Traditionally septic system design has been pervue of the Health Dept. namely public health.  This because of diseases associated with human wastes.  Victims of our own success, we no longer have common disease outbreaks and so it seems not so important to insure that those who design these systems that scientifically treat wastewater to a degree that it can be released -day after day into the environment again.  This is a job that only a trained, certified individual can perform not because it's design is complicated engineering but because of the variety and subtle differences in soils.  The general public, as in most science, has no idea of the complexity of soils and their characteristics.  Appropriately trained and certified individuals must be employed for this task.