that contain large concentrations of excess sodium can, however, be detrimental to solids settling and, therefore, produce a lower quality effluent. Such higher concentrations can be from the operation of a softener in an inefficient manner (</= 1000 gr/lb) in relation to the excessive use of salt for the removal of hardness.
Even before this study was done, California initially in 2000 and 2002 mandated that residents only use salt efficiency certified softeners to address the potential effluent concerns arising due to excess discharge of sodium. Prior to this regulation softeners were very inefficient.
California code related to softeners;
"An appliance installed on or after January 1, 2000, shall be certified by a third party rating organization using industry standards to have a salt efficiency rating of no less than 3,350 grains of hardness removed per pound of salt used in regeneration. An appliance installed on or after January 1, 2002, shall be certified by a third party rating organization using industry standards to have a salt efficiency rating of no less than 4,000 grains of hardness removed per pound of salt used in regeneration."
These regulations ensure, softeners certified per California salt efficiency standards far exceed the recommendation from research study which say efficiency less than (</= 1000 gr/lb) are likely contribute to effluent That said, if the purpose of installing softener is to remove the scales, the only technology that physically removes the calcium and magnesium is Salt based softeners. So, it is totally up to the consumer on which way they would like to go. Ideally most I know would like to go with salt less but cost makes even environmentally conscious consumer to stay off them. The good thing is California has a regulation around salt efficiency.
The proponents of salt-based softeners say that there are environmental benefits of using Salt based softeners and there are studies in support of salt based system which primarily talk about lessened consumption of soap or chemicals to wash clothes or dishes etc., thereby reducing chemical pollution into the sewer / drains, savings on electrical bills etc. So, there is a case to be made on both sides. Given this, it is important when changing softener or installation new salt based softener to always go with softener's that meet California salt efficiency standards or higher. Hope this helps.